CA2304083A1

CA2304083A1 - Method for measuring the distance to at least one target

Info

Publication number: CA2304083A1
Application number: CA002304083A
Authority: CA
Inventors: Beat Aebischer; Christian Betschon; Bernhard Gaechter; Georg Orawez; Ernst Ramseier; Esther Rutishauser
Original assignee: Leica Geosystems Inc
Current assignee: Leica Geosystems Inc
Priority date: 1999-04-06
Filing date: 2000-04-05
Publication date: 2000-10-06
Anticipated expiration: 2020-04-05
Also published as: ATE232306T1; EP1043603A1; JP2000304862A; JP4843128B2; DE50001197D1; CA2304083C

Abstract

To measure the distance to targets, local maxima exceeding a threshold (.epsilon.) and being associated with a correlation function (f) are determined, the reference values of which are obtained from scaler products of a received signal (e), produced by reflections of an optical pulse by the targets, and a number of comparison functions, copies of a reference function (r) which are shifted as a function of time, and the transit times of the reflections are derived from the time-related shifts of the corresponding comparison functions. By interpolation with respect to the time-related shifts or optimization thereof, for example by adjusting them so that the orthogonal distance of the received signal (e) from the vector space which is spanned by the corresponding comparison functions is a minimum, the transit times can be determined even more accurately.

Claims

1. A method for measuring the distance to at least one target, in which an optical pulse is sent by a transmitter toward said target, reflections of the at least one pulse are picked up by a receiver and converted into a received signal (e) and stored and in each case a transit time is determined from a reception time of the input signal and the distance is calculated therefrom, wherein, to determine the at least one reception time, the input signal (e) is compared with comparison functions which are copies, shifted by various time intervals, of a previously determined and stored reference function (r), and the reception times are at least approximately determined from the time intervals which correspond to the most similar comparison function.

2. The method as claimed in claim 1, wherein the at least approximate determination of the at least one reception time is based on a set of comparison functions which correspond to predetermined fixed time intervals.

3. The method as claimed in claim 2, wherein the predetermined fixed time intervals are integral multiples of a fixed basic interval.

4. The method as claimed in claim 2 or 3, wherein only those comparison functions where in each case the similarity function (f) representing a measure of similarity with the received signal (e) forms a local maximum which exceeds a specific threshold (.epsilon.) are used.

5. The method as claimed in claim 4, wherein the similarity function used is a correlation function (f).

6. The method as claimed in claim 5, wherein the function values of the correlation function are calculated in each case as the scaler product of the received signal (e) and one of the comparison functions.

7. The method as claimed in any of claims 2 to 6, wherein exact reception times are determined starting from the approximately determined reception times, by interpolation between the time intervals.

8. The method as claimed in any of claims 4 to 6 and claim 7, wherein the interpolation is effected by determining t:he local maximum of the correlation function (f) expanded between reference values to give a continuous function.

9. The method as claimed in claim 7 or 8, wherein the interpolation is carried out at least in part by minimizing the distance of the received signal (e) from a vector space which is spanned by the comparison functions corresponding to the reception times, starting from the vector space which is spanned by the comparison functions which correspond to the approximately determined reception times.

10. The method as claimed in any of claims 1 to 9, wherein the received signal (e) is scanned at times which in each case follow one another at intervals of a fixed scanning increment (.DELTA.).

11. The method as claimed in any of claims 1 to 10, wherein the reference function (r) is determined at a number of points which follow one another at intervals of reference increments (.delta.).

12. The method as claimed in claims 10 and 11, wherein the reference increment (.delta.) corresponds in each case to the scanning increment (.DELTA.) divided by an integer >1.

13. The method as claimed in any of claims 1 to 12, wherein, to determine the reference function (r), at least one optical pulse is sent toward a reference target (10), and the corresponding reference input signal is scanned for determining the reference values of the reference function (r).

14. The method as claimed in claim 13, wherein, to improve the resolution as a function of time, the reference input signal is shifted as a function of time by various delay values.

15. The method as claimed in claim 14, wherein the shifting of the reference input signal as a function of time is effected by delaying the emission of the optical pulse by the transmitter (6) or of the reference input signal.

16. The method as claimed in claim 14, wherein the shifting of the reference input signal as a function of time is effected by changing the distance to the reference target (10).

17. The method as claimed in any of claims 13 to 16, wherein, to improve the signal-to-noise ratio, the emission of the optical pulse and the scanning of the reference input signal are repeated and, for the formation of the reference values of the reference function (r), the scanning values corresponding to identical transit times are summed or averaged.

18. The method as claimed in any of claims 10 to 17, wherein, starting from reference values, the reference function (r) is expanded by interpolation to give a continuous function preferably continuously differentiable at least once.

19. The method as claimed in any of claims 14 to 17, wherein a formulation according to which the reference function (r) can be fixed by a finite number of parameters and the parameters are determined from a comparison of the reference input signal with copies of the reference function which are shifted as a function of time by the delay values is postulated for the reference function (r).

20. The method as claimed in claim 19, wherein, to determine the parameters, the distance of the reference received signal from a superposition of the copies of the reference function (r) which are shifted as a function of time by the delay values is minimized.