On Mon, 15 Feb 2010 12:03:06 -0800, Jim Stewart wrote:> linnix wrote: >> On Feb 15, 11:26 am, rickman <gnu...@gmail.com> wrote: >>> On Feb 15, 2:07 pm, Jim Stewart <jstew...@jkmicro.com> wrote: >>> >>> >>> >>>> Paul Keinanen wrote: >>>>> On Mon, 15 Feb 2010 09:29:10 -0600, Vladimir Vassilevsky >>>>> <nos...@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
measuring 10 km with 1m accuracy
Started by ●February 15, 2010
Reply by ●February 15, 20102010-02-15
Reply by ●February 15, 20102010-02-15
On Mon, 15 Feb 2010 11:26:06 -0800, rickman wrote:> On Feb 15, 2:07 pm, Jim Stewart <jstew...@jkmicro.com> wrote: >> Paul Keinanen wrote: >> > On Mon, 15 Feb 2010 09:29:10 -0600, Vladimir Vassilevsky >> > <nos...@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
Reply by ●February 15, 20102010-02-15
On Feb 15, 12:12=A0pm, Tim Wescott <t...@seemywebsite.com> wrote:> On Mon, 15 Feb 2010 11:26:06 -0800, rickman wrote: > > On Feb 15, 2:07=A0pm, Jim Stewart <jstew...@jkmicro.com> wrote: > >> Paul Keinanen wrote: > >> > On Mon, 15 Feb 2010 09:29:10 -0600, Vladimir Vassilevsky > >> > <nos...@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 woul=d> >> >> 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. > > =A0-- snip -- > > A GPS works by measuring the time of flight from the visible satellites > > and triangulating. =A0Clearly if one or more measurements are off becau=se> > of reflections, it will mess up the results. =A0I 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. =A0But a $150 GPS receiver is a pretty amazing exercise in cos=t> 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. =A0The receiver has to solve for three spatial dimensions and time. ==A0> Solving those four equations demands four unknowns. =A0Height is generall=y> 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
Reply by ●February 15, 20102010-02-15
On Mon, 15 Feb 2010 11:07:28 -0800, Jim Stewart <jstewart@jkmicro.com> wrote:>Paul Keinanen wrote: >> On Mon, 15 Feb 2010 09:29:10 -0600, Vladimir Vassilevsky >> <nospam@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.
Reply by ●February 15, 20102010-02-15
On Mon, 15 Feb 2010 11:49:57 -0800 (PST), linnix <me@linnix.info-for.us> wrote:>On Feb 15, 11:43�am, Jon Kirwan <j...@infinitefactors.org> wrote: >> On Mon, 15 Feb 2010 11:26:06 -0800 (PST), rickman >> >> <gnu...@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.No firearms necessary. With the explosion of the number of GPS units in use in non-permissive environments, lots of the units -- especially handheld and vehicle mounted -- use "black" (unencrypted) keys. For example, see http://www.mil-embedded.com/articles/holm/ -- Rich Webb Norfolk, VA
Reply by ●February 15, 20102010-02-15
On Feb 15, 1:33=A0pm, Rich Webb <bbew...@mapson.nozirev.ten> wrote:> On Mon, 15 Feb 2010 11:49:57 -0800 (PST), linnix <m...@linnix.info-for.us= > > wrote: > > > > >On Feb 15, 11:43=A0am, Jon Kirwan <j...@infinitefactors.org> wrote: > >> On Mon, 15 Feb 2010 11:26:06 -0800 (PST), rickman > > >> <gnu...@gmail.com> wrote: > >> ><snip> > >> >A GPS works by measuring the time of flight from the visible > >> >satellites and triangulating. =A0Clearly if one or more measurements =are> >> >off because of reflections, it will mess up the results. =A0I don't k=now> >> >how they compensate for this, or if they even do. =A0I would think th=ey> >> >would toss out one or two outliers if they had more than half a dozen > >> >or so satellites in view. =A0I think it takes a minimum of 4 to get a=3D> >> >fix and the more measurements included in the calculations, the bette=r> >> >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. =A0They can give you the key, > >and then shoot you. > > No firearms necessary. With the explosion of the number of GPS units in > use in non-permissive environments, lots of the units -- especially > handheld and vehicle mounted -- use "black" (unencrypted) keys. For > example, seehttp://www.mil-embedded.com/articles/holm/ > > -- > Rich Webb =A0 =A0 Norfolk, VAPPS data are encrypted and still subject to strict guide lines. Civilian codes are unencrypted, but subject to availability (can be turned off) in war threaters. "After manufacturing the SAASM unit, the GPS receiver manufacturer ships the SAASM hardware to the KLIF for the loading of the Key Data Processor (KDP) crypto software"
