Detection of spread spectrum signals

Details Detection of spread spectrum signals Detection of spread spectrum signals

1997-03-19

1999-09-21

Issing, Gregory C.

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

Directive

Beacon or receiver

342424, 359306, 708816, G01S 502, G02F 133

Patent

active

059559934

DESCRIPTION:

BRIEF SUMMARY
The invention relates to the detection of low power spread spectrum signals as used, for example, in many modern radars and in particular though not exclusively to direction finders using the cross correlation of signals received from two spatially separated receivers.
Many modern radars use spread spectrum techniques. This enables them to achieve good range resolution with low power outputs, and it also gives them LPI (Low Probability of Intercept) properties. The radar receiver, with its matched filter, can make use of the full processing gain available from a pulse of CW signal with large time-bandwidth product. An ESM receiver can not normally take advantage of this potential processing gain, and is additionally hampered by having to have a relatively wide bandwidth with a consequent noise penalty.
If a system attempting to detect and locate spread spectrum signals uses two spatially separated receivers and cross correlates the two received signals, it can achieve much of the processing gain available to the matched filter receiver. The two signals will contain both antenna noise and internal receiver noise. The internal noise in the two receivers will be independent and will not correlate. Much of the two sets of antenna noise comes from the same sources. However, if these sources are spatially distributed then the signals from these sources arrive at the two antennas with a distribution of time delays, and the antenna noise signals are effectively uncorrelated. Any fixed signal source (emitting a spread spectrum signal) will, however, emit a signal that will be present in both receiver inputs with a single fixed time delay. The cross correlation function of the two receiver outputs will contain the autocorrelation function of the signal coming from any fixed source, shifted along the time axis according to the time difference of arrival at the antennas. It will also contain two cross correlations of the signal in one receiver with the noise in the other, and the cross correlation of the two sets of noise. For large input signal to noise ratios the noise-noise cross correlation will be insignificant and the correlator output will effectively contain the signal autocorrelation function plus the two signal-noise cross correlations.
However, we are generally more interested in the case where the input signal to noise ratio is small. The noise-noise cross correlation will then dominate and the system will perform much worse than the matched filter, unless we can find some other way of rejecting a significant portion of the noise. This is possible. If the noise signals in the two receivers are indeed uncorrelated, then the signal-noise and noise-noise cross correlations will simply be noise signals spread, more or less uniformly, over the full length of the cross correlation function. However, the signal autocorrelation function will, for a spread spectrum signal be concentrated in the centre. If we take just the central portion of the cross correlation function, therefore, we can reject most of the noise energy present. This does not help us if we are concerned with making measurements, such as threshold detection, on the time domain cross correlation function.
It should also be noted that, because it has no a-priori timing information, the ESM system cannot integrate over several pulses in the way that the radar can.
If we wish to direction find on a fixed signal source, we can obtain the time difference of arrival at the antennas by direct measurements on the cross correlation function. However the resolution of this is limited by the width of the main lobe of the signal auto correlation function, which is approximately the inverse of the signal bandwidth. For example, a system using a baseline of 50 m to direction find on a signal of bandwidth 10 MHz would have a bearing resolution of about 34.degree. at best. Such direct measurement of the position of the autocorrelation function using threshold detection is, of course, very crude and fails to make use of the phase information present in the cross cor

REFERENCES:
patent: 2962714 (1960-11-01), Meixell et al.
patent: 4225938 (1980-09-01), Turpin
patent: 4297704 (1981-10-01), Marom et al.
patent: 4326778 (1982-04-01), Berg et al.
patent: 4468093 (1984-08-01), Brown
patent: 4558925 (1985-12-01), Casseday et al.
patent: 4652817 (1987-03-01), Yarborough et al.
patent: 4845502 (1989-07-01), Carr et al.
patent: 4847862 (1989-07-01), Braisted et al.
patent: 5016256 (1991-05-01), Stewart
patent: 5121248 (1992-06-01), Mohon et al.
patent: 5296861 (1994-03-01), Knight
Richard G. Wiley, "Electronic Intelligence: The Interception of Radar Signals", 1985, pp. 233-238.
IEEE Transaction on Microwave Theory and Techniques, Jun. 1994, USA, vol. 42, No. 6, ISSN 0018-9480, pp. 951-955, Soares S. et al, "A wide band tracking delay for radio astronomy".
RADAR 92 pp. 180 to 183 (133 Conference Publication No. 365): B Bashegi: A radar countermeasures receiver.

Affiliated with

Also associated with

Rating

Detection of spread spectrum signals does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Detection of spread spectrum signals, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Detection of spread spectrum signals will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-84463