very low power gps application

Hi everybody,

I'd like to build a very-low power gps tracker for "off-line"
application (no
need for real time positioning). I need a fix every 1
to 15min (user-dependant),
and a precision of 10m would be acceptable
(5m good, 2m wonderful).

First idea: buy a low-power GPS chip (Skytraq Venus 6 or u-blox 5)
But they have a long long long cold start time to download ephemeris.
I made a PCB with Skytraq Venus 6 GPS IC and I'm able to store position
on a
memory and transfer logs from card to a PC.

Second Idea: store only raw data and post-process it in combination
with
ephemeris downloaded from a web-server. This solution soons very
good for me but
it's technically harder...

I have some questions:
- Could this second solution works?
- How much time should I wait between GPS start-up and first complete
raw
measurement with all sats?
- What software should I use?

Thanks a lot!




--
bonelli

The 2nd definitely works.
Depending on your algorithm.
We are developing a very efficient algorithm. I will post an
advertisement here once it is available for the market.

Johnson

bonelli wrote:

Second idea is very good, but depends on your application.
If you need a fix every 15 minutes, only the first one will have a
very long startup time to download ephemeris.
And refreshing them every 15 minutes, you just need to be "ON" for
30/40 seconds (even less, actually)
You could also use ephemeris extension that gives you ephemeris for a
week, so you have always 10/15 seconds of TTFF.



Sure.


You shoudl ask yourself how much data can you store... Raw data is a
lot of data :)
You have to collect very high speed samples.


Software to do what? Postprocessing?
If you use a ublox chipset... give a look here:
http://www.u-blox.com/en/capture-process.html

I suggest to think carefully about your requirement to find the best
solution.
If you can satisfy your requirements with solution (1), that would be
simpler to implement.

Actually I would like to use something like:


Code:
--------------------

while(1)
{
gpsPowerOn();
gpsWaitForFirstRawAvailable();
gpsReadRaw();
gpsPowerDown();
wait_15min();
}

--------------------


Here "raw" means pseudorange, doppler measurment... it didn't means ADC
output.

A software to process input data :
- Input 1 : raw measurment from GPS (pseudoranges...)
- Input 2: ephemeris from a web server
- Output : 3D position

U-blox "capture 'n' process" store ADC output. But the RF front end and
YUMA
service are only available for huge productions. My project
concerns 10-100
items...

Thanks for your help guys.




--
bonelli

[cut]

Mmmm... here we have a problem: the pseudorange is the corrected
distance between the receiver and the satellite.
But to compute this distance you need the satellite position, that is
obtained using ephemeris data.
So... you always need to keep your GPS receiver "ON" enough to get the
ephemeris.



No way... I think... no ephemeris, no pseudoranges.
At least you could try to do it with almanacs... but the accuracy will
drop dramatically


I see.