CYGNSS (Cyclone Global Navigation Satellite System)
CYGNSS is part of the NASA ESSP (Earth System Science Pathfinder) program. The overall objective of CYGNSS is to improve extreme weather predictions. The mission is focused on tropical cyclone (TC) inner core process studies. CYGNSS attempts to resolve the principle deficiencies with current TC intensity forecasts, which lie in inadequate observations and modeling of the inner core. The inadequacy in observations results from two causes: 1) 2)
1) Much of the inner core ocean surface is obscured from conventional remote sensing instruments by intense precipitation in the eye wall and inner rain bands.
2) The rapidly evolving (genesis and intensification) stages of the TC life cycle are poorly sampled in time by conventional polar-orbiting, wide-swath surface wind imagers.
CYGNSS is specifically designed to address these two limitations by combining the all-weather performance of GNSS bistatic ocean surface scatterometry with the sampling properties of a constellation of eight satellites. The use of a dense constellation of microsatellites results in spatial and temporal sampling properties that are markedly different from conventional imagers.
CYGNSS will use a constellation of eight small satellites in LEO (Low Earth Orbit) carried to orbit on a single launch vehicle. In orbit, CYGNSS’s eight microsatellite observatories will receive both direct and reflected signals from GPS (Global Positioning System) satellites. The direct signals pinpoint CYGNSS observatory positions, while the reflected signals respond to ocean surface roughness, from which wind speed is retrieved. 3) 4) 5)
The mission will study the relationship between ocean surface properties, moist atmospheric thermodynamics, radiation and convective dynamics to determine how a tropical cyclone forms and whether or not it will strengthen, and if so by how much. This will advance forecasting and tracking methods.
NASA selected and funded the CYGNSS mission proposal in June 2012. The eight CYGNSS satellites will be built by SwRI ( Southwest Research Institute). SSTL of Colorado, the U.S. subsidiary of the British spacecraft-builder, will provide the GPS receivers for the mission, and SNC (Sierra Nevada Corporation) will provide the DM (Deployment Module). 6) 7) 8)
The University of Michigan (UM) is leading the NASA hurricane prediction project. The CYGNSS science team consists of the following institutions: UM, Ann Arbor, MI (Christopher S. Ruf, PI); SwRI, Boulder, CO; NOAA/AOML (Atlantic Oceanographic and Meteorological Laboratory), Miami, FL; University of Miami, Coral Gables, FL; NOAA/NESDIS, Silver Spring, MD; Ohio State University, Columbus, OH; Purdue University, Lafayette,IN; and the NOAA/ESRL ( Earth System Research Laboratory), Boulder, CO.
Table 1: CYGNSS science goals and objectives
Secondary science: Support the operational hurricane forecast community by producing and providing ocean surface wind speed data products, and helping to assess the value of these products for use their retrospective studies of potential new data sources.
The eight CYGNSS microsatellites can pass over the ocean more frequently than one large satellite could. This allows the satellites to capture a detailed view of the ocean’s surface. The observatory satellites are able to capture data from the inner core of tropical cyclones because the satellite signals can travel through extreme rainfall. 9) 10)
The number of satellites, their orbit altitudes and inclinations, and the alignment of the antennas are all optimized to provide unprecedented high temporal-resolution wind field imagery of TC (Tropical Cyclone) genesis, intensification and decay.
The satellites are designed and developed at SwRI. Each CYGNSS observatory consists of a microsatellite platform hosting a GPS receiver modified to measure surface reflected signals. Each observatory simultaneously tracks scattered signals from up to four independent transmitters in the operational GPS network. The number of observatories and orbit inclination are chosen to optimize the TC sampling properties. The result is a dense cross-hatch of sample points on the ground that cover the critical latitude band between ±35° with an average revisit time of 4.0 hrs.
The CYGNSS observatory is based on a single-string hardware architecture (Figure 4) with functional and selective redundancy included for critical areas. The microsatellite has been designed from the beginning for ease of manufacture, integration, and test to provide a low-risk, cost-effective solution across the constellation.
SMT (Structure, Mechanisms, and Thermal): The SMT subsystem design leverages SwRI's instrument and avionics SMT heritage and capabilities to meet SMT requirements.
Structure: The microsatellite structure requirements are driven by physical accommodation of the DDMI antennas, the S/As(Safe/ Arms), and launch configuration constraints. The design uses milled AI piece parts bolted together to provide an integrated, mass efficient solution for CYGNSS. The spacecraft shape is specifically configured to allow clear nadir and zenith FOV for the DDMI antennas, while its structure integrates the microsatellite and instrument electronic boards directly by creating avionics and DMR (Delay Mapping Receiver) "bays" (Figure 1).
