Megha-Tropiques (Meteorological LEO Observations in the Intertropical Zone)
Megha-Tropiques (or MT) is a cooperative experimental mission of ISRO and CNES, the space agencies of India and France, with the objective to study the convective systems (water cycle and energetic exchanges) that affect the ITCZ (Intertropical Convergence Zone), in particular in the latitudes between 10º and 20º, with satisfactory temporal sampling. The most energetic tropical systems, such as the cloud clusters of the ITCZ, the monsoon systems and the tropical cyclones, extend over hundreds of kilometers. Hence, a ground resolution of about 10 km is adequate for these observations. Megha-Tropiques, with its unique combination of scientific payloads and its special near-equatorial orbit (offering improved data sampling of the ITCZ), is expected to provide valuable data for climate research. - Note: Megha means "cloud" in Sanskrit; Tropiques is the French word for "tropics."
Data gathered by the mission will be used to study the tropical climate system (complementary to the data of the other weather satellites in sun-synchronous orbits). The goals are to:
• Study the life cycle of the convective systems and their interactions with the environment. This requires to obtain the simultaneous observations of the different parameters like: precipitation, 3D water vapor distribution, radiative budget. The present LEO weather satellites only allow an approximate matching of the evaluations done during the same day, but at different hours (the repetitive cycle is much too low).
• Study scale interactions: convective systems - regional scale weather regimes - links with the global tropical system and climate.
The principles of the mission are to provide frequent sampling of the intertropical convergence zone measuring:
- Cloud properties and precipitation (MADRAS instrument)
- Water vapor horizontal and vertical distribution (SAPHIR instrument)
- Radiative fluxes (ScaRaB instrument).
Within the initial project agreement and work-sharing allocations (Joint decision of mission in 1998), the satellite used the French Proteus bus, the S/C integration was to be performed at CNES while the payload integration was to be done at ISRO. On the operational side, CNES was to provide the S/C operations via its own ground station; ISRO's ground station at Bangalore was to be used as the Mission Science Center. The science mission was also supported by CNRS/LMD (Laboratoire de Météorologie Dynamique) of Palaiseau, France. A statement of intent was signed on Nov. 21, 1999 in Bangalore between ISRO and CNES. A further ISRO/CNES mission agreement was signed in May 2001. 1) 2) 3) 4) 5) 6) 7) 8)
After much delay and many changes (in particular the reorganization of CNES along with a very tight budget constraints), India and France signed a MOU (Memorandum of Understanding) on Nov. 12, 2004 in Bangalore, India - to proceed with the development and implementation of the joint atmospheric satellite mission. Under the revised work-sharing arrangement, the role of CNES is to provide only two of the three meteorological payloads to accommodate the tighter budget requirements of CNES. ISRO will built and operate the spacecraft for the mission, and also receive, process and distribute the scientific data. ISRO will also launch the satellite using its PSLV (Polar Satellite Launch Vehicle) launcher. 9) 10) 11) 12)
Figure 1: Artist's rendition of the Megha-Tropiques spacecraft in orbit (image credit: ISRO, CNES)
The spacecraft consists of two major modules: the spacecraft bus, a cuboid of IRS satellite series heritage, and PIM (Payload Instrument Module). The PIM consists of a set of CFRP (Carbon Fiber Reinforced Plastic) based panels with appropriate interfaces for mounting onto the main platform and for mounting of the payloads and associated elements. Three deployment mechanisms are included - solar array deployment after separation from the launcher, MADRAS hold-down and release mechanism, and the MADRAS scan mechanism. 13) 14) 15) 16) 17) 18)
The spacecraft features a passive thermal control system augmented with heaters radiating the heat out of the spacecraft and highly reflective MLI (Multi-Layer Insulation) blankets, to maintain the required temperature distribution in the spacecraft. The power subsystem consists of an unregulated direct energy transfer dual bus system with a bus voltage varying from 28 - 42 V. The solar array is divided into two equal sections connecting two raw buses. Two 24 Ah NiCd batteries are directly connected to solar array.
A centralized BMU (Bus Management Unit), designed with a MAR31750 microprocessor, supports all functions of AOCS (Attitude Orbit Control Subsystem), sensor processing, TT&C handling, thermal management, on-board data storage logistics, and AHM (Ampere Hour Meter) processing.
The AOCS implementation uses a 3-axis body stabilized spacecraft with four reaction wheels mounted in tetrahedral configuration. The AOCS consists of various types of sensors (star sensors, digital sun sensors, 4π sun sensors, magnetometer, solar panel sun sensors, IRU (Inertial Reference Unit) with 3 DTGs (Dry Tuned Gyros) for the measurement of attitude errors, control electronics and different types of actuators such as reaction wheels, magnetic torquers and reaction control thrusters to impart thrust/torque to the spacecraft in the desired direction. A 10-channel SPS (Satellite Positioning System) is used, consisting of a C/A code GPS receiver at L1 (1575.42 MHz), providing both position and velocity, improving the overall orbit determination accuracy.
