The ISRO (Indian Space Research Organization) spacecraft OceanSat-2 is envisaged to provide service continuity for the operational users of OCM (Ocean Color Monitor) data as well as to enhance the application potential in other areas. OCM is flown on IRS-P4/OceanSat-1, launched May 26, 1999. The main objectives of OceanSat-2 are to study surface winds and ocean surface strata, observation of chlorophyll concentrations, monitoring of phytoplankton blooms, study of atmospheric aerosols and suspended sediments in the water. OceanSat-2 will play an important role in forecasting the onset of the monsoon and its subsequent advancement over the Indian subcontinent and over South-East Asia. - The OceanSat-2 mission was approved by the Government of India on July 16, 2005. 1) 2) 3)
Coverage of applications:
• Sea-state forecast: waves, circulation and ocean MLD (Mixed Layer Depth)
• Monsoon and cyclone forecast - medium and extended range
• Observation of Antarctic sea ice
• Fisheries and primary production estimation
• Detection and monitoring of phytoplankton blooms
• Study of sediment dynamics
Figure 1: Illustration of the deployed OceanSat-2 spacecraft (image credit: ISRO)
OceanSat-2 is a three-axis stabilized spacecraft configured around the proven IRS bus along with improved mission-specific subsystem designs. The main structure is made up of a CFRP (Carbon Fiber Reinforced Plastic) composite cylinder with a PSLV interface ring. Three deployment mechanisms are included: a) solar panel auto-deployment after separation from the launcher, b) OCM hold-down-release-tilt mechanism, and c) OSCAT antenna hold-down-release mechanism. The thermal design of the spacecraft employes both passive and active control elements. 4) 5) 6)
The EPS (Electrical Power Subsystem) uses two solar arrays with silicon cells, the size of the arrays is identical to those of IRS-P6/P5. A power storage capacity of 24 Ah is provided by 2 NiCd batteries for eclipse operations. All onboard subsystems are supplied with two raw buses of 28 - 42 V, and DC-DC converters are used to derive required voltage lines. A centralized BMU (Bus Management Unit), designed with a MAR31750 microprocessor, provides the service functions for AOCS (Attitude and Orbit Control Subsystem), sensor processing, TT&C (Telemetry, Tracking & Command), auto-temperature control, and for PSK demodulation of the TT&C uplink carrier. Attitude sensing is provided by Earth horizon sensors, digital sun sensors, triaxial magnetometers, sun sensors with a FOV of 4π sr, and a and gyroscope-based inertial reference unit. Actuation is provided by four reaction wheels (5 Nms, 0.1 Nm) mounted in a tetrahedral configuration, , two magnetic torquer coils, and monopropellant hydrazine thrusters. An 8-channel SPS (Standard Positioning Service) GPS receiver provides both position and velocity, improving the overall orbit determination accuracy.
The payload data handling system is a new design; it is configured to transmit OCM and scatterometer data on a single carrier with QPSK modulation. The OCM data will be transmitted on the I-channel, while the OSCAT / ROSA data will be transmitted on the Q-channel. An indigenous onboard SSR (Solid-State Recorder) of 64 Gbit capacity is used to record the processed data of OSCAT and ROSA continuously; the OCM data is being recorded per requirement.
RF communications: The payload telemetry data transmission system is configured using SSPAs (Solid-State Power Amplifier) with a conventional X-band antenna. In addition, there is a TT&C subsystem in S-band for spacecraft control.
The ground segment elements for OceanSat-2 include the SCC (Spacecraft Control Center) at ISTRAC (ISRO Telemetry, Tracking and Command Network), Bangalore; the payload data reception station at NRSA (National Remote Sensing Agency) Shadnagar; data processing, data product generation and dissemination to users at NRSA, Balanagar, Hyderabad; data product software development at SAC (Space Application Center), Ahmedabad; and development of mission software, flight dynamics software and mission management at ISAC (ISRO Satellite Center), Bangalore, India.
