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- Question-About-Garmin-Hot-Fix-Data
- 08-02-2010
   Re: Question About Garmin "Hot Fix Data"
| Harvey Gratt | 08-04-2010 |
  Re: Question About Garmin "Hot Fix Data"
| Greg Russell | 08-04-2010 |
  Re: Question About Garmin "Hot Fix Data"
| Gene E. Bloch | 08-05-2010 |
 Re: Question About Garmin "Hot Fix Data"
| Harvey Gratt | 08-05-2010 |
| Re: Question About Garmin "Hot Fix Data"
| Darren Dunham | 08-16-2010 |
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Harvey Gratt wrote:
The question was answered many years ago in the original GPS newsgroup:
http://groups.google.com/groups/search?safe=off&q=doppler+group%3Asci.geo.satellite-nav&btnG=Search&sitesearch=
(it was/is sci.geo.satellite-nav FWIW)
If you feel like experienting, go outdoors, turn on your GPS.
Stay still. Speed should indicate 0.0
With your arm extended, start spinning as fast as you can. Note the
speed on your GPS, and how quickly it updates.
Remember that your position is updated once every second on the typical
GPS. And if you rotate at 1rpm, each reading should be at exact same
location, yet your GPS still shows you are moving at a certain speed.
wrote:
>JF Mezei wrote:
>> Harvey Gratt wrote:
>>> My guess is that the less stressful (vehicle dynamics)automotive case
>>> simply utilize a least squares and simple filtering algorithm to
>>> determine vehicle position and speed from position only measurements.
>>> simply utilize a least squares and simple filtering algorithm to
>>> determine vehicle position and speed from position only measurements.
>> Garmin has had doppler calculations in its algorythms for more than a
>> decade. It wouldn't make sense to produce a separate version of its core
>> GPS engine for different models.
>> A car system may have an accelerometer in it, so perhaps they had to
>> tweak the engine to factor this because in urban canyons, you may lose
>> sufficient GPS fix at which point the accelerometre provides information
>> that allows the GPS to guess your speed/direction and hence position.
>> But once it has a GPS fix, it would use the standard engine which
>> includes doppler.
>> decade. It wouldn't make sense to produce a separate version of its core
>> GPS engine for different models.
>> A car system may have an accelerometer in it, so perhaps they had to
>> tweak the engine to factor this because in urban canyons, you may lose
>> sufficient GPS fix at which point the accelerometre provides information
>> that allows the GPS to guess your speed/direction and hence position.
>> But once it has a GPS fix, it would use the standard engine which
>> includes doppler.
>The question is whether the "Doppler calculations" are used in aiding
>the position measurements or is it used only for the satellite search
>and tracking functions. Maybe someone can ask Garmin (or Tom-Tom, etc).
>the position measurements or is it used only for the satellite search
>and tracking functions. Maybe someone can ask Garmin (or Tom-Tom, etc).
Several have tried, but so far no one has provided convincing evidence
that Doppler is used in calculating the position.
wrote:
>Several have tried, but so far no one has provided convincing evidence
>that Doppler is used in calculating the position.
>that Doppler is used in calculating the position.
I don't think it is. The position is calculated using the received time
differences.
=20
--=20
Alan White
Mozilla Firefox and Forte Agent.
Twenty-eight miles NW of Glasgow, overlooking Lochs Long and Goil in =
Argyll, Scotland.
Webcam and weather:- http://windycroft.co.uk/weather
On Fri, 06 Aug 2010 07:51:18 +0100, Alan
>wrote:
>>Several have tried, but so far no one has provided convincing evidence
>>that Doppler is used in calculating the position.
>>that Doppler is used in calculating the position.
>I don't think it is. The position is calculated using the received time
>differences.
>differences.
Thank you.
> Aren't they transmitting a CDMA signal? =A0A kind of Spread Spectrum
> signal?
> If so, even relative frequency measurement would be difficult. =A0It is m=
y
> signal?
> If so, even relative frequency measurement would be difficult. =A0It is m=
> understanding that all calculations are based on time-of-arrival
> compared to the local GPS clock.
> compared to the local GPS clock.
One of the inputs to the signal correlator is the frequency. This
frequency has to be "close enough" to the received frequency for the
signal to appear. If you're too far out, you just get noise. That's
why a GPS with a faulty almanac can take so long to acquire.
Once the channel has signal lock-on, the correlator tries to keep the
signal strength maximized by adjusting the frequency. By comparing
this "steered" frequency with what would be expected for a satellite
with zero radial motion, the doppler adjustment to the sat frequency
can be calculated directly.
The doppler signal as calculated by a standard consumer unit (not
doing carrier-phase readings) will be pretty noisy, so it's not going
to display or use it directly. It is probably fed into the Kalman
filter though, since the whole point is to use multiple noisy signals.
This article has been posted to this group in the past by Mike Lane.
It details some experiments that use the GPS-derived doppler for
calculating speed.
http://gauss.gge.unb.ca/papers.pdf/ionntm2004.serrano.pdf
--
Darren
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> the position measurements or is it used only for the satellite search
> and tracking functions. Maybe someone can ask Garmin (or Tom-Tom, etc).