Figure 2: Deployed view of the Megha-Tropiques spacecraft (image credit: ISRO)
Figure 3: Exploded view of the Megha-Tropiques spacecraft (image credit: ISRO) 19)
Table 1: Overview of spacecraft characteristics
Figure 4: Photo of the deployed Megha-Tropiques spacecraft at ISRO (image credit: ISRO)
Figure 5: Photo of the Megha-Tropiques spacecraft during testing at SDSC SHAR (image credit: ISRO)
Orbit: Circular orbit, altitude = 865 km, inclination = 20º, period ~ 102 minutes (~14 rev./day). Repetivity: 97 orbits in 7 days. An equatorial region up to about ±23º can be visited at least three times daily (important to follow the life cycle of the mesoscale convective systems). 20)
Ground track pattern: With the inclination of 20º, the orbit regresses towards west by 6º per day and hence the orbit is not sun-synchronous as the sun moves by 1º towards east/day. This means that unlike sun-synchronous missions, only the set of ground traces repeat, but not the local time. The number of orbits in one solar day is 14.12. As the repeat cycle is 97 orbits in 7 days, a set of 97 ground traces go on repeating every 7 days. Since it is not the image data like optical missions, the reference scheme in terms of paths and rows is not required in the case of Megha-Tropiques.
Repeate observations: 6 times a day over the 10º – 20º latitude band, 4 times at many other latitudes. 21)
Figure 6: Observation coverage showing the nadir track of one day (image credit: LMD)
Secondary payloads on the flight were:
• SRMSat, a nanosatellite (10.9 kg) of SRM (Sri Ramaswamy Memorial) University, Chennai, India.
• Jugnu (the Hindu word for firefly), is a nanosatellite (3 kg) of the Indian Institute of Technology (IIT), Kanpur.
• VesselSat-1, a microsatellite (29 kg) of LuxSpace, Luxembourg (a company of OHB Technology AG). VesselSat-1 carries an AIS (Automatic Identification System) payload for the detection of ships in the ground segment. Orbcomm is the exclusive licensee for the AIS data collected by VesselSat-1. 25)
Figure 7: Illustration of PSLV-C18 flight payloads in deployed configuration (image credit: ISRO) 26)
• On October 6, 2016, the presidents of CNES and ISRO, Jean-Yves Le Gall and AS Kiran Kumar, signed at CNES's Head Office in Paris a four-year extension of the French-Indian Megha-Tropiques climate-monitoring satellite mission that has already delivered exceptional results for research and operational applications since its launch five years ago. 27)
- The Megha-Tropiques mission is delivering a dynamic three-dimensional picture of different states of water in the atmosphere. The main feature of the satellite is the combination of its instruments and its position over the inter-tropical belt, in a low-inclination orbit enabling up to five revisits to the same location every day. This unique capability has allowed it to achieve remarkable progress in estimating rainfall and forecasting cyclones, monsoons and droughts. Continuous monitoring of the turbulent giant tropical convection spots, where most extreme weather phenomena form, provides a better understanding of the cycles affecting mid-latitude countries and is extremely valuable for the global climate science community.
• The Megha-Tropiques mission is operational in 2016.
• October 2015: Megha-Tropiques nominal life: 3 years + 2 years extension up to end 2016. In mid-2016, Senior Review for further extension 2017-2018. 28)
- The mission has been through the 4th year of operation (October 12, 2015). SCARAB and SAPHIR are in perfect health .SAPHIR data are available in NRT via EUMETCast since summer 2014. Leading centers are already or plan to assimilating the SAPHIR data.
- Megha-Tropiques/SAPHIR showcases the proof of concept and the added value of a tropical orbit for constellation based rainfall estimation, assimilation and storms life cycle monitoring.
• May 2015: After 3 years on orbit, the scientific activity of the Megha Tropiques mission was reviewed. The instruments SAPHIR and SCARAB are functioning nominally and exhibit instrumental performances well within the original specifications. - The microwave imager, MADRAS, stopped acquisition of scientific data on January 26th, 2013 due to a mechanical failure. During its 16 months of operation, this radiometer experienced electrical issues making its usage difficult and delayed its validation. - A suite of geophysical products has been retrieved from the Megha-Tropiques payloads, ranging from TOA radiative flux to water vapor profiles and instantaneous rain rates. Some of these geophysical products have been merged with geostationary data to provide, for instance, daily accumulation of rainfall all over the intertropical region. These products compare favorably with references from ground based or space-borne observation systems. 29)
- Low inclination orbit: The latitudinal sampling statistics further indicate that up to 5 (3) observations per day are provided at the same location in the best (worst) case over the Tropics (Figure 8). The inclination and the height of the orbit yield the 2 peaks patterns seen in Figure 8. Poleward of the maximum, the number of samples decreases with latitude. Note that the most poleward measurement for MADRAS is at 27°. The same kind of statistics holds for the across track scanning instruments SCARAB although due to the enlarged swath (~2200 km) more observations are harvested on each day with up to 6 overpasses per day and a minimum of 4 per day over the 22°S-22°N band. The most poleward measurement for SCARAB is 30º. The SAPHIR statistics are similar although with a narrower swath (~1700 km).