Table 1: Overview of spacecraft parameters
Figure 2: Stowed configuration of OceanSat-2 (image credit: ISRO)
Secondary payloads (6) on the flight are the following CubeSats:
• UWE-2 (University of Würzburg), Würzburg, Germany
• BeeSat (Technical University of Berlin), Berlin, Germany
• SwissCube, a CubeSat of Ecole Polytechnique Federale de Lausanne, Lausanne, Switzerland
• ITUpSat (Istanbul Technical University PicoSatellite-1), Istanbul, Turkey.
• Rubin-9.1 and Rubin-9.2 nanosatellites of OHB-System, Bremen, Germany (each with a mass of 8 kg). Both nanosatellites are equipped with newly developed types of spaceborne AIS receivers.
The secondary payload (CubeSat) launch service provider (supporting integration and contracting efforts) is ISIS (Innovative Solutions In Space), Delft, The Netherlands. All of the four CubeSats are equipped with the SPL (Single Picosatellite Deployer) system of Astrofein (Astro und Feinwerktechnik Adlershof GmbH), Berlin, Germany.
Orbit: Sun-synchronous near circular orbit, altitude ~720 km, inclination = 98.28º, period = 99.31 min, the LTAN (Local Time on Ascending Node) is at 12:00 hours ±10 minutes, revisit cycle of 2 days.
The OceanSat-2 tracking system is S-band tone ranging from ISTRAC (ISRO Telemetry Tracking and Command Network) ground stations. The ranging system is CORTEX. Tracking measurements are two-way range, Doppler and angles (Azimuth and Elevation). 9)
• September 7, 2017: OceanSat-2 L2 25 km and 50 km winds CDR (Climate Data Records) have been created covering the period from 15 December 2009 to 20 February 2014, using the latest state-of-the-art wind retrieval algorithms. 10)
- Scatterometer data have been collocated with ECMWF (European Centre for Medium-Range Weather Forecasts) ERA-Interim winds and scatterometer level 2 wind products have been created at 25 km and 50 km swath grid.
- The data records, OSCAT L2 25 km winds data record release 1 - OceanSat and OSCAT L2 50 km winds data record release 1 - OceanSat, in BUFR and NetCDF format, will be available soon from the EUMETSAT Data Center.
• June 9, 2017: ESA is pleased to announce that an improved access to the collections distributed online by ESA through the TPM (Third Party Missions) program has been deployed. Two enhanced features have been set up to ease access to these collections: 11)
- A TPM online access list has been created to provide users with a centralized access to all the TPM online collections dissemination systems (17 TPM collections)
- A new online dissemination system providing a geographical catalogue, a folder tree access and a filename search has been deployed for 13 TPM collections.
From the new TPM L-OADS (Online Dissemination System), the products can be downloaded immediately if the user is logged in and authorized for the specific product download. The below collections are currently available; ESA will, however ,continuously populate this new dissemination system with new collections.
- IKONOS ESA archive
- Image 2006
- KOMPSAT-2 ESA archive (NEW)
- Oceansat-2 NRT data
- Pleiades ESA archive (NEW)
- RapidEye ESA archive
- RapidEye South America
- RapidEye time series for Sentinel-2
- Seasat ESA archive
- SPOT-1 to 5 ESA archive
- SPOT-6 and 7 ESA archive (NEW)
- Tropforest Dataset
- WorldView-2 European Cities.
• May 2017: ISRO report on Ocean Color Activities. 12)
Figure 3: OCEANSAT-2 OCM Feb 6, 2017 Irrawaddy River Delta, Bay of Bengal (image credit: ISRO/SAC, Prakash Chauhan)
• April 26, 2016: ESA, in collaboration with GAF AG (Gesellschaft für Angewandte Fernerkundung, Munich), acquires and process every day OceanSat-2 passes over the Neustrelitz reception station from 1 January 2016. All passes are systematically processed to levels 1B, 2B and 2C, and available for users in NRT (< 3 hours). Products are available in: 13)
1) Level 1B: Geophysical Data containing Radiance Data for all 8 Bands of OCM-2.
2) Level 2B: Geophysical Data L2B for given Geo physical parameter. Geo physical parameters: Chlorophyll, Aerosol Depth, Different Attenuation, Total Suspended Sediments.
3) Level 2C: Georeferenced Radiance Data for given geo physical parameter. Geo physical parameters: Chlorophyll, Aerosol Depth, Different Attenuation, Total Suspended Sediments.
