Jason-2 / OSTM
Jason-2 is a follow-on satellite to the joint CNES/NASA oceanography mission Jason (or Jason-1, with a launch Dec. 7, 2001). Jason-1, in turn is a follow-on mission of TOPEX/Poseidon, the ocean surface topography since its launch in 1992 (operations ceased on Oct. 9, 2005).
At NASA and at NOAA, the Jason-2 mission is also referred to as OSTM (Ocean Surface Topography Mission). Initially, NASA/JPL planned OSTM to come with a promising new experimental instrument and interferometric radar configuration (an additional feature to the core mission) called WSOA (Wide Swath Ocean Altimeter). The objective was to demonstrate ocean topography over 200 km swaths (parallel to the regular altimeter mission). 1) 2) 3)
However, this WSOA option was cancelled by NASA in the spring of 2005 due to project cost overruns. Hence, the agreed-upon Jason-2 mission under development in 2005 will simply consist of the core mission configuration. 4)
The science objectives of Jason-2/OSTM are to extend the time series of ocean surface topography measurements to: a) obtain a continuous record of observations (with the previous missions), b) to determine the variability of ocean circulation at decadal time scales from combined data record with T/P and Jason, c) improve the measure of the time-averaged ocean circulation, d) improve the measure of global sea-level change, and e) improve open ocean tide models.
The mission objectives call for the provision of the same measurement accuracy of Jason (3.3 cm) with a goal of achieving 2.5 cm, and to maintain the stability of the global mean sea level measurement with a drift less than 1 mm/year over the life of the mission. The overall goal is to better understand the forces behind global changes of climate and to predict seasonal anomalies in weather patterns; this is vital to understand the physics of the ocean.
Introduction of new operations concept for Jason-2/OSTM:
While spacecraft operations of Jason-1 are still being conducted by NASA and CNES - the operational function of Jason-2 will be carried out by NOAA and EUMETSAT, respectively (the operators of US and European weather satellites). This programmatic switch of service support from research institutions to operational institutions is a definite sign of a service mature enough to become operational. 5) 6) 7) 8)
The four partners of the Jason-2 mission have agreed on mutual cooperation and division of responsibilities. Project management is still in the hands of NASA and CNES, but now with the support of NOAA and EUMETSAT as partners. This new program organization is expected to reinforce the transition of the Jason-2 mission towards operational applications in the fields of oceanography, marine meteorology, seasonal prediction and climate monitoring.
CNES is providing the Proteus bus of Jason-2, including the altimeter and the DORIS receiver, while NASA delivers the radiometer, the GPS Receiver, and the LRA (Laser Retroreflector Array). NASA procures the launcher and launch services and CNES is in charge of overall system integration. EUMETSAT and NOAA provide Earth terminals and part of the ground network. In addition, they are responsible for near-real-time product processing, archiving, product dissemination and user support. The OSTM Science Working Team selected by the four agencies assists the project in science algorithm development and further calibration and validation activities.
Figure 1: Artist's view of the Jason-2 spacecraft (image credit: CNES)
Jason-2 employs the Proteus bus of CNES/Thales Alenia Sapce and a payload module, with Thales Alenia Space as the prime contractor of the spacecraft. Jason-2 is three-axis stabilized and nadir pointing - maintained by reaction wheels and magnetic torque rods. Power (580 W) is provided by two solar panels. A hydrazine propellant system is being used for orbital maintenance. Jason-2 has a launch mass of about 550 kg; the design life is 5 years. 9)
Table 1: Parameters of the Jason-2 spacecraft
Figure 2: Alternate view of the Jason-2 spacecraft (image credit: CNES, NASA)
RF communications: Downlink data rate at 838 kbit/s (S-band, QPSK modulation), uplink at 4 kbit/s (S-band). The CCSDS communication protocol standard is used in the forward and return link mode (use of virtual channels). Convolutional coding is also applied to telemetry.
Figure 3: Schematic view of Jason-2 instrument allocations (image credit: CNES/AVISO)
Launch: A launch of Jason-2/OSTM took place on June 20, 2008 on a Delta-2-7320-10 launch vehicle from VAFB, CA, USA. The launch provider was ULA (United Launch Alliance).