- A further original aspect of the Megha Tropiques orbit is the distribution of the overpasses in time as revealed in Figure9 for the MADRAS case(the same results apply to the other scanning instruments).The precession cycle is clearly seen from these figures where the local time of observations shifts backwards as the satellite progresses throughout the month. The precession cycle is responsible for the complex aliasing between the Megha Tropiques orbit and the diurnal cycle. Figure9 shows that during the first 10 days of the month,at13ºN,no observations will be performed during nighttime. At 0ºN,the orbits come in small packets 12 h apart yielding a more even sampling of the diurnal cycle. At 13ºN,the various orbits come in one single packet following each other ~100 min apart. On the edges of the region of interest (25ºN),the sampling goes down to 2–3 consecutive overpasses a day separated by roughly 20–24 h.
Figure 9: 51days of local time sampling for the MADRAS instrument on-board Megha Tropiques for the Equator (left),a 13ºN position (center) and 25ºN position (right), image credit: Megha-Tropiques Science Team
• In October 2014, the Megha-Tropiques spacecraft was 3 years on orbit. The mission was extended for 2 years. 30)
- The MADRAS instrument was operational up to the end of January 2013.
- The SAPHIR instrument is fully operational. Since May 2013 the SAPHIR L1A data are available in realtime to registered users thanks to the EUMETCast link operated by EUMETSAT with the support of CNES/ISRO.
- The ScaRaB instrument is fully operational. 31)
Figure 10: Scaled representation of the payloads footprints. The L1A2 products correspond to brightness temperature of pixels for MADRAS and SAPHIR radiometers based on original L1 samples from the instruments. Note that the L1A2 product for Scarab is identical to L1A. The L1A3 format corresponds to SAPHIR, The SCARAB pixels and all MADRAS pixels are projected onto the conical scan 89 GHz MADRAS grid. Data are interpolated from L1A for SCARAB and L1A2 for SAPHIR inside a fixed grid related to the current orbit and defined by the location of MADRAS 89 GHz pixels centers. Note that the level 1A2 and 1A3 products are identical for MADRAS.
• January 2014: Typhoon Bejisa took place in the western part of the Indian Ocean from December 28th, 2013 to January 4th, 2014. The storm center passed approximately 100 km off La Réunion Island in the afternoon of January 2nd. The SAPHIR instrument was able to capture 21 brightness temperature "images" showing the evolution of the storm structure from its early stage as a tropical depression, to the full cyclone, and then to the dissipating stage. The image (Figure 11) shown here is the 183±11 GHz image on January 1 at 20:45 UTC, showing the storm at one of its most intense moments while moving toward La Réunion. The eye and eyewall structure are clearly visible and also the convective rain bands extending to the south-east and affecting la Reunion and Mauritius. 32) 33)
Figure 12: Megha-Tropiques image of the Typhoon Bejisa acquired on January 01, 2014 (image credit: LATMOS)
• Fall 2013: The Megha-Tropiques project of ISRO studied and tested the data of the ROSA (alias GPS-ROS) receiver for POD (Precise Orbit Determination). The ROSA instrument is able to work in two main modes: 34)
1) Navigation - in which the normal navigation functions of a space GPS receiver are carried out
2) Observation - in which the occultation measurements are carried out.
The POD antenna measurements (pseudo range and phase) of ROSA instrument can be used for orbit determination. The ROSA data for Megha-Tropiques was accessed and processed for various orbits for orbit determination. The measurements were corrected for atmospheric and clock errors. The orbit determinations were carried out using dual frequency L1 & L2 pseudo-range and Doppler data. Carrier phase measurements were processed and cycle slips were removed. Single difference method was used to remove the ambiguity. The dynamic model used for orbit determination consists of all the dominant perturbing forces including asphericity of the Earth, aerodynamic drag, lunar-solar gravitation attraction, solar radiation pressure.
An analysis of the POD results obtained from ROSA data were compared with those from an onboard 10-channel SPS (Satellite Positioning System). It was observed that the achieved orbit determination accuracy with Megha-Tropiques ROSA data is about 5 m in position and 1 cm/s in velocity (Ref. 34).
