IRS (Indian Remote Sensing Satellites) - Overview and early LEO Program of ISRO

Following the successful demonstration flights of Bhaskara-1 and Bhaskara-2 - experimental Earth observation satellites developed and built by ISRO (Indian Space Research Organization) - and launched in 1979 and 1981, respectively, India began the development of an indigenous IRS (Indian Remote Sensing Satellite) program. India realized quite early that sustaining its space program in the long run would depend on indigenous technological capabilities (in particular, US export restrictions made this clear). Keeping this in mind, besides building satellites, India embarked as well on satellite launch vehicle development in the early 1970s. As a consequence, India has two very capable launch systems at the start of the 21st century, namely PSLV (Polar Satellite Launch Vehicle) and GSLV (Geosynchronous Satellite Launch Vehicle). ¹⁾

IRS is the integrated LEO (Low Earth Orbit) element of India's NNRMS (National Natural Resources Management System) with the objective to provide a long-term spaceborne operational capability to India for the observation and management of the country's natural resources (applications in agriculture, hydrology, geology, drought and flood monitoring, marine studies, snow studies, and land use). The intend of the program is to create an environment of new perspectives for the Indian research community as a whole, to stimulate the development of new technologies and applications, and to utilize the Earth resources in more meaningful ways.
Note: The INSAT system is India's GEO (Geosynchronous Earth Orbit) element, providing for simultaneous domestic communications and earth observation functions.

The IRS program started in the mid 1980s. Eventually, a continuous supply of synoptic, repetitive, multispectral data of the Earth's land surfaces was obtained (similar to the US Landsat program). In 1995, IRS imagery was made available to a larger international community on a commercial basis. The initial program of Earth-surface imaging was extended by the addition of sensors for complementary environmental applications. This started with the IRS-P3 satellite which is flying MOS (Multispectral Optoelectronic Scanner) for the measurement of ocean color. The IRS-P4 mission is dedicated to ocean monitoring. ^{2) 3) 4)}

Note: The availability of Landsat imagery created a lot of interest in the science community. The Hyderabad ground station started receiving Landsat data on a regular basis in 1978. The Landsat program with its design and potentials was certainly a great model and yardstick for the IRS program.

• The first generation satellites IRS-1A and 1B were designed, developed and launched successfully during 1988 and 1991 with multispectral cameras with spatial resolutions of 72.5 m and 36 m, respectively. These early satellites were launched by Russian Vostok boosters from the Baikonur Cosmodrome.

• Subsequently, the second generation remote sensing satellites IRS-1C and -1D with improved spatial resolutions have been developed and successfully launched in 1995 and 1997, respectively. IRS-1C/1D data has been used for cartographic and town planning applications.

Table 1: Chronology of ISRO polar-orbiting imaging missions and instruments (starting with IRS series)

IRS-1A (Indian Remote Sensing Satellite-1A)

The spacecraft bus is box-shaped (1.6 m x 1.6 m x 1.5 m) with two solar panels (8.6 m²) as shown in Figures 1 and 4. The S/C structure is made of aluminum/aluminum honeycomb. The satellite is three-axis stabilized utilizing a zero momentum system. Hydrazine thrusters (80 kg fuel) are also used for control and momentum dumping. The IRS series satellites are built around a zero-momentum reaction wheel based system. Gyro-based attitude reference using quaternion propagation with attitude updates from Earth sensors and sun sensors (CCD-based is used for yaw angle measurements) provide the high-pointing accuracy and stability required for the imaging payload. Attitude is sensed by Earth sensor, sun sensor, star sensor and dynamically tuned gyros. The actuators are reaction wheels (4), magnetic torquers, and hydrazine thrusters (sixteen 1-newton thrusters). A pointing accuracy of 0.3º is achieved in pitch/roll and 0.5º in yaw. Attitude determination accuracy of ±0.1º. Total S/C mass = 975 kg (at launch), power = 700 W, two NiCd batteries (40 Ah) provide power for the eclipse phase of the orbit. The design life is three years. ^{5) 6) 7) 8) 9)}

Figure 1: Illustration of the IRS-1A spacecraft (image credit: ISRO)

Launch: March 17, 1988 on a Russian launch vehicle Vostok-2M from the Baikonur Cosmodrome, Kazhakstan.

