Newsgroups: comp.robotics
Path: brunix!uunet!gatekeeper.us.oracle.com!decwrl!netcomsv!netcom.com!poliquin
From: poliquin@netcom.com (Tom Poliquin)
Subject: Re: Laser Range Finding
Message-ID: <1993Feb7.072817.27415@netcom.com>
Organization: Netcom - Online Communication Services  (408 241-9760 guest) 
References: <1993Feb6.231545.18477@CSD-NewsHost.Stanford.EDU>
Date: Sun, 7 Feb 1993 07:28:17 GMT
Lines: 97

Two quick notes ....

1) I read an article in "Microcornocopea" (sp?) about two years
ago (I think the mag is out of biz now) about using the intensity
of two optosensors to measure distance. The sensors were about
6 cm apart along the line of detection. The actual distance
of an object was computed using the two intensity values and some
function of 1/r^3 .... I don't remember the details... The results
(according to the article) were very gud.

2) I happened to come across a thread in sci.optics about 
rangefinding (mainly long stuff ) and included an article 
(I'm new to the INTERNET
and I hpe this doesn't violate any mores) ...

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From: sprent@sol.surv.utas.edu.au (Tony Sprent)

There are four basic techniques for distance measurement using electro
magetic radiation.

These are 
1 Pulse Timing
2 Phase Comparison
3 Doppler Methods
4 Interferometry 

All are used in practice for distance measurement depending on the particular
application.

Pulse timing as the name suggests involves measuring the round time for a 
signal to be transmitted to a reflective surface and return. 

This is the principle used in Radar, DME for aircraft, LORAN, Satellite 
Altimetry, Airborne RADAR Altimetry, Lunar Laser Ranging etc. Some of the newer
EDM instruments used by surveyor are also using pulse timing and accuracies of
+/- 5mm are possible. Most of the military range finders also use pulse timing. 
The GPS system uses pulse timing for coarse distance measurement.
Very Long Base Interferometry (VLBI) is also a pulse timing technique where 
signals from pulsars are timed from two or more radio telescopes and the 
difference in times of arrival are converted to intercontinental distances with
a precision of a few centimetres.

Phase difference involves the use of a carrier wave which may be modulated at 
different wavelengths. By measuring the diference in phase between the 
transmitted signal and the received signal after it has been reflected from the 
other end of the line, the distance can be determined as an integer number 
(unknown) of wavelengths plus a fraction of a wavelength which is known from the
phase comparison. By using a range of modulation frequencies the ambiguity can be
resolved. There are many applications of this technique. A wide range of carrier 
frequencies are used ranging from visible through infra red to microwave and right
down to VLF. Typical instruments used by surveyors have accuracies of +/-(1to2 mm
+1to3 parts per million) and use infra red as the carrier. Precise positioning 
using GPS can be achieved by phase comparison of the carrier wave signals of the 
various satellites. Accuracies in position of better than 1 part per million can
be achieved.

Doppler techniques were used in the earlier satellite positioning 
systems. The received frequency of a low orbit satellite is compared with the 
actual transmitted signal as a function of time. The rate of change of frequency
gives the slant range between the satellite and the observer while the instant
when the two freqencies are the same gives the point of closest approach. By
knowing the orbital parameters of the satellite which are transmitted, the observers 
position can be determined.

Interferometric methods are the same as those used in the original Michelson 
Interferometer. It is used for metrology, high precision distance measurement over 
short distances (up to 60 metres) and in the definition of the metre. 

An excellent reference on this subject is Electronic Distance Measurement by 
JM Rueger (Springer-Verlag pub.) 

 ________________________________________________________
| Tony Sprent                                             |
| Department of Surveying and Spatial Information Science | 
| University of Tasmania                                  |     
| GPO Box 252C Hobart        _--_|\                       |
| Tasmania*                 /      \                      |
| Australia   7001          \_.--._/   )                  |
|                                 *   /                   |
| Phone 61-02-202110 Fax 61-02-240282  International      |
|         002-202110       002-240282  Australia          |
| email  sprent@surv.utas.edu.au                          |
|_________________________________________________________| 

..... end of artricles.

Gud luck

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Tom Poliquin                                            "All great truths -
Dedicated Systems                                        begin as         -
San Jose, CA                                             blasphemies"     -
(408) 779-2808                                                            -
Internet: poliquin@netcom.com                        GEORGE BERNARD SHAW  -
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