Orbit: Circular non-sun-synchronous orbit; 1336 km altitude (2 hour period), inclination = 66.038º, 9.9-day repeat orbits (127 revolutions), ground track repeatability = ±1 km cross-track at the equator. The drift of the orbital plane with respect to the inertial reference frame is -2º per day.
Jason-2 is scheduled to join Jason-1 in the same orbit with a 10 day repeat observation cycle(9.9156 days to be precise, i.e., 10 days minus two hours). Both satellites will pass within minutes of each other over the same ocean surface, thus enabling verification and cross-calibration of the collected data. Together, they will provide a vital contribution to the expanding network of global ocean observations and their application in meteorology, operational oceanography and climate monitoring.
Figure 4: Orbits of the Topex/Poseidon, Jason-1 and Jason-2 satellites, and the mission control center at CNES in Toulouse (image credit: NASA/JPL)
• The Jason-2/OSTM spacecraft and its payload are operating nominally in 2014.
• June 2013: The 2013 Senior Review evaluated 13 NASA satellite missions in extended operations: ACRIMSAT, Aqua, Aura, CALIPSO, CloudSat, EO-1, GRACE, Jason-1, OSTM, QuikSCAT, SORCE, Terra, and TRMM. The Senior Review was tasked with reviewing proposals submitted by each mission team for extended operations and funding for FY14-FY15, and FY16-FY17. Since CloudSat, GRACE, QuikSCAT and SORCE have shown evidence of aging issues, they received baseline funding for extension through 2015. 10)
- The mission extension of Jason-2/OSTM is rated as Low Risk. The Technical Review Panel has identified 2 Major Strengths, 2 minor strengths, no Major Weaknesses and no minor weaknesses that influence the risk determination. The Jason-2/OSTM instrument systems continue to perform very well and all retain full redundant. The OSTM bus has operated exceptionally well providing high confidence that it will remain fully functional during the proposed mission extension period until the end of 2015. - Also, the 3 passenger payloads: Carmen-2 (Environment Characterization and Modelisation-2), LPT (Light Particle Telescope), and the T2L2 (Time Transfer by Laser Link) continue to perform very well with redundancy.
• June 20, 2013, marked the fifth anniversary of the launch of the OSTM (Ocean Surface Topography Mission) on the Jason-2 satellite from Vandenberg Air Force Base in California. In just a few hours after its early morning launch, the spacecraft had made its first full revolution around Earth. The satellite has now made more than 25,550 orbits around our planet, keeping its highly accurate radar altimeter tuned to measure changes in the dynamic topography of our ocean. Day-by-day, month-by-month, and year-after-year, Jason-2 has continued to seamlessly add to the more than 20-year record of global sea surface height measurements begun by TOPEX/Poseidon in 1992. 11)
• The Jason-2/OSTM spacecraft and its payload are operating nominally in early 2013.
• Sept. 2012: The current OSTM/Jason-2 mission status is OK. 12)
- Stationkeeping maneuvers: The equatorial nodal crossing requirement calls for ±1 km deviation (max) from reference nodes. The Jason-2 stationkeeping maneuvers are made with only one thrust above land on any orbit. The remaining propellent is > 23 kg.
- The core payload () Poseidon-3, DORIS, AMR, GPS) is fully operational after > 4 years on orbit; the passenger payloads (T2L2, CARMEN-2, LPT) perform satisfactorily.
- The global Jason-2 system availability is 99.9%.
• June 20, 2012 marked 4 years on-orbit of the Jason-2/OSTM spacecraft. 13)
- Several significant events occurred which impact the altimetry constellation. The Envisat mission of ESA ended on April 8, 2012, exactly two years after the launch of their Cryosat-2 satellite. Jason-1 suffered a pair of safehold events in the spring of 2012, and for safety reasons wasmoved to a new geodetic orbit with a long 406-day repeat period. Routine generation of Jason-1 data resumed on May 7, 2012. Operational users, as well as researchers, have had to adapt their processing to exploit Jason-1 data in its geodetic orbit as well as Cryosat-2 data in lieu of Envisat.