The avionics and DMR bays form the core of the microsatellite; all other components are mounted to this backbone with structural extensions included to accommodate the Aluminum honeycomb-based S/As and DDMI nadir antenna assemblies. The structural configuration allows easy access to all observatory components when the nadir DDMI antenna panel assemblies and the microsatellite endplates are removed for observatory AI&T (Assembly, Integration and Test).
Figure 1: Structural element of a microsatellite (image credit: SwRI)
TCS (Thermal Control Subsystem): Thermal control is provided by heaters and MLI. The primary radiator is located on zenith surface in the S/A gap along the observatory centerline, with a second radiator on the nadir baseplate. These locations are chosen to provide a direct, cohesive thermal conductive path to the primary observatory dissipative loads. The radiators are coated with 5 mil ITO/Tef/Ag, while MLI is used on nonradiating external surfaces. All materials used in the thermal design are flight qualified and compatible with the minimal CYGNSS contamination control requirements.
Figure 2: Observatory and component definition (image credit: SwRI)
Figure 3: Illustration of the CYGNSS spacecraft (image credit: SwRI)
ADCS (Attitude Determination and Control Subsystem): The spacecraft are 3-axis stabilized (momentum biased). Attitude sensing is provided with MAI horizon sensors (0.5º accuracy, ±5º range) and a Honeywell magnetometer (model: 230212, 10 nT sensitivity, ±50,000 nT range). Actuation is provided by a Sinclair momentum wheel (30 mNms @ 5600 rpm, 2 mNm torque) and by SatServ torque rods (1 Am2, residual moment < 0.1 Am2).
The ADCS has three primary states of operation: rate damping, nadir acquisition, and normal pointing. The rate damping state is used initially after separation from the LV (Launch Vehicle) and for anomaly recovery if rates exceed normal state capabilities. Rate damping uses a "B-dot" algorithm to command magnetic dipole moments opposed to the rate of change of the magnetic vector, both measured in body coordinates. It only uses the sensed magnetic field, and does not rely on a correct orbital ephemeris or magnetic field model. The wheel speed is off for launch and initial tip-off recovery, or set to its nominal value during anomaly recovery.
After the body rates are damped, the system transitions into nadir acquisition, which monitors the pitch/roll horizon sensors to determine a rough Earth vector. The sensors are not assumed to be in their linear range; simple "on-Earth" and "off-Earth" measurements are used to establish slow roll and pitch rates to bring the sensors into their linear range (±5°). The momentum wheel is also maintained relatively close to its commanded nominal speed, with a desaturation gain much lower than normal.
EPS (Electrical Power Subsystem): The EPS is designed to perform battery charging without interrupting science data acquisition. It is based on a 28±4 V dc primary power bus with electrical power generated by a 8-panel rigid S/A (Safe/Arm). The 0.71m2 total area S/A provides a 30.3% margin during max eclipse periods (35.8 minutes). The design provides 43.4% margin during these periods. When stowed, the z-fold design of the S/A allows the solar cells to face outward, combining with the two supplemental ram/wake S/As to power the microsatellite indefinitely in standby mode before the S/A deployment (22% margin). Electrical power storage for eclipse operations is provided by 2 ABSL 1.5 Ah Li-ion 8s1p batteries connected directly to the primary power bus. The batteries are "build-to-print" and configured for 3 Ah (EOL) at 28.8 V nominal. Battery charge regulation for the CYGNSS EPS is a PTT (Peak Power Tracking). The EPS battery charging and power distribution hardware operates independent of FSW (Flight Software) except for configuration commanding and status reporting. Over-current protected switched power services are provided for the DDMI and initial microsatellite power application. A power of 70 W (30% margin)is generated with triple junction solar cells.
RF communications: The S-band transceiver is a single card communication solution developed by SwRI to provide a low-cost, radiation-tolerant, communication system. The core of the transceiver is a SDR (Software Defined Radio) architecture configured to provide S-band (2 GHz) communications. The transceiver provides OQPSK (Offset Quadrature Phase Shift Keying) encoded transmit data at 1.25 Mbit/s with a FSK (Frequency Shift Keying), the uplink receiver is supporting data rates up to 64 kbit/s.