• Sept. 24, 2013: The MADRAS instrument was declared non-operational. After several months of investigations, ISRO and CNES declared the MADRAS instrument non-operational due to an anomaly on the scan mechanism. The instrument doesn't produce valid data since January 26, 2013. 35) 36)
• May 2013: Good stability of the SAPHIR instrument. All instrument performances checked parameters introduced in L1 SW and IODD. 37)
- MARFEQ (MADRAS RF EQuipment) performances are nominal and stable since launch , effort on processing has to be continued. 38)
• January 2013: ISRO and CNES decided to open the access to SAPHIR and SCARAB level 1A and 1A2 data to all users either from the MOSDAC or the ICARE dissemination centers. 39)
• Dec. 2012: After a few months of in orbit operation, due to a suspected electrical interference, the MADRAS instrument data are affected by random channel mixing. It is found that intrinsic data of the 9 channels are generally not affected but located at different positions in the transmitted data stream. - A methodology has been worked out by the CNES and ISRO Project teams for realignment of the data. With this additional processing, a significant amount of data is recoverable. 40)
• July 2012: The SAPHIR level 1A dump-type products are transmitted in a continuous manner since July 6, 2012 to the ICARE Data and Service Center located in Lille (Belgium) from ISSDC (Indian Space Science Data Center), Bangalore, India. The processing software has been validated by CNES and ISRO prior to implementation at ISSDC.41)
- According to the aggreements signed by CNES and ISRO, during 6 months, these first data is restricted to the French and Indian mission teams as well as to scientists selected through the Megha-Tropiques International Announcement of Opportunity (21 scientific teams from Australia, Brazil, Italy, Japan, Korea, Niger, Sweden, UK and USA are using the initial data for research purpose under International Announcement of Opportunity). 42)
- All calibration and validation activities will be completed by January 15, 2013 (Ref. 42).
• In late March 2012, the data of SAPHIR and ScaRAB are being made available to AO ( Announcement of Opportunity ) PI's (Principal Investigators). The MADRAS and ROSA instruments are functioning well and are undergoing detailed assessment. 43)
• On January 5, 2012, an anomaly was detected on the MADRAS L0 data stream (likely a multiplexing problem). 44) The anomaly is cleared after a reset of the MADRAS electronics. Cause: Sensitivity of the electronics to heavy ions.
• The cyclone Thane in late December 2011over the Indian Ocean was observed by the SAPHIR and MADRAS instruments. 45)
Figure 13: Evolution of cyclone Thane on December 29, 2011 as seen by the SAPHIR instrument (image credit: CNES)
Figure 14: Representation of the 6 SAPHIR channels on one orbit (image credit: CNES)
Figure 15: The cyclone Thane on Dec. 31 as observed by the MADRAS instrument (image credit: ISRO, CNES)
Legend to Figure 16: The quicklooks were made from raw data: raw numeric data without instrument geometry correction (conic scanning)
• Soon after separation of the Megha-Tropiques spacecraft from PSLV, ISTRAC (ISRO's Telemetry Tracking and Command Network) of Bangalore confirmed that the satellite had been placed very precisely into its intended circular orbit.
Sensor complement: (MADRAS, ScaRaB, SAPHIR, GPS-ROS, ADCS)
The PIM (Payload Instrument Module) provides accommodation for three payload instruments (Figure 18): MADRAS, SAPHIR and ScaRaB. The GPS-ROS instrument is included on the platform itself. The bottom deck of PIM is attached to the main bus along the central cylinder through an aluminum alloy ring. Four additional gussets are provided connecting the edges of the PIM bottom deck and main bus top deck.
Figure 17: General observation geometry of the Megha-Tropiques payload (image credit: CNES, ISRO)
MADRAS (Microwave Analysis and Detection of Rain and Atmosphere Systems):
MADRAS is a multi-frequency radiometer/imager designed and developed by ISRO with CNES providing the RF assembly (dish, horns, receivers). The low-frequency receivers are "direct detection" type employing MMIC (Microwave Monolithic Integrated Circuit) technologies. The main objective of the microwave imager is to measure precipitation and cloud properties (integrated column precipitation content and integrated water vapor content, aerial distribution and intensity of precipitation, determination of convective cells). 48)
MADRAS is a five-frequency (9 channel) mechanical conical-scanning passive microwave radiometer providing brightness temperature measurements (the instrument is of the same type as SSM/I or TMI, the TRMM radiometer). The frequencies of 89 and 157 GHz are responding to ice particles in cloud tops, thus permitting the detection of convective rain regions over land and sea. The lower frequency channels are employed over oceanic regions for the measurement of cloud liquid water and precipitation (absorption at 18.7 and 36.5 GHz, integrated water vapor at 23.8 GHz, and estimation of the sea surface wind speed with the 18.7 GHz channel. The polarization is in H+V, except for the 23.8 GHz channel which is only V polarized.
Table 2: Specification of the MADRAS channel characteristics
A scene is observed at a constant incidence angle of about 49º using conical scanning of narrow antenna beams. The incidence angle complies with the Brewster angles obtaining a swath width of 1700 km. The radiometer measures the scene radiation only when the onboard scan angle is between ±65º with respect to the S/C velocity vector (the rest of the scan revolution is used for calibration). The constant scan speed is 24.6 rpm. The cross-track spatial resolution of the antenna main lobe (FWHM) is about 40 km for the low-frequency channels, 10 km for the 89 GHz, and about 6 km for the 157 GHz channels.