Figure 4: One day acquisition sample (3 passes) - Circle is Neustrelitz acquisition mask (image credit: DLR, ESA)
• Feb. 11, 2016: The OceanSat-2 spacecraft is still operational after the OSCAT instrument was switched-off in April 2014. It continues to operate with the other two sensors: OCM-2 and ROSA. 14)
- OCM-2 (Ocean Color Monitor-2), a multispectral (eight bands in visible and near infrared) electrooptical payload operating in two modes:
1) LAC (Local Area Coverage), with a GSD of 360 m. The LAC data is received in realtime with provision for reception in SSR mode as well. In LAC mode, the data is received over two passes per day.
2) GAC (Global Area Coverage) with a GSD of 1 km. In GAC mode, the data is received only in SSR mode with full globe coverage in 8 days.
The data is received at the following ground stations: NRSC Hyderabad (LAC and GAC Mode); INCOIS, Hyderabad (LAC Mode); Mauritius (LAC Mode); Euromap, Germany (LAC Mode).
ROSA (Radio Occultation Sounder for Atmospheric studies), a GPS occultation receiver of ASI (Italian Space Agency), is operational with four dumps of data received daily at NRSC, Hyderabad where Level-1 processing of the data is carried out.
OSCAT (OCEANSAT-2 Scatterometer) proved to be an excellent instrument and the widespread acceptance and usage of the OSCAT data by various international agencies like NASA/JPL, NOAA, KNMI, EUMETSAT and ECMWF, for both research and operational purposes, is a testimony to its quality and utility. OSCAT had developed snags in the transmitter subsystem system of primary chain and scan mechanism of the redundant (backup) chain during the end of its design life period (around 4½ years from launch), forcing the ISRO to suspend its operation in April 2014. During its life time, OSCAT has demonstrated the importance and significance of the ocean wind vector data in weather modeling and prediction and laid the ground for continuation of the global ocean surface wind vector data acquisition. Based on this experience, ISRO developed the SCATSAT-1 (Scatterometer Satellite) spacecraft, which is scheduled for launch in July 2016.
• August 2015: The OceanSat-2 spacecraft and its sensor complement (OCM-2 and ROSA) are operating nominally in 2015; the OSCAT instrument is unavailable since Feb. 2014. ISRO terminated the operation of OSCAT in April 2014. 15) 16) 17)
- Following the loss of OSCAT, only ASCAT data from MetOp-A and -B of EUMETSAT are currently available in near realtime. Since both MetOp satellites are on the same orbit, there is now a lack of scatterometer data on a well separated orbital plane.
- The PO.DAAC regrets to inform its users of the discontinuation of data retrieval for its OSCAT Level 2B dataset produced by the QuikSCAT science data systems team at JPL. The OSCAT scatterometer onboard the OceanSat-2 spacecraft experienced failure in its traveling wave tube amplifier (TWTA) toward the end of February 2014. As a result, science data processing of ocean surface wind vector data has permanently ceased. The last available file from the QuikSCAT science data systems team provided to the PO.DAAC is effective for 20 February 2014 corresponding to orbital revolution 23370 ending at 2330 UTC. The data retrieved over the course of the mission will remain available at the PO.DAAC for continued public distribution. 18)
Figure 5: CGMS (Coordination Group for Meteorological Satellites), image credit: CEOS Ocean Surface Vector Winds Virtual Constellation, April 2014 19)
• On Feb.20, 2014, the OSCAT (OceanSat-2 Scanning Scatterometer) instrument experienced an irrecoverable malfunction in its TWTA (Traveling Wave Tube Amplifier) and is not operating anymore. As a result, science data processing of ocean surface wind vector data has permanently ceased. 20) OSCAT failed shortly before the completion of the intended five year instrument lifespan. The OSCAT scatterometer on OceanSat-2 was ISRO's first active microwave payload. 21)
• The OceanSat-2 spacecraft and its sensor complement are operating nominally in 2014.
