Complex homodyned FSK diplex radar

Details Complex homodyned FSK diplex radar Complex homodyned FSK diplex radar

2001-02-16

2002-08-06

Lobo, Ian J. (Department: 3662)

Communications: directive radio wave systems and devices (e.g.,

Determining distance

Phase comparison

C342S118000, C342S145000, C342S070000

Reexamination Certificate

active

06429806

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to radar systems (and sonar and ladar) and methods for determining the range of objects, and more particularly to radar systems and methods for accurately determining the range of objects having little or no relative velocity.
2. Description of Related Art
Radio Detection and Ranging (“Radar”) is commonly employed to detect and determine the range of objects or targets relative to the radar system.
FIG. 1
is a diagram of a general radar system
1
and a channel or medium
2
that includes a target
30
. As shown in
FIG. 1
, the radar system includes a transmitter
10
having a transmit antenna
12
and a receiver
20
having a receive antenna
22
. In simple terms, the transmitter
10
generates a signal s(t) that is converted to an electromagnetic wave
14
by the transmit antenna
12
. The signal travels at the speed of light, c away from the transmit antenna
12
in the medium of the channel
2
. The signal may reflect off targets or objects such as the target
30
in the channel
2
. The receive antenna
22
receives the reflected electromagnetic waves and generates a signal Sr(t), which is processed by the receiver
20
. It is noted that the transmit antenna
12
and the receive antenna
22
may be in close proximity (monostatic radar systems). Alternatively, the transmitter
10
and the receiver
20
may be separated by a large distance (e.g., in bistatic radar systems).
In radar systems, if s(t) is a pulsed signal, the received signal s
r
(t) is nominally equal to &agr;s(t−t
r
). In such systems, t
r
is the round trip delay or the time required for the electromagnetic wave to travel from the radar transmit antenna to the target and back to the receive antenna and &agr; is an amplitude scaling coefficient. In such systems the range of the target is nominally equal to c x t
r
/2 where c is the speed of light (approximately equal to 3(10
8
)m/s in a vacuum). If the target is moving away from or toward the radar system (i.e., has a non-zero relative velocity), the relative velocity of the target may be determined by calculating the frequency or Doppler shift of s(t). In particular, it is well known that the velocity of the target, v, is nominally equal to −f
d
*c/f
0
where f
d
is the Doppler frequency and f
0
is the frequency of the transmitted wave
14
of s(t). These principles also apply to sonar and ladar (laser-based) target detection and ranging systems. In ladar the velocity of propagation is also the speed of light (the same as for radar). In sonar the velocity of propagation is the speed of sound (which varies with the nature of the medium in the channel). Various radar systems and methods are developed to exploit these well-known attributes to measure the range or velocity of targets in different environments. For example, a prior art system
100
that is used to measure the range and velocity of objects is shown in FIG.
2
. As is described below in more detail, the radar system
100
is a homodyned frequency shift keyed (“FSK”) diplex radar system. As shown in
FIG. 2
, the system
100
includes a signal generator or oscillator
101
, a transmit antenna
102
, a transmit coupler
103
, a receive antenna
106
, a mixer
104
, a switch
108
, a dual anti-alias filter
105
, and a signal processor
107
. The signal generator
101
alternately generates two transmit signals: s
1
(t)=Cos((&ohgr;
0
+&ohgr;
1
)t−&thgr;
0
) and s
2
(t)=Cos((&ohgr;
0
−&ohgr;
1
)t−&thgr;
0
). The signal generator
101
is thus a diplexed signal generator that alternates between the generation of the s
1
(t) and s
2
(t) signals. The transmit signals s