MADRAS instrument: The MADRAS conically-scanning microwave radiometer realized in a total power configuration with highly stable low-noise front-end receivers. The scanning is implemented using a mechanically-scanning reflector at about 25 rpm with constant incidence angle of about 49º. The instrument is made up of the following elements: 49)
• MARFEQ A (MADRAS RF EQuipment A). MARFEQ-A consists of the receivers and the main antenna.
• MARFEQ B (MADRAS RF EQuipment B). Note: the MARFEQ A and B elements are involved with the radiometric part of the instrument. MARFEQ-B consists of the calibration units, including a sky-looking reflector and a blackbody target.
• MSM (MADRAS Scan Mechanism). MSM rotates the entire MARFEQ-A assembly at 25 rpm on a conical surface.
• MCW (Momentum Compensating Wheel). MCW is used for neutralizing the disturbances induced by the rotation of MADRAS.
• MBE (MADRAS Back-end Electronics) divided in 2 units, rotating and static. MBE forms the tail end of the MADRAS payload. It mainly carries out the data handling and payload control functions for the MADRAS payload.
• MSM/MCW electronics module
• PSU (Power Supply Unit), and DC-DC converter. The PSU is used for powering only MARFEQ-A which is a moving part and is located on PIM (Payload Instruments Module). The PSU and PIM are provided by ISRO. The PSU and MARFEQ are electrically interfaced via PSTD (Power and Signal-Transfer Device) which also mechanically couples MARFEQ and MSM. A precision optical encoder is used to precisely measure the MADRAS scan position.
The radiometers are of the self-calibrating type involving a two-point onboard calibration (sky and hot load during each scan). The hot (ambient) load is precisely measured using platinum resistance thermometers while a sky reflector measuring the cosmic background radiation at 2.7 K serves as the lower end calibration point.. The total power configuration using MMIC receivers for 18 to 36 GHz (LF) channels provides high temperature sensitivity. Inter-channel calibration is 0.5 K. Automatic gain and offset corrections are implemented in the back-end electronics for optimum quantizer utilization. The higher channels at 89 and 157 GHz (HF) are realized in superheterodyne configuration.
The parabolic dish effective diameter is 65 cm. The beam efficiency is 0.95. A single horn is used for the three channels: 18.6, 23.8 and 36.5 GHz. The mass of MADRAS is 162 kg, power consumption is 153 W.
Figure 19: Schematic view of the MADRAS microwave equipment (MARFEQ A&B modules), image credit: CNES
MBE-R (MADRAS Back-end Electronics) rotating unit: The MBE-R unit is a multi-channel data acquisition unit based on high precision digitizers and digital signal processor FPGA. MBE-R consists of three separate chains of front end signal conditioners and moderate speed 12-bit digitizers (AD674) followed by Xilinx Virtex (XQVR600) FPGA based digital integrator, data formatter, MBE-S RS422 interface and associated control and timing logic. It also carries out acquisition of temperature telemetry parameters from radiometer receiver channels using thermistors. 50)
A single MBE-R module, housed in the rotating part of MADRAS payload along with the total power radiometer receivers, handles the complete MADRAS payload data acquisition and processing requirements. The main function of the FPGA-based onboard control and signal processor unit for MBE-R is to generate the control signals for various onboard devices, perform the integration on the digitized data and serialize the output data to send to the payload controller subsystem. Here operating modes are kept programmable, which select between nominal operating mode and test mode.
Figure 20: Onboard control and signal processor FPGA (image credit: ISRO)
Figure 21: Photo of the MBE-R module of the MADRAS radiometers / sounders (image credit: ISRO)
Figure 22: Illustration of the MADRAS 23.8 GHz direct detection receiver (image credit: CNES)
Figure 23: View from left of the MADRAS control electronics, scan mechanism and momentum compensation wheel (image credit: ISRO)
Figure 24: Electrical architecture of the MADRAS instrument (image credit: ISRO, CNES)
Figure 25: Diagram of the atmospheric opacity (image credit: ISRO, CNES)
Figure 26: Imaging geometry of the Megha-Tropiques instruments (image credit: CNES, ISRO)
Figure 27: Photo of the MADRAS instrument under thermo-VAC tets in LSSC (Large Scale Simulation Chamber), image credit: ISRO
ScaRaB (Scanner for Radiation Budget):
ScaRaB is sponsored by CNES and developed at CNRS/LMD, France. ScaRaB is of Meteor-3-7 and Resurs heritage (Russian missions) with 16 months of data collection. Its overall objective is the collection of data on shortwave and longwave radiation (reflected solar and emitted thermal radiation) to estimate the Earth's radiation budget at the top of the atmosphere on global and regional scales. Specific objectives are: 51) 52) 53) 54) 55) 56) 57) 58) 59) 60) 61)
- To observe simultaneously the radiation fluxes and the water cycle components (water vapor, clouds, precipitation, ..) to support studies of the water and energy balance in the Tropics (mean accuracy of 10 Wm-2 is sought for the instantaneous fluxes)
- To extend the time-series of data from broadband and well calibrated radiometers (ERBE, ScaRaB, CERES), i. e., direct survey of climate parameters (mean accuracy of 5 Wm-2 is sought for the regional monthly means, and up to 2 Wm-2 for zonal monthly means).