- The contributions of Jason-2 to the altimetry constellation have become more important than ever, as the Jason-3 and Jason-CS (Jason-Continuity of Service) missions are being developed to ensure the continuity of observational services for operational applications as well as climate assessment. 14)
• The Jason-2/OSTM spacecraft and its payload are operating nominally in early 2012 after more than 3 ½ years on orbit. The global Jason-2 system availability is 99.9%. The passenger (auxiliary) payloads (T2L2, Carmen-2, LPT) are performing satisfactorily. The DORIS receiver is functioning nominally. 15) 16)
- The Jason-2/OSTM mission extension is rated as Low Risk for a two-year and four-year extension. In June 2011, the NASA Earth Science Senior Review recommended an extension of the Jason-2/OSTM mission as baseline to 2013 and for another baseline to 2015. 17)
The Technical Review team has identified 2 major strengths, 3 minor strengths and 3 minor weaknesses that influence the risk determination. All of the Jason-2 primary instruments have operated without incident for the 3-year primary mission and retain full as-launched redundancy. The spacecraft is in excellent health, remains fully redundant and is expected to survive for several more years. The DORIS instrument is performing well. The AMR instrument has operated without incident to date and retains full redundancy (Ref. 17).
• Jason-2 is operating nominally in 2011. NOAA continues to operate the satellite from Suitland , MD. Data is processed by NOAA, EUMETSAT, and CNES and the products are being provided to the users well above the requirements. 18)
On June 20, 2011, Jason-2 was 3 years on orbit - providing state-of-the-art data to more than 2,000 teams around the world for use in a wide range of studies and applications.. Since the beginning of the mission data availability is very high thanks to a very reliable system and associated ground procedures. In average, only 0.18% of data are missing since launch - and this very low number is mainly related to planned operations (altimeter software upload, routine calibrations, ...). 19) 20)
Figure 5: Mean sea level measurements since 1993 (image credit: CLS, CNES, LEGOS, NASA)
• The Jason-2 spacecraft and its payload are operating nominally in 2009. 24)
• The Jason-2 ground tracks are maintained within ±1 km from the reference grid. Improvement wrt Jason-1: for Jason-2 station keeping, maneuvers are made with only one thrust above Earth on any orbit (with 2 thrusts and on the last orbit of the 10-day cycle for Jason-1). 25) 26)
• Starting shortly after the launch of Jason-2 in June 2008, both spacecraft, Jason-1 and Jason-2 attained tandem orbits with about 1 minute shift. Jason-1 and Jason-2 started their coordinated science operations on July 12, 2008 with cycle 240 (Jason-1) and cycle 01 (Jason-2).
However, in mid-February 2009 (starting on repeat cycle 262), Jason-1 assumed a new orbit midway between its original ground tracks but with a time lag of approximately 5 days with Jason-2 (hence, the start time of Jason-1 and Jason-2 differ now by about 5 days). This new tandem configuration better suits for real-time applications. The former TOPEX/Poseidon ground tracks are now being overflown by the Jason-2 spacecraft. The orbit change implies also that Jason-2 is from now on regarded the prime spacecraft of the mission.
• Jason-2 mission declared operational: On Dec. 15, 2008, the new Jason-2 OGDR (Operational Geophysical Data Record) service started. After five months of calibration and validation activities an international team of scientists, including representatives from NOAA, EUMETSAT and CNES, declared the near real-time Jason-2 data were ready for public distribution.
Jason-2 products are now available and distributed to operational meteorology users in near-real time (~ three hours delay after reception). Climate users will have access to offline data later in 2009.
• On Oct. 29, 2008, CNES handed over to NOAA the monitoring and control function of Jason-2 for the start of routine operations. This was preceded by four months of tests and qualification of the entire satellite and the ground segment by CNES. 27) 28)
• As of early August 2008, Jason-2 begins mapping the oceans. The first complete maps were calculated from the first 10 days of Jason-2's operational orbit starting on July 4, 2008.
• On July 4, 2008, Jason-2 reached its operational orbit some 1,336 km above the Earth at a 66º inclination. The Jason-2 orbit is just 58 seconds behind that of Jason-1. Jason-1 and Jason-2 will now fly in formation for a few months, making nearly simultaneous measurements to allow scientists to precisely calibrate Jason-2's instruments. This will allow Jason-2 to become fully operational by the end of 2008.