1
(t) and s
2
(t) are transmitted by the transmit antenna
102
via the transmit coupler
103
. The receive antenna
106
receives the reflected signals s
r
(t) from target objects where the signals are in the form of s(t−&tgr;) (switching between s
1
(t−&tgr;) and s
2
(t−&tgr;)). Accordingly, s
r
(t) is equal to either:
Cos((&ohgr;
0
+&ohgr;
1
) (
t
−&tgr;)−&thgr;
0
)
or
Cos((&ohgr;
0
−&ohgr;
1
)(
t
−&tgr;)−&thgr;
0
).
The received signal s
r
(t) and the transmit signals s
1
(t) and s
2
(t) are downconverted (mixed and low-pass-filtered) by the mixer
104
with the “local oscillator” (“LO”) signal Cos((&ohgr;
0
+&ohgr;
1
)t) and Cos((&ohgr;
0
−&ohgr;
1
)t). The variable &thgr;
0
represents the phase delay of the signal between the transmit antenna
102
and the mixer
104
LO signal. The resultant signal is the low pass filter (“LPF”) Of s
r
(t) x s
1
(t) or s
2
(t), which is either:
LPF
{Cos((&ohgr;
0
+&ohgr;
1
)
t
)Cos((&ohgr;
0
+&ohgr;
1
)(
t
−&tgr;)−&thgr;
0
)}=Cos((&ohgr;
0
+&ohgr;
1
)&tgr;+&thgr;
0
)  Eq. 1
LPF
{Cos((&ohgr;
0
−&ohgr;
1
)
t
)Cos((&ohgr;
0
−&ohgr;
1
)(
t
−&tgr;)−&thgr;
0
)}=Cos((&ohgr;
0
−&ohgr;
1
)&tgr;+&thgr;
0
).  Eq. 2
The switch
108
is synchronized to the changes in frequency at the diplexed transmit signal generator
101
and thus generates two different outputs at ports
110
and
112
having signals, F
1
and F
2
nominally equal to Eq. 1 and Eq. 2 after anti-alias filtering by the dual anti-alias filter
105
.
In the above equations, “&tgr;” is the round trip propagation delay to the target. By substituting &tgr;=(2/c)(R+Vt) and by letting &ohgr;
d
=&ohgr;
0
(2V/c) (note that the Doppler frequency is f
d
=2Vf
0
/c), &thgr;
0
′=&ohgr;
0
(2R/c)+&thgr;
0
, &ohgr;
1
′=&ohgr;
1
(1−(2V/c))≈&ohgr;
1
, then &ohgr;
0
&tgr;+&thgr;
0
=&ohgr;
0
(2V/c)t+&ohgr;
0
(2R/c)+&thgr;
0
=&ohgr;
d
t+&thgr;
0
′ and &ohgr;
1
&tgr;+&thgr;
1
=&ohgr;
1
(2V/c)t+&ohgr;
1
(2R/c)+&thgr;
1
=&ohgr;
1
(2Vc)t+&thgr;
1
+2&ohgr;
1
R/c=&thgr;
1
+2&ohgr;
1
R/c. Therefore the equations that were written in terms of &tgr; can also be written as:
F
2
=Cos(&ohgr;
d
t+&thgr;
0
′+2&ohgr;
1
R/c
))
and
F
1
=Cos(&ohgr;
d
t+&thgr;
0
′−2&ohgr;
1
R/c
)).
Thus, the F
1
and F
2
signals of the radar system
100
have the same amplitude and frequency but have a different phase. The phase difference between the F
1
and F
2
signals is &Dgr;&phgr;=2&ohgr;
1
&tgr;=2(2&ohgr;
1
R/c)=(4&pgr;(2f
1
)R/c). Accordingly for this system
100
, the range R is computed by the signal processor
107
as follows: R=(&Dgr;&phgr;)c/(4&pgr;(&Dgr;f)) where &Dgr;f=2f
1
is commonly called the “deviation frequency”. Targets of the prior art system (real FSK diplex Doppler radar) appear as signals of the form Cos(&ohgr;
d
t+&thgr;
0
′−2&ohgr;
1
R/c))=Cos(&ohgr;
0
(2V/c)t+&thgr;
0
′−2&ohgr;
1
R/c)).
For outbound targets, i.e., targets with increasing range with time, the Doppler shift f
d
is negative. For inbound targets, i.e., targets with decreasing range with time, the Doppler shift f
d
is positive. The FFT spectrum for real receivers, however, is always symmetrical about its origin. Specifically, the negative frequency portion of the spectrum is equal to the complex conjugate of the positive frequency portion of the spectrum. It is because of this symmetry that target Doppler signals appearing in any Doppler bin may either be inbound targets or outbound targets, thus there exists a velocity direction ambiguity.
Since the two halves of the spectrum in real receivers contain essentially the same information it is customary in real receivers to only process target information in only one half of the spectrum, e.g., in the positive frequency portion of the spectrum. In the prior art system
101
the direction ambiguity is resolved by observing the polarity of the measured delta phase. Since it is known that target ranges must always be positive it can be inferred whether the target information co

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