Table 2: Overview of spacecraft parameters and developers
Figure 4: Block diagram of a microsatellite with a highlight on avionics (image credit: SwRI)
Figure 5: Overview of the CYGNSS mission timeline (image credit: UM)
Launch: A single launch of the CYGNSS mission is planned for the fall of 2016.
DM (Deployment Module): The DM serves as the constellation carrier during launch and then deploys the observatories into their proper orbital configuration once on orbit. The DM is provided by SNC (Sierra Nevada Corporation).
The DM consists of 2 AL cylindrical sections or tiers, each with 4 mounting/separation assemblies (Figure 6). The tier design approach simplifies observatory-DM integration by enabling easy access of GSE while minimizing potential for damage inherent in a single core structure. The mounting/separation assemblies are positioned 90º apart to release the observatories in pairs opposite each other, balancing deployment forces and keeping disturbance torques well within LV capabilities. Tier 2 is clocked 4º from Tier I to provide proper orbital dispersal vectoring.
Deployment is initiated using flight-proven, high-reliability Frangibolts. Observatory separation tip-off errors are minimized by averaging 4 push springs (Figure 6) to reduce the microsatellite CG (Center of Gravity) location criticality and to minimize the effects of spring tolerances. The tip off errors are further reduced by screening the springs during DM assembly. Each observatory is secured to the DM by torquing the Frangibolt actuator into the microsatellite nadir baseplate, compressing the separation springs to achieve the desired spring load for observatory ejection. The tapered alignment pins, combined with the Frangibolt actuator, rigidly constrain each observatory to the DM for launch.
Figure 6: The 2- tier DM provides balanced separation forces by using a matched spring deployment mechanism (image credit: CYGNSS project)
DM avionics: The DM uses a heritage electronic sequencer to release the observatories in a pre-determined sequence stored within the sequencer memory. The sequence is initiated via a standard LV discrete signal when the LV arrives at the required orbit. The sequencer then performs the deployment sequence by actuating the Frangibolt actuators. The sequence timing incorporates the constellation separation requirements and deployment actuation tolerances. Hardware safety is ensured through the use of a 2-stage command, single-fault tolerant actuator driver design that includes a pre-flight S/A (Safe/ Arm) connector to fully disarm the system.
A 28 VDC DC 140 Wh Li-ion battery is used to power the DM avionics and activates the deployment Frangibolt actuators. The battery is fully charged at launch with <5% of capacity required to complete the orbit insertion and deployment sequence.
Figure 7: Illustration of the complete flight segment with Deployment Module (image credit: CYGNSS project)
Orbit: Non-synchronous near-circular orbit (all spacecraft in a single plane), altitude = 500 km, inclination = 35º. Period = 90 minutes.
Figure 8: Each LEO CYGNSS observatory will orbit at an inclination of 35º and be capable of measuring 4 simultaneous reflections, resulting in 32 wind measurements/s across the globe (image credit: UM, NASA)
Legend to Figure 8: The orbit inclination was selected to maximize the dwell time over latitudes at which hurricanes are most likely to occur. The result will be high-temporal-resolution wind-field imagery of tropical cyclone genesis, intensification, and decay. Shown here are planned CYGNSS ground tracks for 90 minutes (top) and a full 24-hour period (bottom). 11)
Constellation orbital configuration:
After deployment, there are a number of different options for how the constellation of eight observatories can orbit Earth relative to each other. One option is not to control the configuration and allow the different velocities imparted by deployment determine their orbit state. This would result in all satellites having slightly different orbit periods and result in a dynamic configuration of satellites that constantly changes over time. The other option is to specify a desired and relatively fixed configuration and control to this desired end state. A controlled configuration offers a number of spacing options that can be exploited to address specific mission science objectives and requirements (Ref. 9).
The benefits of an uncontrolled constellation are that there are no maneuvers required to establish the desired constellation configuration or to maintain it. The downside is that over time the constellation configuration is undetermined and the science coverage will change over time in an unpredictable fashion. When the Observatories cluster together, they start measuring similar areas over the ocean, thereby reducing science coverage and resulting at times in coverage that does not meet mission requirements (Figure 9). The controlled constellation requires the calculation of maneuvers to establish and then maintain the configuration, but provides predictability to the constellation for science coverage. The configuration can also be selected to “tune” competing science coverage metrics such as percent area coverage and revisit rate. The CYGNSS team has developed a set of tools, based on high fidelity STK ® (Satellite Tool Kit) scenarios that have been used to explore a number of potential configurations to assess the impact on science coverage.
