INSAT-3DR (Indian National Satellite-3D Repeat)
INSAT-3DR, similar to INSAT-3D, is an advanced meteorological satellite of ISRO (Indian Space Research Organization) configured with an imaging System and an Atmospheric Sounder. The significant improvements incorporated in INSAT-3DR are:
• Imaging in Middle Infrared band to provide night time pictures of low clouds and fog
• Imaging in two thermal infrared bands for estimation of SST ( Sea Surface Temperature) with better accuracy
• Higher spatial resolution in the visible and thermal infrared bands.
And, like its predecessor INSAT-3D (launched on July 25, 2013), INSAT-3DR carries a DRT (Data Relay Transponder) as well as a Search and Rescue Transponder. Thus, INSAT-3DR will provide service continuity to earlier meteorological missions of ISRO and further augment the capability to provide various meteorological as well as search and rescue services. 1)
Figure 1: Image of the INSAT-3DR spacecraft in the clean room at ISRO with the solar panel deployed (image credit: ISRO) 2)
INSAT-3DR is a meteorological satellite and will join the operational search and rescue service provided by INSAT 3D to various users, including the Indian Coast Guard, Airport Authority of India, Shipping, and Defense Services. The spacecraft was built by ISRO/ISAC (ISRO Satellite Center), Bangalore, and by ISRO/SAC (ISRO Space Applications Center) ofAhmedabad, India.
The structure is a cuboid with a central cylinder which accommodates two propellant tanks. The bipropellant propulsion system uses fuel (MMH) and oxidizer (N2O4) for various attitude and orbital maneuvers of the spacecraft. The structure is made of honeycomb panels with aluminum/CFRP face-sheets to make the structure lightweight.
INSAT-3DR has dimensions of 2.4 m x 1.6 m x 1.5 m; it has a liftoff mass of 2211 kg, which includes about 1255 kg of propellant (dry mass of 956 kg) . The propellant carried by INSAT-3DR is mainly required to raise the satellite from the GTO (Geosynchronous Transfer Orbit) to its final Geostationary Orbit and to maintain the satellite in its orbital slot during its life. The satellite has a solar array generating 1.70 kW of power. The nominal in-orbit design life of INSAT-3DR is 10 years.
The DRT (Data Relay Transponder) system onboard INSAT-3DR will be used for receiving meteorological, hydrological and oceanographic data from remote locations over the coverage area from DCPs (Data Collection Platforms) like AWSs (Automatic Weather Stations), ARGs (Automatic Rain Gages) and AMSs (Agro Met Stations).
TCS (Thermal Control Subsystem): Thermal control of the spacecraft is a passive control with heaters mounted at various locations for controlling temperature. The temperature control of the payload infrared detectors is important for the success of the payload. The required detector temperature of less than 95 K is achieved by employing a passive cooler. The achievement of 95 K is not only dependant on cooler design but also on the heat load falling in the cooler FOV (Field of View). This heat load is minimized by not allowing any object to come into the FOV of the coolers and also by incorporating a bi-annual yaw rotation of the spacecraft so that the cooler face is not exposed to sun's radiation during solstices.
BMU (Bus Management Unit): The BMU is a microprocessor system with redundancies based on a MAR 31750 processor and MIL-STD-1553 interface. It is used for achieving the functions of telemetry, telecommand and control electronics. The system interfaces with all other subsystems for telemetry and telecommand functions. It interfaces with sensors, inertial systems and the propulsion systems for attitude and orbit control functions. The BMU software has been developed in ADA language.
EPS (Electrical Propulsion Subsystem): The spacecraft has a deployable solar panel. The panel uses high efficiency ATJ (Advanced Triple Junction) solar cells and produces 1164 W of power. The power to the spacecraft during eclipse is provided by two 18 Ah NiCd chemical batteries. The power electronics provide the bus regulation from which power is taken to the various subsystems. To get the various voltage and current values, high efficiency distributed DC/DC converters are used. The SADA (Solar Array Drive Assembly) technique is implemented to reduce the spacecraft disturbances (micro stepping device).