Some key requirements are:
- Precise spectral response of the broadband channels (in the SW-Shortwave and LW-Longwave domains)
- Absolute radiometric calibration, 1% (LW), 2% (SW), compared to about 5% for most other SW radiometers
- Robust and qualified data processing for levels 2 & 3: to resolve the triple sampling issue (viewing angles, space and time averaging) and to provide detailed description of the observed scenes.
ScaRaB instrument: The instrument is a cross-track scanning radiometer featuring four channels. Channels 2 and 3 are considered the main channels, while channels 1 and 4 are auxiliary channels. The optical subsystem features four parallel telescopes, one telescope per channel, they are identical except for their filters.
ScaRaB uses BARNES pyroelectric detectors for all bands (placed at the focus of a spherical aluminum mirror), which are sensitive only to the AC component of the signal (i.e., the modulated energy). Hence, chopping is needed for each pixel. This reduces the influence of the self radiation of the telescope and filters. Two mechanical choppers are used (one for two channels), providing a 10 Hz chopping frequency. The four channels, the two choppers, and a filter wheel dedicated to channel 2 and 3, are mounted on a scanning optical bench (rotor). The telescopes are swiveled by the optical bench so that no extra mirror for the scanning is needed. This reduces the likelihood of offsets dependent on the scanning angle.
Table 3: Spectral bands of ScaRaB
The spatial resolution of ScaRaB data is 48 x 48 mrad, scan angle=97.82º, swath width = 3300 km. ScaRaB points to nadir and scans the full field of view (FOV) within six seconds. In this cross-track mode data are generated continuously.
Table 4: ScaRaB instrument parameters
Figure 28: Functional block diagram of ScaRaB (image credit: CNES)
Figure 29: Photo of the ScaRaB optical head (image credit: CNES)
Onboard calibration subsystem: Gray lamps and blackbodies are used for onboard gain calibration; deep space is used for offset calibration. That subsystem comprises a set of two reference blackbodies for channels 3 and 4, and a set of gray calibration lamps for channels 1, 2 and 3. There is continuous thermal control of the blackbodies. The gray lamps are turned on during the calibration session (typically once per day). In addition, there are short wave references, consisting of two lamps for the calibration of channels 2 and 3 (typical use is once per month). On the ScaRaB/Meteor-3-7 mission, however, the lamp system was damaged so that actual calibration was performed by using the instrument temperature and a pre-launch established gain-temperature law. The remaining lamps were then used to verify this calibration. During one year of operation, no significant sensor degradation was observed.
ScaRaB has a duty cycle of 100%, data rate=3 kbit/s, data volume=18 Mbit/orbit. An instrument mass memory provides data storage for up to 12 hours. The mass of the instrument is 20 kg, the maximum power use is 33 W.
The data processing system is based on algorithms for transforming the instantaneous measurements of radiances, filtered by the optics and detectors, into estimates of the monthly mean values of the radiant excitations in the solar and thermal domains, at the top of the atmosphere. This requires corrections for non-flat spectral response, anisotropic, and diurnal variations. The estimates are provided on a spatial grid of 250 km.
SAPHIR (Sondeur Atmospherique du Profil d'Humidite Intertropicale par Radiometrie):
SAPHIR, or (Sounder for Atmospheric Profiling Sounder of Humidity in the Intertropics by Radiometry), is developed by CETP (Centre d'etude des Environnements Terretre et Planetaires), by DEMIRM (Department de Radioastronomie Millimetrique de l'Observatoire de Paris), and by LMD. SAPHIR is a multi-channel cross-track millimeterwave sounding instrument with the objective to measure water vapor profiles in the troposphere (water absorption band at 183.3 GHz) in six layers from 2-12 km altitudes and spatial resolutions of 10 km. The two objectives are: 62) 63) 64)
• Analysis of the diurnal cycle of the water vapor distribution to evaluate the vertical transport associated with convective structures at the mesoscale and the large scale and to study the scale-to-scale interactions in the flux.
• Study of the space-time humidity distribution and its effect on the development of deep convection.
The SAPHIR sounder employs a total-power type microwave radiometer design to achieve high-sensitivity measurements. The sounding principle is similar to that used on AMSU-B and SSM/T2 instruments. The measurement sensitivity is given in Table 5 with an inter-channel calibration of 0.5 K. The measurement polarization is H. The radiometric sensitivity is specified for an antenna temperature of 300 K and an integration time of 7.34 ms for all channels.
The SAPHIR instrument is composed of two units:
• The electronic unit (EM): the electronic module manages the interfaces with the platform (power, telemetry, commands) and drive the RF unit for the science data acquisition.
• RFU (Radio Frequency Unit). This module contains the scanning mirror protected by a shroud, the on-board calibration target, the front-end, the intermediate frequency processor and manages the RF signal acquisition.
