RISAT-1. Radar Imaging Satellite

Spacecraft
Characteristics
Radar Imaging Satellite
RISAT-1 spacecraft mainframe
bus is configured by deriving
heritage from previous IRS missions
as well as mission specific new subsystem
designs. The prism shape of
the satellite allows stowing of the
active antenna in three folds around
the prism structure. The prism
structure is built around a central
cylinder. Most of the spacecraft
subsystems and the complete
payload are integrated in the prism
structure and the central cylinder.
The solar panel and rest of the
spacecraft subsystems are mounted
on the cuboid portion of the
satellite. Two solar panels with high
efficiency multi-junction solar cell
charge Ni-H2
battery of 70 AH
capacity. The satellite has an onboard
recorder with storage
capability of 240 Gbits of data. The
on-board data transmitter can
transmit with maximum data rate of
640 Mbits/sec in X-band on two
polarizations (RHC and LHC) by
reuse of X-band carrier. In the nonoperating
condition, the active
antenna looks at nadir. Prior to
operation, the spacecraft will be roll
0
tilted by ±34 to enable viewing
either right or left side of the flight
track. The satellite will also have
yaw steering capability to minimize
earth rotation effects.
Attitude and Orbit Control
System (AOCS) caters to various
modes of SAR payload operation.
The pointing accuracy requirement
of RISAT is 0.05deg (3 σ); and
-5
attitude rate specification is 5x10
deg/sec (3 σ) for frequencies
-4
0 - 0.2 Hz and 1x10 deg/sec (3 σ)
for frequencies ≥ 2 Hz; and the
post-facto attitude knowledge
requirement is 0.02deg. One of the
payload operating modes, namely
'spot tracking' calls for imparting a
large rate (around 0.42deg/sec)
about pitch axis and imaging for a
few seconds in the step and stare
mode. The basic attitude reference
during spot, strip, and ScanSAR
modes of payload operation will be
from gyros. For strip and ScanSAR
modes, the updates will be from
star sensors.
The attitude sensors will include
star sensors (18 deg.x18 deg.
FOV), 4π
sun sensors,
magnetometers, Inertial Reference
Unit (IRU) with DTGs, and conical
earth sensors. The actuators will
include RCS thrusters (eight 11N
thrusters for attitude control and
one 11N thruster for orbit control),
eight 1N canted thrusters to
produce control torques around all
axes, and four 11N thrusters for
orbit control with off-modulation
capability); reaction wheels (0.3 NM
and momentum storage capacity of
50 NMs); and magnetic torquers of
2
60 Ampm capacity. A bus
management unit (BMU) is
designed around the MA31750
processor which implements the
MIL-STD –1750 architecture. The
BMU interfaces with TTC (RF),
sensors, power, thrusters, MTCs,
DTG, WDE and payload for
command and house keeping
telemetry functions. It has MIL-
STD-1553 interface with star sensor
and SAR payload.