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Annex A Radar system
A. I Background
It is possible to establish the feasibility in principle of
opcrating a radar on a TACSAT. by examining in outline
the pwcr budgets and hence signal/noisc ratio that might
be achieved using an advanced airborne radJr. The model
that will Dc uscd here is the Advanced Tactical Fighter
(ATF) radar currently under development. A tlcscription of
the this development programme5 included thc following
quotations:
- The USAF has pursued X-band AESA [Active
Electronically Scanned AnayJ technology since, the
early 1980s for three main rciuons: pciwcrlwcight rlitio.
agility and rcliahility. With more than a thousand
transmitlrcceive (T/R) modules. each capable of
generating around IOW of powct, Ihc F.22 has a peak
pwcr in the megawatt range. should that hc rcquircd.
-
Ultimately. a maintenance-free life of 20.0(N h . the
lifetime of the airaaft - is possible.
Synthetic apcrtiire radar (SAR) ... is not in ttic baseline
but he hardware can do it ...
A.2 Radar equation
The following assumptions will be made:
- number of T/K modules: , 4000
* module plwcr: IOW
- square anay. half-wave clement spacing
- . wavelength: 30 nini
Th,is indicatcs nn antenna gain of -39 dR and an cffcclivc
anicnnr area of - 1 In2. If \he spacecraft is at 400 km
altitude and a target of cross section Im2,is located 300 km
from the satellite track. then the received power will bc
-165 dRW. If a receiver noise figure of S dR bntl dctcction
threshold of 10 dB arc assumed. then the integrating time
will nwd to be - 3ms.
The bcam width will hc - IS km at SW km range. An area
of SO0 km square will cornprise of the ortlcr of 1000
resolution cells. A transmitter power of 40 kW and
integrating time of 3 ms pcr cell will r6quire - 0.1 MJ of
transmitted energy. The total energy available ' to the
payload is of the order of 50 MJ pcr orbit.
A ;3 Operational modes
AI; surveillance
Any look-down radar suffers from ground clutter and the
system dcscribcci here will be particularly badly affcc>cd
bccsuse of the relfltivcly widc beam widh. Two techniques
thai might be employed to improve the dctcclability of
airborne targets are:
- DopplCr hlTli
- mntched illutnination.
Doppler MTI may bc employed provided that thc radar is
directed across track to detect targets with a significant
velocity component towards the satellite. The pcrfnrmance
of MTI with the widc beam of this 'radar needs further
analysis.
Matched illurninatibn uses modulation schemes matched to
the characteristic dimensions and resonances of the target.
This has been u:.cd to idrntify specific aircraft typcs6. No
trials have been reported of the use of this technique to
reject ground clutter but the selectivity shown'in he
identification trials suggests hat it might be effective.
Ground su rvelllnnce
Ground surveillance to detect and locate vehicles could be
carried out using spotlight SAR. Typical performance
would be to form a SAR image of a single 15km diameter
region (corresponding to the beam width) on each satellite
pass. A synthetic apcrturc of 15 km (corresponding to 2 s of
illumination) would allow a SAR image of I m resolution
to be constructed.
Current real-time SAR processors capable of this level of
processing have a mass and power consumption suitable foi
airborne use and might feasibly be carried on-board the
satellite. Alternatively. the raw data could be broadcast for
ground processing.
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4-5
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