• On Nov. 6, 2013, the OSCAT instrument of OceanSat-2 measured Haiyan's surface winds as shown in Figure 6. The arrows indicate wind direction while the colors indicate wind speed, with darker shades of purple indicating stronger winds (the strongest ones are shown in red). As is typical of cyclones in the northern hemisphere, the area of strongest winds was northeast of the storm center. 22)
According to the OceanSat-2 data, which was processed by scientists at NASA/JPL (Jet Propulsion Laboratory) using an experimental technique, the storm's winds peaked at 206 km/hr at the time of measurement—strong enough to devastate the landscape.
Figure 6: Assessing Haiyan's Winds with OSCAT on OceanSat-2 (image credit: NASA Earth Observatory, acquired on Nov. 6 and released on Nov. 13, 2013)
• The OceanSat-2 spacecraft and its sensor complement are operating nominally in 2012 (3rd year on orbit). 23)
- Progress is being reported in the context of the ROSA-ROSSA processing chain. 24)
• Since NASA's SeaWinds scatterometer on the QuikSCAT spacecraft ceased nominal operations in November 2009, scientists and engineers from NASA, JPL, and NOAA (National Oceanic and Atmospheric Administration) have collaborated with ISRO in ongoing efforts to calibrate and validate OSCAT (OceanSat-2 Scanning Scatterometer) measurements in order to ensure continuous coverage of ocean vector winds for use by the global weather forecasting and climate community.25) 26)
The satellite image of Hurricane Irene (Figure 7), showing the storm's ocean surface wind speed and direction, was acquired at 1:07 a.m. EDT on Aug. 27, 2011 approximately six hours before it hit the North Carolina coast. The data are provided courtesy of the Indian Space Research Organization (ISRO) from the OSCAT instrument on ISRO's OceanSat 2 spacecraft, launched in September 2009. Wind vector data processing was performed at NASA/JPL, Pasadena, CA. The OSCAT winds are obtained at a resolution of 25 km x 25 km and do not resolve the hurricane's maximum wind speeds, which occur at much finer scales.
Figure 7: NASA-ISRO OSCAT image shows Irene's winds before landfall on Aug. 27, 2011 (image credit: NASA, ISRO)
Legend to Figure 7: Hurricane Irene made landfall early Saturday morning, Aug. 27, 2011, just west of Cape Lookout, NC (USA), as a category one hurricane with maximum sustained winds of 136 km/h (75 knots). It is currently over eastern North Carolina and is forecast to gradually weaken as it moves northward along the East Coast of the United States over the next two days.
• NOAA has been receiving day-old OSCAT data via the ISRO dedicated FTP server since September 2010. 27)
• At KNMI (Royal Netherlands Meteorological Institute), the OSCAT (OceanSat-2 Scatterometer) instrument data are processed towards a 50 km product for the EUMETSAT OSI SAF (Ocean and Sea Ice - Satellite Applications Facility). 28)
KNMI hosts the operational deployment of this product as part of the EUMETSAT OSI SAF, a project by Météo France, the Norwegian Meteorological Institute, the Danish Meteorological Institute, the Swedish Meteorological Institute, Ifremer and the Royal Netherlands Meteorological Institute (KNMI). 29)
At KNMI, OSCAT data are acquired at the Svalbard ground station and sent to India for further processing, with a backup facility at the EUMETSAT headquarters. EUMETSAT then makes available near real-time Level 2a scatterometer products through EUMETCast. These products are used as basis for further processing at KNMI. The wind products are distributed in one resolution with 50-km cell spacing. The product has a timeliness of approximately 1-1.5 hours from the last sensing time in a product file.
• The OceanSat-2 spacecraft and its sensor complement are operating nominally in 2011.
Figure 8: OCM-2 image of phytoplankton bloom in the Arabian Sea (image credit: ISRO, Ref. 23)
Figure 9: OCM-2 LAC data downloaded over Indian & International Ground Stations (USA, South Korea, Europe, Malaysia, Thailand, Australia), image credit: ISRO 30)
• The in-flight commissioning phase of ROSA was completed in July 2010 (Ref. 47).
• In May 2010, the first ROSA-ROSSA release was installed into the Italian operative ROSA Ground Segment (the same processing chain will be also installed into the Indian ground segment). ROSA-ROSSA scientific validation was performed using observations collected by other Radio Occultation mission data and it was carried out following two procedures. The input dataset was defined considering one day of real observations carried out in the framework of the operative COSMIC, CHAMP and GRAS on-board MetOp-A radio occultation missions (Ref. 49).
