CryoSat-2 (Earth Explorer Opportunity Mission-2)
CryoSat-2 is the follow-on Earth Explorer Opportunity Mission in ESA's Living Planet Program. It replaces CryoSat, which was selected for development in 1999 and lost as a result of launch failure on October 8, 2005. CryoSat-2 will have the same mission objectives as the original CryoSat mission; it will monitor the thickness of land ice and sea ice and help explain the connection between the melting of the polar ice and the rise in sea levels and how this is contributing to climate change.
The original CryoSat mission was proposed by Duncan Wingham of the University College London (UCL) and an international science team. Duncan Wingham is also the mission PI. A nominal mission duration of three years is planned (excluding the commissioning and validation phases, which may last up to six months).
In Feb. 2006, ESA received the green light from its Member States to build and launch a CryoSat recovery mission, CryoSat-2, based on the same objectives as the original CryoSat mission. However, the design of the CryoSat-2 spacecrasft is being updated. The changes required to the design of CryoSat-2 were scrutinized from December 2006 to January 2007. The Δ-CDR (Critical Design Review) was completed on Feb. 1, 2007. 1) 2) 3) 4) 5)
A total of 85 improvements/modifications have been approved in the design of CryoSat 2 (of which 30-40% have been small software changes that make the satellite much easier to operate). The new key features of CryoSat-2 include the following items:
• The SIRAL-2 (SAR/Interferometric Radar Altimeter-2) design includes a full backup SIRAL system, in case the primary payload malfunctions. Once in orbit, a special algorithm will be used to convert data collected by the CryoSat-2 satellite to create more accurate ice maps. - As a result of the dual SIRAL payload and associated interfaces, and other improvements to reliability, there has been a knock-on effect to the design of the satellite. For example, the backup SIRAL system has to be kept warm while it is switched off - the additional heater power is provided by increasing the size of the satellite's battery.
• A heat-radiating panel is being added. The path of CryoSat-2's orbit means it will face extremes of temperature. The panel ensures the electronics are protected
• Solar panels on the satellite's back are being added to account for the additional power requirements. Unlike many spacecraft, CryoSat 2 does not have solar wings.
In addition to building the new satellite, a number of field experiments to support the CryoSat-2 mission, were conducted or are getting underway in the Arctic. First is the Arctic Arc Expedition, part of the IPY (International Polar Year) 2007-2008. 6) 7)
- Antarctic 2008/9 CroVEx campaign in the blue ice region (see Figure 1) in December 2008: German scientists from the Technical University of Dresden and the AWI (Alfred Wegener Institute) are spending up to four months venturing out onto the vast frozen reaches of what is known as the 'blue ice' region near the Russian Novo airbase in Dronning Maud Land in Antarctica. The aim is to take very accurate measurements of the surface topography, both from the air and on the ground to contribute to the validation program for CryoSat-2. 8)
In parallel to the efforts on the ground, the Alfred Wegner Institute (AWI) will be flying their POLAR5 aircraft across the blue ice site – starting just before Christmas and finishing before the New Year. From the plane, the AWI team will collect laser and radar height measurements along the very same tracks as the ground team. To do this they are using ESA's ASIRAS (Airborne Synthetic Aperture and Interferometric Radar Altimeter System), which simulates the measurements CryoSat.
Figure 1: Antarctica showing the location of the blue ice region where validation activities to support ESA's CryoSat mission (image credit: ESA)
- CryoVEx (CryoSat Validation Experiment) 2008 (3-week campaign in May 2008 in the far north of Greenland and Canada). CryoVEx 2008 is a continuation of a number of earlier campaigns that focus on collecting data on the properties snow and ice over land and sea. This year's campaign is a huge logistical undertaking as airborne, helicopter and ground measurements are being taken simultaneously in three different locations - out on the floating sea-ice north of the Canadian Forces Station Alert, on the Devon ice cap in Canada and on the vast Greenland ice cap. A Twin Otter is carrying two key instruments: ASIRAS (Airborne SAR/Interferometric Radar System), a radar altimeter that mimics the radar altimeter onboard CryoSat-2, and a laser scanner which maps the surface beneath the plan, and a helicopter with an on-board sensor that measures sea-ice thickness. 9)
- In the spring of 2007, an international team of scientists stationed in Svalbard, Norway and two polar explorers are crossing the North Pole on foot. Both teams are part of a common effort to collect vital data on the ground and from the air in support of ESA's ice mission CryoSat-2. The expedition's two Belgian explorers, Alain Hubert and Dixie Dansercoer, 'stepped' onto the sea ice off the coast of Siberia on March 1, 2007 each pulling a 130 kg sledge holding supplies and equipment. A parallel campaign by scientists from Germany, Norway and the UK is unfolding in the extreme northern archipelago of Svalbard, Norway. As part of the CryoVEx 2007 campaign (CryoSat-2 Validation Experiment), they are spending one month making measurements of snow and ice properties along long transects that crisscross the ice sheet surface.
- As the ground experiments are carried out, measurements are also being taken from the air by the Alfred Wegner Institute (AWI), Bremerhafen, Germany. The Dornier-228 aircraft carries the ASIRAS (Airborne SAR/Interferometric Altimeter System) instrument, which is an airborne version of the radar altimeter instrument onboard CryoSat-2. By comparing the airborne data with ground measurements scientists will test and verify novel methods for retrieving ice-thickness change from the CryoSat-2 satellite mission ahead of the launch. - ASIRAS was built by Radar Systemtechnik (RST) of Switzerland with the support of the AWI and Optimare for the implementation and operation on an aircraft. It was test flown in March 2004 over the snow and ice expanses of Svalbard.
- The CryoVEx 2006 campaign took place April/May, 2006 and consisted of coordinated airborne and ground activities in support of CryoSat-2 validation goals over three land validation sites (Devon Island in Canada, Central Greenland and Svalbard, Spitzbergen, Norway) and a series of ice experiments over Alert / Ellesmere Island, Canada.
- LaRA (Laser and Radar Altimeter) campaign in the Arctic region of Greenland and Svalbard: The D2P (Delay-Doppler Phase-monopulse Radar) instrument of JHU/APL participated in this campaign which took place in May 2002 under joint NASA/ESA sponsorship to support calibration and validation activities, and science investigations in advance of the CryoSat and ICESat missions. The D2P radar altimeter was flown aboard the NASA-P3 aircraft along with the ATM (Airborne Topographic Mapper) laser altimeters to collect simultaneous laser and radar altimeter (hence, the LaRA campaign) measurements over land and sea ice.
- CryoVEx (CryoSat Validation Experiment) campaign: As a follow-on to the LaRA campaign, the D2P system was flown again in 2003 under joint NASA/ESA sponsorship as part of the CryoVEx field campaign. As in 2002, simultaneous laser and radar altimeter measurements were collected in the Arctic.
Such painstaking ground work is necessary to be able of measuring ice thickness down to centimeter level (1-3 cm average) from space. This in turn may lead to a better understanding of the impact that changing climate is having on the polar ice fields. See also the D2P and ASIRAS descriptions on the eoPortal (along with the campaigns for the validation of the SIRAL instrument).
