Short range ranging system

An accurate ranging system for position determination of a mobile unit using trilateralization. A pair of active reflectors return signals transmitted by the mobile unit in phase at the reflectors, after compensating for interval circuit delay. The mobile unit compares the phases and determines mutual distances and thus its position. The invention includes a two frequency technique for compensating for inaccuracies introduced at distances in excess of one wavelength and for inaccuracies observed at a large multiple of wavelengths, and also includes means for dynamically compensating for drift.

We claim:

1. A ranging system comprising:

(a) first means for transmitting a first continuous wave (CW) signal for a predetermined period of time, and for generating a second CW signal for a predetermined period of time at a frequency which is a very large fraction of the first CW signal,

(b) an active remote reflector for generating and transmitting a first reflection signal at least initially in phase at the reflector with the first CW signal, and for generating and transmitting a second reflection signal initially in phase at the reflector with the second CW signal following transmission of the second CW signal,

(c) means at the first means for receiving the first reflection signal and comparing its phase with the phase of the first CW signal whereby an apparent distance between the first means and reflector can be determined,

(d) means at the first means for receiving the second reflection signal and comparing its phase with the second CW signal, and

(e) means at the first means for providing a distance indicating signal corresponding to a result obtained by subtracting the phase differences of the first CW signal and received first reflection signal and the second CW signal and received second reflection signal and multiplying the result by the wavelength of the first CW signal divided by the difference between the wavelength of the first CW signal divided by the difference between the wavelengths of the first and second CW signal, the distance indicating signal representing an integral number of wavelength in excess of a fraction at which the first means is distance from said remote reflector.

2. A ranging system as defined in claim 1 including means for transmitting the second CW signal following completion of the first reflection signal.

3. A ranging system as defined in claim 2 including means for retransmission of the first CW signal following completion of the second reflection signal, and means at the first means for averaging the phase difference of the first CW signal and the first reflection signal resulting from the first and repeated transmission of the first CW signal and for using the average phase differences as the difference between the first CW signal and first reflection signal in providing said distance indication signal.

4. A ranging system comprising:

(a) a first transmitter for transmitting a first continuous wave (CW) signal for a predetermined period of time,

(b) an active reflector remote from said transmitter comprising:

(i) means for receiving said first CW signal,

(ii) phase locked loop means for receiving a signal corresponding to said CW signal from the receiving means and locking its phase thereto;

(iii) firsts compensation means for actively determining circuit phase delay within the active reflector,

(iv) a second transmitter for transmitting a first CW reflection signal at a predetermined frequency with initial phase defined by the phase locked loop means, compensated by the circuit phase delay, after termination of said first CW signal,

(c) means at said first transmitter for receiving said first CW signal and the reflection signal, for comparing the phases of said CW signal and the reflection signal, and for providing a signal corresponding to the apparent distance between the transmitter and active reflector related to the phase difference thereof, and

(d) compensation means for actively determining the circuit phase delay within the first transmitter and receiving means at the first transmitter and for subtracting the latter circuit phase delay from the phase of the first CW signal upon comparing it with the received reflection signal.

5. A ranging system comprising:

(a) a first transmitter for transmitting a first continuous wave (CW) signal for a predetermined period of time,

(b) an active reflector remote from said transmitter comprising:

(i) means for receiving said first CW signal,

(ii) phase locked loop means for receiving a signal corresponding to said CW signal from the receiving means and locking its phase thereto;

(iii) first compensation means for actively determining circuit phase delay within the active reflector,

(iv) a second transmitter for transmitting a first CW reflection signal at a predetermined frequency within initial phase defined by the phase locked loop means; compensated by the circuit phase delay, after termination of said first CW signal,

(c) means at said first transmitter for receiving said first CW signal and the reflection signal, for comparing the phases of said CW signal and the reflection signal, and for providing a signal corresponding to the apparent distance between the transmitter and active reflector related to the phase differences thereof,

(d) means at the first transmitter for transmitting a second CW signal having a frequency which is a very large fraction of the frequency of the first CW signal for a predetermined period of time,

(e) means at the active reflector for receiving said second CW signal, phase locked loop means for receiving a signal corresponding to said second CW signal from the latter receiving means and locking its phase thereto, means for actively determining circuit phase delay within the active reflector, and means for transmitting a second CW reflection signal at said second predetermined frequency with the initial phase defined by the latter phase locked loop means compensated by the circuit phase delay after termination of said first CW signal,

(f) means at said first transmitter for receiving the second CW signal and the second reflection signal, for comparing the phases of the second CW signal and the second reflection signal, and

(g) means at said first transmitter for providing a distance indicating signal corresponding to a result obtained by subtracting the phase differences of the first CW signal and first reflection signal and the second CW signal and second reflection signal and multiplying the result by the wavelength of the first CW signal divided by the difference between the wavelength of the first and second CW signal, the distance indicating signal representing an integral number of wavelengths in excess of a fraction at which the first means is distant from said remote reflector.