Figure 9: Observatory separation distances for mission given no orbital position control. Points of conjunction are identified with significant clustering occurring several times during the mission (image credit: CYGNSS project)
Orbital control: Cost constraints of the CYGNSS mission preclude inclusion of velocity thrusters on the Observatories Although the satellites do not have thrusters, a ΔV can be realized by pitching the vehicle to increase its drag area relative to the other vehicles. This technique, known as "differential drag", results in an increased drag profile that results in an acceleration opposite the velocity vector which can be used to adjust the relative position of the satellites and to avoid potential conjunctions. The unique physical configuration of the CYGNSS Observatory provides an excellent 7:1 drag profile that can be exploited to control the constellation orbital configuration with minimal impact to CYGNSS operational life expectations. 12)
Constellation maintenance: After the constellation orbital configuration has been established, it will be disturbed in one of two ways; atmospheric drag variations and space object avoidance.
1) Atmospheric drag variance: The uncertainties in vehicle state and the imperfect ability to control the vehicle state mean that there will be a very small relative drift rate between the observatories that will eventually grow to a large error that we would want to correct. This will occur over a period of a number of months to years depending on selected tolerance.
2) CA (Conjunction Assessment): The other source of disturbance is maneuvers required to avoid conjunctions with other space objects. Studies have shown that conjunction events will happen frequently enough to be the dominant factor in determining when CYGNSS Observatory orbital maneuvers will occur. In 2013, there are currently over 13,700 objects in the unclassified space catalog of which about 1900 of them are presumed to be active. Most of these objects reside in LEO (Low Earth Orbit, below 2000 km) with a peak density around 800 km caused by a combination of the Fengyun-1C debris and the Iridium-Cosmos collision. Monte Carlo studies have been performed to assess the expected frequency of conjunction events within a given threshold minimum range. The study assumed that the current catalog of space objects is representative of the environment that CYGNSS will encounter during operations. The results of the study show that there will be regular (i.e. weekly) encounters within 2 km and somewhat more infrequent encounters (monthly or less) within 0.5 km.
Due to the large uncertainty in object state, there will likely be a daily process to assess incoming data from the JSpOC (Joint Space Operations Center) for potential conjunctions even though actual conjunctions that require maneuvers will be rare.
Science operations: Following commissioning, the instrument is set to science mode for the duration of the mission, except for brief returns to engineering verification performed bi-annually. In science mode, science measurements are acquired and downlinked with 100% duty cycle. The Observatories are designed to implement nominal Observatory operations and science data collection without on-board schedule command sequences.
Sensor complement: (DDMI)
Background: The UK-DMC-1 spaceborne demonstration mission of SSTL (Surrey Satellite Technology Ltd., launch Sept. 27, 2003) with the GPS reflectometry receiver onboard, showed that a microsatellite-compatible passive instrument (SRG-10), potentially could make valuable geophysical measurements using GPS reflectometry. The left side of Figure 10 illustrates how the process works. The direct GPS signal is transmitted from the orbiting GPS satellite and received by a RHCP (Right-Hand Circular Polarization) receive antenna on the zenith (i.e., top) side of the spacecraft that provides a coherent reference for the coded GPS transmit signal. The quasi-specular, forward-scattered signal that returns from the ocean surface is received by a nadir- (i.e., downward-) looking LHCP (Left-Hand Circular Polarization) antenna on the nadir side of the spacecraft. The scattered signal contains detailed information about its roughness statistics, from which the local wind speed can be derived (Ref. 11). 13)
Figure 10: GPS signal propagation and scattering geometries for ocean surface bistatic quasi-specular scatterometry (left). Spatial distribution of the ocean surface scattering measured by the UK-DMC-1 demonstration spaceborne mission – referred to as the Delay Doppler Map (image credit: SSTL)
The image on the right of Figure 10 shows the scattering cross section as measured by UK-DMC-1 and demonstrates its ability to resolve the spatial distribution of ocean surface roughness. This type of scattering image is referred to as a DDM (Delay Doppler Map). There are two different ways to estimate ocean surface roughness and near-surface wind speed from a DDM. The maximum scattering cross-section (the darkest shades in the graph) can be related to roughness and wind speed. 14) 15)
This, however, requires absolute calibration of the DDM, which is not always available. Wind speed can also be estimated from a relatively calibrated DDM, using the shape of the scattering arc (the lighter shades in the graph). The arc represents the departure of the actual bistatic scattering from the theoretical purely specular case—i.e., scattering from a perfectly flat ocean surface—which appears in the DDM as a single-point scatterer. The latter approach imposes more-relaxed requirements on instrument calibration and stability than does the former. However, it derives its wind speed estimate from a wider region of the ocean surface, and thus has lower spatial resolution.