AOCS (Attitude and Orbit Control System): INSAT-3DR is a momentum-biased 3-axis stabilized spacecraft using star trackers for precise pointing control. There are stringent payload pointing requirements which the control system has to provide. The payload short term and long term stability are required to be 2 visible pixels (56 µrad) and 4 visible pixels (112 µrad) for navigation. These requirements are met by providing compensation to scan mirror servo in terms of MMC (Mirror Motion Compensation) and IMC (Image Motion Compensation). The algorithm for these compensations are stored in control electronics, errors are computed and real-time corrections are carried out.
The various sensors used for attitude sensing are ES (Earth Sensors), DSS (Digital Sun Sensors), CASS (Coarse Analog Sun Sensor), SPSS (Solar Panel Sun Sensors) and the Star Sensors. The star sensors have been configured to provide higher accuracies for image processing. The sun sensors are used for initial attitude acquisitions and other maneuvers while the earth sensors are prime sensors for on-orbit attitude control.
LAM (Liquid Apogee Motor): The spacecraft is injected into GTO (Geosynchronous Transfer Orbit) and then raised into GEO, using the 440 N LAM system. There are twelve 22 N bipropellant thrusters for initial attitude maneuvers and for on-orbit station keeping. The two momentum wheels and one reaction wheel provide on-orbit attitude stabilization.
RF communications: Due to high data rate from the Imager payload, the bandwidth requirement increases for the meteorological transmitter; a total 30 MHz bandwidth is allocated for the communication payload in INSAT-3DR from 4770 - 4800 MHz in the Extended C-band. This will also provide for growth of MET payload data rate in future. The transponders consist of redundant transmitters, redundant receivers and associated antenna. The antenna system consists of an omnidirectional antenna system for both up and down links and additional global horn for downlink.
The 4500-4510 MHz downlink transmit frequency band is allocated for DRT (Data Relay Transponder) and SAS&R (Satellite Aided Search & Rescue) transponder to maintain the present service network requirements and also to avoid interference from the spectrum roll off from the MET modulated carriers to ensure good C/I at the ground receivers for the demodulation of the carrier. The DRT will receive the signals from various DCPs (402.65 to 402.85 MHz UHF band) in random ALOHA mode. These uplink carriers are located within ±100 kHz band around the center frequency. The translated downlink frequency is at 4506.5 MHz with a bandwidth of 200 kHz. 3)
Table 1: Payload frequencies of INSAT-3D 4)
Figure 2: Illustration of the INSAT-3D configuration (image credit: ISRO)
The communication payload components are: Meteorological Transmitter, Data Relay Transponder, SAS&R (Satellite Aided Search and Rescue) Transponder & S-band Broadcast Satellite Services Transponder. All communication components have a mass of ~70 kg.
Figure 3: Photo of the INSAT-3DR seen with the two halves of the payload faring of the GSLV-F05 rocket (image credit: ISRO)
Launch: The INSAT-3DR spacecraft was successfully launched on September 8, 2016 (11:20 UTC) from SDSC (Satish Dhawan Space Center SHAR), the main launch center of ISRO on the south-east coast of India, Sriharikota. The launch vehicle was India's GSLV-F05 (Geosynchronous Launch Vehicle).
GSLV-F05 is the flight in which the indigenously developed CUS (Cryogenic Upper Stage) is being carried onboard for the forth time during a 3-stage GSLV flight. The GSLV-F05 flight is significant since it is the first operational flight of GSLV carrying CUS. The CUS is more efficient and provides more thrust when compared to solid and earth-storable liquid propellant rocket stages. The main and two smaller steering engines together provide the CUS a nominal thrust of about 73.55 kN in vacuum. During the flight, the CUS fires for a nominal duration of approximately 720 seconds. The metallic payload fairing of GSLV-F05 has a diameter of 3.4 m. The overall length of GSLV-F05 is 49.1 m with a lift-off mass of 415.2 tons. 5)
Figure 4: Indigenously developed CUS (Cryogenic Upper Stage) undergoing testing (image credit: ISRO)
Figure 5: Image of the fully integrated GSLV-F05 carrying INSAT-3DR (image credit: ISRO)
Orbit: After reaching GTO (Geosynchronous Transfer Orbit), INSAT-3DR will use its own propulsion system to reach its final geostationary orbit (~35786 km) and will be stationed at 74º East longitude over the equator.