The CryoSat-2 spacecraft is being built and integrated by EADS Astrium GmbH of Friedrichshafen, Germany, as prime contractor of a consortium. The spacecraft structure consists of a long rectangular main platform body, surmounted by fixed solar arrays in the form of a tent. The spacecraft has neither deployable appendages nor any other moving parts except for thruster valves. The lower surface of this structure is permanently earth facing. All electronics are mounted on the nadir plate acting as radiator. The antennas used for radio communication, and the Laser Retroreflector, are mounted on this surface; an emergency antenna for command and monitoring is also fitted on top of the satellite between the solar arrays. The two SIRAL instrument antenna dishes are mounted on a separate rigid bench in the forward section of the S/C. In addition, a dedicated SIRAL radiator is mounted at the nose tip. 10)
Figure 2: Illustration of the CryoSat-2 spacecraft with thermal covers on the SIRAL antennas (image credit: ESA)
Figure 3: Alternate view of the CryoSat-2 spacecraft (image credit: EADS Astrium)
The spacecraft is 3-axis stabilized. A slight nose-down attitude of the S/C (6º) is chosen (using magnetorquers and 10 mN cold-gas thrusters) to ensure minimum attitude correction due to gravity-gradient disturbances. The S/C has dimensions of 4.6 m x 2.34 m x 2.20 m. The S/C mass is about 720 kg (including 36 kg propellant), the design life is 3.5 years (goal of 5.5 years). Spacecraft power generation is provided by two triple-junction GaAs solar arrays with an efficiency of 27.5% (two oriented solar panels), each panel provides power of 850 W at normal solar incidence. A PCDU (Power Control and Distribution Unit) provides onboard distribution. The energy is stored by a lithium-ion battery (60 Ah capacity).
The pointing requirements are the main design drivers for the AOCS: 11)
• High precision cross-track pointing knowledge of < 10 arcsec for SARIn mode (SARIn refers to the SAR interferometric mode).
• S/C attitude maintenance with a pointing accuracy of < 0.2º per axis and a pointing stability of < 0.005º for 0.5 s in the nominal Earth-pointing phase of the mission
• Provision of very low disturbances due to thruster activity to meet the very high precise orbit determination (POD) accuracy of the CryoSat orbit.
• The AOCS (Attitude Orbit and Control Subsystem) comprises the following elements:
- A cold gas system (RCS) for attitude control and orbit transfer and maintenance maneuvers, 16 attitude control thrusters (10 mN) and 4 orbit control thrusters (40 mN). Nitrogen is used as propellant (132 l tank). 12)
- A set of 3 magnetorquers is used for compensation of environmental disturbance torques in support of RCS. The MT30-2-GRC, originally developed and qualified for the GRACE mission by ZARM Technik GmbH, has been selected for CryoSat.
- A set of three star tracker heads (also a part of the payload) providing autonomous inertial attitude determination for the spacecraft. The multiple configuration makes the sensor system one-failure tolerant, except for the rare occurrence of simultaneous sun and moon blinding of two heads, to which the system software is tolerant. Consequently, two camera heads are operated in parallel at all times to cope with sun-blinding. In its acquisition mode, which takes 2-3 seconds, the star tracker calculates a coarse attitude by matching triangle patterns of stars with patterns stored in its catalog. Subsequently, in attitude update mode it calculates precise attitude at a rate of 1.7 Hz.
The star tracker attitude serves also as reference for determining the orientation of the SIRAL interferometric baseline. The orientation of the interferometric baseline needs to be very accurately measured in-flight: small errors in knowledge of the roll-angle translate into substantial errors in the elevation of off-nadir points. The HE-5AS star tracker of Terma A/S, originally developed and qualified for the NEMO (Navy EarthMap Observer) and FCT (Foreign Comparative Test) projects, is selected for CryoSat. It is a fully autonomous star tracker capable of delivering high-accuracy inertial attitude measurements from a lost-in-space condition with no external attitude information. The EOL performance of the star tracker is < 3.2 arcsec in the lateral axes and < 16 arcsec about the roll axis under worst-case conditions. 13)
The star tracker baffle has been designed to meet the required sun exclusion angle of 30º and the moon exclusion angle of 25º. These exclusion angles ensure together with the star tracker accommodation on the antenna bench that during the whole mission sun and moon can only blind one star tracker head at any time.
Figure 4: Photo of one star tracker camera head unit (image credit: ESA)
• A DORIS receiver is part of an overall system, for real-time measurements of satellite position, velocity and time. DORIS measures the Doppler frequency shifts of UHF and S-band signals transmitted by ground beacons. Its measurement accuracy is < 0.5 mm/s in radial velocity allowing an absolute determination of the orbit position with an accuracy of 2-6 cm (see DORIS and LLR description under sensor complement). - The DORIS system comprises a network of more than 50 ground beacons, a number of receivers on several satellites in orbit and in development, and ground segment facilities. It is part of the International DORIS Service, the IDS, which also offers the possibility of precise localization of user beacons.
• CESS (Coarse Earth-Sun Sensor) of CHAMP and GRACE heritage (a patented design of Astrium GmbH). Provides attitude measurements (<5º) with respect to the sun and Earth for initial acquisition and coarse pointing. The FOV of CESS is a full spherical one, i.e. no special search maneuvers are necessary to find the Earth or the sun. Its measurement principle is based The concept is based on temperature differences measured by 6 omnidirectional arranged sensor heads (PT1000 thermistors).
• A set of three three-axis fluxgate magnetometers are used for magnetorquer control and as rate sensors. They provide a measurement range of at least ± 60.000 nT with an accuracy of better than 0.5 % full scale.
The AOCS provides high pointing accuracy (a few tens of an arcsecond), knowledge and stability in nominal Earth-pointing. It also has to perform the orbit changes between the science and validation phase orbits. The AOCS uses inertial attitude measurements from the set of 3 star tracker camera head units and DORIS real-time navigation to convert the inertial attitude into Earth referenced attitude (star sensor FOV of 22º x 22º, ). - The RCS (Reaction Control Subsystem), developed at PolyFlex Space Ltd. (a Marotta UK Ltd. company), is a cold gas propulsion system for auxiliary attitude control (in which it provides deadband protection around the axis defined by the instantaneous geomagnetic field) and for orbit transfer and maintenance maneuvers. It has 16 attitude control thrusters of 10 mN each and 4 orbit control thrusters of nominally 40 mN each. A single high-pressure tank stores 36.2 kg of nitrogen gas at 278.6 bar. 14) 15)
The CDMU (Control and Data Management Unit), consisting of a processor and a hardware-based fault detection system, handles all on-board command and control functions including telecommand decoding and the AOCS processing functions (the OBC is based on the ERC-32 microprocessor). A MIL-STD-1553B communications bus is used as payload interface (for SIRAL and DORIS). The on-board solid-state memory has a capacity of 2 x 128 Gbit.
Figure 5: Block diagram of the major elements od the CryoSat-2 spacecraft (image credit: ESA)
• Experimental Rate Sensor: CryoSat-2 carries a small technology experiment as a passenger. This device is an attitude rate sensor based on MEMS (Micro-Electro-Mechanical-Systems) technology in which microelectronics and mechanical devices (in this case a sensor) are fabricated on the same substrate. The MEMS sensor detects attitude rate to provide the same function as a more traditional gyro and is based on a device widely used in in-car navigation systems. Three orthogonal MEMS sensors are mounted in the experiment, to measure 3-axis attitude rates. The unit is called MRS (MEMS Rate Sensor) in the CryoSat context. The goal is to provide a low-cost rate-sensor or gyro. The device is provided free of charge to CryoSat-2 in exchange for the flight opportunity (Ref. 10).
The measurement data are not used on-board and only sent in housekeeping telemetry to the flight control centre. Here they will be used as an additional data type in monitoring satellite dynamics during attitude transitions.
In the context of the technology program in which the MRS has been developed, it is called SiREUS-FExp, for European Silicon Rate Sensor Flight Experiment. - SiREUS is a compact and lightweight solid-state MEMS rate sensor which was developed in the context of an ESA technology technology program. The UK development team consisted of the following partner organizations: AIS (Atlantic Inertial Systems - formerly BAE Systems of Plymouth), SEA (Systems Engineering & Assessment Ltd. of Bristol), and SELEX-GALILEO a Finemeccanica owned company (formerly BAE Systems of Edinburgh). This development is based on the established BAE SYSTEMS automotive MEMS detector, however significant developments were required to meet the performance requirements while achieving compatibility of the electronics to the space environment and ensuring low recurring price. The partnership with a significant commercial provider such as AIS should be emphasized as a critical aspect of the program. 16) 17) 18)
Table 1: MRS (MEMS Rate Sensor) key requirements and current (2008) status
The SiREUS unit has met or exceeded the key performance requirements set at the start of the program. The unit does not contain any software; all control loops are implemented digitally in an FPGA. The SiREUS unit is fairly compact, but its size is currently dominated by analog electronics, not the MEMS. It may be cost effective to achieve a significant reduction in mass and volume, if this results in a match with many more customer requirements.
