Communications: directive radio wave systems and devices (e.g. – Determining distance – With frequency modulation
Patent
1998-03-09
2000-07-11
Sotomayor, John B
Communications: directive radio wave systems and devices (e.g.,
Determining distance
With frequency modulation
342118, 342127, G01S 1336
Patent
active
060879793
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a device for measuring range.
The prior art Meinke, Gundlach, Taschenbuch der Hochfrequenztechnik, 5th edition, Springer-Verlag, pages S3-S4 discloses a continuous-wave radar, which is also known as FM-CW radar. FM stands for frequency modulation and CW stands for continuous wave. A continuous-wave radar of this typo continuously transmits a radar signal whose frequency is varied continuously by the frequency modulation. The radar signal reflected by an object arrives at the radar sensor after the signal propagation time t.sub.L ##EQU1## where c=propagation velocity of the radar signal, has been varied in the meantime by the frequency modulation. The radar signal which, for example, has been transmitted at time t.sub.1 at the frequency f(t.sub.1), in, at the time of its return t.sub.2 =t.sub.1 +t.sub.L, compared with the transmitted signal with the frequency f (t.sub.2). The two signals are compared using a mixer. The frequency obtained at the output of the mixer is the mixing frequency .DELTA.f of the two signals:
The mixing frequency .DELTA.f is proportional to the range L between the object and the sensor. So that, for a given optimum object range L, the mixing frequency .DELTA.f remains constant during a time interval, it is necessary for the frequency modulation to be linear as a function of time during the relevant interval, i.e. the frequency increases or decreases linearly as a function of time:
An example of a suitable modulating signal would be a sawtooth oscillation.
Complex evaluation of the mixing frequency .DELTA.f, that is to say of the FM Doppler signal, gives rise, depending on whether the instantaneous frequency is increasing or decreasing, to positive or negative distance-proportional frequencies which can be distinguished from velocity-proportional frequencies due to the object's motion.
Problems arise when measuring very distant objects on account of the limited coherence length of the transmitted signal. This means, even if the frequency modulation of the transmitted signal is exactly linear as a function of time, statistical phase and frequency fluctuations f.sub.x (t) are superimposed on the transmitted signal:
Fluctuations of this type result from the phase noise of the transmission oscillator. This means that, between two instants, the phase of the received signal can deviate from the expected value by a particular value which is commensurately greater as the phase noise of the oscillator becomes stronger and the time between the two instants becomes longer. The result of this is that, depending on the phase noise of the signal source, there is a time period after which the phase difference fluctuates so greatly that a meaningful measured value can no longer be determined. The propagation length of the microwave signal (=radar signal) corresponding to this time period is referred to as the coherence length. In the came of object ranges for which the signal propagation length exceeds the coherence length, it becomes very difficult if not impossible to measure the range.
The problem can be solved by equipping the transmitter with a low-noise oscillator. This implies a largo coherence length. However, the production of an oscillator of this type requires the use of expensive components.
Range finders having FM-CW radar are also described in U.S. Pat. Nos. 4,205,316 and 4,360,812.
SUMMARY OF THE INVENTION
The object of the invention is to provide a device for measuring range having improved coherence length.
The object is achieved by a device according to patent claims 1 or 2.
Advantageous refinements of the invention are given by the subclaims.
If the isolation of the transmit/receive switch is insufficient, a bistatic antenna arrangement may be provided. The transmit/receive switch may, for example, have a circulator or a directional coupler. If uncontrolled coupling of the transmitted power to the mixer is to be avoided, then the transmit/receive switch may be connected between the signal source, the mi
REFERENCES:
patent: 4054878 (1977-10-01), Diehl
patent: 4205316 (1980-05-01), Peperone
patent: 4360812 (1982-11-01), Peperone
patent: 4682175 (1987-07-01), Lazarus
patent: 4860017 (1989-08-01), Grudkowski
patent: 4895441 (1990-01-01), Allen, Jr.
"Signalverarbeitung mit akustischen Oberflachenwellen", Peter Schmitt Bauelemente der Elektronik, nachrichten elektronik 34, 1980 vol. 8, pp. 263-266.
"Taschenbuch der Hochfrequenztechnik", K. Lange et al., Springer Verlag, Berlin Heidelberg, New York, fifth edition, p. 305.
Heide Patric
Magori Valentin
Greenberg Laurence A.
Lerner Herbert L.
Siemens Aktiengesellschaft
Sotomayor John B
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