The target GTO parameters of the INSAT-3DR mission are:
- Perigee: 170±5 km
- Apogee: 35,975±675 km
- Inclination: 20.61º
After reaching GTO onboard the GSLV-F05, the solar panels of the satellite will be deployed immediately. Following this, ISRO's MCF (Master Control Facility) in Hassan takes control of the satellite and performs the initial orbit raising maneuvers using the LAM (Liquid Apogee Motor) of the spacecraft, finally placing it in the circular geostationary orbit. Later, INSAT-3DR will be put into its final orbital configuration and positioned at 74º East longitude.
• January 30, 2017: A team of the satellite meteorology division of IMD (India Meteorological Department), housed in New Delhi's Mausam Bhawan, has amassed a huge tranche of data relayed by INSAT-3DR over the past few months, at an average rate of a whopping 142 GB per day. Launched on September 8, 2016, INSAT-3DR works in tandem with INSAT-3D, operational since 2014, in sending raw data and high-resolution images, zoomed up to 1 km near the Earth's surface, every 15 minutes. 6) 7)
- Sunil Peshin, who heads the division, stated that while storing and archiving data was itself a challenge, the IMD shares information relayed by these satellites with international agencies like the US-based NOAA (National Oceanic and Atmospheric Administration). Peshin said with the operationalizing of INSAT-3DR, night time monitoring of atmospheric phenomenon like cloud cover, fog, haze and snow among others has become possible.
- "Within the next few months we hope to equip ourselves with the ability to detect farm fires as well which the NASA (National Aeronautics and Space Administration) does currently. It is just a matter of developing the right tools and algorithm which will take a little time," he said.
- This assumes importance against the backdrop of the Delhi government blaming seasonal agro-residue burning in the fields of Haryana and Punjab and the subsequent emission of smoke for the city's foul air, especially during October and November. A. K. Mitra, a scientist with the satellite research and calibration/validation unit of the division, explained that apart from night-time monitoring, INSAT-3D/3DR has enabled forecasters here to prepare a ‘vertical profile' of weather data which comes in handy in case of extreme weather events, picking last year's Chennai floods as a case in example.
- "The ‘Sounder' payload of INSAT-3D/3DR provides vertical distribution of temperature and moisture which also gives more information on the nature of fog and its potential of remaining suspended or lifting. It can be put to great use by the railways and the airlines," Mitra said.
- Both Peshin and Mitra concurred that the advanced weather forecasting capabilities could be put to greater use by training staff of the public transport sector like railways in handling the RAPID (Real-time Analysis Product Information Dissemination) tool, available on the IMD website. The officials said the other set of weather data that INSAT-3DR measures using its ‘Imager' payload include sea surface temperature, snow cover, snow depth, smoke, aerosols, water vapor, wind, flash floods.
Figure 6: Image acquired by INSAT-3D (image credit: IMD)
Sensor complement (Imager, Sounder, DRT, SAS&R)
INSAT-3DR will provide meteorological services to India using a 6-channel imager and a 19-channel atmospheric sounder. It will also deliver rescue services thanks to its DRT (Data Relay Transponder) instrument and the SAS&R (Satellite Aided Search & Rescue) transponder. The payload was developed at ISRO/SAC.
For meteorological observations, INSAT-3DR carries a multispectral imager (optical radiometer) capable of generating Earth imagery in six wavelength bands. The Imager will generate of the Earth's disk an image every 26 minutes and provide information on various parameters, namely, outgoing longwave radiation, quantitative precipitation estimation, SST (Sea Surface Temperature), snow cover, cloud motion winds, etc. The Imager is an improved design of VHRR/2 (Very High Resolution Radiometer) heritage instrument flown on the Kalpana-1 and INSAT-3A missions.
The Imager consists of an EO (Electro-Optics) module and a set of electronics packages including power-supply modules. The EO module, containing the telescope, scan assembly, and detectors along with cooler, is mounted external to the spacecraft. The electronic packages are mounted on an internal panel of the spacecraft. The complete instrument has a mass of ~ 130 kg, a power consumption of 85 W and a data rate of ~3.9 Mbit/s.
Table 2: Spectral parameters of the Imager
Figure 7: Illustration of the Imager (image credit: ISRO)
Table 3: Key parameters of the imager
The incoming radiation is reflected on to a 310 mm aperture telescope with a Silicon Carbide (SiC) scan mirror mounted at 45° to the optical axis of the telescope. The optical system includes primary mirror, secondary mirror, a specially designed beam splitting assembly for efficiently steering radiant energy simultaneously to the respective band focal planes.
