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