Figure 30: Functional block diagram of SAPHIR (image credit: CNES)
Front End Unit: The millimeter Front End is composed of a local oscillator, a mixer and a low noise amplifier. The mixer associated with the local oscillator will perform the down conversion of the signal.
IFU (Intermediate Frequency Unit). The IFU will de-multiplex the signal of the various bandwidths, perform amplification and filtering for each channel. After amplification, analog power detection of the signal is performed for each channel. The IFP unit includes sampling and integration of radiometer data: The 6 video data flows will be sampled and integrated using analog to digital converters.
The instrument performs a continuous cross-track scanning while the satellite is moving along-track. Every scan period, the antenna reflector performs a complete rotation. Part of the period will be devoted to the collection of Earth atmosphere temperature data. During the scan period, when the reflector is properly oriented, acquisition of cold sky temperature measurements will be done. During part of the scan period, acquisition of hot target temperature measurements will also be done. The horn will focus the free space radiation collected by the antenna reflector.
The free-space radiation from the atmosphere is collected by the antenna reflector and focused into the horn using quasi-optics techniques. The signal is detected and separated into 6 bands. The narrow beam of the antenna provides cross-track scanning within an angle of ±42º with respect to nadir. The incidence angle is < 52º. - The SAPHIR instrument has a mass of 18 kg and a power consumption of 30 W.
Figure 31: The RFU of the SAPHIR instrument (image credit: CNES)
Figure 32: The 6 channels of SAPHIR positioned versus the water vapor absorption line at 183,31 GHz (image credit: CNES)
Figure 33: Schematic view of the SAPHIR instrument (image credit: CNES)
Table 6: Overview of some SAPHIR instrument parameters
Figure 34: Scan pattern of the SAPHIR instrument (image credit: CNES)
GPS-ROS (GPS Radio Occultation Sensor):
GPS-ROS is a dual frequency system of ROSA (Radio Occultation Sounder for Atmosphere) heritage provided by the Italian Space Agency (ASI) in collaboration with ISRO. The instrument enables the measurement of water vapor and temperature profiles in the tropics. The GPS-ROS payload will also be there to supplement/complement the mission for the atmospheric studies. It has a fore and an aft antenna facilitating measurements in both directions, thus allowing a large number of observations. GPS-ROS features two receiving frequencies at 1575.42 MHz (L1) and at 1227.60 MHz (L2).
The GPS signals are acquired through multiple antennas: a zenith antenna is used for navigation purpose, while two sounding antennas, pointed toward the velocity and anti-velocity satellite vectors, respectively, are used for Earth limb rising-setting occultation observations for scientific purpose.
Megha Tropiques periodically performs a yaw axis rotation causing ROSA to exchange velocity RO antenna with antivelocity RO antenna, the RO Antennas are 6-patch panels with FOV ±35° in azimuth.
Figure 35: Elements of the GPS-ROS instrument (image credit: ISRO)
ROSA is flown on-board of the following three missions: 65)
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.
ROSA instrument: The ROSA receiver is a GPS receiver for spaceborne applications, specifically conceived for atmospheric sounding by radio occultation, which is able to determine position, velocity and time using GPS signals. Besides providing real-time navigation data, ROSA is able to accurately measure pseudoranges and integrated carrier phase (raw data), to be later processed on ground for scientific purposes. ROSA processes the received GPS signals in both the L1 and L2 frequency bands, allowing compensation of ionospheric delays. A codeless tracking scheme is included, in order to process the encrypted P(Y) signals transmitted in the L2 frequency band. The ROSA instrument, in its complete configuration, is composed by the follow parts (Ref. 65):
• One navigation antenna to acquire the GPS signals to determine position, velocity and time of the object where there is the receiver.
• Two radio occultation antennas to acquire the GPS signals used in the calculus of all the parameters used in the atmospheric sounding (for complete instrument). The Oceansat-2 tailored version features only the velocity antenna.
• RF (Radio Frequency) cables connecting the antennas (for navigation and radio occultation) to the receiver box.
• Receiver box which processes the GPS signals from all the antennas.
Legend to Figure 36: The OceanSat-2 tailoring features only one occultation antenna (the velocity antenna).
The instrument is equipped with one hemispherical-coverage antenna that is mounted with boresight direction equal to the zenith direction and is used to track the GPS signals for navigation purpose and for POD (Precise Orbit Determination). In addition, one directive velocity antennas is mounted on the OceanSat-2 spacecraft. This antenna is oriented in such a way to be able to track signals from GPS satellites in Earth occultation. Sixteen dual-frequency channels (4 AGGA chips) are available in the ROSA Receiver, and can be freely assigned to any combination of satellites.
ROSA is provided with a MIL-STD-1553 communication interface over which telecommand, telemetry and measurement data are exchanged. The receiver digital section is based on an ADSP 21020 processor and four AGGA-2a channels ASIC. In summary, the ROSA receiver performs the following main operations:
• Receives L1 C/A, L1 P(Y) and L2 P(Y) signals from GPS satellites and automatically allocates HW channels to GPS satellites.
