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uk.rec.gps (UK Sat Nav) (uk.rec.gps) for the discussion of all aspects of the UK use of Global Positioning Systems and any other satellite positioning/navigation systems which may be developed. Also any improvements, or extensions to the above and radio navigation systems.

Variation in speed reported by GPS



 
 
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  #1  
Old December 30th 12, 08:20 PM posted to uk.rec.gps
NY
external usenet poster
 
Posts: 10
Default Variation in speed reported by GPS

Am I correct in understanding that GPS receivers calculate the speed of the
receiver (eg when travelling in a moving vehicle) by calculating the
distance moved between one GPS reading and the next?

The timestamp of each sample is accurate to the nearest second and the
distance readings will be accurate to around 5 - 50 m (typically, depending
on DOP at the time).

Does this explain why when travelling at a constant speed (according to a
car's speedometer) the speed reported by the GPS (either as an ever-changing
live display or by software that analyses the track afterwards) fluctuates
so much?

I suppose the only way to improve the steadiness is to do a moving average
of several consecutive speed calculations.

Or is there a better way than this? Do GPS receivers for scientific or
engineering purposes have a more elaborate way of smoothing out errors in
the speed which are due to errors in the distances and imprecision (only to
the nearest second) in the time?

  #2  
Old December 30th 12, 10:45 PM posted to uk.rec.gps
Steve Firth
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Posts: 281
Default Variation in speed reported by GPS

"NY" wrote:
Am I correct in understanding that GPS receivers calculate the speed of
the receiver (eg when travelling in a moving vehicle) by calculating the
distance moved between one GPS reading and the next?


No.

--
•DarWin|
_/ _/
  #3  
Old December 31st 12, 10:59 AM posted to uk.rec.gps
NY
external usenet poster
 
Posts: 10
Default Variation in speed reported by GPS

"Steve Firth" wrote in message
...

"NY" wrote:
Am I correct in understanding that GPS receivers calculate the speed of
the receiver (eg when travelling in a moving vehicle) by calculating the
distance moved between one GPS reading and the next?


No.


Ah.

The GPX files that Viewranger returns on various mobile phones and tablets
that I've looked at just contain lat/long, elevation and time - no speed.
That's what led me to think that speed was a property that was derived by
the display software rather than a parameter that was returned by the GPS.
So does the GPS receiver chip actually calculate it by some means other than
distance/time since last fix - such as doppler shift? Is the speed that is
reported by the GPS generally less prone to random errors than the speed
that is derived from distance/time since last fix?

I've now installed some software that records system logs and extracted NMEA
data, and the GPRMC sentence does contain plausible values for speed in
knots. Doing further research it seems that GPX files don't contain speed
info so any software such as MemoryMap which takes its input from GPX files
has to derive the speed rather than using the value that the GPS has
returned.

As a matter of interest, does the internal track file used by Viewranger
(before export to GPX) contain speed information?

  #4  
Old December 31st 12, 11:29 AM posted to uk.rec.gps
Roger Mills[_2_]
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Posts: 17
Default Variation in speed reported by GPS

On 30/12/2012 21:20, NY wrote:
Am I correct in understanding that GPS receivers calculate the speed of
the receiver (eg when travelling in a moving vehicle) by calculating the
distance moved between one GPS reading and the next?

The timestamp of each sample is accurate to the nearest second and the
distance readings will be accurate to around 5 - 50 m (typically,
depending on DOP at the time).

Does this explain why when travelling at a constant speed (according to
a car's speedometer) the speed reported by the GPS (either as an
ever-changing live display or by software that analyses the track
afterwards) fluctuates so much?

I suppose the only way to improve the steadiness is to do a moving
average of several consecutive speed calculations.

Or is there a better way than this? Do GPS receivers for scientific or
engineering purposes have a more elaborate way of smoothing out errors
in the speed which are due to errors in the distances and imprecision
(only to the nearest second) in the time?


I don't know exactly how it's done, but I assume that there must be some
smoothing. The speed reported by my TomTom when I'm using cruise control
to maintain a constant speed is steady as a rock. But there does appear
to be some lag during acceleration and deceleration.
--
Cheers,
Roger
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checked.
  #5  
Old December 31st 12, 11:40 AM posted to uk.rec.gps
Alan White[_2_]
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Posts: 8
Default Variation in speed reported by GPS

On Sun, 30 Dec 2012 21:20:48 -0000, "NY" wrote:

Am I correct in understanding that GPS receivers calculate the speed of the
receiver (eg when travelling in a moving vehicle) by calculating the
distance moved between one GPS reading and the next?


The GPS receiver calculates what is called the PVT, (Position, Velocity,
Time) solution based on the received signals from the satellites. The
velocity bit uses Doppler shift of the received frequency from those
satellites.

--
Alan White
Mozilla Firefox and Forte Agent.
By Loch Long, twenty-eight miles NW of Glasgow, Scotland.
Webcam and weather:- http://windycroft.co.uk/weather
  #6  
Old December 31st 12, 01:15 PM posted to uk.rec.gps
Steve Firth
external usenet poster
 
Posts: 281
Default Variation in speed reported by GPS

"NY" wrote:
"Steve Firth" wrote in message
...

"NY" wrote:
Am I correct in understanding that GPS receivers calculate the speed of
the receiver (eg when travelling in a moving vehicle) by
calculating the distance moved between one GPS reading and the next?