After UK-DMC-1, the development of wind-speed retrieval algorithms from DDMs became an active area of research and resulted in the design of a new instrument. called the SGR-ReSI (Space GNSS Receiver – Remote Sensing Instrument). For the development of SRG-ReSI, SSTL and the University of Surrey teamed with the National Oceanographic Centre in Southampton, U.K., the University of Bath, and Polar Imaging Ltd.
Like its predecessor, the SRG-ReSI instrument can make valuable scattering measurements using GPS, but it has greater onboard data storage capacity and can process the raw data into DDMs in real time. It also has been designed with flexibility so it can be programmed while in orbit for different purposes — e.g., tracking new GNSS signals when needed, or applying spectral analysis to received signals. 16)
In effect, the SGR-ReSI fulfils in one module what has historically been handled by three separate units on earlier spacecraft (Figure 11). Specifically:
• It performs all the core functions of a space GNSS receiver, with front-ends supporting up to eight single or four dual-frequency antenna ports
• It is able to store a quantity of raw sampled data from multiple front ends, or processed data in its 1 GB solid-state data recorder
• It has a dedicated reprogrammable field-programmable gate array (FPGA) coprocessor (a Xilinx Virtex 4).
The coprocessor was specifically included for the real-time processing of the raw reflected GNSS data into DDMs. However, it has flexibility to be programmed in orbit as required for different purposes, for example to track new GNSS signals, or to apply spectral analysis to received signals.
For the coprocessor to generate DDMs of the sampled reflected data, it needs to be primed with the PRN, the estimated delay and the estimated Doppler of the reflection as seen from the satellite. These are calculated by the processor in conjunction with the main navigation solution - the data flow for this is shown in Figure 12. Direct signals (from the zenith antenna) are used to acquire, track GNSS signals. From the broadcast Ephemerides, the GNSS satellite positions are known. Then from the geometry of the position of the user and the satellites, the reflectometry geometry can be calculated, and hence an estimate of the delay and Doppler of the reflection.
Figure 11: Schematic view of the SGR-ReSI instrument (image credit: SSTL, UM)
Figure 12: GNSS reflectometry data flow (image credit: SSTL, UM)
The processing of the DDM (Delay Doppler Map) is performed on the coprocessor using data directly sampled from the nadir antenna (Figure 13). In common with a standard GNSS receiver, the local PRN is generated on board the coprocessor. As an alternative to synchronizing and decoding the reflected signal in a standalone manner, the direct signals can be used to feed the navigation data sense, and assist the synchronization. The sampled data is multiplied by a replica carrier and fed into a matrix that performs an FFT (Fast Fourier Transform) on a row by row basis of the DDM, to achieve in effect a 7000 channel correlator, integrating over 1 ms. Each point is then accumulated incoherently over hundreds of milliseconds to bring the weak signals out of the noise.
Figure 13: The DDM (Delay Doppler Map) processing scheme (image credit: SSTL)
This processing is performed in real-time on board the satellite and greatly reduces the quantity of data required to be stored and for the satellite's downlink, enabling a larger number of reflections to be captured across the globe. The initial implementation has been to predict and track a single reflection from a single downward pointing antenna. It is planned, however, to implement in the Flight Model (Figure 5) the prediction and mapping of four reflections simultaneously from two nadir antennas giving an increased swath.
Figure 14: Photo of the SRG-ReSI flight model (image credit: SSTL)
Each CYGNSS observatory will be equipped with a DDMI (Digital Doppler Mapping Instrument), based on the SGR-ReSI design (Ref. 11).
The TechDemoSat-1 of SSTL (launch planned for late 2013, with SGR-ReSI onboard) 17) will act as a valuable precursor and validation of the concept of GNSS Reflectometry for the NASA CYGNSS mission. As the CYGNSS constellation is optimized for the cyclone germination zones near the equator, it has a coverage that is limited globally by the selected inclination of 35°. Future GNSS reflectometry missions covering higher inclinations are likely to make a valuable contribution towards weather knowledge at higher latitudes.