• Obtains and maintains code lock and carrier lock, demodulates and decodes data message and recovers navigation data from each received GPS satellite, in navigation and occultation, the latter using a robust PLL (Phase Locked Loop) aided by a FLL (Frequency Locked Loop).
• When at least 4 GPS satellites are in view, performs position, time and velocity calculation based on a least square algorithm. In parallel, it performs Kalman filtered solutions.
• Uses calculated position information to establish geometrical line of sight information of each acquired GPS satellite with respect to the receiver platform and maintains a tracking list of visible satellites.
• Performs autonomous forthcoming occultation event prediction and channel allocation.
• When carrier lock is not possible, performs open loop high-rate sampling of raw observables (I, Q) for carrier phase reconstruction on ground.
• Provides observable data for each GPS satellite in lock.
• Monitors and maintains receiver health status.
• Allows receiver control from ground through user commands and provides telemetry data.
The antenna for navigation (used also for POD) is a dual-band L1/L2, with hemispherical coverage pattern; the gain is greater than – 4 dBic for elevations > 5° and RHCP (Right Hand Circular Polarization). The ROSA radio occultation antenna is of critical importance due to the fact that its scope is to receive the strongly attenuated signals coming from the GPS constellation during their occultation time window. The electromagnetic signals passing through the low atmosphere during the satellite occultation phase are subject to increased propagation losses with respect to the navigation antenna atmospheric losses.
The antenna is connected to the ROSA receiver unit by two different RF cables for L1 and L2 frequency signals directly to the LNA's, without passing through the diplexer used for the navigation one. Two BPF (Band-Pass Filter) boxes are mechanically attached to the antenna, centered around the GPS carriers L1/L2.
ADCS (Argos Data Collection System):
ADCS is provided by CNES (under study). Further information will be provided when available.
The ground segment elements for Megha-Tropiques include the SCC (Spacecraft Control Center) at ISTRAC (ISRO Telemetry, Tracking and Command Network), Bangalore, for payload data reception, data processing and data dissemination to CNES, France and MOSDAC (Meteorology & Oceanography Satellite Data Center) at ISRO/SAC (ISRO Space Applications Center), Ahmedabad (India), and development of spacecraft health monitoring software, flight dynamics software and mission management at ISAC (ISRO Satellite Center), Bangalore.
The existing ISTRAC stations at Bangalore, Lucknow, Bearslake, Mauritius and Biak are being used for telemetry, tracking and command support under the control of SCC.
There is no real-time data transmission of the payload data to the ground stations. All payload data will be recorded onboard onto the SSR (Solid State Recorder) with a capacity of 8 Gbit memory. The instrument data rates are: 35.65 kbit/s for MADRAS, 1.5 kbit/s for SAPHIR, and 12.66 kbit/s for ScaRaB. The payload data are downlinked in CCSDS format. These data will be played back by the Bangalore ground station. The playback is at a data rate of 2.6 Mbit/s using S-band.
Figure 37: Ground segment configuration (image credit: ISRO, CNES)
The operation of all the payload data is continuous throughout the mission life. The Level `0' and Level `1' processing is done at ISTRAC. Level `1' data products are further transmitted to CNES, France and MOSDAC, SAC for further processing and dissemination. Data products include 3-hourly, 6-hourly and daily products. Higher level data products as well as value-added services are provided at MOSDAC. MOSDAC is the data dissemination agency to science data users within India and external users. Ground communication links are planned between ISTRAC and MOSDAC. The data will be shared between the two agencies (ISRO, CNES). In case of CNES, it is planned to transmit the data by placing it into an ftp-site with password protection.
Data distribution policy agreement as of 2008:
A Joint Working Group meeting between ISRO and CNES took place in Goa, India on July 5-6, 2008 to review the progress of the on-going collaborative programs Megha-Tropiques and SARAL. During the meeting, an agreement between ISRO and CNES on the data policy for distribution of data received from Megha-Tropiques was signed. 66) 67) 68)
This new policy agreement enables the global scientific community to have free access to Megha Tropiques data after calibration and evaluation of the payloads by scientists from both the agencies for weather and climate change studies. These data are expected to enhance a better understanding of the tropical weather phenomena including the monsoons.
The ICARE Data & Services Center is in charge of Megha-Tropiques French scientific data processing and distribution (level 2 and higher). 69)
- collect Level-1 and ancillary data
- development of production framework : the framework approach allows parallel development of science codes and operational codes
- streamlining of science codes, development of I/O modules, integration into production framework, coordinated with scientists
- production and distribution of L2/L3/L4 products
- product documentation and users support.
Note: ICARE is hosted at USTL (Université des Sciences et Techniques de Lille), Lille, France. ICARE is a research structure set up in 2003 on a national level and consisting of CNES, INSU, USTL, etc. (all research laboratories) - to study aerosol-cloud-radiation interactions and the water cycle (cloud properties, atmospheric chemistry) and using data from various missions (PARASOL, Calipso, Megha-Tropiques, etc.).
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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 (firstname.lastname@example.org).