- Firstly, ROSA-ROSSA products at each level were statistically compared with the corresponding products generated by the other processing software, giving in input the raw data observed by the other occultation mission.
- Secondly, ROSA-ROSSA and COSMIC original refractivity and temperature profiles (obtained giving in input to ROSA-ROSSA the corresponding COSMIC raw observations) were individually compared on a statistical base with co-located ECMWF 91 level analysis.
• In April 2010, the data from the OceanSat-2 payloads ((OCM-2 and OSCAT) have started flowing to the users. Actually, the OCM-2 data products are available from January 1, 2010 onwards.
• In early 2010, the spacecraft and two payloads (OCM-2 and OSCAT) are operating nominally. 31)
Figure 10: Distribution of chlorophyll-a concentration retrieved from OCM-2 data on Sept. 24, 2009 in the north-eastern Arabian Sea (image credit: ISRO, Ref. 5)
Figure 11: OCM-2 GAC image of the global oceans observed in the timeframe September 2-9, 2010 (image credit: ISRO/NRSC)
• All the three payloads on-board the OceanSat-2 have been successfully turned on providing good quality data. 32)
Sensor complement: (OCM-2, OSCAT, ROSA)
OCM-2 (Ocean Color Monitor-2):
OCM-2 is an improved version of the one flown on OceanSat-1. OCM-2 is a solid-state radiometer providing observations in eight spectral bands in the VNIR region. The instrument employs pushbroom scanning technology with linear CCD detector arrays (191 6K CCD) of 6000 elements (3730 active detectors in the center are used to cover the image field, the rest are used to correct for dark current). A swath width of 1420 km is provided. An along-track instrument tilt capability of ±20º is provided to avoid sun glint. OCM optics is based on one lens per band (wide angle telecentric lens design, refractive system). The ground resolution is 360 m in the along-track and 236 m in the cross-track direction. 33) 34) 35) 36) 37) 38)
The processing electronics consists of a video processor, timing logic and interface circuits. An onboard calibration scheme, using light emitting diodes (LEDs) mounted near each CCD, is incorporated to study long-term stability of the radiometric performance.
Figure 12: Illustration of OCM-2 spectral bands (image credit: ISRO)
Two modes in ground resolution are supported:
- LAC (Local Area Coverage): 360 m (cross-track) x 236 m (along-track)
- GAC (Global Area Coverage): 1 km.
Table 2: Specification of the OCM instrument
The configuration of the OCM payload is identical to the one flown in IRS-P4 (OceanSat-1) except that the spectral band is modified for band 6 and band 7. For band 6, the center wavelength is shifted from 670 nm to 620 nm to improve the reflectance from suspended sediments; for band 7, the center wavelength is shifted from 760 nm to 740 nm to avoid oxygen absorption. However, the bandwidth remains same in both cases.
Table 3: Spectral bands of OCM-2 and their applications
Figure 13: Illustration of the OCM instrument (image credit: ISRO)
Figure 14: Chlorophyll-a distribution by OCM-2 (LAC, Sept. 27, 2009) over the parts of the western Arabian Sea (image credit: ISRO)
OSCAT (OceanSat-2 Scanning Scatterometer):
OSCAT is an active microwave device designed and developed at ISRO/SAC, Ahmedabad. The objective is to monitor ocean surface wind speed and directions. The instrument is a pencil beam wind scatterometer operating at Ku-band of 13.515 GHz. OSCAT is being utilized for the estimation of the radar backscattered power and subsequent local and global wind vector (velocity magnitude and direction) retrieval over the ocean, from the normalized radar cross-section (σo), for cell resolution grids of 25 km x 25 km over a swath of 1400 km. The aim is to provide global ocean coverage and wind vector retrieval with a revisit time of 2 days. 39) 40) 41)
The scanning configuration of OSCAT, similar in design to Seawinds of NASA, offers the advantages like simpler onboard payload, better radar backscatter cross section (σo) measurement and directional accuracy, continuous and wider swath with no nadir gaps, less complex signal processing and reduced data rates, smaller and lighter onboard instrument and simplified wind retrieval model compared to conventional multiple fan beam scatterometers.