Figure 15: The CYGNSS Constellation (left); the CYGNSS observatories are shown as yellow spheres. The white lines represent direct GPS signals and the blue ocean surface scattered signals. The lighter blue circles on the Earth surface represent individual samples of the Delay Doppler Map. -At right, the full constellation of GPS transmitters and CYGNSS receivers in the bistatic radar constellation are shown (image credit: CYGNSS project, Ref. 4)
DDMI (Delay Doppler Mapping Instrument)
The instrument will be collecting signals directly from GPS satellites and the reflected GPS signal off the ocean surface. The DDMI measures the ocean surface wind field with unprecedented temporal resolution and spatial coverage, under all precipitating conditions, and over the full dynamic range of wind speeds experienced in a TC. It does so by combining the all-weather performance of GPS-based bistatic scatterometry with the sampling properties of a dense satellite constellation.
The GPS receiver performs standard GPS navigation and timing functions, and provides digital signal processing. The Doppler effect processed and mapped aboard each satellite will support up to 4 simultaneous measurements per satellite per second. The maps generated from the GPS signals scattered from the ocean surface are called DDMs (Delay Doppler Maps). - The method of measuring wind speed is not affected by precipitation states, support a continual collection of data through the entire area of interest. The number of satellites and the orbit chosen will result in a full map of the area between the Tropic of Cancer and the Tropic of Capricorn every day (24 hour period).
The DDMI instrumentation consists of the Surrey DMR (Delay Mapping Receiver), plus a zenith and two nadir antennas, also provided by SST-US LLC (Surrey Satellite Technology US LLC), Englewood, CO.
The DDMI will generate DDMs continuously at a low data rate, which will provide a source for ocean roughness measurements across the ocean. In special situations, such as when passing over an active tropical cyclone, the instrument can be operated in Raw Data Mode, where 60 seconds of raw sampled data is accumulated. This allows researchers to fully analyze and re-analyze the acquired data using different processing schemes to ensure that the nominal DDM mode of operation is not losing important geophysical data.
Figure 16: Illustration of the DDMI components (image credit: Surrey, SwRI)
1) MOC (Mission Operations Center), located at the SwRI (Southwest Research Institute) Planetary Science Directorate in Boulder, CO
2) SOC (Science Operations Center), located at the University of Michigan’s Space Physics Research Laboratory in Ann Arbor, MI
3) GDN (Ground Data Network), operated by USN (Universal Space Network), consisting of existing Prioranet ground stations in South Point, HI, in Santiago, Chile, and in Western Australia, some 400 km south of Perth, and at the MOC facility.
Figure 17: Illustration of the CYGNSS ground system components (CYGNSS project)
• MOC: During the mission, the CYGNSS MOC is responsible for mission planning, flight dynamics, and command and control tasks for each of the observatories in the constellation. These primary MOC tasks include:
- coordinating activity requests
- scheduling ground network passes
- maintaining the CCSDS/FDP (Consultative Committee for Space Data Systems/File Delivery Protocol) ground processing engine
- collecting and distributing engineering and science data
- tracking and adjusting the orbit location of each observatory in the constellation
- trending microsatellite data
- creating real-time command procedures or command loads required to perform maintenance and calibration activities
- maintaining configuration of onboard and ground parameters for each observatory.
• SOC: The CYGNSS SOC will be responsible for the following items related to calibration/validation activities, routine science data acquisition and special requests, and data processing and storage:
- supporting DDMI testing and validation both prelaunch and on-orbit
- providing science operations planning tools
- generating instrument command requests for the MOC
- processing Levels 0 through 3 science data
- archiving Level 0-3 data products, DDMI commands, code, algorithms, and ancillary data at a NASA DAAC (Distributed Active Archive Center).
Figure 18: Overview of the CYGNSS SOC (image credit: CYGNSS project)
• GDN: CYGNSS selected USN to handle ground communications because of their extensive previous experience with missions similar to CYGNSS. Collocation of a backup CYGNSS MOC server at the USN/NMC (Universal Space Network/Network Management Center) can also be supported.
Note: Universal Space Network, Inc. (USN) is a U.S. based independent subsidiary of SSC with US government approval and oversight. USN is a leader in space operations and ground network services (GNS).USN provides unparalleled coverage through PrioraNet, a seamless network of worldwide satellite tracking and communications assets. These assets include both those owned by USN and those of our collaborative partners. Founded by the aerospace pioneer Charles “Pete” Conrad, Jr., USN reflects his leadership, innovative spirit and dedication to excellence. From the network management centers in Horsham, PA, and Newport Beach, CA, to the global network of ground stations. 19)
Each of the observatories in the CYGNSS constellation will be visible to the three ground stations within the USN for periods that average between 470 and 500 seconds of visibility per pass. Each observatory will pass over each of the three ground stations 6-7 times each day, thus providing a large pool of scheduling opportunities for communications passes. MOC personnel will schedule passes as necessary to support commissioning and operational activities. High-priority passes will be scheduled to support the solar array deployment for each observatory.