SiREUS has demonstrated that it is possible to construct multi-lateral programs to spin-in technology from non space industry organizations and to make significant improvements in the performance of the 'spin-in' technology. There are positive signs for the wider application of this technology in 'spin-off' programs. The instrument has a size of 100 mm x 100 mm x 70 mm.
Figure 6: Top view of MRS FExp front end PCBs (left) and view of the MRS Exp unit on the CryoSat-2 nadir panel (right), image credit: SEA
Table 2: Overview of spacecraft parameters
Launch: The CryoSat-2 spacecraft was launched on April 8, 2010 on a Dnepr vehicle from the Baikonur Cosmodrome, Kazakhstan.. The launch provider was ISC (International Space Company) Kosmotras. 19) 20) 21)
Note: The technical issue with the second stage of the Dnepr rocket that delayed the launch of ESA's Earth Explorer CryoSat-2 satellite in February 2010 has now been resolved – and the new launch date of 8 April has been set. The fuel reserve problem of the second stage surfaced a week before the scheduled launch date and after the 'space head module', encasing the CryoSat-2 satellite, had been mated to the rest of the rocket in the launch silo. Consequently, the space head was returned to the integration facilities pending an investigation and new launch date. 22)
The delay, from the planned launch date of Dec. 2009, is due to the limited availability of facilities at the Baikonur launch site in Kazakhstan, which is particularly busy at the moment. 23)
Satellite Orbit: Non sun-synchronous circular LEO orbit, mean altitude = 717 km, inclination = 92º, nodal regression of 0.25º per day. Ground track repeat cycle: 369 days (with 30 day pseudo subcycles). This configuration allows a sufficient coverage for the polar regions. The CryoSat mission requirements include:
• An orbit change is required during the mission with the objective to visit at least twice a validation orbit, approximately 6 km lower in altitude than the science phase orbit
• The payload must be operated in various modes, as a function of geographical region, such that the orbital operations, and data sets collected, on successive orbits are dissimilar
• The payload utilization demands very precise orbit and attitude restitution. Minimum operations of three years are required.
The CryoSat mission is aimed in part at gaining coincident coverage with the GLAS laser altimeter of the NASA ICESat mission. The following support phases are defined:
• Commissioning phase: The nominal duration is two months. During this phase the satellite and its payload are brought into a fully operating condition in its nominal orbit.
• Science phase: This includes a long-repeat cycle [a 369-day orbit (5344 revolutions) repeat phase will be used, with a 30-day subcycle]. The science phase is the nominal operational support mode of the mission. This orbit is designed to provide very dense orbit cross-overs above 72º of latitude, for use over the ice sheets. With coverage to 88º of latitude, all but a very small area of the land and marine ice fields will be within the coverage of the satellite. In addition, the 30 day subcycle provides approximately monthly coverage of the sea ice fluctuations.
• Validation phases: The objective is to conduct calibration or validation experiments that are at a fixed locations on Earth. In these phases the satellite may be placed into a 2-day repeat orbit. A validation phase may have a duration of up to 1 month, and there may be more than one during the mission lifetime. The measurements made by the satellite mission will need to be verified by ground-based experiments.
RF communications: The S-band link is used for all TT&C communications (2 kbit/s uplink and 8 kbit/s downlink). The physical downlink operates at 16 kbit/s but carries an overhead of error correction coding. The X-band downlink (center frequency of 8.100 GHz) provides a payload transfer rate of 100 Mbit/s. All onboard data are stored in the MMFU (Mass Memory and Formatting Unit) of 2 x 128 Gbit (EOL) capacity. Data arrive at the MMFU directly from the SIRAL instrument on a pair of fast IEEE 1355 standard serial links (SpaceWire for the two high-rate interferometric data channels) and via the MIL-STD-1553 bus for the low rate data channels. Data are also transferred from the CDMU and the DORIS over the MIL-STD-1553 bus. About 320 Gbit/day of onboard source data are being generated and transmitted to the ground.
Figure 7: The CryoSat-2 spacecraft and its instruments (image credit: ESA)
• The CryoSat-2 spacecraft and its payload are operating nominally in 2014.
• January 2014: Near the center of Antarctica, measurements from CryoSat-2 show an unusual pattern in the ice sheet’s elevation (Figure 8). Scientists have now found the reason for this pattern – and the discovery is leading to even more accurate measurements from ESA’s ice mission. 24)
CryoSat collects data over Antarctica while passing on northbound and southbound orbits. But the data show an unusual pattern of height differences where these orbit cross, radiating from the South Pole.
Initially it was reasoned that there could be an issue with the satellite itself, such as a miscalculation of the altitude, a timing error or a problem with one of the corrections we apply to the measurements. - After eliminating the possibility of these errors through careful experimentation, scientists discovered that the pattern was caused by the way the satellite signal is scattered from the ice sheet surface.
Antarctica has some of the strongest and most persistent winds on Earth, which leave permanent erosional and depositional features on the surface and in the snow pack. The scientists found that these wind-driven features modify CryoSat-2’s radar measurements in such a way as to produce the pattern that has been detected. - The pattern in Figure 8 is not an ‘error’, but an artefact arising from the interaction of the polarization of CryoSat’s antenna with the structure of the ice surface induced by wind.
It has long been known that wind-driven directional properties of the ice sheet surface can affect the signal received by radar altimeters, but has never been seen so clearly. The most striking feature of the pattern – the diamond ring pattern close to the pole – had not been seen by past altimeter missions because they did not fly far enough south.
Since the pattern appears to be stable over time, the data can easily be corrected, ensuring that CryoSat’s past and future measurements of Antarctica are precise. The discovery also helps scientists better understand the interaction between radar waves and ice sheet surfaces.
Figure 8: Antarctic artefacts: Elevation differences in Antarctica as measured by CryoSat-2 in Nov. 2013 and released on Jan. 20, 2014 (image credit: ESA, MSSL)
Legend to Figure 8: There is a distinct pattern of alternating high and low elevations (shown in red and blue), which inverts closer to the South Pole. After careful analysis, it was discovered that this is an artefact caused by the interaction of the polarization of CryoSat’s antenna with the structure of the ice.
• December 16, 2013: Measurements from CryoSat-2 show that the volume of Arctic sea ice has significantly increased this autumn. In October 2013, CryoSat-2 measured about 9000 km3 of sea ice – a notable increase compared to 6000 km3 in October 2012. This year’s multi-year ice is now on average about 20%, or around 30 cm, thicker than last year. While this increase in ice volume is welcome news, it does not indicate a reversal in the long-term trend. - It’s estimated that there was around 20 000 km3 of Arctic sea ice each October in the early 1980s, and so today’s minimum still ranks among the lowest of the past 30 years. 25)
• December 11, 2013: Three years of observations by ESA’s CryoSat-2 satellite show that the West Antarctic Ice Sheet is losing over 150 km3 of ice each year – considerably more than when last surveyed. The imbalance in West Antarctica continues to be dominated by ice losses from glaciers flowing into the Amundsen Sea. The ice thinning continues to be most pronounced along fast-flowing ice streams of this sector and their tributaries, with thinning rates of between 4–8 m per year near to the grounding lines – where the ice streams lift up off the land and begin to float out over the ocean – of the Pine Island, Thwaites and Smith Glaciers. 26)
The melting of ice sheets that blanket Antarctica and Greenland is a major contributor to global sea-level rise. An international team of polar scientists had recently concluded that West Antarctica caused global sea levels to rise by 0.28 mm each year between 2005 and 2010, based on observations from 10 different satellite missions. But the latest research from CryoSat-2 suggests, that the sea level contribution from this area is now 15% higher.