The OSCAT onboard processing requirements are:
- Digital IQ demodulation and decimation
- Doppler shift computation for received return echo
- Doppler frequency compensation
- Reference chirp generation and de-chirping of echo returns
- Multiple 1 K complex FFTs of the de-chirped data
- Binning for estimation of signal+noise energy for every pulse
- Noise filtering and binning for noise-only estimation for every pulse
- Formatting of processed and payload and spacecraft auxiliary data
- Optional formatting and transmission of sensor raw data for selected acquisitions over Indian visibility regions.
Figure 15: Onboard signal processor implementation of OSCAT (image credit: ISRO)
The OSCAT payload design includes many new elements; it consists of an antenna, rotary joint, scan mechanism and switch assembly, transmitter, receiver, frequency generator, internal calibration unit and digital subsystems, that is to say: DCG (Digital Chirp Generator), DACS (Data Acquisition and Compression Subsystem), and payload controller. The frequency generator provides coherent reference frequencies for other onboard units and also generates LFM (Linear Frequency Modulated) pulses for transmission.
The OSCAT parabolic dish antenna has a diameter of 1 m diameter which is offset-mounted with a cant angle of 46º with respect to the yaw axis (earth viewing axis). The antenna is continuously rotated at 20.5 rpm using a DC motor with the scan axis along the +velocity yaw axis. By using two offset feeds at the focal plane of the antenna, two beams (inner beam and outer beam) are being generated which scan the ground surface in a conical fashion.
The antenna consisting of two off-axis near prime focus feeds along with a 1 m paraboloid reflector creates inner and outer beams, which operate in an interleaved manner with an effective PRF (Pulse Repetition Frequency) of 100 Hz each. The antenna, is conically scanned about the positive yaw axis at 20.5 rpm, by an appropriate scan mechanism. The received signal is amplified and down converted to generate the IF signal. This IF received signal from the receiver is fed to the onboard digital system for subsequent digitization, digital I/Q demodulation and signal processing. The raw and processed data are fed to the spacecraft data handling unit for ground transmission.
Figure 16: Photo of the OSCAT instrument at integration (image credit: ISRO)
The two pencil beams, inner and outer, result in a constant angle of incidence for both beams; this allows σo measurements at multiple (4 or 2) azimuth angles for the same point on the ocean surface. Each point in the inner swath is viewed twice at different azimuth angles by both beams. The region between the inner and outer swath is subjected to two measurements by only the outer beam, and the wind vector there can only be determined with a directional ambiguity of 180º.
Table 4: Specification of the OSCAT instrument
Figure 17: Observation geometry of the OSCAT instrument (image credit: ISRO)
Due to the very low receiver bandwidth of ±800 kHz, a single channel digital I/Q demodulation scheme has been implemented instead of the conventional analog I/Q demodulator. This approach results in compact RF and digitizer hardware and offers better signal fidelity in handling low bandwidth signals.
For the OSCAT instrument, real-time onboard signal processing involving range compression is mandatory considering the global mode of sensor operation, as it reduces the effective output data rate of the sensor by a large factor (~50). Also, the Doppler shift computation and subsequent Doppler compensation (within ±550 kHz) will be carried out in the signal processor itself, prior to range compression. To extract the range information, FFT (Fast Fourier Transforms) are performed on the deramped signal and an average periodogram is formed by applying magnitude-squaring operations. The DSP (Digital Signal Processor) hardware is based on an FPGA implementation.
Table 5: Design specification of the OSCAT digital signal processor
The OSCAT instrument employs linear chirp transmission and digital de-ramping receiver techniques to measure surface backscatter with better accuracy and without compromising on range resolution. It uses digitally generated linear frequency modulation (LFM) transmit signal, having 400 KHz bandwidth and 1.35 ms pulse duration. The return echo signal is subsequently processed by a digital IF Receiver and signal processor based on a high-speed digitizer and FPGA (Field Programmable Gate Array).
