Communications: directive radio wave systems and devices (e.g. – Directive – Beacon or receiver
Reexamination Certificate
2001-09-19
2003-09-09
Phan, Dao (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Beacon or receiver
C342S444000, C342S453000
Reexamination Certificate
active
06618009
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method and apparatus for locating the source of an unknown signal received by a plurality of signal relays.
2. Discussion of Prior Art
In IEEE Trans. on Aerospace and Electronic Systems, Vol. AES-18, No. 2, March 1982, P C Chestnut describes the basic technique of locating the source of an unknown signal such as a ground-based communications antenna; it involves determining the time difference of arrival (TDOA) and/or frequency difference of arrival (FDOA) of two signals from the source relayed to receivers by respective intercept platforms in the form of relay satellites in geostationary or geosynchronous orbits. The signals are relayed by the satellites along two independent signal paths to a receiving station, ie ground-satellite-ground paths. One satellite lies in the main beam or lobe of the source antenna radiation pattern and the other in a sidelobe. Each satellite accepts a signal (uplink) from the source, frequency shifts it using a turnaround oscillator and returns its frequency-shifted equivalent (downlink) to a ground receiver. The two signal path lengths are normally unequal, and this gives two signal arrival times at the receiver differing by the TDOA value. FDOA is due to relay satellite motion relative to the Earth and to one another, which Doppler shifts both downlink signal frequencies. The positions of the two satellites and the receiving station are known, and the locus of points of constant TDOA or FDOA is in each case a surface which intercepts the Earth's surface to define a curve referred to as a line of position (LOP). Two measurements of TDOA or FDOA at different times, or one of each at one or more times, provides two LOPs which intersect at the position of the source to be located.
TDOA is also referred to as differential time offset (DTO) and FDOA as differential frequency offset (DFO) or differential Doppler shift.
The technique of determining TDOA and FDOA from two received signals is described in IEEE Trans. on Acoustics Speech and Signal Processing, Vol. ASSP-29, No. 3, June 1981 by S Stein in a paper entitled “Algorithms for Correlation Function Processing”. It is also described in U.S. Pat. No. 5,008,679 relating to a transmitter location system incorporating two relay satellites and using both TDOA and FDOA measurements. The technique involves deriving the degree of correlation between the signals by multiplying them together and integrating their product. Trial relative time shifts and frequency offsets are introduced in sequence between the signals and their correlation is determined for each. The time shift and frequency offset which maximise the correlation are taken to be the required TDOA and FDOA, subject to correction for signal propagation delays in satellite transponders and frequency shifts in satellites and in processing.
The degree of correlation is determined from what is referred to by Stein as the cross ambiguity function or CAF A(&tgr;,&ngr;) defined by:
A
(
τ
,
v
)
=
∫
0
τ
z
1
*
(
t
)
z
2
(
t
+
τ
)
e
-
2
π
i
v
t
ⅆ
t
(
1
)
A(&tgr;,&ngr;) is the integral of the product of two signals z
1
(t) and z
2
(t) [complex or analytic versions of real signals s
1
(t) and s
2
(t)] after a trial time shift &tgr; and a trial frequency shift &ngr; have been introduced between them in processing after reception at the receiving station. The asterisk in z
1
*(t) indicates a complex conjugate. When a maximum value of the modulus of A(&tgr;,&ngr;), ie |A(&tgr;,&ngr;)|, is obtained, this being a correlation peak in the surface |A(&tgr;,&ngr;)| as a function of the two variables &tgr; and &ngr;, the values of &tgr; and &ngr; for the peak are the required TDOA and FDOA.
The system of U.S. Pat. No. 5,008,679 requires satellite positions and velocities to be known accurately, and needs highly stable phase in ground station and satellite oscillators. It has bandwidth limitations for satellite orbits inclined to the Earth's equatorial plane, and needs two receivers which are on the same site and have common time and frequency references.
U.S. Pat. No. 6,018,312 to Haworth relates to a transmitter location system employing a reference signal passing via the same satellite relays as the unknown signal and processed in phase coherence with it. The reference signal is used to remove a number of sources of error and limitations to which earlier techniques are subject, giving improved accuracy and capability for use under a wider range of conditions. Despite this improvement it has been found surprisingly that from time to time it can be impossible to discern a correlation peak in the CAF surface |A(&tgr;,&ngr;)|-all that can be seen is noise.
A related but different technique for counteracting sources of error using a broad band approach is disclosed in U.S. Pat. No. 5,594,452 to Webber et al.
SUMMARY OF THE INVENTION
It is an object of this invention to provide an alternative method and apparatus for transmitter location.
The present invention provides a method of locating the source of an unknown signal received by a plurality of signal relays, the method including the steps of:
(a) arranging for a plurality of receivers to receive replicas of the unknown signal via respective signal relays; and
(b) subjecting the replicas to correlation processing;
characterised in that correlation processing in step (b) is performed with a complex correlation function (CCF) at least partly compensated for change in the replicas' Differential Frequency Offset (DFO) with time due to relay motion relative to the source and receivers.
In a preferred embodiment of the invention, correlation processing in step (b) is also performed with data sets adapted in phase and subject to data replication or removal to counteract time dilation arising from relay motion relative to the source and receivers.
As indicated above it has been found that the reason for failure to obtain a correlation peak using a prior art ambiguity function is due to relay motion relative to the source and receivers. This is very surprising for geostationary satellite relays in particular, because their motion has hitherto been treated as constant over a measurement interval, which in turn implies that it affects all measurements at a site equally. Satellite motion alone would not have been expected to make a correlation peak obtainable on some occasions but not others. Despite this, in accordance with the invention it has been discovered that signal correlation is affected by velocity and acceleration components of the relay satellite along the lines joining it to the source and receiver, which results in the replicas' DFO and Differential Time Offset (DTO) being time dependent. DFO variation can be compensated as indicated above by adapting the correlation function, and where necessary DTO variation can be compensated also by adapting data samples.
Correlation processing in step (b) may include introducing a trial time offset between the replicas and evaluating their correlation and iterating this to obtain a correlation maximum and derive at least one of the replicas' DTO and DFO; it may be carried out with a CCF containing an exponent of a function of time having a first term which is a constant DFO value and a second term which is a product of time and a constant value for rate of change of DFO with time, ie a constant differential frequency rate offset (DFRO) value, and wherein step (b) also includes introducing a trial value corresponding to DFRO prior to evaluating correlation and iterating one type of trial value for each of the other type, repeating for more values of the other type and determining a DFRO value appropriate to compensate at least partly for change in DFO with time.
The CCF may be expressed as A(&tgr;
0
, b
1
, b
2
) given by:
A
(
τ
0
,
b
1
,
b
2
)
=
∫
0
τ
z
1
*
(
t
)
z
&a
Duck Simon R.
Edmonds Paul R.
Griffin Christopher
Nixon & Vanderhye P.C.
Phan Dao
Qinetiq Limited
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