Figure 9: Three years of measurements from CryoSat show that the West Antarctic Ice Sheet is estimated to be losing over 150 km3 of ice each year (image credit: CPOM, ESA)
• During December 5-6, 2013, a major storm passed through northern Europe causing flooding, blackouts, grounding flights and bringing road, rail and sea travel to a halt. ESA’s CryoSat-2 satellite measured the storm surge from the recent North Sea storms, as high waters passed through the Kattegat sea between Denmark and Sweden. Since the storm coincided with a period of high tides in the North Sea, there were extremely high sea levels – a ‘storm surge’. In the UK, sea levels were at their highest since the 1953 North Sea Floods, while in Germany, parts of Hamburg were flooded. 27)
- On Dec. 6, CryoSat-2 passed over the Kattegat, providing an estimate of total water levels. The observations matched predictions, helping to confirm these models. The measurements were made by CryoSat’s radar altimeter that – although designed to measure sea-ice thickness – is providing outstanding results over sea and, especially, coastal areas.
- Until recently, altimeter measurements of sea-level height could only be made over open oceans, because of land interference closer to the coast. In the last few years, however, progress has been made in reducing these effects, also thanks to the new generation of radar altimeters being heralded by CryoSat-2. This has allowed scientists not only to map water levels closer to the coast, but also profile land surfaces and inland water targets such as small lakes, rivers and their intricate tributaries.
- Altimeter measurements from space can be used to validate storm surge models as well as provide near-realtime information that can be incorporated into predictions. Under ESA’s Data User Element, the 'eSurge' project is helping to optimize the use of altimetry and other types of satellite data to improve storm surge forecasting.
• Sept. 2013: CryoSat-2 has been in orbit since 2010 and with the satellite still in excellent health it is now set to continue providing precision measurements until 2017.
ESA’s CryoSat-2 mission has provided three consecutive years of Arctic sea-ice thickness measurements, which show that the ice continues to thin. Along with observations of ice extent, The measurements of CryoSat-2 thickness now span from October 2010 to April 2013, allowing scientists to work out the real loss of ice, monitor seasonal change and identify trends. CryoSat-2 continues to provide clear evidence of diminishing Arctic sea ice. 28)
Figure 10: Variations in spring ice thickness: Changes in ice thickness for March/April 2011, 2012 and 2013 as measured by CryoSat-2 (image credit: A. Ridout,UCL)
Figure 11: Variations in autumn ice thickness: Changes in ice thickness for October/November 2011, 2012 and 2013 as measured by CryoSat-2 (image credit: A. Ridout, UCL)
• In early July 2013, ESA is reporting that CryoSat-2 has found a vast crater in Antarctica’s icy surface. Scientists believe the crater was left behind when a lake lying under about 3 km of ice suddenly drained. 29)
Far below the thick ice sheet that covers Antarctica, there are lakes of fresh water without a direct connection to the ocean. These lakes are of great interest to scientists who are trying to understand water transport and ice dynamics beneath the frozen Antarctic surface – but this information is not easy to obtain.
By combining new measurements acquired by CryoSat-2 with older data from NASA’s ICESat satellite, the Cryosat team has mapped the large crater left behind by a lake, and even determined the scale of the flood that formed it.
Figure 12: Location of the crater in Antarctica (image credit: ESA)
The CryoSat science team analyzed the data acquired by the SIRAL (SAR Interferometer Radar Altimeter) instrument on CryoSat-2 and demonstrated its novel capability to track topographic features on the Antarctic Ice Sheet. The perimeter and depth of a 260 km2 surface depression was mapped above an Antarctic SGL (Subglacial Lake) and, in combination with ICESat laser altimetry data,the decadal changes were charted in SGL volume. During 2007-2008, between 4.9 and 6.4 km3 of water drained from the SGL, and the peak discharge exceeded 160 m3/s. The flood was twice as large as any previously recorded, and equivalent to ~ 10 % of the meltwater generated annually beneath the ice sheet. - The ice surface has since uplifted at a rate of 5.6 ± 2.8 m/yr. Our study demonstrates the ability of CryoSat-2 to provide detailed maps of ice sheet topography, its potential to accurately measure SGL drainage events, and the contribution it can make to understanding water flow beneath Antarctica. 30)
• The CryoSat-2 spacecraft and its payload are operating nominally in 2013. 31)
An international team of scientists using new measurements from Europe’s ice mission has discovered that the volume of Arctic sea ice has declined by 36% during autumn and 9% during winter between 2003 and 2012. A team of scientists led by University College London has now generated estimates of the sea-ice volume for the 2010–11 and 2011–12 winters over the Arctic basin using data from ESA’s CryoSat-2 satellite. This study has confirmed, for the first time, that the decline in sea ice coverage in the polar region has been accompanied by a substantial decline in ice volume. Since 2008, the Arctic has lost about 4300 km3 of ice during the autumn period and about 1500 km3 in winter. 32) 33) 34)
The team confirmed CryoSat-2 estimates using independent ground and airborne measurements carried out by ESA and international scientists during the last two years in the polar region, as well as by comparing measurements from NASA’s Operation IceBridge.
• December 2012: ESA’s ice mission is now giving scientists a closer look at oceans, coastal areas, inland water bodies and even land, reaching above and beyond its original objectives. The satellite’s radar altimeter not only detects tiny variations in the height of the ice, it also measures sea level and the sea ice’s height above water to derive sea-ice thickness with an unprecedented accuracy. At a higher precision than previous altimeters, CryoSat’s measurements of sea level are improving the quality of the model forecasts. Small, local phenomena in the ocean surface like eddies can be detected and analyzed. 35)
Taking CryoSat a step further, scientists have now discovered that the altimetry readings have the potential to map sea level closer to the coast, and even greater capabilities to profile land surfaces and inland water targets such as small lakes, rivers and their intricate tributaries.
• The CryoSat-2 spacecraft and its payload are operating nominally in 2012.
- May 2012: While the main objective of the CryoSat-2 mission is to measure the thickness of polar sea ice and monitor changes in the ice sheets that blanket Greenland and Antarctica, the radar altimeter, SIRAL (SAR Interferometer Radar Altimeter), is not only able to detect tiny variations in the height of the ice but it can also measure sea level. 36)
Recent studies at the Scripps Institution of Oceanography in San Diego, USA, found that the range precision of CryoSat-2 is at least 1.4 times better than the US's GEOSAT or ESA's ERS-1. They estimate that this improved range precision combined with three or more years of ocean mapping will result in global seafloor topography – bathymetry – that is 2–4 times more accurate than measurements currently available.
Most satellite radar altimeters, such as the one on the joint CNES/NASA/Eumetsat/NOAA Jason-2, follow repeated ground-tracks every 10 days to monitor the changes in ocean topography associated with ocean currents and tides. - On the other hand, the 369-day repeat cycle of CryoSat-2 provides a dense mapping of the global ocean surface at a track spacing of over 4 km. Three to four years of data from CryoSat can be averaged to reduce the ‘noise’ due to currents and tides and better chart the permanent topography related to marine gravity (Ref. 36).
