Communications: directive radio wave systems and devices (e.g. – Directive – Beacon or receiver
Reexamination Certificate
2000-01-19
2001-11-06
Phan, Dao (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Beacon or receiver
C342S442000, C342S444000, C342S445000
Reexamination Certificate
active
06313794
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and apparatus of detecting and determining an angular location of frequency agile emitters, and more particularly, to a method and apparatus which allows Electronic Warfare Support Measures (ESM) intercept receivers to identify and locate in angle frequency agile emitters from nonmonopulse interferometer phase measurements.
BACKGROUND OF THE INVENTION
Radio frequency interferometer design techniques, e.g. as described by Robert L. Goodwin, in “Ambiguity-Resistant Three and Four Channel Interferometers” (Naval Research Laboratory, Washington, D.C. Report 8005 Sep. 9, 1976), typically assume the same RF carrier frequency for all pulses used to resolve the phase ambiguities and locate the transmitter in angle. This assumption is always valid if monopulse techniques are used in making the phase measurements. However, monopulse measurements require a separate receiver pair and phase detector for each interferometer baseline (FIG.
1
), and are therefore expensive in terms of both weight and cost.
As depicted in
FIG. 1
, one such system includes the separate receiver pair and phase detector and includes antennas
40
,
42
,
44
,
46
,
48
coupled to frequency preselectors
50
,
52
,
54
,
56
,
58
, respectively. Receivers
60
,
62
and
64
,
66
form receiver pairs. The frequency preselectors
52
,
54
,
56
,
58
are in turn coupled to RF/IF receivers
60
,
62
,
64
,
66
,
68
, respectively, which are in turn coupled to phase detectors
70
,
72
,
74
,
76
, respectively. Power divider
78
splits the received signal and is coupled to an instantaneous frequency measurement device (IFM)
80
and to phase detector
76
. Phase detectors
70
,
72
,
74
and
76
are coupled to phase storage
84
. The phase difference &phgr; induced by the signal direction-of-arrival is measured modulo (2&pgr;)
86
by the individual phase detectors associated with each particular antenna, with antenna
40
as the reference. The measured phase is then stored in memory
84
. The IFM
80
provides measurements to a frequency storage
82
. The &phgr; mod (2&pgr;) measurements
86
are resolved at a resolve mod(2&pgr;) &phgr; ambiguity
101
process, and then the spatial signal angle of arrival (AOA) derived in a COS (AOA) process
102
, which is performed using frequency storage
82
and the resolved mod(2&pgr;) &phgr; ambiguity
101
. At process
103
the AOA and associated pulse data records or PDRs are provided to an active emitter file
104
.
FIG. 2
illustrates a conventional approach to reducing the number of receivers and phase detectors required by using a baseline switch in the interferometer implementation. The conventional interferometer includes a plurality of antennas
210
. The antennas
210
are coupled to an RF switch
200
. The RF switch
200
is coupled to a phase detection section
209
which includes a preselector
220
coupled to an RF/IF receiver
222
which is coupled to a splitter or power divider
224
. Splitter
224
is in turn coupled to an IFM
226
and a phase detector
230
. The RF switch
200
is also coupled to a second preselector
232
which is also coupled to an RF/IF receiver
234
. Both splitter
224
and RF/IF receiver
234
are coupled to the phase detector
230
. The phase detector
230
is coupled to a sort on frequency process
201
. The IFM
226
is coupled to a frequency storage device
240
. The sort on frequency process
201
is coupled to a resolve mod (2&pgr;) ambiguity
204
. The resolve clusters are forwarded to process
205
angle measurement extraction
205
. The extracted AOA is forwarded to a sort on AOA process
203
. The frequency storage
240
provides frequency measurements to processes
201
and
205
. A bit bucket
208
contains pulses from frequency agile radars which are unassociated with previously detected frequency agile emitters and end up in a pulse residue collection. The I-file
207
provides known frequency agile patterns.
Baseline switching, e.g., using an RF switch
200
(
FIG. 2
) to connect a single pair of receivers and phase detector
209
sequentially between interferometer antennas
210
can be used to save cost and weight. When implementing the baseline switching approach, intrapulse switching retains the monopulse performance. However intrapulse switching has drawbacks due to increased vulnerability to multipath. Multipath corrupts the trailing edge of received radar pulses, but not the leading edge; therefore it is most desirable to use phase measurements near the leading edge of a pulse. But typically the cumulative time required to sequentially sample the phase in intrapulse switching means at least some baseline phase measurements are made well back from the leading edge.
To allow phase measurements on the pulse leading edge, while still reducing the number of receivers used, interpulse switching can be employed. This alternative to monopulse switches the single receiver-pair and phase detector set between baselines to catch the leading edge of each pulse used to make the phase measurement. In this method the minimum number of pulses collected for a single emitter equals the number of interferometer baselines. But more pulses than this minimum number are typically collected. For example the intercept receiver tune strategy can be structured with dwells long enough to detect at least two pulses for the longest emitter pulse repetition interval expected.
Although interpulse switching saves weight and cost while reducing vulnerability to multipath, it introduces new problems. In particular, nonmonopulse switching makes the identification and location of frequency agile radars difficult. As used herein, nonmonopulse switching is identical to interpulse switching. Radars use frequency agility either as an electronic counter countermeasure (ECCM), or to enhance performance. As an example of performance use, many sea borne radars use frequency change every 10 ms to 100 ms to electronically steer the antenna beam. An ECCM application is frequency hopping within a bandwidth, possibly extending over 1 GHz, to reduce the vulnerability of surface-to-air missile systems to jamming.
Thus the change in transmitted frequency can be on a pulse-by-pulse or pulse-batch to pulse-batch basis with the RF carrier frequency of the pulse perturbed in either a random or preprogrammed fashion. But even if deterministic at the transmitter, the frequency change schedule is typically such that the frequency of the next pulse or pulse group cannot be reliably predicted from the frequency of the current pulse by ESM processing. A consequence of this unpredictability is that frequency hopping creates two serious problems for the intercept receiver: the inability to do pulse cluster-to-emitter association, and the inability to do precision emitter location in angle.
These problems arise because in a single frequency dwell the wideband ESM receiver collects pulses from many different emitters within tune bandwidths typically covering 500 MHz to 800 MHz. These pulses must be sorted or clustered by correctly associating the pulses with the individual radars. Hence, with both fixed-frequency and frequency-agile emitters, the interpulse switched system must augment the basic interferometer signal processing elements of resolving phase measurement ambiguities and extracting the signal angle-of-arrival or AOA measurements (
101
and
102
FIG. 1
) with the additional processing
201
and
202
shown in FIG.
2
. For the monopulse system, linking pulses to emitters can simply be done using angle measurements and angle predictions alone in
103
, but in the interpulse system, location in angle (processes
204
and
205
) and pulse-to-emitter association
203
through angle comparison, must be proceeded by pulse frequency clustering
201
.
This preliminary frequency sort occurs first in an interpulse switched system because it must compensate for the nonmonopulse interferometer baseline phase sampling, associating all the pulses from a fixed-frequency emitter so that the ambiguity resoluti
Litton Systems Inc.
Lowe Hauptman & Gilman & Berner LLP
Phan Dao
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