measuring 10 km with 1m accuracy
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From: Jon Kirwan on 15 Feb 2010 14:43 On Mon, 15 Feb 2010 11:26:06 -0800 (PST), rickman <gnuarm(a)gmail.com> wrote: ><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 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. 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
From: linnix on 15 Feb 2010 14:49 On Feb 15, 11:43 am, Jon Kirwan <j...(a)infinitefactors.org> wrote: > On Mon, 15 Feb 2010 11:26:06 -0800 (PST), rickman > > <gnu...(a)gmail.com> wrote: > ><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 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. > > 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.) Yes, the military uses encrypted codes. They can give you the key, and then shoot you.
From: linnix on 15 Feb 2010 14:53 On Feb 15, 11:26 am, rickman <gnu...(a)gmail.com> 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. > > 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. 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.
From: Jim Stewart on 15 Feb 2010 15:03 linnix wrote: > On Feb 15, 11:26 am, rickman <gnu...(a)gmail.com> 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. >> 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. > > 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. 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.
From: Tim Wescott on 15 Feb 2010 15:08
On Mon, 15 Feb 2010 12:03:06 -0800, Jim Stewart wrote: > linnix wrote: >> On Feb 15, 11:26 am, rickman <gnu...(a)gmail.com> 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. >>> 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. >> >> 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. > > 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. That could happen too, but the strength of the reflection would be low unless the soil were particularly conductive. Multipath off of buildings can cause significant elevation errors, too. -- www.wescottdesign.com |