- April 2012: After nearly a year and a half of operations, CryoSat has yielded its first seasonal variation map of Arctic sea-ice thickness (Figure 13). Results from ESA’s ice mission were presented at the Royal Society in London as part of the events celebrating the 50th anniversary of the UK in space. 37)
Figure 13: Produced from CryoSat-2 data, this map shows Arctic sea-ice thickness, as well as the elevation of Greenland ice sheet, for March 2011. For sea ice, green indicates thinner ice, while yellow and orange indicate thicker ice (image credit: CPOM/UCL/Leeds/ESA/PVL) 38)
- February 2012: Ocean measurements from ESA’s CryoSat-2 mission are being exploited by the French space agency, CNES, to provide global ocean observation products in near-real time. Understanding sea-surface currents is important for marine industries and protecting ocean environments. 39)
- January 2012: Although the primary objective of CryoSat-2 was to measure the thickness of ice, fast data delivery was not initially intended. The CryoSat team has changed this to demonstrate that CryoSat-2 can deliver marine information in near-real time from most of its orbits around Earth. Up to now, this new product called 'fast delivery mode' has only been provided to organizations such as the US NOAA (National Ocean and Atmospheric Organization). This is about to change: marine information is expected to be available systematically to all users from February 2012 onwards. 40)
At NOAA’s LSA (Laboratory for Satellite Altimetry), the CryoSat-2 data are processed to estimate wind speed and wave height, which are then provided to forecasters at NOAA’s NCEPs (National Centers for Environmental Predication). LSA combines CryoSat-2 data with information from other organizations such as CNES of France, the ECMWF (European Centre for Medium-Range Weather Forecasts) and NASA. This processing takes a matter of only three days. NOAA delivers these data to ocean modelers and forecasters worldwide. For example, Australia’s Integrated Marine Observing System now uses CryoSat observations of sea level to monitor surface currents.
Figure 14: The NOAA fast delivery product displays the estimate of wind speed over oceans using data from ESA’s CryoSat-2 mission from 17 Nov. to 13 Dec. 2011 (image credit: NOAA)
• Antarctic measurement campaign: In early December 2011, a team of Australian and German scientists from the University of Tasmania, the Australian Antarctic Division and the Alfred Wegner Institute (AWI, Bremerhaven) has just finished the first leg a remarkable measurement campaign. The campaign is being carried out in East Antarctica around Law Dome and the Totten Glacier. Law Dome is relatively stable but features steep surface slopes and Totten Glacier is changing rapidly – so both offer ideal locations for validating CryoSat-2 data. 41)
The campaign involves taking measurements from a Polar-6 aircraft. It carries the ASIRAS radar, which mimics CryoSat’s SIRAL. Ground-truth measurements are also collected for comparison. The skidoos drag GPS to map the height of the ice, which are later compared to the aircraft and satellite measurements. The skidoo team gathered ground measurement over about 250 km transects.
Figure 15: The plot shows preliminary processing of the ASIRAS data from a short 2 km section across Law Dome, Antarctica (image credit: ESA)
Legend to Figure 15: Strong layering as a result of seasonal changes in snow accumulation is clearly visible down to about 20 m. In combination with snow pit and firn-core data, scientists can determine the spatial variability of the accumulation rate (Ref. 41).
• The first map of sea-ice thickness from ESA’s CryoSat-2 mission was revealed at the Paris Air and Space Show on June 21, 2011. This new information is set to change our understanding of the complex relationship between ice and climate. CryoSat-2 has spent the last seven months delivering precise measurements to study changes in the thickness of Earth’s ice. 42)
CryoSat-2’s exceptionally detailed data have been used to generate this map of sea-ice thickness in the Arctic (Figure 16). Data from January and February of 2011 have been used to show the thickness of the ice as it approaches its annual maximum. Thanks to CryoSat-2’s orbit, ice thickness close to the North Pole can be seen for the first time.
CryoSat measures the height of the sea ice above the water line, known as the freeboard, to calculate the thickness. The data are exceptionally detailed and considerably better than the mission’s specification. They even show lineations in the central Arctic that reflect the ice’s response to wind stress.
Figure 16: The first map of sea ice thichness in the Arctic ocean (image credit: ESA, UCL)
A new map of Antarctica has also been created showing the height of the ice sheet (Figure 17). This is more preliminary because more data are needed here to see what CryoSat-2 can do. Even so, the extra coverage that CryoSat-2 offers near the poles can be demonstrated: parts of Antarctica can now be seen for the first time from space.
In addition, detail of edges of the ice sheet where it meets the ocean can now closely be monitored thanks to CryoSat’s sophisticated radar techniques. This is important because this is where changes are occuring.
Figure 17: Preliminary map of the Antarctica ice sheet (image credit: ESA, UCL)
• A month-long common ESA/NASA Arctic campaign was conducted in the spring of 2011 in support of ESA's CryoSat mission. The one-month Arctic expedition is a major undertaking, with scientific teams from numerous organizations braving temperatures of –30ºC in central Greenland, Svalbard and the Fram Strait, Devon Island and offshore from Alert, Ellesmere Island, in northern Canada. 43)
To ensure that CryoSat is delivering accurate data, the scientists are gathering a wealth of ice and snow measurements on the ground and from the air. These in situ measurements will be compared with measurements delivered by CryoSat, thereby guaranteeing that the mission is delivering the best quality data possible. Data on changes in the thickness of ice floating in the polar oceans and in the vast ice sheets on land are vital in the quest to deepen our understanding of the delicate relationship between ice, climate change and sea-level rise.
In parallel, NASA is also in the Arctic, surveying the polar ice cover from the air for their IceBridge operation (see IceBridge below).
Figure 18: Illustration of CryoSat-2 and aircraft flight lines (image credit: ESA)
This involved coordinated ESA–NASA flight activities and orbital ground track of CryoSat on 17 April. In Figure 18, the straight red and black thick line indicates CryoSat's ground track and the color changes where the measurement mode switches to pass over the steep ice sheet margin. The dark green straight line shows the 15 April CryoSat track, with the yellow line indicating the NASA P-3 aircraft underflight. The white line shows the paths taken by the Twin Otter carrying the ASIRAS instrument and the green lines show the AWI aircraft carrying the electromagnetic sensor, which take measurements of the area for comparison.
NASA IceBridge campaigns: IceBridge, a six-year NASA mission, is the largest airborne survey of Earth's polar ice ever flown. It will yield an unprecedented three-dimensional view of Arctic and Antarctic ice sheets, ice shelves and sea ice. These flights will provide a yearly, multi-instrument look at the behavior of the rapidly changing features of the Greenland and Antarctic ice. IceBridge will use airborne instruments to map Arctic and Antarctic areas once a year. The first IceBridge flights were conducted in March/May 2009 over Greenland and in October/November 2009 over Antarctica. Other smaller airborne surveys around the world are also part of the IceBridge campaign. 44) 45)
- IceBridge has flown more than 200 hours during science flights with the P-3 during the 2011 Arctic deployment, and only one canceled flight day due to weather. On April 29, 2011, the P-3 flew a medium-priority mission over the catchment area of Petermann Gletscher that when combined with two other surveys will result in a 10-kilometer grid over the glacier’s entire catchment area. 46)
The ground measurements, multiple airborne measurements and the CryoSat-2 overpass will create a landmark dataset to shed light on fundamental issues in remotely sensing sea ice.
• On Feb. 1, 2011, ESA announced at the CryoSat Validation Workshop (Frascati, Italy, Feb. 1-3, 2011) the release of the CryoSat-2 ice data. This means that the international science community will have free and easy access to all of the measurements from CryoSat-2. This will amount to a unique dataset to determine the impact climate change is having on Earth's ice fields. 47)
• The CryoSat-2 mission was declared operational on Nov. 19, 2010. - The results of the intense commissioning phase were presented to more than 80 scientists and engineers from ESA, industry and universities at the CryoSat-2 Commissioning Results Review, held in Noordwijk, the Netherlands, on 22 October. 48) 49)
• The commissioning phase of the CryoSat-2 mission is expected to last for about 6 months.
