measuring 10 km with 1m accuracy

Hi,

What would be best technology (GPS,IR,Radio/freq,ultrasonic etc) for doing distance measurements upto 10km with accuracy of 1 meter?

GPS could be good for greater distance but accuracy seems coarse.

- Surinder

Reply to
Surinder Singh
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Surinder Singh wibbled on Monday 15 February 2010 12:13

Laser I would have thought - eg google for Laser Total Station (an electronic theodolite with laser rangefinder). I'm not sure if those can do

10km in one go but some of the Leica machines with high powered lasers can do fair distances...

It might help if you said what the application was eg surveying (ie operator based stationary measurements), unattended, moving targets, moving measuring station?

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Tim Watts

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Reply to
Tim Watts

There are survey grade GPS units with centimeter accuracy. And, of course, there is also the traditional method using theodolites. It all depends on how you define "best."

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Rich Webb     Norfolk, VA
Reply to
Rich Webb

If you are not in a hurry, GPS using the geodetic methods will give accuracies well under a meter with static relative measurement. The needed measurement time will be about an hour, simultaneously at both ends of the measurement distance.

With top-end geodetic receivers and sufficient measurement time, an accuracy of a centimeter or less is attainable. The position is the phase centerpoint of the antenna.

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Tauno Voipio
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Reply to
Tauno Voipio

Measure 10km of what? The purpose of this measurement is what?

Use a long enough measuring tape.

VLV

Reply to
Vladimir Vassilevsky

While ago I did a plot of a common GPS module readings taken at every second. The distribution was clearly not Gaussian; it was asymmetrical and skewed. I am not sure if it would be possible to improve the accuracy by averaging and how much of averaging it would take.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

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Reply to
Vladimir Vassilevsky

I guess it would be difficult to get 1 m accuracy with a 10 km measuring tape (non-planar ground, tape stretching etc.).

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Reply to
Frank Buss

The original baseline of the Ordnance Survey was measured with glass tubes laid end to end.

Mind you, they had a team of squaddies to do it for them, a much better activity than coming off second best against an army of

3rd World peasants in Afghanistan.
Reply to
invalid

It also depends on what one considers distance to be. Is it the straight cord between the points? Or the great circle distance? Or the measured ground distance? Or something else? There are statutory definitions for civil survey work but those aren't appropriate in all cases.

Actually, I would have guessed that the difference between the arc length and chord would have been greater but imagination isn't always a good guide at these scales. Turns out that it's only about 1 mm over a

10 km arc, so probably not too significant in the OP's 1 m requirement.
--
Rich Webb     Norfolk, VA
Reply to
Rich Webb

That's why I said 'static relative measurement'. It means that the skew will be about the same at both ends. To get geodetic quality measurements, the raw data has to be adjusted so that the points will be taken at the same time (and plenty of them).

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Tauno Voipio
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Reply to
Tauno Voipio

I.e. cheap and low resolution GPS.

but identical at both end.

say that to the military.

not by averaging, but other means.

or Differential GPS.

Reply to
linnix

How did the displayed elevation behave ?

If it is violently jumping up and down, this may be a symptom of a ground reflection., i.e. the distance to one (or more) satellites would appear to bee too large, i.e. going through the ground reflection.

Using an antenna with a radiation pattern that is receiving signals only directly from the sky above (thus blocking ground and building reflections) might help.

Reply to
Paul Keinanen

So the accuracy requirement is 100 ppm.

Is this measurement in vacuum or in the atmosphere and if so, what is the refractive index during the measurement. Any mirages ?

Even when using lasers or some microwave measuring devices, the propagation in typical atmospheric conditions will bend the beam slightly downwards, thus forming an arc and thus the reading would also be slightly too big.

The speed of light is about 300 ppm lower at sea level than in vacuum, so also the elevation and hence air density will affect the speed of light and this may also cause errors.

Reply to
Paul Keinanen

Does this really happen or are you speculating? No disrespect intended, I've just never heard of this issue.

Reply to
Jim Stewart

Yes, the chicken in the microwave could reflect the energy back at you. It could happen one in a billion.

I am just speculating.

Reply to
linnix

You've never heard of GPS receivers picking up reflected signals in place of direct? Yes, I can assure you that it can happen. I have used handheld GPS receivers when geocaching and in cities with "urban canyons" you can get readings that are 100 or even 200 feet off and wander like crazy. One particular time I was trying to measure a coordinate pair of a marker on a street downtown. I took a dozen readings at different times, each one averaged over 3 minutes. They were off by over 80 feet from one another, each set taken at the same time bunching together. I had to measure another point which was in an area with a wider view of the sky, but still close to buildings which can reflect the signal and got a similar, erratic location. Most of the time this same unit is within 10 to 20 feet of the spot measured by someone else using a different receiver at another time.

A GPS works by measuring the time of flight from the visible satellites and triangulating. Clearly if one or more measurements are off because of reflections, it will mess up the results. I don't know how they compensate for this, or if they even do. I would think they would toss out one or two outliers if they had more than half a dozen or so satellites in view. I think it takes a minimum of 4 to get a 3D fix and the more measurements included in the calculations, the better the result... as long as they are not reflections.

Rick

Reply to
rickman

I don't know what kinds of cross-correlators might exist for GPS wavelengths (or even if down-conversion and digital ones might be useful), but digital cross-correlation in areas where I've used it provided phase information not just of one but also many of the reflections, all in one go.

You've made me wonder if any of the commercial units include such a capability, or even if the US military does it in theirs (probably yes, if at all possible.)

Anyone know?

Jon

Reply to
Jon Kirwan

Yes, the military uses encrypted codes. They can give you the key, and then shoot you.

Reply to
linnix

y

al

Yes, it is possible. Just like the chicken in the microwave can bounce some micro-tron targeting back at you. So, my theory is that the more you microwave the chicken, the more you get microwaved. On the other hand, the chance of you measuring ionic conditions of the atmosphere is much higher.

Reply to
linnix

Ok, I think we were thinking of different things. I know that the gps signal can reflect off of buildings and such, but by ground reflection, I thought he was talking about the ground underneath or immediately adjacent to the the gps.

Reply to
Jim Stewart

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