Orbit: sun-synchronous orbit, nominal altitude = 904 km, inclination = 99.049º^, period = 103.2 minutes; the repeat cycle = 22 days; equator crossing at 10:26 hours on descending node.

Application: Land use, agriculture, forestry, hydrology, soil classification, coastal wetland mapping, natural resources (in particular pinpointing likely groundwater locations), disaster monitoring, cartography, etc.

RF communications: Payload downlink in S-band and X-band. The satellite carried a real time LISS-2A/B data downlink in X-band with a transfer rate of 10.4 Mbit/s (2 links). The data was downlinked to a 10 m dish antenna at Shadnagar on 20 W. The LISS-I data downlink was on 5 W S-band at a rate of 5.4 Mbit/s. There was no onboard recorder. The payload data were PCM/BPSK modulated.

The IRSO S/C control center is in Bangalore. TT&C function is provided by ISTRAC (ISRO Tracking Network), supported by DLR (GSOC, Weilheim), NOAA (Fairbanks), ESA (Malindi) and the USSR (Bearslake) ground stations.

Mission status: Spacecraft operations were ended in 1992.

Sensor complement:

LISS-I, -II (Linear Imaging Self-Scanning Sensor), with a total of three cameras. LISS-I and LISS-II are two multispectral camera assemblies, each with a different resolution providing a swath of about 150 km. Each LISS camera consists of the collecting optics, imaging detectors, inflight calibration system, the processing electronics, and data formatting electronics.

LISS-I employs four 2048-element linear CCD detector arrays with spectral filters (Fairchild CCD 143A). All cameras use refractive type collecting optics with spectral selection by appropriate filters. The refractive optics were chosen to obtain a large FOV (Field of View). A lens assembly for each spectral band is used for better performance and effective utilization of the full dynamic range of the CCDs. Two LEDs (Light Emitting Diodes) per band are provided for inflight calibration.

A LISS-I scene is 148 km x 174 km. The LISS-II A/B assembly features eight 2048-element linear CCD detector arrays with spectral filters (2 parallel swaths of 74 km each for the LISS-II A/B assembly with 3 km overlap, the total swath is 145 km). Four LISS-II scenes cover the area of one LISS-I scene. ^{10) 11) 12) 13)}

Table 2: Specifications of the LISS-I and -II instruments

Figure 2: Illustration of the LISS-I camera (image credit: ISRO)

Figure 3: Illustration of the LISS-II camera (image credit: ISRO)

IRS-1A data products are being acquired, processed and disseminated by NRSA (National Remote Sensing Agency) Data Center, Hyderabad. These data products compete directly with Landsat TM and MSS data as well as with SPOT Image data on the market.

IRS-1B (Indian Remote Sensing Satellite-1B)

IRS-1B is a follow-up satellite of IRS-1A. The IRS-1B spacecraft and instruments are practically identical with those of IRS-1A (satellite mass = 975 kg).

Launch of the S/C was by a Russian Vostok-2M vehicle from Baikonur on August 29, 1991 (same tracking support configuration as for IRS-1A).

Orbit: Polar sun-synchronous orbit; altitude = 904 km, inclination = 99.49^º, period = 103.2 minutes. Repeat cycle: 22 days.

ISRO's policy allows ground stations from other countries to have direct access to the Indian satellite imagery. Any existing ground station equipped to receive data from SPOT or from Landsat will be able to receive IRS-1B satellite data with very minor changes (ISRO can supply the upgrade). ^{14) 15)}

Figure 4: Line drawing of the IRS-1B spacecraft

Mission status: The IRS-1B satellite far outlived its design live of 3 years - being operational on its 10th anniversary after launch (Aug. 29, 2001). ¹⁶⁾

Afterwards, the S/C was being used for conducting various experiments including studying the design margins of the various subsystems and to test different modes of operation using the available fuel.