Figure 8 depicts the scanning geometry of the INSAT-3D Imager for full disk and program mode in reference to FOR (Field of Regard). During a scan, the detector outputs for all the channels are sampled at a uniform rate. The sampling rate of detector output is 5460 samples/s for MIR, WV, TIR-1 and TIR- 2 channels and 21840 samples/s for the VIS and SWIR channels. This combined with scan rate of 20°/s (optical ) results in over-sampling of the detector output in fast scan direction by a factor of 1.75 for all channels except WV. The over-sampling ratio for WV channel is 3.5. Thus, each VIS and SWIR sample is 16 µrad E/W and each IR sample is 64 µrad E/W.
The spectral splitting scheme employed in the Imager payload is shown in Figure 9. The Imager has two different modes of operation – the "full frame" mode and the "program" mode.
Figure 10: Illustration of the Imager spectral bands and their applications (image credit: ISRO) 8)
INSAT-3DR also carries a 19-channel sounder, which was earlier flown on INSAT-3D. The Sounder has 18 narrow spectral channels in the SWIR (Short-Wavelength Infrared), MWIR ( Mid-Wavelength Infrared) and LWIR (Long-Wavelength Infrared) regions and one channel in the visible region. The objective is to provide vertical profiles of temperature, humidity and integrated ozone. These profiles will be available for a selected region over the Indian landmass every hour and for the entire Indian Ocean Region every six hours.
The Sounder measures radiance in eighteen IR and one visible channel simultaneously over an area of area of 10 km x 40 km at nadir every 100 ms. Using a two-axes gimballed scan mirror, this footprint can be positioned anywhere in the FOR. A scan program mode allows sequential sounding of a selected area with periodic space and calibration looks. In this mode, a ‘frame' consisting of multiple ‘blocks' of the size 640 km x 640 km, can be sounded. The selected frame can be placed anywhere within a 24º (E-W) x 19º (N-S) FOR. It takes almost three hours to sound an area of 6400 km x 6400 km in size. As with the Imager, the Sounder provides an adequate radiometric resolution for the intended science applications.
Infrared radiometric fidelity is maintained by timed interval views of the space for reference (approximately every 120 s). And the full-aperture internal blackbody (at 30 minutes interval or whenever commanded) location of the space view (east or west) is ground command selectable. The blackbody view establishes a high-temperature baseline for in orbit calibration for IR channels. Also during space views, an electrical calibration (E-cal) signal, consisting of a sixteen step staircase, is injected every time. Eventually, this staircase signal covers the full dynamic range of the video processors. The E-cal signal helps in checking the stability of the amplifiers and of the data stream.
Instrument operation: The scan mirror motion, synchronized with the filter wheel, determines the sounding operation. At a particular mirror position, the filter wheel rotates and sequentially brings all 18 IR spectral filters into the optical paths of the three bands. This activity takes around 80 ms, after which, the filter-less ( blank region) of the wheel starts. During this period of about 20 ms, the scan mirror steps to the next location in East-West direction and settles before the start of occurrence of the first filter in the optical path. This time is also used for the DC restoration of the IR detector output. Thus, the total time for one filter-wheel revolution is 100 ms (600 rpm). It is possible to carry out multiple (up to four) measurements before the scan mirror steps to a new location. The VIS channel is sampled independently of the filter-wheel position.
The operation of the Sounder is controlled by ground commands in terms of configuration, gain, sounding area location and other such parameters. The sounding area is defined in terms of east –west and north –south ‘blocks'. A step in the east-west direction is 10 km and such 64 steps make a block in the east-west direction. - Similarly, each step into the north-south direction is of 40 km, and 16 steps make a block in the north-south direction.