For all subsequent stages, the MOC schedules nominal passes for the USN stations for each observatory in the constellation per the USN scheduling process. Each observatory can accommodate gaps in contacts with storage capacity for greater than 10 days’ worth of data with no interruption of science activities.
The data returned from CYGNSS are expected to expand our knowledge of the rapidly changing environment in the core of a developing tropical cyclone. The SOC is responsible for data product development and dissemination. After science commissioning is complete and the mission enters its nominal science operations stage, the L2 data will be made available for public release.
The CYGNSS science team members will use the fully calibrated L2 data for their own research and make it available to the external user science community and eventually to operational users. Calibration/validation assessment of L2 data quality continues for the life of the mission using an updated version of the same wind field intercomparison database used during science commissioning. Twice a year, nominally at the beginning and end of the Atlantic hurricane season, engineering performance will be verified by a brief (approximately two-week) repeat of the instrument calibration activities performed during engineering commissioning.
The primary goals of CYGNSS are to measure ocean surface wind speeds in all weather conditions — including those inside the eyewall—and measuring wind speed with sufficient frequency to resolve genesis and rapid intensification in the inner core of a tropical cyclone. In addition to success with these two primary objectives, there is likely to be a secondary benefit with direct societal relevance: The CYGNSS team will produce and provide ocean surface wind speed data products to the operational hurricane forecast community and help them assess the value of these products for use in their retrospective studies of potential new data sources. In time, this information will be incorporated into models used to predict the evolution of hurricanes.
While improved hurricane forecasting is not the CYGNSS mission’s primary objective, it is hoped that hurricane prediction—in particular, hurricane intensity forecasts— will improve as a result of the data that the CYGNSS mission returns.
1) Christopher Ruf, “The NASA EV-2 Cyclone Global Navigation Satellite System (CYGNSS) Mission ,” August 27, 2012, URL of abstract: http://svcp.jpl.nasa.gov/cgi/mtgabstract.cgi?series=earth&meetingfile=../meetings/2012/es2012082701.txt; URL of presentation: http://svcp.jpl.nasa.gov/.../CYGNSS_27Aug2012_Ruf_CYGNSS_JPL_Seminar.pdf
2) Christopher S. Ruf, Scott Gleason, Zorana Jelenak, Stephen Katzberg, Aaron Ridley, Randall Rose, John Scherrer, Valery Zavorotny, “The CYGNSS Nanosatellite Constellation Hurricane Mission,” Proceedings of IGARSS (International Geoscience and Remote Sensing Symposium), Munich, Germany, July 22-27, 2012
3) “NASA’s Weather Prediction Project,” URL: http://aoss-research.engin.umich.edu/missions/cygnss/
4) “CYGNSS fact sheet,” UM, URL: http://aoss-research.engin.umich.edu/missions/cygnss/docs/CYGNSS_factsheet.pdf
5) John Dickinson, Chris Ruf, Randy Rose, Aaron Ridley, Buddy Walls, “CYGNSS: The Cyclone Global Navigation Satellite System's CubeSat Foundations,” 12th Annual JACIE (Joint Agency Commercial Imagery Evaluation) Workshop, St. Louis, MO, USA, April 16-18, 2013, URL: http://www.cubesat.org/images/stories/Spring_Workshop_2013/Dickinson_CYGNSS.pdf
6) J. D. Harrington, “NASA Selects Low Cost, High Science Earth Venture Space System,” NASA, June 18, 2012, URL: http://www.nasa.gov/home/hqnews/2012/jun/HQ_12-203_Earth_Venture_Space_System_CYGNSS.html
