Communications: directive radio wave systems and devices (e.g. – Determining velocity
Patent
1997-03-10
1998-05-12
Sotomayor, John B.
Communications: directive radio wave systems and devices (e.g.,
Determining velocity
342111, 342112, 342115, 342116, 342196, G01S 1360
Patent
active
057512418
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to methods for measuring the speed of a moving body, and to apparatuses for implementing said methods.
BACKGROUND OF THE INVENTION
More particularly, the invention relates to a method of measuring the speed v of a body, in particular a vehicle, moving in a direction parallel to the ground, said measurement being performed by means of a Doppler radar having a transmitter/receiver fixed to the moving body at a certain height h above the ground and designed to transmit a radar beam towards the ground along a mean axis extending forwards or backwards relative to the direction of movement, said method including the following steps: with a first sinusoidal control electrical signal at a first frequency f.sub.1 to cause the transmitter/receiver to transmit a first incident radar wave at the same frequency f.sub.1 ; radar wave being reflected on the ground, and generating from said first reflected radar wave a first received electrical signal; received electrical signal, thereby generating a first multiplied electrical signal having a high frequency component and a low frequency component; filtered signal proportional to the low frequency component of the first multiplied signal; and instant t.sub.1, determining a first frequency spectrum corresponding to the first filtered signal.
Document EP-A-0 534 056 describes one example of such a method.
That mode of measurement is more accurate than using conventional revolution counters or angle sensors which measure the speed of rotation of the wheels of the vehicle, insofar as variations in wheel diameter and the wheels slipping or skidding on the ground give rise to relatively large errors in speeds measured by means of revolution counters or the like.
In addition, the Doppler effect radar merely needs to be fixed to the structure of the vehicle, and is simpler to install than a revolution counter which implies a mechanism connected to the moving parts of the wheels.
In known methods of the kind in question, the frequency spectrum of the filtered signal has a peak corresponding to a certain frequency, and that frequency can be used to compute the speed of the moving body relative to the ground, given an angle .alpha. formed between the travel direction of the moving body and the transmit-and-receive direction of the radar wave relative to the transmitter/receiver.
If the selected angle .alpha. is 90.degree., then the transmitted wave is indeed reflected by the ground, even if the ground presents no particular irregularity, but in this case the frequency shift due to the Doppler effect is nil.
In contrast, if the angle .alpha. is small relative to 90.degree., the frequency shift due to the Doppler effect is large, providing it is possible to receive a reflected wave from ground roughnesses extending perpendicularly to the reflected wave.
The frequency spectrum of the first filtered signal therefore depends strongly on the angle .alpha..
For that angle to be well defined, it is necessary for the radar beam to be very narrow, so that known methods of the kind in question operate poorly when the body is moving over relatively smooth ground, as applies for example to a road surface, particularly in the event of rain or ice.
Because the radar beam is narrow, there is then low probability of the beam encountering a reflecting obstacle capable of returning a reflected wave to the transmitter/receiver to enable the speed of the moving body to be measured.
In addition, if the beam is highly directional, then it is necessary to use a transmit-and-receive antenna that is directional, which antenna must therefore have lateral dimensions that are large relative to the wavelength used, and that constitutes a handicap both in terms of cost and in terms of ease of implementation.
Also, known methods of the kind in question generate measurement errors when the above-mentioned .alpha. varies unintentionally, e.g. when the moving body is a vehicle that is tilted forwards or backwards to a greater or lesser extent depending on its loading.
A
REFERENCES:
patent: 4302758 (1981-11-01), Tomasi
French Search Report dated 20 Mar. 1995, French Appl. No. FR9408346
Carreel Eric
Lewiner Jacques
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