measuring 10 km with 1m accuracy
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From: Paul Keinanen on 15 Feb 2010 13:07 On Mon, 15 Feb 2010 09:29:10 -0600, Vladimir Vassilevsky <nospam(a)nowhere.com> wrote: >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. 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.
From: Paul Keinanen on 15 Feb 2010 13:22 On Mon, 15 Feb 2010 04:13:09 -0800 (PST), Surinder Singh <gogreenmiles(a)gmail.com> wrote: >What would be best technology (GPS,IR,Radio/freq,ultrasonic etc) for >doing distance measurements upto 10km with accuracy of 1 meter? 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.
From: Jim Stewart on 15 Feb 2010 14:07 Paul Keinanen wrote: > On Mon, 15 Feb 2010 09:29:10 -0600, Vladimir Vassilevsky > <nospam(a)nowhere.com> wrote: > > >> 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. > > 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. Does this really happen or are you speculating? No disrespect intended, I've just never heard of this issue.
From: linnix on 15 Feb 2010 14:16 On Feb 15, 11:07 am, Jim Stewart <jstew...(a)jkmicro.com> wrote: > Paul Keinanen wrote: > > On Mon, 15 Feb 2010 09:29:10 -0600, Vladimir Vassilevsky > > <nos...(a)nowhere.com> wrote: > > >> 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. > > > 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. Yes, the chicken in the microwave could reflect the energy back at you. It could happen one in a billion. > > Does this really happen or are you speculating? I am just speculating. > No disrespect intended, I've just never heard > of this issue.
From: rickman on 15 Feb 2010 14:26
On Feb 15, 2:07 pm, Jim Stewart <jstew...(a)jkmicro.com> wrote: > Paul Keinanen wrote: > > On Mon, 15 Feb 2010 09:29:10 -0600, Vladimir Vassilevsky > > <nos...(a)nowhere.com> wrote: > > >> 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. > > > 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. > > Does this really happen or are you speculating? > No disrespect intended, I've just never heard > of this issue. 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 |