The number of blocks can be independently selected in both directions. For example, 1 block in E-W and 1 block in N-S direction will scan 640 km x 640 km on the ground at the subsatellite location. Depending upon the scan offset selection: the maximum number of blocks in each direction can be 15. The dwell time for each sounding can be selected from 100 ms to 400 ms in steps of 100 ms. The sounding is bi-directional in the East-West direction. After every 1216 filter wheel revolutions, the mirror slews into the east-west direction to a location 9º away from nadir for a space look. The direction of this slewing (East or West) is ground command selectable so as to optimize the sounding time and to avoid any sun-moon intrusion. After the commanded ‘N' steps into the E-W direction are traversed, the scan mirror steps to South by 1120 µrad and starts scanning in the reverse direction. The south stepping takes 200 ms including the stabilization time. The sounding operation described so far is periodically (every 30 minutes), interrupted for views of the internal blackbody. The blackbody view sequence is similar to that for the Imager.
The signals from the detectors are processed and readout in synchronization with the filter wheel rotation. Thus, the data format for the Sounder, which includes the sounding data as well as other auxiliary information, repeats every 100 ms.
Figure 11: Illustration of the Sounder (image credit: ISRO)
Optics subsystem: The optical scheme for the Sounder is shown in Figure 12. The telescope for the Sounder is identical to that of the Imager. Dichroic beam splitters separate the scene radiance into various spectral ranges of interest. There are a total of three dichroic beam-splitters in the instrument. The first one, placed behind the primary mirror, separates the VIS channel from the IR channels. The VIS beam is transmitted through the dichroic and after subsequent folding, is measured by the VIS detector array. The reflected IR radiation is collimated after passing through a collimating lens. A set of two dichroic systems and a fold mirror separate the LWIR, SWIR and MWIR bands, in that order. These beams, corresponding to the three bands, are passed through the filter wheel assembly, which defines the channels in each band.
Figure 12: Block diagram of the Sounder optics subsystem (image credit: ISRO,Ref. 9)
The filter wheel has 18 filter windows distributed into three concentric rings, one each for the LWIR (7 channels), the SWIR (6 channels) and the MWIR (5 channels). The filter lengths and inter filter gaps are optimized to get the best possible performance of all channels with this size of the wheel. Nearly 20% of the wheel area is kept blank, i.e. without any filters. When this portion blocks the optical path, the scan mirror stepping takes place. Thus, the mirror is held stationary during sounding. The filter wheel rotates at a uniform speed of 600 rpm, completing one revolution in 100 ms.
The filters on the wheel function as the spectral defining elements for each of the 18 IR channels. Since the filter wheel is placed away from the detectors in the cooler, its temperature has major a effect on the radiometric stability and background radiation. To minimize these effects, the wheel is cooled to 213K using a separate cooler. The temperature of the wheel casing is controlled with a stability of ±1 K.
The 19 channels of the Sounder are acquired by use of four distinct detector head assemblies. Each detector assembly consists of an array of four detector elements arranged in N-S direction. Each detector element is of the size 10 km x 10 km on the ground (280 µrad IGFOV) at Nadir. Thus, each detector head assembly provides a footprint of 10 km x 40 km.
The detector for the VIS channel is very similar to that of the Imager, except that the element size is larger commensurate with the IGFOV, and each array contains only four detectors instead of eight for the Imager. This is the only channel with redundancy for the detector and the signal processor.
The 3 detector assemblies catering to the LWIR, MWIR and SWIR channels, sense the 18 IR channels. Each detector assembly has four detector elements. The LWIR and MWIR detectors are HgCdTe detectors operated in PC mode while for SWIR, InSb detectors are operated in PV mode. The first stage of the preamplifier is integrated within the SWIR detector package, similar to the MWIR detector package for the Imager These detectors are mounted in a passive cooler similar to the one used in the Imager and operated at a nominal temperature of 95 K with a stability of better than ±0.25 K. 9)
Figure 13: Illustration of the Sounder filter wheel for spectral selection (image credit: ISRO)
Figure 14: Scan modes of the Sounder (image credit: ISRO)
Figure 15: Sounder spectral bands and their applications (image credit: ISRO)
INSAT-3DR adds a new dimension to weather monitoring through its Atmospheric Sounding System, which provides vertical profiles of temperature (40 levels from surface to ~ 70 km), humidity (21 levels from surface to ~ 15 km) and integrated ozone from surface to top of the atmosphere. INSAT-3DR provides continuity to earlier and ongoing missions and further augments the capability to provide various meteorological as well as search and rescue services. INSAT-3DR is functioning along with the INSAT-3D mission and hence is providing enhanced imaging periodicity (~15 minutes) for better and accurate whether forecasting.
