| Author |
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il_zio
Guest
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 Posted:
Fri Nov 18, 2005 11:32 pm Post subject:
question about sampling rate |
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I know this is surely a topic already discussed but I wasn't able to find it
reading the newsgroup.
In a thread I read that the satellite sends a time signal every second, so a
sampling rate of the receiver graeter than 1 Hz is useless. Is it true? I
ask this because I read about a gps based seismometer with a 20 Hz sampling
rate: how is it possible if the satellite sends a complete signal only every
second? Are there some "tricks" to bypass this problem and to have the
possibility of sampling at higher rates or were those 20 Hz false and
obtained by interpolation (even though I think it is not so useful an
interpolation in a seismometer)?
Thanks a lot to all those will be so kind to answer. |
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Guest
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| Posted:
Sat Nov 19, 2005 1:08 am Post subject:
Re: question about sampling rate |
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il_zio wrote:
| Quote: | I know this is surely a topic already discussed but I wasn't able to find it
reading the newsgroup.
In a thread I read that the satellite sends a time signal every second, so a
sampling rate of the receiver graeter than 1 Hz is useless. Is it true? I
ask this because I read about a gps based seismometer with a 20 Hz sampling
rate: how is it possible if the satellite sends a complete signal only every
second? Are there some "tricks" to bypass this problem and to have the
possibility of sampling at higher rates or were those 20 Hz false and
obtained by interpolation (even though I think it is not so useful an
interpolation in a seismometer)?
Thanks a lot to all those will be so kind to answer.
|
The signal from the satellite is continuous, not once a second. The
basic underlying repetitive modulation pattern (PRN code) that spreads
the signal repeats at a 1.023 MHz rate (C/A code, P/Y code is ten times
quicker), and individual cycles of the carrier can be tracked to get
even finer grain resolution. A slow data rate (50 bps) signal that is
modulated on top of that speading modulation provides a time reference
label once every 6 seconds. That reference is the zero crossing of the
carrier phase associated with first bit of the next 6 second subframe.
The carrier phase and PRN code modulation are the fine and coarse tick
marks on the time ruler, the timing information in the 50 bps subframe
data are the numerical lables on the ruler.
The limits on sampling rates are a cost, noise, and benefit. |
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Sam Wormley
Guest
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| Posted:
Sat Nov 19, 2005 8:47 am Post subject:
Re: question about sampling rate |
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il_zio wrote:
| Quote: |
In a thread I read that the satellite sends a time signal every second....
|
I want you to read these:
GPS User Equipment Introduction
http://www.navcen.uscg.gov/pubs/gps/gpsuser/gpsuser.pdf
GPS SPS Signal Specification
http://www.navcen.uscg.gov/pubs/gps/sigspec/default.htm
NAVSTAR GPS signal are continuous with each satellite transmitting
its ephemeris data every 30 seconds and the whole constellation
almanac every 12.5 minutes. Timing signal patterns arrive every 6
seconds with a GPS Navigation Message Bit Rate of 0.02 seconds.
shorter increments of time can be derived from the the carrier
frequencies. |
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Sam Wormley
Guest
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| Posted:
Sat Nov 19, 2005 9:05 am Post subject:
Re: question about sampling rate |
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il_zio wrote:
| Quote: |
In a thread I read that the satellite sends a time signal every second...
|
I want you to read these:
GPS User Equipment Introduction
http://www.navcen.uscg.gov/pubs/gps/gpsuser/gpsuser.pdf
GPS SPS Signal Specification
http://www.navcen.uscg.gov/pubs/gps/sigspec/default.htm
NAVSTAR GPS signal are continuous with each satellite transmitting
its ephemeris data every 30 seconds and the whole constellation
almanac every 12.5 minutes. Timing signal patterns arrive every 6
seconds with a GPS Navigation Message Bit Rate of 0.02 seconds.
shorter increments of time can be derived from the the carrier
frequencies. |
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il_zio
Guest
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| Posted:
Sat Nov 19, 2005 5:08 pm Post subject:
Re: question about sampling rate |
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Thanks a lot for the answers, they have been very useful! I'll search for a
receiver with a sampling rate greater than 1 Hz, hoping it isn't too
expensive. |
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Rotareneg
Guest
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| Posted:
Sat Nov 19, 2005 5:08 pm Post subject:
Re: question about sampling rate |
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The Garmin GPS 18 5Hz has, not surprisingly, a 5 Hz sampling rate, and
"only" costs $200.
http://www.garmin.com/products/gps185hz/ |
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MikeLee
Guest
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| Posted:
Sun Nov 27, 2005 1:08 am Post subject:
Re: question about sampling rate |
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The 1Hz update rate (sampling rate is not the correct term) that has
been discussed is due to GPS receiver firmware. Since the GPS receiver
integrate one CA code period and process from there, theoretically the
highest update rate possible is 1KHz, taking measurement and compute
PVT solution every millisecond. Due to CPU processing power limitation
and actual application requirements, maximum rate seen are in order of
several 10Hz by Javad, Novatel, ... etc at the higher end. These rates
higher than 1Hz are not extrapolated.
Mke |
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