• In mid-July 2010, the project is already releasing access data to about 150 scientists of around 40 research institutes (users outside the project team) as part of the calibration and validation procedure. The intent is to help ensure that these measurements meet the mission's exacting standards before the data are released to the wider scientific community later this year. 50)
• Taking advantage of NASA's 'Operation Ice Bridge' campaign in April 2010, measurements of Arctic sea ice have been made from an aircraft flying directly under CryoSat-2's orbital path. These measurements offer an early opportunity to check the quality of the newly launched CryoSat-2 satellite data over sea ice. The campaign uses a DC-8 aircraft carrying the ATM (Airborne Topographic Mapper) laser, which sends pulses of light in circular scans to the ground. The pulses reflected back to the aircraft are converted into elevation maps of the ice surface below.
Since the campaign is being carried out in the Arctic, ESA and NASA seized the opportunity to collaborate by timing one of the DC-8's flights to coincide with CryoSat-2 orbiting above. The NASA aircraft flew from Thule in northwestern Greenland over the Arctic Ocean on 20 April to pass directly under the satellite orbiting close to the North Pole. In parallel, ESA's European Spacecraft Operations Centre (ESOC) in Germany switched CryoSat-2's SIRAL from 'health monitoring mode' into its sea-ice measuring 'SAR mode'. 51)
- The first IceBridge underflight of CryoSat-2 took place on April 20, 2010 along a 670 km track over the Arctic Ocean. The flight was carried in using a NASA DC-8 as part of NASA’s Operation IceBridge and was designated Sea Ice 07. The aircraft intercepted the CryoSat-2 groundtrack near 87º latitude, 205º longitude approximately 10 minutes after the satellite passed over (13:33:06 GMT) as shown in Figure 19. The DC-8 followed the groundtrack westward at an altitude of 7000 m for about one hour, covering 670 km. The DC-8 carried the usual complement of IceBridge instrumentation, including the DMS (Digital Mapping System) camera, and two laser altimeters: 1.) the LVIS ( Land, Vegetation and Ice Sensor), usually flown at high altitudes, and 2.) the ATM (Airborne Topographic Mapper), usually flown at lower altitudes. 52)
Figure 19: Arctic map showing flight path of Operation IceBridge Sea Ice 07 flight. Upper horizontal portion of red line follows the path of the CryoSat-2 groundtrack flown by the NASA DC-8 (image credit: NOAA, NASA)
• On April 11, 2010, the LEOP (Launch and Early Orbit Phase) was formally ended. The spacecraft is in excellent condition. Later on April 11, SIRAL (Synthetic Aperture Interferometric Radar Altimeter) was switched on for the first time and started gathering the first radar echo data. 53)
• CryoSat-2 has delivered its first data just hours after ground controllers switched on the satellite's sophisticated radar instrument for the first time. CryoSat-2 was launched on 8 April and has been performing exceptionally well during these critical first few days in orbit.
Sensor complement (SIRAL, DORIS, LRR)
SIRAL (SAR Interferometer Radar Altimeter):
SIRAL is the primary instrument of the mission, designed and developed for ESA by Thales Alenia Space (formerly Alcatel Alenia Space), France. SIRAL is of Poseidon-2 heritage flown on the Jason-1 mission. The objective is to observe ice sheet interiors, the ice sheet margins, for sea ice and other topography. 54)
The SIRAL-2 design is based on existing equipment, but with several major enhancements designed to overcome difficulties associated with measuring ice surfaces. It works by bouncing a radar pulse off the ground and studying the echoes from the Earth's surface. By knowing the position of the spacecraft - achieved with an onboard ranging instrument called DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite) - the signal return time will reveal the surface altitude. Correct antenna orientation is vital for this and is maintained using a trio of star trackers.
The design of SIRAL for the CryoSat-2 mission was made completely redundant. 55)
Figure 20: A perspective view of the nose of the CryoSat-2 with the SIRAL units (image credit: ESA)
The SIRAL instrument design makes use of the DDA (Delay Doppler Altimeter) concept representing a new technology introduction into spaceborne altimetry and permitting that detailed views of irregular sloping edges of land ice, as well as non-homogenous ocean ice, can also be obtained. The new features of SIRAL have been demonstrated with the airborne D2P (Delay-Doppler Phase-monopulse Radar) of JHU/APL, first test flights were conducted in 2000 (for a DDA concept introduction see the last chapter of the description).
The SIRAL design features two receiving antennas forming an interferometer in the cross-track direction with a baseline of 1.2 m (support for SARIn mode). In addition, the return signal in along-track direction is processed to construct a synthetic aperture for enhanced ground resolution. The instrument is a Ku-band radar altimeter which uses the full-deramp range compression technique of conventional altimeters (conventional single frequency pulse-limited altimeter). However, it introduces two features that make it different from previous spaceborne altimeter implementations: 56) 57) 58) 59)
• The instrument has two parabolic antennas (including pulse-to-pulse phase coherence) and two receive chains, permitting an interferometric mode of operation (and interferometric signal processing).
• SIRAL operates at high PRF (Pulse Repetition Frequency), ensuring coherent along-track sampling for aperture synthesis (PRF>Doppler bandwidth). The distinguishing feature of SIRAL compared to conventional altimeter instruments (generally with pulse intervals of about 500 µs) is that it sends bursts of pulses separated by intervals of only 50 µs. Though the return echoes are correlated, the bursts are instead treated using “aperture synthesis” data processing techniques.
Figure 21: Illustration of the SIRAL instrument electronics (image credit: Thales Alenia Space)
The instrument consists of three major subsystems, two of these are in discrete electronic boxes:
• DPU (Digital Processing Unit), it serves all digital altimeter functions, including the digital chirp generation, the full sequencing functions of the altimeter, and the receive and processing functions of the echo
• RFU (Radio Frequency Unit). It contains all analog IF and RF electronics and a solid-state power amplifier with an RF peak power of 25 W.
• The antenna subsystem consists of two Cassegrain antennas, mounted side-by-side and forming the interferometric cross-track. Both antennas are identical; one is used to transmit and receive, whereas the other antenna is used to receive echoes (bistatic configuration). The primary super-elliptic reflectors are about 1.1 m x 1.2 m in size. They are supported by a composite sandwich plate. A high thermoelastic stability is needed to meet the interferometric instrument performance.
Table 3: SIRAL key instrument parameters
Figure 22: Block diagram of the SIRAL instrument (image credit: ESA)
The science requirements demand of CryoSat to measure variations in ice thickness of perennial sea and land ice fields to the limit allowed by natural variability, on spatial scales varying over three orders of magnitude. The natural variability of sea and land ice depends on fluctuations in the supply of mass by the atmosphere and ocean, and snow and ice density. The precisions of the measurements are expressed in terms of cm of yearly ice equivalent thickness variations. These are:
• Arctic sea-ice: 1.6 cm/year vertical measurement accuracy at 105 km2 scale (equivalent to 300 km x 300 km cells). Temporal sampling: 1 month
• Land ice (small scale): 3.3 cm/year at 104 km2 (equivalent to 100 km x 100 km cells). Temporal sampling : 1 year
• Land-ice (large scale): 0.17 cm/year over 13.8 x 106 km2 (about the area of Antarctica). Temporal sampling: 1 year.
Table 4: Overview of measurement goals
The monitoring of the interferometric behavior of the receive chains is ensured by a dedicated interferometric calibration mode that can be used as an operational mode. An additional calibration mode permits the measurement of the amplitude/phase distortions of each receive chain. By using an internal frequency synthesizer, this measurement can be done for several frequencies inside the IF bandwidth. In addition, different gain settings can be used, which makes it possible to accurately determine the gain of the receiver. Either of the receive chain (chain 1 or 2) may be selected in LRM or in SAR support modes. This in-flight capability increases the knowledge of the instrument contribution on the echo measurement.
