[FOSS-GPS] High accuracy positioning with low cost GPS devices:a FOSS project

[Tyler Mitchell (OSGeo)]

Anyone happened to tried any of these modules before?
http://www.sparkfun.com/commerce/categories.php?c=4

They also have some receivers listed with a combined cellular  
module.  Or are all of these out of the league for doing higher  
accuracy stuff?

Tyler

On 17-Oct-08, at 2:20 AM, John Morris wrote:

> If your LEA-4T has early firmware and you want "fix" solutions,  
> then you
> should upgrade. U-blox used to do free upgrades. It involved  
> sending the
> unit to their office for a week or so. The newer firmware does full- 
> cycle
> phase just fine.
>
> One way to think of it ... with half cycles, you have to know your  
> position
> to double the accuracy. This will take at least 4X the measurements,
> assuming the measurements don't have any bias to them. If they do,  
> then you
> can only handle half the bias; you have to stay closer to the  
> reference
> station.
>
>   - John
>
> -----Original Message-----
> From: foss-gps-bounces at lists.osgeo.org
> [mailto:foss-gps-bounces at lists.osgeo.org] On Behalf Of Martin Vermeer
> Sent: Friday, October 17, 2008 12:30 AM
> To: Open Source GPS-related discussion and support
> Cc: eugenio.realini at gmail.com
> Subject: Re: [FOSS-GPS] High accuracy positioning with low cost GPS
> devices:a FOSS project
>
> On Thu, 16 Oct 2008 18:20:28 +0200
> Eugenio Realini <eugenio.realini at gmail.com> wrote:
>
>> We have written something on goGPS here:
>> http://geomatica.como.polimi.it/elab/goGPS/index.php?lang=en
>>
>> John Morris wrote:
>
> ...
>
>>> I sort of got burned out trying to implement a kinematic float  
>>> solution.
> I
>>> eventually concluded it is very difficult to detect or correct phase
> errors
>>> with float solutions. You either have to be stationary and use a  
>>> Kalman
>>> filter, or you have to "fix" your solution so you know your position
> well
>>> enough to catch the inconsistencies. I'm curious to know your  
>>> approach -
> and
>>> I'm really, really glad you've taken on this piece of work.
>> Do you mean cycle slips when you say "phase errors"?
>> At the moment, we set ambiguities as state variables in the Kalman
>> filter, so that they evolve "dynamically" with the system... we
>> initialize them as float values by comparing code and phase  
>> measurements
>> and they converge in not so much time (with good conditions of sky
>> visibility).
>> Then we detect cycle slips by comparing subsequent epochs and when  
>> they
>> occur we just re-initialize the affected ambiguities.
>>
>> ---
>>
>> We have a couple of doubts we'd like to ask the community:
>>
>> 1. up to now we didn't worry about the "half-cycle
>> ambiguities resolved" that appear in the LEA-4T data sheet... do they
>> impact significantly on the receiver performance?
>
> I suppose so, in the sense that the "wavelength" for ambiguity
> resolution will be only half, so it gets harder to do. But when
> successful it shouldn't affect performance.
>
> So this receiver apparently uses a Costas discriminator which doesn't
> "see" a 180 deg phase shift.
>
>> 2. We noticed that the same phase values read from RINEX and from  
>> RTCM
>> (same permanent station: RINEX taken directly from it, RTCM 3.0  
>> stream
>> sent by the GNSS positioning service it belongs to) differ by a  
>> constant
>> value. The results are not affected by it because it goes away in the
>> differential positioning, but we do not understand why it's there.
>>
>> Eugenio
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