measuring 10 km with 1m accuracy
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From: Tim Wescott on 15 Feb 2010 15:12 On Mon, 15 Feb 2010 11:26:06 -0800, rickman wrote: > 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. > -- snip -- > 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 suspect that the cheap handheld units don't -- the cost of the logic would go up as something like N^1 or N^2 as the size of the time window increased, and to catch a reflection you need a time window big enough to see it and the 'correct' signal, or you need a multiplicity of receive channels. > I would think they would toss > out one or two outliers if they had more than half a dozen or so > satellites in view. One hopes. But a $150 GPS receiver is a pretty amazing exercise in cost reduction already -- I wouldn't count on it. > 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. Yes. The receiver has to solve for three spatial dimensions and time. Solving those four equations demands four unknowns. Height is generally known to a much lesser accuracy than horizontal position, because the slant angle to the satellites is often quite shallow, which makes it harder to resolve height. -- www.wescottdesign.com
From: linnix on 15 Feb 2010 15:38 On Feb 15, 12:12 pm, Tim Wescott <t...(a)seemywebsite.com> wrote: > On Mon, 15 Feb 2010 11:26:06 -0800, rickman wrote: > > 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. > > -- snip -- > > 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 suspect that the cheap handheld units don't -- the cost of the logic > would go up as something like N^1 or N^2 as the size of the time window > increased, and to catch a reflection you need a time window big enough to > see it and the 'correct' signal, or you need a multiplicity of receive > channels. > > > I would think they would toss > > out one or two outliers if they had more than half a dozen or so > > satellites in view. > > One hopes. But a $150 GPS receiver is a pretty amazing exercise in cost > reduction already -- I wouldn't count on it. Low resolution GPS chip sets are available for $10 to $20. Logic cost is not the issue. High precision GPS requires high precision clock, but such product would likely be classified as amunitions and sale restricted. > > > 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. > > Yes. The receiver has to solve for three spatial dimensions and time. > Solving those four equations demands four unknowns. Height is generally > known to a much lesser accuracy than horizontal position, because the > slant angle to the satellites is often quite shallow, which makes it > harder to resolve height. > > --www.wescottdesign.com
From: Paul Keinanen on 15 Feb 2010 16:00 On Mon, 15 Feb 2010 11:07:28 -0800, Jim Stewart <jstewart(a)jkmicro.com> wrote: >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. Take a look what happens to the elevation display, when you walk below a bridge, that obscures at least some of the satellites. I also wonder why any tripod mounted GPS antennas intended for geodetic survey clearly try to avoid any ground reflections or signals from low elevation angles (and hence reflected from a building) with a special constructions (e.g. using a ground plane sheet and a few concentric rings). Of course, a receiver capable of receiving more than the minimum of four satellites might be capable of determining, if some of the satellite signals are received through a reflection and hence ignore it.
From: RCIngham on 15 Feb 2010 17:07 >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 Meanwhile, back in a real world, questions to the OP: Are both measurement points static? Or one mobile? Both mobile? How often are you doing the measurement? Every second/minute/hour/day/week/etc.? Is elevation difference important? --------------------------------------- Posted through http://www.EmbeddedRelated.com
From: Cesar Rabak on 15 Feb 2010 18:49
Rich, Em 15/2/2010 14:45, Rich Webb escreveu: > On Mon, 15 Feb 2010 16:32:51 +0100, Frank Buss<fb(a)frank-buss.de> wrote: > >> Vladimir Vassilevsky wrote: >> >>> Use a long enough measuring tape. >> >> I guess it would be difficult to get 1 m accuracy with a 10 km measuring >> tape (non-planar ground, tape stretching etc.). > > 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. > When I started in the profession, the portable [and affordable] computing device was a slide rule, and it had a scale for sine and tangent which was essentially homologous to the arc itself (in radians) for angles lesser than six degrees (for a 360� circumference), so it should not be a surprise. . . [] -- Cesar Rabak GNU/Linux User 52247. Get counted: http://counter.li.org/ |