The onboard range compression signal processor algorithm implemented in Xilinx XQVR600 FPGA is based on periodogram estimation approach, where multiple 1K FFT of contiguous (50% overlapped) data partitions are averaged and binning is performed on the averaged spectrum to obtain signal+noise and noise-only energy estimates.
The digital receiver-signal processor hardware consists of a multi-layered PC Board called DACS (Data Acquisition and Range Compression System) module. It is based on Atmel's (Now E2V) TS8388B ADC and TS81102G0 Demux and Xilinx Virtex XQVR600 FPGA. The DACS unit has a mass of 2.25 kg and consumes 22.5 W of bus power.
Figure 18: Digital receiver/processor of OSCAT (image credit: ISRO)
Figure 19: Photo of the OSCAT onboard processor flight module package (image credit: ISRO)
Figure 20: A schematic view of the OSCAT scanning geometry (image credit: ISRO, Ref. 17)
ROSA (Radio Occultation Sounder for Atmospheric studies):
ROSA is a new GPS occultation receiver provided by ASI (Italian Space Agency). A MOU (Memorandum of Understanding) between ASI and ISRO was signed in Fukuoka, Japan in Oct. 2005. The objective of ROSA is to characterize the lower atmosphere and the ionosphere, opening the possibilities for the development of several scientific activities exploiting these new radio occultation data sets. 42) 43) 44) 45) 46) 47) 48)
The ROSA instrument has been developed by TAS-I (Thales Alenia Space-Italia) formerly Laben. The ROSA payload is a dual channel GPS receiver with two antennae and a receiver package. The accommodation of the ROSA instrument on board the OceanSat-2 spacecraft has been driven by the satellite configuration that allows the possibility to install only one radio occultation antenna in the flight direction of the spacecraft. This means that only rising occultation events can be detected by the ROSA instrument in this mission.
The radio occultation antenna, looking along the satellite velocity vector, receives signals from the 'rising' GPS satellites near the Earth horizon. These signals get refracted by the atmosphere and from the bending angle, the temperature and humidity profiles are derived. The POD (Precise Orbit Determination) antenna, looking at the zenith of the satellite, gives precise position of the receiver.
Table 6: Major parameters of the ROSA instrument
The ROSA instrument, in its complete configuration, is composed of the following parts:
• One hemispherical-coverage navigation and POD (Precise Orbit Determination) antenna to acquire GPS signals to determine position, velocity and time of the LEO satellite.
• Velocity and anti-velocity radio occultation antennas to acquire GPS signals used in the calculus of all parameters used in the atmospheric sounding (for complete instrument).
• The receiver unit which processes L1CA and L2P(Y) codeless GPS signals from all the antennas. 16 dual-frequency channels (implemented in 4 AGGA-2 chips) are available in the ROSA receiver, and can be assigned to different combinations of GPS satellites and POD or RO antennas. A MIL-STD-1553 communication interface is used to exchange telecommand, telemetry and measurement data with the satellite on-board computer.
• The receiver processing is performed by an ADSP-21020 (Analog Devices Signal Processor-21020).
The radio occultation data processing for the ROSA receiver, which is called ROSA-ROSSA (ROSA-Research and Operational Satellite and Software Activities), has been supported by the Italian Space Agency and has been developed by a pool of Italian Universities and Research Centers. 49)
Figure 21: Illustration of ROSA instrument components (image credit: ASI)
The ROSA data will be downlinked to the Indian and the Italian receiving stations - to be processed by the ROSA ground segment, completely developed by Italian universities and research centers (Figure 26). In particular, this ground segment will be implemented at first level in an integrated computing infrastructure installed in Matera (Italy) and mirrored at Hyderabad, India and, at a second level, on a distributed software and hardware infrastructure. This second infrastructure will perform the rapid POD (Precise Orbit Determination) and prediction, the unambiguous bending and impact parameters profiles extraction, the ionospheric correction and the stratospheric initialization, the refractivity, pressure, temperature and humidity profile retrieval, the value added services for meteorology, climate and space weather applications. This will identify a prototype of distributed and Multimission Ground Processing Center distributed through the various research centers and universities involved, connected through a Web-based GRID computing infrastructure.
