Communications: directive radio wave systems and devices (e.g. – Return signal controls external device – Radar mounted on and controls land vehicle
Reexamination Certificate
2000-05-15
2002-06-25
Sotomayor, John B. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Return signal controls external device
Radar mounted on and controls land vehicle
C342S093000, C342S101000, C342S159000, C342S198000
Reexamination Certificate
active
06411250
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an electromagnetic sensor system, and is particularly concerned with dealing with various types of noise which can be received by such a system. The system may be adapted for short range obstacle sensing. Such a system may be installed on a motor road vehicle as part of a collision warning system.
BACKGROUND OF THE INVENTION
A known type of electromagnetic sensor system uses a series of broad band radio frequency pulses to detect the presence and/or motion of objects. Such a system has a pulse generator forming part of a transmitter for transmitting a train of radio frequency pulses. Echoes of those pulses are received by a receiving antenna, the output of which is sampled by a sampler at a succession of sampling periods, each occurring at a predetermined delay after the transmission of a respective pulse. If the reflection of a pulse is received during a given sampling period, this is indicative of the pulse having travelled to the object and returned to the receiver in the predetermined delay, so that it can be deduced that the object is entering or leaving a notional range gate or envelope surrounding the transmitter and receiver.
Examples of such systems are shown in U.S. Pat. No. 5,361,070 (McEwan) and European Patent No EP 469027B (Cambridge Consultants Limited).
In general, the magnitude of the reflected pulses can be small in relation to background noise, and as a result the signals received over the sampling periods can be averaged in order to improve the signal to noise ratio, as is discussed in U.S. Pat. No. 5,361,070.
However such a system is still susceptible to interference from RF spike noise (produced by other systems of the same type, for example) and continuous wave RF signals.
These latter problems are particularly relevant where the system is to be installed in a motor road vehicle, other vehicles may be equipped with similar systems, which generate the noise spikes, and the system is likely to be operated in the vicinity of various sources of continuous wave signals, such as mobile telephone apparatus/vehicle identification systems.
The signals strengths of an echo received by an electromagnetic sensor system from a target of cross-section e at range R is given by the following expression:
S=PGA&sgr;/
(4&pgr;
R
2
)
2
where PGA is power-gain-area product for the system.
If the target is equipped with a similar system, then there is the possibility that a pulse transmitted by the target will arrive at the receiver during a sampling period. The signal strength of such an interfering pulse is:
I=PGA/
(4&pgr;
R
2
)
so that the signal-to-noise ratio, SNR, is:
SNR=&sgr;/
(4&pgr;
R
2
)
For an automotive application typical values might be &sgr;=0.1 m
2
·R=30 m giving:
SNR=−
50 dB
compared with a required signal-to-noise ratio typically of at least +15 dB for acceptable detection performance.
This signal-to-noise ratio will be improved by averaging the signals received over a large number of sampling periods, but even if 10
4
samples are averaged, the processing gain will only be 40 dB. Even if only one interfering pulse is received per averaging period, the averaged SNR will be −10 dB which might still be too low.
An impulse modulated electromagnetic sensor system operates at a pulse repetition frequency (PRF) typically of order 1-10 MHz: both the pulse generator in the transmitter and the sampler in the receiver will operate at this frequency. The audio frequency (AF) output from the sampler will have a bandwidth from DC to the Nyquist frequency (one half of the PRF).
For many applications, and in particular for automotive applications, the target echo occupies only a fraction of the AF bandwidth. The echo bandwidth is related to the wavelength of the signature (typically 3 to 30 cm) and the relative speed of the target (say 0 to 100 m/s) from which the pulses are reflected, giving a bandwidth of order 3.3 kHz. The output signal processing can include a low-pass filtering stage to improve the signal-to-noise ratio by rejecting noise (eg thermal noise in the receiver) outside the AF band.
The receive antenna will pick up any external RF signal, for example from a nearby radio transmitter. Of particular concern in automotive applications are on-car or roadside transmitters such as mobile phones or tolling vehicle identification systems. RF filters in the receiver can suppress any signal outside the operational bandwidth of the system, but other signals will be aliased into the AF output. For example, if PRF is 1 MHz and the operational bandwidth includes frequencies around 2GHz, then a signal at 2.000001 GHz will be aliased to 1 kHz and a signal at 2.000100 GHz will be aliased to 100 kHz. The low-pass filter (presumed DC to 3.3 kHz) in the output processing will reject the second signal but not the first.
In practice, modulated RF signals have finite bandwidth, typically 100 kHz for GSM systems, with corresponding channel separations of 200 kHz. An FM signal with a centre frequency of 2 GHz will appear in the AF output as energy distributed over the DC−50 kHz band, and a proportion of this energy will be passed by the low-pass filter. A signal with a centre frequency around 2.0002 GHz will appear in the AF output as energy distributed over 150-250 kHz, and should be rejected by the low pass filter.
Thus, in an impulse modulated electromagnetic sensor system the reflected signature from a target can be contaminated by continuous wave (CW) or amplitude/frequency /phase-modulated continuous wave (narrowband) RF signals which appear in the AF output. If their amplitude is sufficiently large they will obscure the wanted echo and prevent the target from being detected.
The invention seeks to provide various methods and apparatuses which are less susceptible to interference by either or both these types of noise.
BRIEF DESCRIPTION OF THE INVENTION
According to a first aspect of the invention, there is provided an electromagnetic sensor system comprising transmitting means for transmitting a train of radio frequency pulses, receiving means for receiving reflections of said pulses from remote objects, sampling means for sampling the output of the receiving means, processing means connected to the sampling means, and operable to detect said reflections in the sampled signal, and to determine information on the presence or range of said object, and gating means for preventing radio pulses transmitted by other sources or noise spikes from causing interference which results in spurious detections or indications of range by the processing means.
Thus, a number of such systems may be used in proximity to each other since the gating means prevents the pulses transmitted by one of the systems being mistaken by another system for reflections of pulses transmitted by that other system. Such mistakes would otherwise give rise to spurious detections or other inaccuracies in the output of the processing means.
The gating means may be arranged to operate by monitoring signals received by the receiving means, and preventing or inhibiting the operation of the sampling means when the amplitude of said signals exceeds a threshold.
As is explained above, reflections of signals transmitted by the transmitting means will make a smaller contribution to the amplitude of the output of the receiving means than will transmitted pulses received directly, (i.e. without an intervening reflection) from other systems. Consequently, the above thresholding procedure will discriminate between most genuine reflections and other pulses, which are transmitted by other systems and which are received directly by the receiving means.
Preferably, the gating means comprises a threshold detector for determining whether the received signal is above said threshold and, if it is not, generating an enabling signal for enabling the sampler to operate, and delay means for delaying the passage of the received signal from the receiving means to the sampling means so that the operation of the threshold detector
Kerry Nicholas John
Oswald Gordon Kenneth Andrew
Richardson Alan Trevor
Cambridge Consultants Limited
Lee Mann Smith McWilliams Sweeney & Ohlson
Sotomayor John B.
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