GB2315943A

GB2315943A - Distance measuring system

Info

Publication number: GB2315943A
Application number: GB9616187A
Authority: GB
Inventors: Paul Michael Wood; Anthony John Wood
Original assignee: Individual
Current assignee: Individual
Priority date: 1996-08-01
Filing date: 1996-08-01
Publication date: 1998-02-11
Also published as: GB9616187D0

Abstract

A distance measuring system using modulated radio signals which can be used for eg. car tracking, bird location, sea rescue etc. A method of phase compensation is provided which allows both transponder and measuring devices to have a known fixed phase eg. zero phase. A method of retransmitting the signal with known phase from the transponding device 5 (fig 2) is provided by a 'phase memory' circuit (10) within a switchable phase locked loop (8, 9, 18) in the transponder. A signal is sent from the measuring device 2 (fig 3) to a transponding device 5, then retransmitted from the transponder 5 back to the measuring unit 2 where the group delay of the signal is compared with the reference signal within the measuring device 2, the resulting time delay equates into distance and may be displayed as desired on display 1. Circuitry for removal of phase errors (12, 14, 17, 19 fig 2 ; 22, 27, 28, 29 fig 3) is included in both the measuring device 2 and transponder 5.

Description

DISTANCE MEASURING SYSTEM This invention relates to a Distance Measuring System.

Distance measuring systems involving radio have long been used to determine the distance of a target using various techniques, some more accurate than others. Two methods called 'radio triangulation' and 'RADAR' have always been the only methods to give accurate results. Radio Triangulation is a long established method whereby two (or more) receiving stations can determine, with directional antennas, the direction from each station to a transmitting station, and by drawing two lines on a map from each station, the exact 'cross point' can be shown on the map giving the transmitting stations' location (both bearing and distance). RADAR, a long established method used for both distance and bearing measurement, works on the principal of measuring time delay of a radio pulse echo from a target object.

Triangulation however, does require two or more receiving stations, spaced at some distance. This is a drawback when distance needs to be measured from a single point to another single point. An example would be a mobile situation where both the target (sender) and the measure (receiver) are both on the move. Drawbacks with RADAR include wide frequency consumption, restricted microwave operation only and multiple target echoing making a single target difficult to discriminate.

Another drawback with both these methods is the high cost and large amount of equipment.

According to the present invention there is provided a distance measuring system comprising of a measuring device and a transponder device.

The transponder device is a rebroadcasting transceiver with known accurate group delay, and returns signals with this known group delay back to the measuring device. The signal itself is a modulated carrier and the group delay pertains to the modulated, rather than the carrier signal.

The measuring device determines distance from the measurer to the transponder by comparing a transmitted reference group delay with a received group delay coming back from a transponder. The distance is a function of group delay of the two modulated signals which in turn is dependent on the speed of radio propagation.

specific embodiment of the invention will now be described by way of example with reference to the accompanying drawing in which: Figure 1 shows an overview of the distance measuring system; Figure 2 shows a block diagram of components within a transponder device ,and Figure 3 shows a block diagram of components within a measuring device.

Group delay is herein known as the phase time delay.

Referring to the block diagram in Fig. 1, a known phase time delay signal is transmitted from measuring unit 2 via antenna 3 to antenna 4 and is received at transponder 5. This signal is then re-transmitted in phase by transponder 5 via antenna 4 back to antenna 3 and is in turn received back at measuring unit 2 for a phase time delay comparison with the originally sent signal. This phase time delay is a function of distance and the speed of light and can be calculated for display 1 as desired. The sequence of transmitting and receiving can be repeated for as many times as desired to give an accurate average reading for display 1.

Referring to block diagrams in figures 2 & 3, three state sequences or 'cycles' known as send, hold and measure are referred to. These cycles are logically controlled signals which operate components (in this case, electronic switches) within the circuit. Switching between transmit and receive is controlled by these cycles and is described later. Switching between send and receive can be described as 'half duplex' operation.

eferring to the block diagram in Fig. 2, a RF signal is received by antenna 4 and is routed via an electronic antenna switch 16 to a receiver 15. A demodulated signal from 15 then passes though a variable phase circuit 14 which counter-acts any phase error produced from the receiver or other circuit components within the transponder itself. This self compensation is achieved by phase comparing with comparator 12 between a transmitted signal from transmitter 13 with the phase received locally by receiver 15. Note that transmitter 13 and receiver 15 operate on the same frequency so local reception can work. The phase error voltage from comparator 12 controls the phase variable circuit 14 and so nulls out phase error. Capacitor 19 holds the phase error voltage during the hold cycle, (as opposed to the send cycle when the electronic switch 17 is closed, charging capacitor 19 with the error voltage). The error compensated signal from 14 is then fed to a phase memory circuit 6. The phase memory 6 is a standard basic phased locked loop (pll) with the addition of an electronic hold switch 18. Electronic switch 18 is closed during the hold cycle, charging holding capacitor 10 with phase control voltage from comparator 9. The phase control voltage across holding capacitor 10 controls the frequency (and phase) of oscillation of a voltage controlled oscillator (vco) 8. The vco 8 is then compared with comparator 9 to again produce a phase error voltage across capacitor 10. This loop action continues during the hold cycle and maintains a lock condition within blocks 8, 9 and 10.

