linear fmce radar techniques
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Linear FMCW radar techniques
A.G.
Stove
Indexing
term:
Frequency
modulated
continuous
wove
radar
Abstract:
Frequency modulated continuous wave
(FMCW) radar
uses
a very low probability of
intercept waveform, which is also well suited to
make good
use
of simple solid-state transmitters.
FMCW is finding applications in such diverse
fields as naval tactical navigation radars, smart
ammunition sensors and automotive radars. The
paper discusses some features of FMCW radar
which are not dealt with in much detail in the gen-
erally available literature. In particular, it dis-
cusses the effects of noise reflected back from the
transmitter to the receiver
and
the application of
moving target indication to FMCW radars. Some
of the strengths and weaknesses of FMCW radar
are considered. The paper describes how the
strengths are utilised in some systems and how the
weaknesses can be mitigated. It also discusses a
modem implementation of a reflected power can-
celler, which can be used to suppress the leakage
between the transmitter and the receiver, a well
known problem with continous wave radars.
1
Introduction
As the name suggests, frequency modulated continuous
wave (FMCW) radar is a technique for obtaining range
information from a radar by frequency modulating a con-
tinous signal. The technique has a very long history
[l],
but in the past its use has been limited to certain special-
ised applications, such as radio altimeters. However,
there is now renewed interest in the technique for three
main reasons. First, the most general advantage pos-
sessed
by FMCW is that the modulation is readily com-
patible with a wide variety of solid-state transmitters.
Secondly, the frequency measurement which must be per-
formed to obtain range measurement from such a system
can now be performed digitally, using a processor based
on
the fast Fourier transform
[Z]
(FFT).
A
third reason
for the interest in FMCW radars is that their signals are
very dificult to detect with conventional intercept re-
ceivers
[3].
The frequency modulation used by the radar can take
many forms. Linear and sinusoidal modulations have
both been used in the past. Linear frequency modulation
is the most versatile, however, and is most suitable, when
used with an FFT processor, for obtaining range infor-
mation from targets over a wide range. For that reason
the renewed interest in FMCW radar is almost all con-
centrated on linear FMCW,
on
which this article will
concentrate.
Paper 8922F
(ES),
fin1
recejved
14th
June
1991
and
in revised
form
27th March
1992
The
author
is
with Philips Rcscarch Laboratories, Cross
Oak
Lane,
Redhill, Surrey RHl SHA, United Kingdom
IEE PROCEEDINGS-F,
Vol.
139,
No.
5,
OCTOBER
1992
2
The basic explanation of the working of a linear FMCW
radar is well known and has been described in many
other places, for example in Reference
4.
It is not entirely
obvious what one should
call
the beat signal, the fre-
quency of which carries the range information. In this
paper this signal
will
be called the intermediate frequency
(IF) signal
because
it corresponds most appropriately
with the IF of a pulse radar, although the information is
not modulated onto a conventional carrier. The signal
after the frequency analysis can best be referred to as the
video signal. This signal is indeed an exact analogue of
the video of a pulse radar. If a fast Fourier transform is
used
for the frequency analysis, then the FMCW video is
coherent.
3
Moving target indication
The simple analysis of the FMCW radar is adequate for
a single sweep if the target is not moving. In fact sweep-
to-sweep processing is possible for performing moving
target indication (MTI) and moving target Doppler
(MTD)
[SI
processing with FMCW radars in the same
way as for pulse radars. The Appendix contains a more
detailed analysis of the FMCW signal in this case.
The analysis shows that a moving Target indicator
(MTI) system operates in an almost identical manner for
an FMCW radar as for a pulse radar. There are some
additional second-order factors, but they have
no
practi-
cal effect
on
the MTI performance.
FMCW radars incorporating MTI are not yet
common because FMCW has not often been applied to
systems where MTI
is
needed. However, Fig.
1
shows the
operation of the MTI canceller on an experimental
S-band FMCW radar built at Philips Research Labor-
atories. The upper trace shows the video ‘A-scope’ picture
from one sweep of the radar. The lower trace shows the
signal at the output of a digital three-pulse MTI can-
celler. It can be seen that the static targets are indeed
cancelled and that a static cancellation of better than
40
dB has been achieved.
The analysis shown in the Appendix can be extended
to cover multiple cancellers and staggered sweep repeti-
tion frequencies, although these are best analysed by
using the conventional equations for pulse radar MTI,
and checking that the additional terms in the FMCW
MTI equation are indeed negligible.
Because
the MTI technique analysed in the Appendix
is the same as is used in pulse radars, it can likewise
be
extended to encompass moving target Doppler (MTD)
processing. MTD processing of FMCW signals is imple-
mented by measuring the rate of change
of
phase of the
output of each FFT range bin from one sweep to the
next, the same as is done for pulse radars. Fig. 2 shows a
simple block diagram of such a system. The Signal
SQUIRE FMCW battlefield surveillance
radar
uses
343
Basic analysis
of
the
FMCW
principle
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