Figure 6: Overview of altimetry missions, program status as of Sept. 2012 29)
Sensor complement: (Poseidon-3, AMR, DORIS, TRSR-2/GPSP, LRA)
The sensor complement is based on the one flown on Jason-1 but with significant enhancements to improve performance and reliability to progress the sensor suite towards an operational capability. 30) 31)
Poseidon-3 (Solid-State Radar Altimeter):
Poseidon-3 is funded by CNES and of Poseidon-2 heritage (built by Thales Alenia Space). Poseidon-3 is a dual-frequency (5.3 and 13.6 GHz) nadir-looking radar altimeter with the objective to map the topography of the sea surface for calculating ocean surface current velocity and to measure ocean wave height and wind speed. Poseidon-3 has a measurement precision identical to its predecessor Poseidon-2.
Table 2: Poseidon-2 parameters
In addition, Poseidon-3 features an experimental mode to support measurements closer to coastal zones, as well as on lakes and rivers. This will be achieved by an open loop tracker: the satellite to surface distance will be estimated by the altimeter using the real-time orbit position predicted by DIODE (on board navigator based on DORIS receiver) and using the elevation of the surface with respect to the Earth GRIM5 geoid stored in a DEM (Digital Elevation Model) within the altimeter. The instrument's RFU is recurrent from Poseidon-2, while the PCU (Processing & Control Unit) largely reuses electronics from the SIRAL radar altimeter on the CryoSat mission of ESA.
Figure 7: Illustration of the Poseidon-3 instrument (image credit: Thales Alenia Space)
Figure 8: Principle of altimetry measurement (image credit: EUMETSAT)
AMR (Advanced Microwave Radiometer):
AMR is a JPL instrument of TMR and JMR (Jason Microwave Radiometer) heritage. AMR is a passive microwave radiometer measuring the brightness temperatures in the nadir column at 18.7, 23.8, and 34 GHz, providing path delay correction for the altimeter (the brightness temperatures are converted to path-delay information). The 23.8 GHz channel is the primary water vapor sensor, the 34 GHz channel provides a correction for non-raining clouds, and the 18.7 GHz channel provides the correction for effects of wind-induced enhancements in the sea surface background emission. 32) 33)
The AMR consists of two subsystems: ESA (Electronics Structure Assembly) and RSA (Reflector Structure Assembly). The ESA is developed by JPL, while the RSA is developed by ATK Space Systems, San Diego, CA.
Figure 9: The ESA system of AMR (image credit: NASA/JPL)
Figure 10: The calibration target of AMR (image credit: NASA/JPL)
Figure 11: Photo of the RSA of AMR (image credit: NASA/JPL)
DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite):
DORIS is a CNES/Thomson development. DORIS is a precision orbit determination system providing position and ionospheric correction for Poseidon-2. Doris measurements are also used for geophysical studies, in particular through the International Doris Service (IDS). Doris is a dual-frequency instrument able to determine atmospheric electron content. 34)
The DORIS flight segment consists of a two-channel, two-frequency (401.25 MHz and 2036.25 MHz) Doppler receiver capable of tracking signals from a worldwide network of about 50 ground beacons. The Jason-2 DORIS-DIODE (Immediate Onboard Orbit Determination by Doris) receiver is the same second generation device as the one developed for the ENVISAT mission. Its main functional improvements over first-generation receivers are its capability to receive two beacons simultaneously and to produce onboard the orbit ephemeris in real time with a precision of 1 m. The receiver is controlled by an ultra-stable oscillator delivering the reference frequency with a stability of 5 x 10-13 over a 10-100 second interval and delivering an on-board time output within 0.1 ms accuracy. The DORIS instrument mass is 31 kg, power = 30 W. 35) 36)
• Seven dual-frequency channels providing a capacity to track up to 7 beacons simultaneously (Ref. 25)
- Increases data quantity
- Makes available low elevation measurements
- Improves pass distributions.
• Hardened USO. Frequency stability through SAA providing a better quality of MOE. Jason-2 is useful for beacon locations.