IRS-1E (Indian Remote Sensing Satellite-1E), (P1)

A technological mission with the primary objective to test an ISRO-developed launch vehicle by the name of PSLV-D1 (Polar Satellite Launch Vehicle-Developmental flight 1). The IRS-1E spacecraft is a modified IRS-1A engineering model. ¹⁷⁾

Launch: The launch occurred on Sept. 20 1993 from SHAR (Sriharikota Launching Range, Andhra Pradesh, ISRO's main launch site, India, located on India's east coast at 13.57º N, 80.293º E, about 100 km north of Chennai). Unfortunately, the satellite failed to achieve orbit (PSLV failure of 2nd stage, a newly developed launch vehicle of ISRO - the PSLV control software prevented attaining orbit). The mission ended in the ocean.

Nominal orbit: Sun-synchronous polar orbit with equatorial crossing at 10:30 AM on descending node, altitude = 904 km, period = 103 min, repeat cycle = 22 days.

Sensor complement: (LISS-I, MEOSS)

LISS-I (Linear Imaging Self-Scanning System). Same definition as under IRS-1A.

MEOSS (Monocular Electro-Optical Stereo Scanner), a DLR (Germany) experimental sensor with pushbroom CCD technology. MEOSS is a stereo camera system capable of recording three images simultaneously with a single lens by means of linear scanning (3-line stereo system). MEOSS has a single lens with a focal length of 61.6 mm and 3 CCD line sensor arrays. Each CCD array comprises 3236 sensor elements with 10.7 µm in size. The scanner operates in the spectral range of 0.57-0.7 µm. Resolution = 50 m along track, 158 m in cross-track, 45 m in vertical direction; swath width = 510 km, 8 bit quantization. Application: stereo view capability to study topography, geology, terrain analysis and modeling, snow/ice mapping, meteorology (cloud height and movement), etc. ^{18) 19) 20) 21)}

The MEOSS concept had already been demonstrated on an airborne platform of DLR in 1986 and in 1989. Prior to the IRS-1E flight, MEOSS was also launched on the SROSS-2 (Stretched Rohini Satellite Series) mission of ISRO (July 13, 1988 on ISRO's ASLV launcher). However, SROSS-2 experienced also a launcher failure.

Table 3: Some basic characteristics of the MEOSS instrument

Figure 5: Observation geometry of the MEOSS 3-line stereo camera (image credit: DLR)

IRS-P2 (Indian Remote Sensing Satellite-P2)

An ISRO mission with the objective to acquire remote sensing data for oceanographic, land, and atmospheric applications (demonstration of the IRS-1C/1D bus). The spacecraft is three-axis stabilized using conically scanning Earth sensors and dynamically tuned gyros for attitude sensing, and reaction wheels, magnetic torquers and monopropellent hydrazine thrusters as actuators. Inertial attitude referencing with the star sensor updates is one of the major ISRO developments for astronomical X-ray observations. The attitude control system of IRS-P2 provides multipurpose functions by pointing for remote sensing, and inertial pointing for the X-ray observations. S/C mass = 870 kg, solar power = 510 W, two 21 Ah NiCd batteries. ²²⁾

Launch: October 15, 1994 from SHAR (Sriharikota Launching Range) with a PSLV-D2 launcher. A major objective of this launch was to demonstrate/qualify the newly developed launch vehicle PSLV (Polar Satellite Launch Vehicle) of ISRO. This time around, the PSLV functioned correctly, allowing IRS-P2 to assume its orbit.

Orbit: Sun-synchronous circular orbit with an equatorial crossing at 10:30 AM descending node, altitude = 817 km, inclination = 98.7^º, repeat cycle = 24 days, period = 101.35 min.

RF communications: No onboard data storage capability. Downlink broadcast of R/T science data in X-band (8.316 GHz, PCM/QPSK/PM modulation, data rate = 2 x 10.4 Mbit/s) to a dedicated ground station network and to a general user community. TT&C operations in S-band.