Figure 23: SAR observation principle of SIRAL (image credit: ESA)
Figure 24: The footprint of the radar beam in the target region (image credit: ESA)
The chirp generator is composed of a digital pulse generation section, operating with a sampling rate of 160 MHz, followed by an analog multiplier section expanding the pulse bandwidth by a factor of 16, up to 350 MHz. This configuration ensures the pulse-to-pulse coherence required for the SAR modes. Parameters: chirp frequency = 4.08 GHz; bandwidth = 350 MHz; signal duration = 51 µs; SNR=30 dB.
The SSPA (Solid-State Power Amplifier) is composed of four parallel hybrid amplifiers in PHEMT technology. Parameters: frequency = 13.575 GHz; peak power >25 W; gain = 9 dB; amplitude ripple = 0.2 dB in 350 MHz; efficiency = 24%. - The FFT (Fast Fourier Transform) module, needed for the on-board tracking algorithm to estimate range and gain commands, makes use of the existing FFT module of POSEIDON 2.
SIRAL operational modes:
SIRAL provides the following operational modes for different observational support types. The complex waveform data stream from the CryoSat altimeter requires a sophisticated processing scheme in particular for exploiting the synthetic aperture and interferometry techniques over ocean and ice surfaces.
1) LRM (Low Resolution Mode) operation support: LRM uses a single receive channel and low PRF for conventional pulse-limited operation for ice sheet interiors/open oceans. The transmitted pulse length and the transmitted bandwidth are set to the same value as that for Envisat in a similar mode (51 s, 320 MHz). The PRF is kept constant over the orbit at a value around 2 kHz to ensure the decorrelation of received echoes. The averaging for tracking and ground processing is performed after the FFT (Fast Fourier Transform).
The LRM mode is useful over surfaces where the topography is homogeneous, at least as large as the antenna footprint of about 15 km. The altimeter echoes have a predictable shape and the mean surface level of this area can be derived by an appropriate model.
2) SARM (Synthetic Aperture Radar Mode) support mode (also referred to as advanced SAR mode): SARM uses a single channel and a high PRF. Closed burst timing is employed to ensure a high along-track resolution. The PRF is chosen higher than the Doppler bandwidth over the half-power beamwidth to avoid aliasing in the ground processing of the data (the PRF is about 10 times higher than that of LRM to ensure coherence between the echoes of successive pulses). Bursts of 64 pulses at a PRF of 18.5 kHz with a burst repetition frequency of 85 Hz are transmitted.
In SARM, the resolution of the radar is improved in the along-track direction. This is achieved by exploiting the Doppler properties of the echoes as they cross the antenna beamwidth. The result is equivalent to decomposing the main antenna beam into a set of 64 narrower synthetic beams in the along-track direction. The footprints of the different sub-beams over a flat surface are adjacent rectangular areas, about 250 m wide in along-track and as large as the antenna's cross-track footprint (up to 15 km). Hence, a larger number of independent measurements are available over a given area; this property is used to enhance the accuracy of the measurements over sea ice. The echoes are transmitted to the ground segment in the time domain, prior to any averaging. Hence, the data rate in SARM is significantly higher than that for LRM.
3) SARIn (SAR Interferometric) support mode. The objective is to provide improved elevation estimates over variable topography. This mode is used mainly over ice sheet margins with high surface slopes. Both receive channels are operating simultaneously at high PRF to ensure the availability of a high cross-track resolution used for ice sheet margins and coastal areas (accurate determination of the arrival direction of the echoes in along-track and in cross-track). This is needed to derive the height of the surface from the range measurement of the radar. Narrow-band tracking pulses, transmitted in-between successive wide-band measurement bursts are used in this range-tracking concept to cope with abrupt height variations.
In the SARIn mode, the addition of the interferometric feature to the SAR further improves the echo localization capabilities, as the cross-track direction angle of the echoes can be determined. This is achieved by comparing the phase of one receive channel with respect to the other.
The innovative technical features of SIRAL are:
• The capability to operate in all measurement modes
• Digital chirp generation with pulse-to-pulse coherence for Doppler processing
• Solid State Power Amplifier (SSPA) in Ku-band with high performance (25 W),
• Dual antennas forming an interferometer, mounted on an optical bench together with star-tracker heads, ensuring the accurate knowledge and stability of the interferometric baseline orientation
• Two receive chains matched together with very low distortions.
The novel feature of SIRAL, as compared with conventional altimeters, is the capability to locate a resolution cell in the 3 dimensional space. The SIRAL concept is based on a Ku-band nadir-looking radar which can be operated in the conventional mode over oceans. Over terrain (ice or land) the “advanced SAR mode” uses Doppler filtering for the enhancement of the along-track resolution. A second antenna and receiving channel provides a second take of the scene which is used for surface height retrieval as it is usually done with SAR interferometry.
Table 5: Summary of instrument parameters for operational mode support
Operations principle: Conventional radar altimeters send pulses with a long interval : about 500 µs. SIRAL sends a burst of pulses with an interval of only 50 µs between them. The returning echoes are thus correlated, and by treating the whole burst of pulses in one operation, the data processor can separate the echo into strips arranged across the track by exploiting the slight frequency shifts (caused by the Doppler effect) in the forward- and aft-looking parts of the beam. The strips laid down by successive bursts can therefore be superimposed on each other and averaged to reduce noise. This mode of operation is called the SARM (Synthetic Aperture Radar Mode). 60)
Figure 25: Schematic view of conventional LRM operations (left) and Delay-Doppler SARM operations of SIRAL (right), image credit: R. K. Raney, JHU/APL
In interferometric mode (SARIn), a second receiving antenna is activated to measure the arrival angle. It enables to receive some radar echos coming from a point not directly located beneath the satellite. The difference in the path-length time of the radar echos is tiny between radar echos on the track and radar echoes out of the track. The measure of the angle between the baseline joining the antennas and the echo direction is essential and must be very accurate. The baseline orientation is so operated by three star trackers.
The Cryosat-2 mission is the first altimeter mission to operate the SARM (SAR mode), next to the LRM (Low Resolution Mode), in the SIRAL (SAT Interferometer Radar Altimeter) instrument.
Figure 26: Artist's view of the CryoSat-2 observation concept (image credit: EADS Astrium)
DORIS (Doppler Orbitography and Radiopositioning Integration by Satellite):
DORIS measures the Doppler frequency shifts of both VHF and S-Band signals transmitted by ground beacons. Its measurement accuracy is better than 0.5 mm/s in radial velocity allowing an absolute determination of the orbit position with an accuracy of 2-6 cm. DORIS is an uplink radio frequency tracking system based on the Doppler principle. The CNES instrument provides accurate measurements for a precise orbit determination. Knowledge of the orbit is essential for exploitation of the altimeter data and the overall performance. The onboard receiver measures the Doppler shift of uplink beacons in two frequencies (2.03625 GHz for Doppler measurement and 401.25 MHz for the ionospheric correction) which are transmitted continuously by the ground stations. One measurement is used to determine the radial velocity between spacecraft and beacon, the other to eliminate errors due to ionospheric propagation delays. The 401.25 MHz frequency is also used for measurements of time-tagging and auxiliary data transmission. The DORIS instrument comprises:
• A fixed omni-directional dual-frequency antenna
• A receiver performing the Doppler measurements every ten seconds. The nominal mode of operation is an autonomously programmed mode in which the receiver tracks the beacon signals according to information provided by the navigation software (DIODE) based on an on-board table of beacon data.
• An USO (Ultra Stable Oscillator) delivering the reference frequency with a stability of 5 x 10-13 over a period of 10 to 100 s.
The mass of DORIS is 15 kg (including the antenna of 160 mm diameter). The instrument requires 20 W of power, the data rate is 4 kbit/s.
The following DORIS services are used for CryoSat operations:
- Real-time orbit determination for spacecraft attitude and orbit control (on-board)
- Provision of a precise time reference based on TAI (International Atomic Time); in addition a precise 10 MHz reference signal is used (on-board)
- Provision of on-ground POD (Precise Orbit Determination) and ionospheric modelling.