6. A system as defined in claim 5 including means at the first transmitter for transmitting the second CW signal following termination of the first reflection signal.

7. A system as defined in claim 6 including means for repeating transmission of the first CW signal following termination of the second reflection signal, and further means at the first transmitter for averaging the phase differences of the first CW signal and the first reflection signal resulting from the first and repeated transmission of the first CW signal and for using the averaged phase differences as the difference between the first CW signal and first reflection signal in providing said distance indication signal.

8. A ranging system comprising:

(a) a first transmitter for transmitting a first continuous wave (CW) signal for a predetermined period of time,

(b) an active reflector remote from said transmitter comprising:

(i) means for receiving said first CW signal,

(ii) phase locked loop means for receiving a signal corresponding to said CW signal from the receiving means and locking its phase thereto;

(iii) first compensation means for actively determining circuit phase delay within the active reflector,

(iv) a second transmitter for transmitting a first CW reflection signal at a predetermined frequency with initial phase defined by the phase locked loop means, compensated by the circuit phase delay, after termination of said first CW signal, signal,

(c) means at said first transmitter for receiving said first CW signal and the reflection signal, for comparing the phases of said CW signal and the reflection signal, and for providing a signal corresponding to the apparent distance between the transmitter and active reflector related to the phase difference thereof, and

further including means at the first transmitter for impressing a first discontinuity on the CW signal and for starting a first counter upon transmission of said discontinuity, means at the reflector for sensing the discontinuity and for starting a second counter in response thereto, means at the reflector for impressing a second discontinuity on the reflection signal and for stopping the second counter upon transmission of the second discontinuity, means at the first transmitter for stopping the first counter upon reception of the second discontinuity, means at the reflector for transmitting a signal coded to the value held by the second counter, and means at the first transmitter for receiving the latter signal and for subtracting the value held by the second counter from the value held by the first counter to obtain a distance indicating signal corresponding to twice the distance between the first transmitter and the reflector.

9. A system as defined in claim 8 in which the discontinuity compressing means are means for causing abrupt phase reversal in the signal.

10. A ranging system comprising:

(a) a first transmitter for transmitting a first continuous wave (CW) signal for a predetermined period of time,

(b) an active reflector remote from said transmitter comprising:

(i) means for receiving said first CW signal,

(ii) phase locked loop means for receiving a signal corresponding to said CW signal from the receiving means and locking its phase thereto;

(iii) first compensation means for actively determining circuit phase delay within the active reflector,

(iv) a second transmitter for transmitting a first CW reflection signal at a predetermined frequency with initial phase defined by the phase locked loop means, compensated by the circuit phase delay, after termination of said first CW signal,

(c) means at said first transmitter for receiving said first CW signal and the reflection signal, for comparing the phases of said CW signal and the reflection signal, and for providing a signal corresponding to the apparent distance between the transmitter and active reflector related to the phase difference thereof, and

(d) said comparing means comprising means for comparing the phases of the reflection signal and the CW signal multiple times in succession to determine phase differences and for interpolating the phase differences to the time just at or prior to the completion of said predetermined period of time,

whereby a phase comparison of the CW signal and the reflection signal can be made devoid of said drift in phase.

11. A ranging system comprising:

(a) a master unit for transmitting a first CW signal during a predetermined interval,

(b) an active reflector for transmitting a second CW signal initially in phase at the reflector with the first CW signal following aid interval,

(c) said master unit including means for measuring the phase of the second CW signal received at the master unit multiple times in succession, determining any drift of its phase over time, and for interpolating its phase to the time just at or prior to the completion of said interval,

whereby a phase determination of the second CW signal devoid of said phase drift is obtained at the master unit.

12. A ranging system as defined in claim 11 including means for comparing the first CW signal with the phase determination of the second CW signal to provide a signal corresponding to the apparent distance between the transmitter and active reflector.