• New DIODE navigation software
- No more numerical limitation thanks to ERC32 processor
- Improved accuracy better quality of NRT products (OGDR).
TRSR-2 (Turbo Rogue Space Receiver-2):
TRSR-2 is also referred to as GPSP (Global Positioning System Payload). The instrument is of GPS/MET (Microlab) heritage (of a design as flown on CHAMP) and is being provided by NASA/JPL and built by Spectrum Astro Inc. of Gilbert, AZ. BlackJack is a 16-channel GPS receiver with the objective to provide supplementary positioning data to DORIS in support of the POD (Precision Orbit Determination) function and to enhance and/or improve gravity field models. Radial accuracies of 1-2 cm are obtained in post-processing. TRSR-2 is a fully redundant unit (two independent receivers operating in cold redundancy). Each unit is comprised of an omnidirectional antenna, low-noise amplifier, crystal oscillator, sampling down-converter, and a baseband digital processor assembly, communicating through a 1553 bus interface. Instrument mass = 10 kg (2), power = 17.5 W.
In its current configuration, the TRSR-2 on Jason-2 can track up to 16 GPS satellites simultaneously in dual-frequency mode. From these signals, BlackJack acquires measurements of the GPS carrier phase providing range measurements with an accuracy of about 1 mm; the absolute pseudo range (defined as the absolute range plus receiver time offset from GPS time) has an accuracy of about 10 cm. TRSR-2 provides also onboard solutions for S/C position and time, accurate to about 50 m and 150 ns, respectively. 37)
Figure 12: Photo of the TRSR-2 instrumentation (image credit: NASA)
LRA (Laser Retroreflector Array):
LRA is a JPL instrument of TOPEX/Poseidon heritage, built by ITE Inc. under NASA/GSFC contract. LRA provides a reference target for satellite laser ranging (SLR) measurements, which are necessary to calibrate the POD system and the altimeter throughout the mission. The LRA is placed on the nadir face of the satellite. It is a totally passive unit that consists of nine quartz corner cubes arrayed as a truncated cone with one in the center and the other eight distributed azimuthally around the cone. This arrangement allows laser ranging at FOV (Field-of-View) angles of 360º in azimuth and 60º elevation around the perpendicular. The retroreflectors are optimized for a wavelength of 532 nm (green), offering a FOV of about 120º. The LRA instrument mass is 2.2 kg.
Figure 13: Illustration of LRA (image credit: NASA)
The LRA is a passive instrument that acts as a reference target for laser tracking measurements performed by ground stations. Laser tracking data are analyzed to calculate the satellite's altitude to within a few millimeters. However, the small number of ground stations and the sensitivity of laser beams to weather conditions make it impossible to track the satellite continuously. That is why other onboard location systems are needed.
Figure 14: The Jason-2/OSTM spacecraft with the payload accommodation (image credit: NASA/JPL, Ref. 7)
Auxiliary instruments: (Carmen-2, LPT, T2L2)
Carmen-2 (Environment Characterization and Modelisation-2):
These are radiation detectors developed by CNES. The Carmen-2 instrumentation is dedicated to study the influence of space radiation on advanced components to measure e-, p+ (high-energy particle flux of electrons and protons) and ion fluxes in the energy ranges responsible for component effects (as ionizing dose, single event effect and displacement damage), to measure associated effects on test components, to characterize the local radiation environment for DORIS USO and to evaluate its potential drifts inside the SAA.
The instrument is composed of a spectrometer (detectors and acquisition chains) and an experiment module (MEX). The energy range for electrons is expected to be 250 keV to 4 MeV. One for protons is 8 to 120 MeV/(mg/cm2). MEX includes three types of dosimeters to measure accurately the exposed dose and several components under test which are memories, linear and optoelectronic devices, power MOSFETs and so on. 38) 39)
The instrument was powered on on June 22, 2008 and is performing well since then (Ref. 25).