Reply by ●February 15, 20102010-02-15
>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. > >- SurinderMeanwhile, 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
Reply by ●February 15, 20102010-02-15
On Mon, 15 Feb 2010 13:58:59 -0800 (PST), linnix <me@linnix.info-for.us> wrote:>> No firearms necessary. With the explosion of the number of GPS units in >> use in non-permissive environments, lots of the units -- especially >> handheld and vehicle mounted -- use "black" (unencrypted) keys. For >> example, seehttp://www.mil-embedded.com/articles/holm/ >> >> -- >> Rich Webb � � Norfolk, VA > >PPS data are encrypted and still subject to strict guide lines. >Civilian codes are unencrypted, but subject to availability (can be >turned off) in war threaters. > >"After manufacturing the SAASM unit, the GPS receiver manufacturer >ships the SAASM hardware to the KLIF for the loading of the Key Data >Processor (KDP) crypto software"Absolutely, and I mis-typed up above. The keys are *encrypted* and thus they can be distributed as black or *unclassified* material, which is what I meant to say. The earlier generation of keys were themselves classified, so if your battery backup failed during a power outage, it was time to go humbly to the classified material custodian and 'fess up to needing somebody to come around with the magic juujuu. Do that too often and you'd better have fresh donuts waiting... -- Rich Webb Norfolk, VA
Reply by ●February 15, 20102010-02-15
On Feb 15, 2:48=A0pm, Rich Webb <bbew...@mapson.nozirev.ten> wrote:> On Mon, 15 Feb 2010 13:58:59 -0800 (PST), linnix <m...@linnix.info-for.us= > > wrote: > > >> No firearms necessary. With the explosion of the number of GPS units i=n> >> use in non-permissive environments, lots of the units -- especially > >> handheld and vehicle mounted -- use "black" (unencrypted) keys. For > >> example, seehttp://www.mil-embedded.com/articles/holm/ > > >> -- > >> Rich Webb =A0 =A0 Norfolk, VA > > >PPS data are encrypted and still subject to strict guide lines. > >Civilian codes are unencrypted, but subject to availability (can be > >turned off) in war threaters. > > >"After manufacturing the SAASM unit, the GPS receiver manufacturer > >ships the SAASM hardware to the KLIF for the loading of the Key Data > >Processor (KDP) crypto software" > > Absolutely, and I mis-typed up above. The keys are *encrypted* and thus > they can be distributed as black or *unclassified* material, which is > what I meant to say. The earlier generation of keys were themselves > classified, so if your battery backup failed during a power outage, it > was time to go humbly to the classified material custodian and 'fess up > to needing somebody to come around with the magic juujuu. Do that too > often and you'd better have fresh donuts waiting... > > -- > Rich Webb =A0 =A0 Norfolk, VAEven the "unclassified keys" are distributed on "need to know" or "need to have" basis. However, there are ways to build precision devices without the keys altogether. I am sure such projects exists in friendly and unfriendly countries. In a way, PDW (Precision Destructive Weapons) are more dangerous than MDW.
Reply by ●February 15, 20102010-02-15
Rich, Em 15/2/2010 14:45, Rich Webb escreveu:> On Mon, 15 Feb 2010 16:32:51 +0100, Frank Buss<fb@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/
