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

[Eugenio Realini]

We have written something on goGPS here: 
http://geomatica.como.polimi.it/elab/goGPS/index.php?lang=en

John Morris wrote:
> I did a zero baseline test with the LEA-4T, and the carrier phase looked
> pretty good. The LEA-4T needs some interpolation tricks to get things to
> work.
Which kind of interpolation tricks?
> The other inexpensive receiver is Thales AC-12. This one provides
> carrier phase within a couple mm. My understanding it is the same GPS engine
> which is used in the Promark II for doing single frequency RTK. (BTW, is
> there a cheaper LEA-4T than the u-blox demo unit?)
>   
As far as I know the u-blox ev. kit is the cheapest ready-to-use LEA-4T
solution around.
> 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?

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