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27-9
Different data rate: can be associated
with the SAR depending on whether
range ccpmwpreskm IS conducted on-bonrd
or on the Pround. With the long duty
cycles envisaBed for use in Spotlight
mode, there is a need to receive radar
returns, not ody for the duration of the
time delay from near to far sides of the
image regon, but also for the duration of
the transmit puk.
The classical technique for reducing SAR
down linEc rates is to accept dl the returns
from a given single puke emission into a
register (during a receive 'window period
which is mucpI longer than the transmit
, and then to transmit the
data pulse scam pepio3 ated in that register during
the longer period associated with one
pulse repetition interval. However, when
the receive window is shorter tban the
transmit pulse, there is less opportunity
for data rote compression. if, on the other
hand, pulse compression is conducted
on-had, then the duration of the receive
window - post range aom ression - is
identical with the time delay i etween near
and far sides OP the image region. This
time difference can be significantly
smaller ohan the interpulse period and
appreciable reductions made to the data
rate.
Using such techniques, it is possible to
limit the data rate t0 values ips small as
13Q MWs for operations at C-band
where he $Mi chirp bandwidth is limited
to lOOM.Wz, and to BQQMbit/s for
operotions a~ X-band where the chirp
bandwidth is lidacd to 34#)MWz. Data
links with these $r;sradwidohs exist but the
problem of Rmdin these quantities and
rates of data WI 7 I require further
examimaioa