The entire DORIS system comprises a network of more than 50 ground beacons, a number of receivers on several satellites in orbit and in development, and ground segment facilities. It is part of IDS (International DORIS Service), which also offers the possibility of precise localization of user-beacons.
LRR (Laser Retroreflector):
LRR is a passive optical device. The objective is to use LRR as an additional tool and backup for precise orbit determination with the aid of the international laser tracking network. LRR is accommodated in the nadir plate of the spacecraft, its FOV of ±57.6º is suitable for range measurements above 20º elevation angles at all azimuths from the ground. For any aspect angle the predicted rms target error is below 6 mm.
Figure 27: Illustration of the LRR system (image credit: ESA)
Table 6: Performance characteristics of one LRR
Introduction of the DDA (Delay Doppler Altimetry) technology into the SIRAL design
The concept of DDA, initially proposed by Keith Raney of JHU/APL (Johns Hopkins University/Applied Physics Laboratory), represents a new technology introduction with the potential to greatly increase the value of observations from satellite radar altimetry. The DDA scheme takes advantage of the Doppler shift of the pulse frequency in the along-track direction to allow for an increase in pulse repetition frequency and a subdivision of the illuminated area along-track into discrete Doppler bins to provide a dramatic improvement in efficiency and precision. 61) 62)
A conventional pulse-limited altimeter independently averages many radar pulses as the spacecraft moves along its track during the averaging time window and its illuminated area becomes defocused with increasing significant wave height. The relatively slow repetition of pulses and the impact of the waves limit the available resolution of the instrument.
Figure 28: Comparison of a conventional pulse-limited radar altimeter’s (a) illumination geometry (side view) and footprint (plan view) and (b) impulse response, with a delay/Doppler altimeter’s (c) illumination geometry and footprint and (d) impulse response (image credit: JHU/APL)
The DDA (Delay-Doppler Altimeter) differs from a conventional radar altimeter concept in that it exploits coherent processing of groups of transmitted pulses and the full Doppler bandwidth is exploited to make the most efficient use of the power reflected from Earth's surface. While the conventional altimeter technique is to measure the distance between the satellite and the mean ocean surface, the DDA method differs from those instruments in two ways: 63) 64) 65) 66)
- Pulse-to-pulse coherence and full Doppler processing to allow for measurement of the along-track position of the range measurement
- Use of two antennas and two receiver channels that allow for measurement of the across-track angle of the range measurement.
This is a significant improvement over conventional Doppler beam sharpening. To exploit this full bandwidth, the range variation that exists across the Doppler bins is removed as part of the data processing. The reflected pulses from a given area of the observed surface are integrated over the entire time that the target area is within the radar beamwidth. As a result, much more of the reflected energy is captured and a smaller transmitted power is required to obtain a given level of performance.
The DDA concept (Figure 28 b) retains the inherent advantages of a pulse-limited altimeter with its spherical wavefront always providing a nadir component, thus avoiding instrument nadir-pointing errors. In addition, the DDA exploits the faster pulse repetition frequency by binning the Doppler frequency shifts in the along-track direction. These bins appear as narrow strips orthogonal to the satellite ground track. As the DDA moves along its path, the leading edge Doppler bin illuminated during the first pulse becomes the second Doppler bin during the next pulse and receives a second “look” by the instrument. This process repeats as long as the bin remains within the DDA footprint. Each pulse defines a new leading edge Doppler bin, re-samples each bin within the footprint, and integrates the retrievals as the satellite moves along its track. Since each bin is sampled many times, the samples can be coherently processed and the higher pulse repetition frequency provides for a higher resolution footprint along-track that is independent of the significant wave height. For example, a 30 Hz altimeter pulse provides a signal integration length that results in widths of the Doppler bins as narrow as 250 m.
The DDA technology provides several advantages over conventional altimetry. The sea surface height precision available from this type of instrument is approximately twice that of existing sensors. Simulations of the associated signal processing concepts have produced 0.5 cm precision in a calm sea, with precision remaining better than 1.0 cm even in significant wave heights as great as 4 m. The DDA technique is much less sensitive to errors induced by ocean waves. For a calm sea, DDA and conventional altimetry experience comparable levels of random noise; however, as the waves grow, a conventional altimeter experiences a dramatic noise level increase. With the coherent processing of the DDA, only a slight increase in random noise with wave height is experienced. This makes the DDA particularly well suited for geodetic applications where the random error due to ocean waves is the dominant error source. Wind speed and wave height retrievals from the DDA have twice the precision of current sensors.
Another advantage of DDA is the ability to sample the coastal ocean where conventional altimeters experience signal contamination from land. As the spacecraft approaches or departs a coastline where the angle of intersection with the satellite ground track is nearly orthogonal, on board processing can identify individual Doppler bins close to the coast and continue to sample it as the satellite passes over the boundary.
From a system architecture perspective, the efficiency of the DDA provides for less transmitted power by the instrument and the potential for smaller and lighter spacecraft components - and thus a less costly mission - when compared to conventional altimeters with a similar design life.
The CryoSat mission will be operated from ESA/ESOC, Darmstadt, Germany. The Kiruna ground station in Sweden functions as the prime command and data acquisition facility. The payload data segment (data processing, archiving and distribution) function is also located at the Kiruna station. The ground segment makes use of the existing infrastructure. All user interfaces are coordinated via ESA/ESRIN with dissemination of data from Kiruna.
An important aspect of the CryoSat-2 ground segment is that it had been designed for operations with a low level of manpower. Furthermore, remote operations and troubleshooting can be performed on all systems located in Kiruna.
The major elements of the ground segment are:
• RPF (Reference Planning Facility): responsible for the planning of the payload and the satellite resources verification.
• FOS (Flight Operations Segment): responsible for the telecommand scheduling, satellite command and control, and telemetry acquisition.
• PDS (Payload Data Segment): responsible of scientific data processing, archiving and distribution
• MF (Monitoring Facility): responsible for providing measures of the performance of the system, and in particular of the instruments.
• Complementary supporting ESA elements, shared with other missions:
- USF (User Services Facility)
- LTA (Long-Term Archive)
• Other elements outside ESA:
- DORIS control and processing centre (SSALTO) which provides precise orbits
- Auxiliary data providers (e.g. meteorological data)
- SLR (Satellite Laser Ranging) stations.
• User community, calibration, validation and retrieval.
Figure 29: Overview of the ground segment infrastructure (image credit: ESA)
5) Richard Francis, “ESA's Ice Mission, CryoSat: more important than ever,” ESA Bulletin Nr. 141, Feb. 2010, pp. 10-19
6) “Scientists and polar explorers brave the elements in support of CryoSat-2,” SpaceRef, April, 19, 2007, URL: http://www.spaceref.com/news/viewpr.html?pid=22417
10) “CryoSat Mission and Data Description,” Jan. 2, 2007, ESA/ESTEC, URL: http://esamultimedia.esa.int/docs/Cryosat/Mission_and_Data_Descrip.pdf
11) T. , N. Duske, S. Schulz, “The CryoSat AOCS - A cost-efficient design for small Earth Observation satellites,” 5th International ESA Conference on Guidance Navigation and Control Systems, Frascati, Italy, Oct. 22-25, 2002, ESA SP516
12) Stefano Pessina, Susanne Kasten-Coors, “In-Flight Characterization of CRYOSAT-2 Reaction Control System,” Proceedings of the 22nd International Symposium on Space Flight Dynamics (ISSFD), Feb. 28 - March 4, 2011, Sao Jose dos Campos, SP, Brazil, URL: http://www.issfd22.inpe.br/S7-Attitude.Dynamics.1-AD1/S7_P4_ISSFD22_PF_041.pdf
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.