Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite
Reexamination Certificate
2002-11-19
2004-01-13
Blum, Theodore M. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a satellite
C342S357490, C342S444000
Reexamination Certificate
active
06677893
ABSTRACT:
BACKGROUND OF THE INVENTION
This application is the U.S. national phase of International Application No. PCT/GB02/01255, filed Mar. 18, 2002, which designated the U.S., the entire content of which is hereby incorporated by reference.
This invention relates to a method and an apparatus for locating the source of an unknown signal received by a plurality of signal relays.
The invention is particularly relevant to communications using Earth-orbiting satellite relays. Interference occurring in a satellite communications channel is a serious problem that can deny use of the channel to a legitimate user. Occurrences of interference number thousands annually, and are likely to grow due to the proliferation of satellite-based services, the emergence of personal satellite communications, and the ever-increasing congestion of the geostationary arc. Interference may result from equipment failure or human error such as incorrect orientation of an antenna, but it may also represent deliberate unauthorised use of a satellite communications channel or an attempt to deny it to other users.
In IEEE Trans. on Aerospace and Electronic Systems, Vol. AES-18, No. 2 March 1982, P C Chestnut describes the basic technique of locating an unknown signal source: 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. The signals are relayed along two independent signal paths to a receiving station. TDOA and FDOA are also known as differential time offset (DTO) and differential frequency offset (DFO) or differential Doppler. The technique of determining DTO and DFO 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 Ambiguity Function Processing”. 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 DTO and DFO, subject to correction for signal propagation delays in satellite transponders and frequency shifts in satellites and in processing.
U.S. Pat. No 5,008,679 relates to a transmitter location system incorporating two relay satellites and using both DTO and DFO measurements. The relay satellites are in geostationary or geosynchronous orbits and they relay signals along two independent signal paths to a receiving station, i.e. ground—satellite—ground paths. Each satellite accepts a signal (uplink) from the source, frequency shifts it using a turn-round 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 Doppler shifts are normally unequal because the satellites' velocities differ, so the signals' frequencies differ after they have passed via respective satellites. There is also a contribution to signal frequency difference from the difference between the frequencies of the two satellites' respective frequency translation or turnround oscillators used for mixing uplink signals before retransmission for downlink. The positions and velocities of the two satellites and the receiving station's position 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.
In the prior art, TDOA is also referred to as differential time offset (DTO) and FDOA as differential frequency offset (DFO) or differential Doppler shift, and the expressions DTO and DFO will be used hereinafter.
The degree of correlation is determined from what is referred to as the cross ambiguity function or CAF A(&tgr;,&ngr;) defined by:
A
(
τ
,
v
)
=
∫
-
T
/
2
T
/
2
s
1
*
(
t
)
s
2
(
t
+
τ
)
ⅇ
-
2
π
ⅈ
vt
ⅆ
t
(
1
)
A(&tgr;,&ngr;) is the integral of the product of two complex 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 s
1
*(t) indicates a complex conjugate. A maximum value of the modulus of A(&tgr;,&ngr;), i.e. |A(&tgr;,&ngr;)| is a peak in the surface |A(&tgr;,&ngr;)| as a function of the two variables &tgr; and &ngr;, and the values of &tgr; and &ngr; yielding this peak are the required DTO and DFO.
Since |A(&tgr;,&ngr;)| is a function of two variables &tgr; and &ngr;, it is two-dimensional and defines a surface referred to as the Ambiguity Surface: it may be calculated using a Fast Fourier Transform (FFT) technique. In one such approach a succession of lines in the Ambiguity Surface are calculated with varying &ngr; (trial DFO) and respective constant values of &tgr; (trial DTO): This effectively decomposes the surface into a series of 1-dimensional slices perpendicular to the &tgr; axis and referred to as ‘DFO Slices’. An efficient operation to compute a DFO Slice is FFT (s
1
*(t)s
2
(t+&tgr;)). Performing this computation for each practical value of &tgr; and combining slices gives the Ambiguity Surface.
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 sources of error and operational limitations: it gives improved accuracy and extends the range of conditions over which measurements can be made. Another 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.
International Patent Application No. GB 00/02940 relates to a modification to the technique of U.S. Pat. No. 6,018,312 to deal with the problem of time-varying DTO and DFO.
There is particular difficulty in locating a source of interference which is frequency agile, i.e. interference that is subject to changes in carrier frequency. The reason for this is as follows: the performance of the correlation process expressed by the Complex Cross Ambiguity Function depends on achieving an output signal-to-noise ratio (SNR), which is defined by
SNR
=
2
BT
snr
1
snr
2
1
+
snr
1
+
snr
2
,
(
2
)
where B is the acquisition sample bandwidth of primary and secondary receiver channels used to receive signals from respective satellites, and T is the integration time as defined in Equation (1) for the CAF A(&tgr;,&ngr;). The acquisition sample bandwidth is the bandwidth within which a signal must lie to be detectable by a receiver, and is defined by the receiver's signal processing system. The primary channel is associated with the ground-based receiver or antenna directed at an interference-affected satellite, and the secondary channel is associated with another receiver directed at a further satellite via which an unknown transmitter causing the interference is also detectable. The terms snr
1
and snr
2
are respectively the input signal-to-noise ratios in the primary and secondary channels. The term 2BT is called the Processing Gain.
To achieve reliable detection of a correlation peak in the modulus |A(&tgr;,&ngr;)| of the CAF A(&tgr;,&ngr;), the SNR in Equation (2) should exceed~100 (i.e. 20dB): if snr
1
and snr
2
are
Duck Simon R
Edmonds Paul R
Griffin Christopher
Haworth David P
Rideout Robert M
Blum Theodore M.
Nixon & Vanderhye P.C.
QinetiQ Limited
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