Once a lock condition is complete, the transponder then changes into the send state, sending the signal back to the measuring unit. The electronic switches 16,17 and 18 are operated to their opposite states. Electronic switch 18 (now open) disables any further variation to the vco 8 phase and also disconnects the holding capacitor 10 allowing the voltage to stay as constant as possible. Although capacitor 10 will discharge because of internal leakage current, it is not detrimental to operation due to a short time required to take a suitable phase time delay measurement back at the measuring unit figl block 2. This memorised phase signal held in vco 8 is fed to transmitter 13 where it is modulated and the RF signal is fed via electronic antenna switch 16 to antenna 4 for re-transmission. This completes the operation on the transponder. All electronic switches 16,17 and 18 are controlled by logic using timing or lock detect techniques.

Referring to the block diagram in Fig.3, A stable reference oscillator 25 is fed to transmitter 23 where the signal is modulated and RF signal passed to antenna 3 during a send cycle via an electronic antenna switch 20. Also during a send cycle, phase compensation takes place with electronic switch 28 closed. Holding capacitor 29 is charged with phase error voltage from comparator 27. Comparator 27 detects phase error between the reference oscillator 25 and locally received compensated signal after phase shift circuit 22. The phase error voltage across holding capacitor 29 is applied to a variable phase circuit 22. The action of the variable phase circuit 22 and comparator 27 nulls out any phase error within the receiver 21 and other internal component errors within the measuring unit fig 1 block 2. Once a sending cycle is over, electronic switches 20 and 28 change state for a measure cycle. During a measure cycle, electronic switch 28 is open disconnecting holding capacitor 29 from phase comparator 27. This allows holding capacitor 29 to hold its charge without any further charging from comparator 27 ensuring a stable phase error compensation voltage to the variable phase circuit 22 during a measure cycle. The last operation in the measure cycle is when the received RF signal at antenna 3 is fed to receiver 21 via electronic antenna switch 20 where it is demodulated and resulting signal phase error is corrected though the action of phase variable circuit 22.

Finally, measurement takes place, measured by timer/counter 26 which will time the cycle edge (rise or fall) difference between the reference oscillator 25 and the phase compensated received signal coming from the variable phase circuit 22. The time delay information from timer/counter 26 can then be fed to display circuit 1 where distance information may be displayed in whatever format desired, or fed to another system.

order to use a full duplex system so that the switching logic can be avoided, known group delay transmitters and receivers may be used in either or both the transponder and measuring devices.

In order to avoid the use of the phase compensation loop (12,17,19 and 14) in the transponder device fig.2, or phase compensation loop (27,28,29 and 22) in the measuring device fig.3, fixed or known group delay transmitters and receivers may be used.

In order to provide a means for an indirect or larger distance measuring path, one or more radio reflectors may be set up, such as a sheet of metal or other radio reflective surface positioned at a strategic position in the path between 3 & 4 in fig. 1.

In order to provide a means for indirect or larger distance measuring path, one or more active radio repeaters (including satellite communication) may be used, using known or compensated group delay methods, in the path between 3 & 4 in fig 1.

In order to provide a means for indirect or larger distance measuring path, one or more passive radio repeaters may be used, placed at a strategic position in the path between 3 & 4 in fig 1.

Claims

1. A distance measuring system comprising of a transponder device and a measuring device and a means for measuring group delay of a modulated radio carrier.

2. A distance measuring system as claimed in Claim 1 wherein a means is provided for converting or calibrating the group delay measurement into one of distance.

3. A distance measuring system as claimed in Claim 1 or Claim 2, wherein a half-duplex alternating transmit/receive circuit is provided in both the transponder and measuring devices.

4. A distance measuring system as claimed in Claim 1 or Claim 2, wherein a full-duplex transmit/receive circuit is provided in both the transponder and measuring devices.

5. A distance measuring system as claimed in Claim 3 or Claim 4, wherein a group delay calibration circuit is provided to automatically reduce group delay errors however caused.

6. A distance measuring system as claimed in Claim 3 or Claim 4, wherein a phase memory circuit is provided in the transponder to sample and then hold the group delay information for a time for retransmission.

7. A distance measuring system as claimed in Claim 5 or Claim 6, wherein a set of electronic switches are provided to switch between the states required for half duplex operation.

A A distance measuring system as claimed in Claim 5 or Claim 6, wherein a set of electronic switches are provided to switch between the states required for automatic reduction of group delay errors.

9. A distance measuring system as claimed in Claims 3 or 4 wherein known or calibrated group delay transmitters and / or receivers are provided.

10. A distance measuring system as claimed in any preceding claim, wherein radio reflectors are provided in the path of the radio carrier.

11. A distance measuring system as claimed in any preceding claim, wherein active radio repeaters (including satelite) are provided in the path of the radio carrier.

12. A distance measuring system as claimed in any preceding claim, wherein passive radio repeaters are provided in the path of the radio carrier.

13. A distance measuring system substantially as described herein with reference to Figures 1-3 of the accompanying drawing.