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Title
Proceedings of the Symposium on Global and Environmental Monitoring

737
over a particular latitude approximately at the same local time enabling the ground
illumination conditions at sub-satellite regions to be constant throughout the
mission. The satellite makes 14 orbits per day with a repetitive cycle of 22 days.
The orbit of IRS-1A is maintained so that the ground track at the equator will be
within 14.8 km across the track from the nominal value. Considering the data
quality, attitude pointing accuracies and attitude rate specifications were arrived
at (DOS, 1988B).
1.2 The Sensors
The imaging sensors onboard IRS-1A and their characteristics in terms of spatial,
spectral, ground swath etc. were arrived based on the application needs of various
users, and keeping in view the state-of-art technology, consideration of future
growth potential, and the need to provide continuity of services to the Landsat MSS
and TM users in India and its neighbours. The IRS-1A sensors operate in pushbroom
scanning mode using linear CCD arrays and there are four spectral bands in the
wavelength regions of 0.45 - 0.52, 0.52 - 0.59, 0.62 - 0.68 and 0.77 - 0.86
microns. There are two types of imaging sensors known as Linear Imaging Self-
Scanning Sensor (LISS) : one with a spatial resolution of 72 meters and a swath of
148 kms (designated LISS-I) and other with two separate imaging sensors with a
spatial resolution of 36 meters and ground swath of 74 kms each (designated as LISS-
IIA & LISS-IIB). A combined swath of 145 kms is provided by the two LISS-II sensors
covering the same ground area of LISS-I sensor. The uniqueness in employing
individual lens assembly for each spectral band optimises the performance in each
spectral band and effectively utilises the full dynamic range of CCDs. Facility
exists for controlling the individual band gain through ground command and four
selectable gains are available. The input signal in each band is amplified and
resolved into 128 radiometric levels.
The overall specifications of IRS-1A are given in Table-1.
TABLE - 1 : IRS-1A CHARACTERISTICS
IRS-1 A SPACECRAFT MAIN FRAME DETAILS
Weight
Solar Array
Battery
Data rate
Modulation
Attitude Sensors
Attitude Control
Orbit Control
Telemetry
Telecommand
Tracking
Approximately 975 Kg
: 8 5 sq metres area, employable and sun-tracking panel
Generation ol 620 watts at EOL
Two Ni-Cd batteries ol 40 AH capacity each
: 5 2 Mbps tor LISS-I
2 x 10 4 Mbps tor LISS-II
: PCM/BPSK lor LISS-I in S-band
PCM/OPSK lor LISS-II in X-band
: IR horizon sensors (conical and static), star sensors,
sun sensors. Dynamically Tuned Gyros (DTG)
: Reaction Wheels (4), magnetic torquers, hydrazine
thrusters
: Monopropellant hydrazine thrusters.
: House Keeping (HK) Information In S-band. PCM/PSK
Real time rate 256 bps and play-back rate 4 Kbps
Onboard storage capacity ot 98 minutes ot HK data
: S-band. PCM/FSK/FM/PM and
VHF; PCM/FSK/AM
Facility for ON/OFF and Data commands
: S-band tone ranging and two way doppler
X-band beacon
IRS-1 A DATA PRODUCTS
TYPE
PROCESSING
LEVEL
FORMAT
(mm)
LOCATION
ACCURACY
(1 sigma)
TURN
AROUND
TIME
Were)
Quick
Look
Ltveuo
(Sun eievaton
and Earih rotation
corrections)
70
Real time
Browse
Level-1
(Earth rotation
and radiometric
corrections)
63x65
HOKms
2 days
Stindard
Lev*?
(Earth rotation.
Radrometnc
& Geometric
corrections)
241
i? 2 Kms
7 days
Precision
Level-3
(Use ot Ground
Control Points)
241
100 mirs
15 days
Special
Levei-4
(Enhanced
Products)
241
Same as
the inputs
15 days
Computer Compai.ble Tepes (CCT) ol 1600 / 6250 BPI
IMAGING SENSORS CHARACTERISTICS
LISS 1
LISS-II
Focal length (mm)
162 2
324 4
Field-ol-view (deg )
Instantaneous Field ol View.
9 4
47 + 47
IFOV (microrad)
80
40
Delector elements — CCD
2048
2048
Scaning mechanism {
Push-Broom
Push-Brocm
Ground resolution (meler)
72 5
36 25
Spectral range (micro meteT)
0 45-0 86
0 45-0 86
Number of bands
4
4
Swath (Kms)
Radiometric resolution
148
74 X 2
(grey levels)
128
128
Data rale (Mbps)
52
104 x 2
Weight (Kgs)
38 5
808 X 2
Power (watts)
34
34 x 2
1RS-1A
MISSION DETAILS
Type
: Polar. Sunsynchronous
Altitude
: 904 km (near circular)
Inclination
: 99.05 DEG
Local time of
Equator crossing
: 10.25 a m., at descening node
Mission Life
: 3 years
Launch Date
: March 17. 1988
Repetivity
: 22 days (307 orbi’s)
Ground Track Control ±14 km ol path