The ROA (ROSA Occultation Antenna) accommodation has been driven by the presence of the scatterometer by tilting of 15º on the satellite yaw axis. Moreover, in January 2010 ISRO decided to rotate on yaw axis in the same direction of other 20º for mission operation reasons. This final accommodation of the ROA antenna affects the number and type of occultation events that can be detected with respect to the optimal, velocity pointing, configuration (Ref. 47).
Figure 22: FOV (Field of View) of the ROA velocity vector (image credit: TAS-I, ASI)
The quantity and quality of the occultations have been affected by the tilting of the ROA antenna of 35º on yaw axis that moreover introduce multipath effect due to the structure of the scatterometer and solar panel. These effects are visible into the following figures that represent the C/No vs azimuth/elevation (Figure 23) and the polar plot of the satellite visibility (Figure 24). The C/No plot shows the presence of two lateral sides with lower gain due to multipath effects.
The polar plot show how the visibility for occultation events (yellow and red) is restricted from -60º to +20º in azimuth, where 0º is the velocity direction. The yellow lines represent the occultation events, the red lines represent the Svs in rising until reaching the 0º of elevation. The blue lines are the satellites tracked by the POD antenna.
The number of occultation per day is also affected by the ROA direction: ~300 occultations/day are expected for a field of view of 90º, while the average value for ROSA on OceanSat-2 is about 255 occultation/day. This number is in line with the expectations for ROSA on OceanSat-2.
Figure 23: L1 C/No in azimuth/elevation of ROA (image credit: TAS-I, ASI)
Figure 24: Polar visibility of ROA (image credit: TAS-I, ASI)
1) OceanSat-2 of ISRO, launched on Sept. 23, 2009.
2) SAC-D of CONAE and NASA, launched on June 10, 2011.
3) Megha Tropiques of ISRO and CNES, launched on Oct. 12, 2011.
Figure 25: Block diagram of the ROSA instrument (image credit: ASI, TAS-I)
Legend to Figure 25: The OceanSat-2 tailoring features only one occultation antenna (the velocity antenna).
The existing ISTRAC stations at Lucknow, Bearslake, Mauritius and Biak will be used for TT&C (Telemetry Tracking and Command) support under the control of SCC which will carry our mission operations, satellite health monitoring and analysis, and payload operations scheduling/programming (Ref. 3).
The NRSA (National Remote Sensing Agency) DRS (Data Reception Station) is located at Shadnagar, Hyderabad with minor augmentation will receive the payload data both in real-time as well as in playback from the on-board memory. The received data will be separated instrument-wise (OCM-2, OSCAT, ROSA)) and recorded on the RAIDS (Redundant Array of Independent Disk Storage) memory of the DAQLB (Data Acquisition and Quicklook Browsing) system. Quicklook display, browse generation, calibration analysis and ADIF (Auxiliary Data File) generation will be carried out here. The data will be transferred to Balanagar on a high-speed data link in offline mode.
Data processing and products generation: The DPS (Data Processing System) at Balanagar will be the nodal center for processing the data from OceanSat-2, with support from SAC (Space Applications Center), Ahmedabad. The DPS is in charge to process the raw science data and generates the ocean color data products with in-built work order generation, online quality control, output media preparations, data quality evaluation, and feedback to mission operations. - Associated development of mathematical formulations, geometric and radiometric look-up tables and their update, associated software tools and geophysical model functions for wind vector derivation from the OSCAT data are part of the overall data products generation at different levels.
OceanSat-2 will provide two types of science data: LAC (Local Area Coverage) at 360 m resolution, and GAC (Global Area Coverage) at 1 km and at 4 km resolutions.
On Sept. 12, 2011, a new state-of-the-art OceanSat-2 ground station (data reception and processing system) was inaugurated in Hyderabad. The OceanSat-2 ground station, fitted with a 7.5 m diameter antenna, is capable to cover an area of 5,000 km in diameter, covering the Bay of Bengal on the east and the Arabian sea in the west. The ground station was established to receive and process data from Ocean Colour Monitor on-board the Indian Remote Sensing Satellite Oceansat-2 in real time. 52)
1) Information provided by V. Jayaraman of ISRO HQ, Bangalore, India
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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 (email@example.com).