Figure 15: Block diagram of Carmen-2 (image credit: CNES)
Figure 16: Illustration of the Carmen-2 instrument (image credit: CNES)
LPT (Light Particle Telescope):
LPT is a detection unit of JAXA (Japan Aerospace Exploration Agency), Tokyo, Japan. LPT complements the radiation measurements of Carmen-2. In June 2006, JAXA and CNES signed a MOU (Memorandum of Understanding) with the intent to load the JAXA instrument LPT (Light Particle Telescope) onto the Jason-2 spacecraft of CNES. 40)
LPT consists of two units, which are LPT-E and LPT-S. Figure 17 shows the external views of LPT-E (left) and LPT-S (right). A block diagram of LPT is shown in Figure 18. LPT-E is mounted inside of the satellite and LPT-S is outside.
LPT-E provides functions of the electrical I/F with the Jason-2 satellite system. It receives primary power supply from satellite system and provides sensors and electrical circuits with secondary power. It also receives telecommands and sends telemetry data via the MIL-1553B bus using protocols specified by the PROTEUS standard satellite bus which is used for the Jason-2.
LPT-S consists of four sensors. Specifications of each sensor are shown in Table 3. Each sensor counts number of interesting particles irradiated from inside of the view angle with the specific energy of each channel every one second (time resolution).
Figure 17: Illustration of the LPT device (image credit: JAXA)
Figure 18: Block diagram of the LPT (image credit: JAXA)
Table 3: Specification of the LPT sensors
The LPT-S device has a FOV in the zenith direction; it is accommodated on the outside of the spacecraft. The LPT-E device is installed inside of Jason-2.
LPT-S consists of 4 sensor units; ELA-A, ELS-B for counting electrons, APS-A and APS-B for protons. Each unit includes a set of radiation detectors, their preamplifiers, high voltage supplies, analog and digital board for data processing and analyzing. They measure energies of incident particles and identify particle species by the ΔE x E method. The counts of each particle are accumulated for a second and transmitted to LPT-E. There is an electrical interface between LPT-S and the satellite bus system. LPT-E includes a CPU board for data handling, receiving commands, and transmitting telemetry data in order to control the LPT-S according to a command. LPT-E also supplies LPT-S with power. 41)
Each sensor has 2 measurement modes. The nominal mode is called “count mode”, which obtains count data in energy bins for each particle. Another “list mode” transmits analog-to-digital converted data indicating energy of incident particles. The list mode is used for checking health and gain drift of the detector and electronics while the volume of data to be transferred is limited.
Initial performance check: LPT was initially checked out from June to November 2008 and the LPT was working correctly. The electrical noise was measured for ELS-A, APS-B and APS-A using regular test pulses. The full width of half maximum (FWHM) derived from the test pulses corresponded to 16.3 keV for ELS-A. For APS-A and APS-B, the electrical noise was smaller than a digit of the ADC (Analog–to-Digital Converter) in LPT. Those FWHMs are consistent with the technical requirement.
A world flux map for electrons measured by ELS-A in the 400 – 490 keV energy range is shown in Figure 19. The map shows averaged data for 4 months from November 2008 to February 2009. It is easily found that the border of SAA (South Atlantic Anomaly) at that altitude of 1336 km is extended from the middle of Indian Ocean to the western edge of Pacific Ocean. The slot region between the inner radiation belt and the outer radiation belt is also seen clearly.
The observational data of LPT helps to improve the radiation environment knowledge and characterize the local radiation environment to evaluate errors of other mission instruments. An improved LPT device will be also onboard JASON-3 which has the same orbit as JASON-2.
Figure 19: Illustration of a world flux map for electrons with 400 keV to 490 keV energy (image credit: JAXA)
T2L2 (Time Transfer by Laser Link):
T2L2 refers to detectors for ultra-precise time transfer. The instrument is provided by the Riviera Observatory OCA (Observatoire de la Côte d'Azur) in Grasse, France, and by CNES. The T2L2 system on Jason-2 will allow the precise characterization of the USO (Ultra-Stable Oscillator) used by the DORIS positioning system. Relying on this clock, T2L2 may also permit to perform some orbit restitutions of Jason-2 uniquely by one-way laser ranging. 42) 43) 44) 45)
The main function of the T2L2 instrument is to allow comparison and follow-up of distant clocks, either of an embarked clock relative to a ground clock or of two (or more) ground clocks. The means used to establish a link between these clocks is the transmission and the dating of laser pulses. For this purpose the system is based on an embarked instrument connected to a clock and on a network of laser telemetry stations equipped with clocks.