8) “SwRI building eight NASA nanosatellites to help predict extreme weather events on Earth,” SwRI, June 21,2012, URL: http://swri.org/9what/releases/2012/nanosatellites.htm
9) Randy Rose, Will Wells, Jillian Redfern, Debi Rose, John Dickinson, Chris Ruf, Aaron Ridley, Kyle Nave, “NASA’s Cyclone Global Navigation Satellite System (CYGNSS) Mission – Temporal Resolution of a Constellation Enabled by Micro-Satellite Technology,” Proceedings of the 27th AIAA/USU Conference, Small Satellite Constellations, Logan, Utah, USA, Aug. 10-15, 2013, paper: SSC13-IV-6, URL: http://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=2937&context=smallsat
10) Randy Rose, Chris Ruf, Debi Rose, Marissa Brummitt, Aaron Ridley, “The CYGNSS Flight Segment; A Major NASA Science Mission Enabled by Micro-Satellite Technology,” IEEE Aerospace Conference, Big Sky, MT, USA, March 2-9, 2013, URL: https://3c.gmx.net/mail/client/dereferrer?redirectUrl=http%3A%2F%2Fktb.engin.umich.edu%2FRSG%2Fpubs_files%2FAeroConf-2013_Rose-etal_CYGNSS-Flight-Segment.pdf&selection=tfol11c44a7b1e85f8f8
11) Christopher Ruf, Allison Lyons, Alan Ward, “NASA Intensifies Hurricane Studies with CYGNSS,” NASA, The Earth Observer, May-June 2013, Volume 25, Issue 3, pp. 12-21, URL: https://3c.gmx.net/mail/client/dereferrer?redirectUrl=http%3A%2F%2Fktb.engin.umich.edu%2FRSG%2Fpubs_files%2FThe-Earth-Observer-2013-25-3_Ruf-etal_CYGNSS.pdf&selection=tfol11c44a7b1e85f8f8
12) T. Finley, D. Rose, W. Wells, J. Redfern, K. Nave, R. Rose, C. Ruf, "Techniques for LEO Constellation Deployment and Phasing Utilizing Differential Aerodynamic Drag," AAS/AIAA Astrodynamics Specialist Conference, Hilton Head Island, SC, USA, August 11-15, 2013.
13) Chris Ruf, Scott Gleason, Zorana Jelenak, Stephen Katzenberg, Aaron Ridley, Randy Rose, John Scherrer, Valery Zavorotny, “The NASA EV-2 Cyclone Global Navigation Satellite System (CYGNSS) Mission,” IEEE Aerospace Conference, Big Sky, MT, USA, March 2-9, 2013, URL: https://3c.gmx.net/mail/client/dereferrer?redirectUrl=http%3A%2F%2Fktb.engin.umich.edu%2FRSG%2Fpubs_files%2FAeroConf-2013_Ruf-etal_CYGNSS-Mission.pdf&selection=tfol11c44a7b1e85f8f8
14) Scott Gleason, “Remote Sensing of Ocean, Ice and Land Surfaces Using Bi-statically Scattered GNSS Signals From Low Earth Orbit,” Ph.D. Thesis, University of Surrey, Gilford, UK, January 2007, URL: http://aoss-research.engin.umich.edu/missions/cygnss/reference/gnss-overview/Gleason_Thesis_GNSS.pdf
15) M. P. Clarizia, C. Gommenginger, S. Gleason, C. Galdi, M. Unwin, “Global Navigation Satellite System-Reflectometry (GNSS-R) from the UK-DMC Satellite for remote sensing of the ocean surface,” Proceedings of IGARSS 2008, Boston, MA, USA, July 6-11, 2008, URL: http://www.sstl.co.uk/getattachment/8ea1074b-b37a-4829-8345-b1bb02656d02/SGR-ReSI
16) Martin Unwin, Philip Jales, Paul Blunt, Stuart Duncan, Marissa Brummitt, Christopher Ruf, “The SGR-ReSI and its application for GNSS reflectometry on the NASA EV-2 CYGNSS mission,” IEEE Aerospace Conference, Big Sky, MT, USA, March 2-9, 2013, URL: http://ktb.engin.umich.edu/RSG/pubs_files/AeroConf-2013_Unwin-etal_SGR-ReSI.pdf
17) “SGR-ReSI Space GNSS Instrument,” SSTL, URL: http://www.sstl.co.uk/Products/Subsystems/Flying-Soon/SGR-ReSI
18) Debi Rose, Michael Vincent, Randy Rose, Chris Ruf, “The CYGNSS Ground Segment; Innovative Mission Operations Concepts to Support a Micro-Satellite Constellation,” IEEE Aerospace Conference, Big Sky, MT, USA, March 2-9, 2013, URL: https://3c.gmx.net/mail/client/dereferrer?redirectUrl=http%3A%2F%2Fktb.engin.umich.edu%2FRSG%2Fpubs_files%2FAeroConf-2013_Rose-etal_CYGNSS-Ground-Segment.pdf&selection=tfol11c44a7b1e85f8f8
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.