THE SEASAT - A SYNTHETIC APERTURE RADAR 
EXPERIMENT 
P.G.Teleki, U.S.Geological Survey, Reston, USA; R.O.Ramseier, 
Environment Canada,. Ottawa, Canada 
INTRODUCTION 
SEASAT-A, the first satellite dedicated to the study of ocean dynamics, 
carries five instruments complementing one another in achieving a data 
ensemble capable of interrelating surface oceanic phenomena (Fig. 1). 
Four of the instruments, the scatterometer (SASS), the five-frequency 
scanning microwave radiometer (SMMR), the altimeter (ALT), and the visible- 
infrared radiometer (VIRR) will be obtaining global data sets. The fifth 
sensor, known as the synthetic aperture radar (SAR) will acquire data only 
in real-time over a network of ground stations limited geographically to 
the viewing capability of the stations' receiving antenna. This horizon- 
tally polarized L-band (1274.8 GHz) radar also differs from the other 
instruments on SEASAT-A by virtue of being flown in space for the first 
time. Hence, although expectations about the application potential of SAR 
data are high, these may have to be tempered by the actual performance of 
the SAR end-to-end system after launch, as all previous knowledge on 
L-band radars has been derived from aircraft flights. 
The SEASAT-A SAR is flown on the starboard side of the spacecraft, viewing 
a 100 km swath continuously 20.5° off nadir (Fig. 2). The spacecraft's 
velocity of 7.5 km/s provides an equivalent ground speed of 6.6 km/s. 
The SAR system consists of five subsystems: the sensor electronics (SES), 
antenna, and data link housed on the spacecraft, the receiving unit located 
at the ground receiving station,and the SAR data-processing system (SDPS) 
at the Jet Propulsion Laboratory. Among the functional aspects of the 
SAR system is the generation of a pulse by the SES, consisting of a chirp 
signal varying linearly from 1265.33 to 1284.33 MHz about a baseband 
carrier of 1274.83 MHz, radiated toward ground by the antenna (Ref. 1). 
The sensor is activated from the ground commands controlling the 
transmitter output pulse-repetition frequency (PRF) and the receiver sensi- 
tivity time control (STC). Radar echoes received by the antenna are com- 
pressed in amplitude by the STC, up-converted from L-band (1274.83) to 
S-band (2265.1 MHz), and telemetered as a wide-band analog signal to the 
ground receiving station at the rate of 117 Mbits/second. 
Stations capable of receiving SAR data contain specialized equipment for 
recording SAR passes (Ref. 1). The multifunctional receiver (MFR) 
locks in the received signal and provides output signals to the demodulator. 
The demodulator obtains and tracks the composite radar return, provides a 
linear transfer function for it, and sends offset video and timing signals 
to the SAR data formatter (SDF). The SDF digitizes the offset video signal 
and formats the data for recording on the high density digital recorder 
(HDDR). The generated high-density digital tape contains echo data, as 
well as housekeeping information (PRF, gain status, STC delay, etc).