measuring 10 km with 1m accuracy

On Feb 15, 9:29=A0am, Tauno Voipio <tauno.voi...@notused.fi.invalid>
wrote:
> Vladimir Vassilevsky wrote:
>
> > Tauno Voipio wrote:
>
> >> 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.
>
> > While ago I did a plot of a common

I.e. cheap and low resolution GPS.

> > GPS module readings taken at every
> > second. The distribution was clearly not Gaussian;

but identical at both end.

> > it was asymmetrical
> > and skewed. I am not sure if it would be possible

say that to the military.

> > to improve the
> > accuracy

not by averaging, but other means.

> > by averaging and how much of averaging it would take.

>
> > Vladimir Vassilevsky
> > DSP and Mixed Signal Design Consultant
> >http://www.abvolt.com
>
> That's why I said 'static relative measurement'.

or Differential GPS.

> 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).
>
> --
>
> Tauno Voipio
> tauno voipio (at) iki fi

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.

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.

On Mon, 15 Feb 2010 04:13:09 -0800 (PST), Surinder Singh
<gogreenmiles@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. 

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.

On Feb 15, 11:07=A0am, 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.

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.

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 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

On Mon, 15 Feb 2010 11:26:06 -0800 (PST), rickman
<gnuarm@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

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 know
> >how they compensate for this, or if they even do. =A0I would think they
> >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 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.

On Feb 15, 11:26=A0am, rickman <gnu...@gmail.com> 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 ever=
y
> > >> second. The distribution was clearly not Gaussian; it was asymmetric=
al
> > >> 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? =A0Yes, I can assure you that it can happen. =A0I 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. =A0One particular time I was trying to measure a
> coordinate pair of a marker on a street downtown. =A0I took a dozen
> readings at different times, each one averaged over 3 minutes. =A0They
> were off by over 80 feet from one another, each set taken at the same
> time bunching together. =A0I 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.

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.