Figure 20: The experiment uses laser ranging stations on ground and a specific timetagging unit on the satellite (image credit: CNES, OCA)
The principle of this link is as follows: A ground station fires laser pulses to the satellite and records the local times of the firings. Aboard the satellite the T2L2 instrument composed of a detector and a dating system connected with the on-board clock, records the arrival dates of photons in the on-board reference time frame. A system of retro-reflectors reflects a certain ratio of the photons back towards the ground station, which, again, records the return dates in local time. Departure and return dates are measured in an accurate way with respect to the ground clock; arrival dates on the satellite are given to an arbitrary reference.
Figure 21: T2L2 space instrument synoptic (image credit: CNES)
Legend to Figure 21: The photo detection unit includes two avalanche photo detectors: One in linear mode for energy measurement and trigger and the other in Geiger mode for precise chronometry.
The T2L2 satellite payload is composed of two subsystems (both subsystems are linked by electric connections and an optical fiber):
- The optical subsystem that insures the functions of electronic activation (linear detection) and collection of the laser pulse for the dating (non-linear optic). It constitutes an assembly of two small cases, one for each function.
- The electronic subsystem that insures the functions of non-linear detection, dating, instrument management and interfacing with the satellite. The set is assembled in a unique case.
The T2L2 tracking data are being collected by participating SLR stations on a global scale. The T2L2 Mission Center (CMT) processes and archives every pass and determines the corresponding time and frequency gaps for every SLR pass.
The T2L2 instrument is fully operational since June 30, 2008 and is working properly. The in-flight measurements are compliant with the instrument and system requirements (Ref. 25).
Figure 22: Block diagram of the T2L2 system (image credit: OCA)
Figure 23: The Optical (left) and Electronic (right) subsystems of T2L2 (image credit: OCA)
The T2L2 instrument has a mass of 8 kg (electronics unit) + 1.1 kg (optics unit). The power consumption is 42 W.
Thales-SESO (Société Européenne de Systèmes Optiques, Aix en Provence, France) was in charge of the design and realization of a demonstrator for a large and massive corner cube on Jason-2. This corner cube was foreseen to reflect the light coming from different (DORIS) laser stations located on ground. The operating mode was requesting a very wide angular field of view (> ±60°) and a unique component. This was possible to achieve only using a massive corner cube (i.e. not a hollow one) and cut from a very high index material (S-LAH65 in the present case). One of the main challenges for the mechanical mount was to design simultaneously: 46)
- a sufficiently light global assembly, as the mass of the optical cube only, with such glass and such optical material, was already important
- having sufficient stiffness to withstand environmental loads
- but also providing sufficient thermal dilatation possibilities (to avoid cube constraints) taking into account the very wide operating temperature range (-40°C to +40°C).
Figure 24: Photo of the large and massive corner cube BBM (Breadboard Model) for T2L2 (image credit: Thales-SESO)
Performances of the Corner Cube:
• High index massive corner cube in order to procure a very wide angular retro reflection possibilities (i.e. at least ±60°) with a unique component
• Deviation accuracy: <2 arcsec
• Dihedral angle: <0,5 arcsec
• Size: Ø180 mm (corner to corner)
• Mounting: Aluminum Housing + flexible annular ring (attachment of cube with space qualified glue).
The Jason-2/OSTM ground segment is comprised of the integrated ground segment capabilities of NOAA, NASA, CNES and EUMETSAT - consisting of the control ground system and a mission ground system. 47) 48) 49) 50)
1) Control ground system:
- The Jason-2 Control Center is located at CNES in Toulouse, France. It monitors and controls the satellite during its entire mission life.
- All flight operations scheduling activities are allocated to SOCC (Satellite Operations Control Center) at NOAA. This includes command plan preparation, command transmission and telemetry acquisition and routing.