Mission status: The IRS-P2 spacecraft continued in operations until September 1997. ²³⁾

Sensor complement:

LISS-IIM (Linear Imaging Self-Scanning System-II Modified). Same definition as under IRS-1A. The LISS-IIM instruments (there are two just as LISS-II A/B) are realized with a single optical feed utilizing the full ±5º FOV (there are 4 lenses in one optical head). The instrument employs pushbroom scanning and a linear CCD line detector array with 4 spectral bands in VNIR (0.45-0.52 µm, 0.52-0.59 µm, 0.62-0.68 µm, 0.77-0.86 µm. Focal length = 324.4 mm. The ground resolution is 32 m (cross-track) x 37 m (along-track). The combined swath is 131 km. The instrument mass is 80.8 kg (x 2), the instrument power is 34 W (x 2).

2) G. Joseph, B. L. Deekshatulu, "Evolution of Remote Sensing in India," Space in Pursuit of New Horizon, National Academy of Sciences publication, (editor: R. K. Verma and others), Allahabad, 1992, pp. 331-354

3) K. Kasturirangan, G. Joseph, et al., "IRS Mission," Current Science, Vol. 61, No. 3 and 4, Aug. 25, 1991, pp. 136-151

4) P. S. Goel, "Spacecraft Technology Development in India," Space Forum, Vol. 5, No 1-3, 2000, pp. 5-38

5) "Indian Remote Sensing Satellite and Associated Data Products," A.K.S. Gopalan, Proceedings of the Twenty-Third International Symposium of Remote Sensing of the Environment, Vol. I, p. 71, ERIM, Ann Arbor, MI, 1990

6) IRS NewsLetter, ISRO, Vol. 2 No. 1, March 1991

7) http://ceos.cnes.fr:8100/cdrom-98/ceos1/isro/eospro/irs1b.htm

8) http://www.nrsa.gov.in/satellites/irs-1a.html

9) http://www.geocities.com/indian_space_story/irs1b.html

10) G. Joseph, IRS-1A Camera - Its Evolution and Realization," brochure of NNRMS (National Natural Resources Management System), Bangalore, India

11) Note: At the time of project initiation, CCD arrays with maturity of production were limited to 2048 elements. Hence the swath of LISS-I was limited to about 150 km. Since LISS-II has a better resolution by a factor of two compared to the LISS-I camera, two LISS-II cameras were needed to produce a swath similar to that of LISS-I.

12) J. Kirshnamurthy, A. S Padmavathy, V. Jayaraman, Uday Raj, "Inter Sensor Comparison of Indian Remote Sensing Satellite - 1A Linear Imaging Self Scanning Sensors for Radiometric behavior," ACRS (Asian Conference on Remote Sensing) 1991, Oct. 30-Nov. 5, 1991, Singapore, http://www.gisdevelopment.net/aars/acrs/1991/mapfs/mapfs04pf.htm

13) http://www.csre.iitb.ac.in/isro/irs-1b.html

14) "India Expands Access to Imagery," Space News Aug. 26 - Sept. 8, 1991, p. 22

15) "India Calls IRS-1B Launch a Success," Space News, September 9-15, 1991, p. 12

16) "IRS-1B Completes 10 years of Remote Sensing Mission," Aug. 29, 2001, URL: http://asia.spaceref.com/news/viewpr.html?pid=5875

17) Note: The satellite designations P1, P2, P3, etc. stand for the launches carried out by the PSLV (Polar Satellite Launch Vehicle), the launch vehicle developed by ISRO

18) IRS-1E MEOSS Utilization Plan, ISRO, July 1991

19) F. Lanzl, "The Monocular Electro-Optical Stereo Scanner (MEOSS) satellite experiment," ISPRS Vol. 26-I, pp. 617-620, Stuttgart, 1986

20) M. Lehner, "Triple stereoscopic imagery simulation and digital image correlation for Monocular Electro-optical Stereo Scanner (MEOSS) project," ESA Proceedings of the International Symposium on Progress in Imaging Sensors, Nov. 1986, pp. 477-484 (SEE N87-24738 18-43)

21) F. Lanzl, "The Monocular Electro-optical Stereo Scanner (MEOSS) satellite experiment," 38th IAF (International Astronautical Congress), England, Oct. 10-17, 1987

22) Document on Configuration of IRS-P2 and MOS and their Interfaces, ISAC, Bangalore, Nov. 1992

23) http://www.geocities.com/indian_space_story/irs.html