About 24 meteorological and geophysical products from INSAT-3DR are derived and ingested into the operational weather forecasting activities. In addition, some of these parameters, particularly the Atmospheric Motion Vector winds from the imager, as well as the temperature and humidity profiles from the Sounder are being ingested in numerical weather forecast models in real time for accurate weather prediction. Temperature maps of a given area in eighteen IR channels of Sounder are shown in Figure 16.
Figure 17: Sounder generated vertical atmospheric profiles of temperature and humidity (image credit: ISRO)
DRT (Data Relay Transponder):
The DRT receives globally metrological, hydrological and oceanographic data from automatic DCPs (Data Collection Platforms) in the ground segment and relays back to downlink in extended C -band.
Figure 18: Illustration of the DRT and the SAS&R transponders (image credit: ISRO)
For extreme weather related disasters such as a cyclone, floods and drought, real time observations of the associated parameters with appropriate network density is very important. Satellite enabled DCPs provide a unique solution for gathering meteorological data from all over the country including remote and inaccessible places. The IMD (India Meteorological Department) and ISRO have established more than 3000 DCPs.
SAS&R (Satellite Aided Search & Rescue):
The SAS&R payload operates at 406 MHz. The objective of SAS&R is to relay a distress signal / alert detection from the beacon transmitters for search and rescue purposes with global receive coverage in UHF band. The downlink operates in extended C-band. The data are transmitted to INMCC (Indian Mission Control Center), located at ISTRAC (ISRO Telemetry, Tracking and Command Network), Bangalore.
The major users of the SAS&R service in India are the Indian Coast Guard, the AAI (Airport Authority of India), Directorate General of Shipping, Defence Services and fisherman. The Indian service region includes a large part of the Indian Ocean Region covering India, Bangladesh, Bhutan, Maldives, Nepal, Seychelles, Sri Lanka and Tanzania for rendering distress alert services.
INSAT-3DR will join INSAT-3A and INSAT-3D to provide the operational Search and Rescue Service.
IMDPS (INSAT Meteorological Data Processing System)
ISRO has taken up the responsibility of end-to-end reception and processing of the INSAT-3DR data and the derivation of the meteorological parameters with IMD (India Meteorological Department), New Delhi. An indigenously designed and developed IMDPS (INSAT Meteorological Data Processing System) is installed and commissioned at IMD, New Delhi with a Mirror Site at SAC (Satellite Applications Center), Ahmedabad.
IMDPS will cater to the processing of all data transmitted by the Imager and Sounder payloads. The data archival and dissemination is through IMD, New Delhi and MOSDAC (Meteorological and Oceanographic Satellite Data Archival Center) websites. IMDPS comprises three major subsystems:
- Data Acquisition and Quicklook Display System
- Data Products System
- Geophysical Parameter Retrieval System.
The geophysical parameters and products will be derived and ingested into the operational weather forecasting activities at IMD. In addition, some of these parameters, particularly the AMVs (Atmospheric Motion Vectors) from the Imager, as well as the temperature and humidity profiles from the Sounder will be ingested in numerical forecast models in real-time for accurate weather prediction.
1) "INSAT-3DR," ISRO, Sept. 8, 2016, URL: http://www.isro.gov.in/Spacecraft/insat-3dr
5) "GSLV-F05/INSAT-3DR," ISRO brochure, August 2016, URL: http://www.isro.gov.in/sites/default/files/gslv_f05_insat_3dr-final.pdf
7) "INSAT-3DR Augments INSAT-3D for Improved Weather Monitoring and Prediction," ISRO, URL: http://www.isro.gov.in/insat-3dr-augments-insat-3d-improved-weather-monitoring-and-prediction
8) Information provided by Somya S Sarkar of ISRO/SAC(Satellite Application Center), Ahmedabad, India.
9) V. R. Katti, V. R. Pratap, R. K. Dave, K. N. Mankad, "INSAT-3D: an advanced meteorological mission over Indian Ocean," Proceedings of SPIE, 'GEOSS and Next-Generation Sensors and Missions,' Stephen A. Mango; Ranganath R. Navalgund; Yoshifumi Yasuoka, Editors, Vol. 6407, Goa, India, Nov. 13, 2006, DOI: 10.1117/12.697880, URL of abstract: http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=1295224
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).