No.


Ah.

The GPX files that Viewranger returns on various mobile phones and
tablets that I've looked at just contain lat/long, elevation and time -
no speed. That's what led me to think that speed was a property that was
derived by the display software rather than a parameter that was returned
by the GPS. So does the GPS receiver chip actually calculate it by some
means other than distance/time since last fix - such as doppler shift? Is
the speed that is reported by the GPS generally less prone to random
errors than the speed that is derived from distance/time since last fix?


GPS chips should return velocity as the pseudo range rate. This is the
velocity estimate for a particle moving across the surface of the GPS datum
ellipsoid. It takes no account of geography hence can differ from actual
velocity over terrain. However it's usually accurate to about 0.1 knots.
The velocity is estimated from the phase of the received signal WRT the
onboard clock and represents the sum of satellite plus GPS platform motion.
Determining the satellite velocity permits determining the velocity of the
GPS platform. Effectively Doppler.

I've now installed some software that records system logs and extracted
NMEA data, and the GPRMC sentence does contain plausible values for speed
in knots. Doing further research it seems that GPX files don't contain
speed info so any software such as MemoryMap which takes its input from
GPX files has to derive the speed rather than using the value that the GPS has returned.

As a matter of interest, does the internal track file used by Viewranger
(before export to GPX) contain speed information?


I know nothing about Viewranger.

--
•DarWin|
_/ _/
  #7  
Old December 31st 12, 02:19 PM posted to uk.rec.gps
Mike Coon[_2_]
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Posts: 144
Default Variation in speed reported by GPS

Alan White wrote:
The velocity bit uses Doppler shift of the received
frequency from those satellites.


I keep reading this, though sometimes people say it does not apply to
consumer units. But while I can understand how doppler for a single
satellite could give motion relative to that satellite (because that is the
simple doppler effect per red-shift etc) I have never understood how it
works with four+ satellites to yield a speed vector. Especially since
although the satellites are slow compared with the speed of light they are
fast compared with vehicle speeds, though I guess that is just a matter of
accurate calculation...

Mike.
--
If reply address is (invalid), remove spurious "@"
and substitute "plus" for +.


  #8  
Old December 31st 12, 03:49 PM posted to uk.rec.gps
Chris J Dixon
external usenet poster
 
Posts: 10
Default Variation in speed reported by GPS

Roger Mills wrote:

I don't know exactly how it's done, but I assume that there must be some
smoothing. The speed reported by my TomTom when I'm using cruise control
to maintain a constant speed is steady as a rock. But there does appear
to be some lag during acceleration and deceleration.


+1

Chris
--
Chris J Dixon Nottingham UK


Have dancing shoes, will ceilidh.
  #9  
Old December 31st 12, 04:08 PM posted to uk.rec.gps
R. Mark Clayton
external usenet poster
 
Posts: 119
Default Variation in speed reported by GPS


"Mike Coon" wrote in message
o.uk...
Alan White wrote:
The velocity bit uses Doppler shift of the received
frequency from those satellites.


I keep reading this, though sometimes people say it does not apply to
consumer units. But while I can understand how doppler for a single
satellite could give motion relative to that satellite (because that is
the simple doppler effect per red-shift etc) I have never understood how
it works with four+ satellites to yield a speed vector. Especially since
although the satellites are slow compared with the speed of light they are
fast compared with vehicle speeds, though I guess that is just a matter of
accurate calculation...

Mike.
--


Police radar guns can give a speed accurate to 0.001mph, although in reality
such accuracy should be taken with a pinch of salt (heartbeat and breathing
would have an effect much greater than that).

The position is resolved by calculating (solving simultaneous equations for)
x, y, z and t from four satellites to get an accurate fix. By corollary the
speed can be calculated as well. The only problem is that the space
vehicles are doing around 18,000mph possibly in opposite directions so the
speed of the receiver is very small by comparison.

In practice taking the difference in two readings some time apart will
probably give a better result than Doppler measurement, however there may be
fluctuations when the constellation used changes.




Using p-code reception t will be accurate to ~10nS and if well separated
satellites are used x, y and z to ~3m.



  #10  
Old December 31st 12, 05:15 PM posted to uk.rec.gps
Mike Coon[_2_]
external usenet poster
 
Posts: 144
Default Variation in speed reported by GPS

R. Mark Clayton wrote:

Police radar guns can give a speed accurate to 0.001mph, although in
reality such accuracy should be taken with a pinch of salt (heartbeat
and breathing would have an effect much greater than that).

The position is resolved by calculating (solving simultaneous
equations for) x, y, z and t from four satellites to get an accurate
fix. By corollary the speed can be calculated as well. The only
problem is that the space vehicles are doing around 18,000mph
possibly in opposite directions so the speed of the receiver is very
small by comparison.
In practice taking the difference in two readings some time apart will
probably give a better result than Doppler measurement, however there
may be fluctuations when the constellation used changes.


I bet if I answered an exam question which said "show working" with the
words "By corollary" I would not get many marks!

Surely the point of speed calculation using doppler is that it is distinct
from position calculations.

I find radar guns even easier to understand than the "doppler" measurements
of blood flow in my legs, in the local maternity clinic, a few years ago...

Mike.
--
If reply address is (invalid), remove spurious "@"
and substitute "plus" for +.


 




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