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

Wed Feb 4 11:49:39 EST 2009

Thanks for the great run-down!

Off topic, notice you are at Warwick and it reminded me that I'm  
planning to be in Nottingham UK for the open source geospatial event  
in June and will be doing some travelling around to other sites too.   
Would love to meet up and talk shop with anyone, especially if you  
have a project you'd like to show off about mapping, analysis,  
imagery or open source GPS.   Just drop me a note privately.

Tyler
On 4-Feb-09, at 1:35 AM, Michael Tandy wrote:

> The key feature of the LEA-4T which makes it interesting for high
> precision applications is that its binary protocol includes a message
> giving raw observables - that is, code and carrier phase pseudorange
> and doppler shift. This is documented in U-blox's protocol
> specification as UBX-RXM-RAW (
> http://www.u-blox.com/customersupport/antaris4_doc.html ). Carrier
> phase information, in particular, is what's interesting.
>
> I have heard the SIRF-II chipset also offered this raw data; but those
> have largely been replaced with SIRF-III which doesn't offer the right
> raw data - Sparkfun does have a few SIRF-II modules in stock though; I
> don't know if sparkfun have a source for new SIRF-II modules after
> those are out of stock. I've dropped Sparkfun an e-mail to ask - if
> they have a source for SIRF-II modules that would be a pretty neat
> find, as $60 for SIRF-II modules is a lot more affordable than $350
> for an AEK-4T evaluation kit from U-blox.
>
> The Trimble Copernicus chip on Sparkfun also got my hopes up with its
> report packet 0x5A - but it doesn't offer carrier phase information
> either. Most of the other GPS modules I can see on Sparkfun either
> don't offer raw data, or at least don't say anything about raw data in
> their documentation, which is as good as not offering it.
>
> Michael
>
>
> 2009/2/3 Tyler Mitchell (OSGeo) <tmitchell at osgeo.org>:
>> 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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>>
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Tyler Mitchell
Executive Director
Open Source Geospatial Foundation
tmitchell at osgeo.org
P: +1-250-277-1621
M: +1-250-303-1831