For command transmission and data acquisition, the CNES control center and the NOAA SOCC rely upon a ground terminal network of Earth terminal/stations suitably located to allow the required orbit coverage compliant with the data latency requirement. NOAA is providing its stations at Wallops Island, VA, and at Poker Flats, AK; EUMETSAT is providing its station in Usingen, Germany.
2) Mission ground system:
- CNES mission center: The mission center functions consist in instrument programming and monitoring (altimeter and Doris), commands requests generation (altimeter and Doris), mission management and operation plan definition, Precise Orbit Determination (POD), algorithm definition and POD data production and validation, scientific altimeter data processing and validation of altimetry product, data distribution and archiving.
- EUMETSAT and NOAA near real-time processing centers: The objective is to process and disseminate the near real-time altimeter product within 3 hours of data acquisition. In particular, EUMETSAT and NOAA will deliver OGDR(Operational Geophysical Data Record). The OGDR will be released within every three hours, containing surface wind speed and wave data and a first SSHA (Sea Surface Height Anomaly) estimate. To meet the timeliness requirements, the OGDR generation will be driven by telemetry availability and the SSHA will be based on the on-board computed DORIS ephemeris data. The OGDR will also be disseminated on the GTS [Global Telecommunications System (of the World Meteorological Organization (WMO)] and EUMETCast. 51)
The data of the three location systems, DORIS, TRSR-2 (GPSP), and LRA are being used in support of the POD (Precision Orbit Determination) function. In addition, three different data products are produced and distributed to the users: 52)
• The Operational Geophysical Data Record (OGDR) with a latency of 3-5 hours
• The Interim Geophysical Data Record (IGDR) with a latency of 1 -2 days
• The Geophysical Data Record (GDR) with a specified latency smaller than 60 days.
The GDR provides fully-validated data produced usually within five to six weeks of the events being recorded and covers sea surface height, principally for climate monitoring and climate modelling. The main users of this product are within the climate research community, for climate model verifications, for routine sea level station validation, and for the International Panel for Climate Change Assessment Report on rising sea levels. The orbits used for generating these products are generated by CNES and are called POE (Precise Orbit Ephemeris). They are computed using all three location systems: DORIS, GPSP, and LRA. They have an RMS orbit accuracy specification of 1.5 cm in the radial direction.
Figure 25: Overview of the Jason-2 /OSTM system elements (image credit: Jason-2 /OSTM collaboration)
The general data flow for the Jason-2 ground system is shown in Figure 26. Raw telemetry data streams from the Jason platform are captured at NOAA’s Wallops and Fairbanks CDAS (Command and Data Acquisition Stations) and at EUMETSAT’s tracking station located at Usingen, Germany. These streams are then forwarded to other Jason-2 subsystems at NOAA’s SOCC (Satellite Operations Control Center), referenced as NOAA Spacecraft Operations in Figure 26) in Suitland, Maryland for further action. 53)
A SOCC server functions as a centralized data exchange and distribution hub for data transfers between NOAA internal subsystems, and between NOAA and its partners. The ESPC (Environmental Satellite Processing Center), collocated with the SOCC in Suitland, generates near real-time operational data products using software provided by CNES. These products are made available to NASA/JPL, EUMETSAT and CNES via the centralized data server located at SOCC, and also staged to the ESPC primary data distribution server for dissemination to the general user community and to the NOAA long-term CLASS (Comprehensive Large Area Data Stewardship System) data archive. A similar process is followed at EUMETSAT for generation and delivery of operational data products to the partners. In addition, data products of improved accuracy are generated by CNES and distributed to NOAA and EUMETSAT in non-real-time fashion, usually with a latency of several days.
As shown in Figure26 , a bi-directional flow of data and information exists between the SOCC and EUMETSAT. The flow from SOCC to EUMETSAT includes the flow of Near-Real-Time operational products, stored payload telemetry, stored housekeeping and real-time housekeeping telemetry. The flow from EUMETSAT to SOCC includes spacecraft raw payload and housekeeping telemetry sent from the Usingen via the EUMETSAT communications network, offline (non-near real-time) geophysical data products generated by CNES, near real-time products generated by EUMETSAT, and earth station telemetry from Usingen.
Figure 26: General data flow of the Jason-2 ground system (image credit: NOAA)
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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.