GPS Data Logger Software - BT747

OK, I'll check if I ever have DGPS with proper satellites used.

Interesting link here showing that SBAS (with all 3 EGNOS satellites) degrades accuracy (the article is not so recent, maybe sats settings have changed since then): http://www.gpspassion.com/fr/articles.asp?id=185

Unfortunately, the test was performed with SIRF 3 and Nemerix chipsets, not MTK like BT747, QSTARZ, BTQ1000...

Note that some GPS were not able at all to calculate DGPS position from EGNOS sats.

Has anyone noted  that our MTK based GPS was capable of DGPS fix (not SPS) with any WAAS/EGNOS satellite ?

Hi

The MTK chipset is capable of DGPS.  I've seen an MTK based logger do that.

I am not surprised that the DGPS positioning is considered not accurate.  That corresponds to my personal observations.

I've recommended a few times here and there to not activate DGPS - but things can have changed since.

Mario

Mario,

I test two iBlue-747 at same time with/without SBA. I have not observed any difference between two track, when I see in Google Map.

Only in the CSV file, in the SBA set, show DGPS, but the accurate it is exacty the same.

Regards from Spain

Manuel

Thanks Manual for this feedback.  I've noticed that one can have considerable position jumps when DGPS becomes active on a track.

The applied correction surely depends on when and where the GPS tracked.

 ! Hola Manuel !

That's interesting, your test. Have you also checked in your Excel file if the altitudes (and VDOP) were also exactly the same ? Normally, SBAS is designed for aircraft final approach paths, and GPS worst accuracy is in vertical mode. Due to the relatively low elevation angle of the geostationnary satellite (it depends on which one(s) you received ?), there should be an improvement on vertical accuracy.

Besides, there should be an improvement on reliability and integrity of the SBAS data. But I have no clue of how to give any evidence for that with the iblue 747.

Hasta pronto ...

Hello Mario,

I think I got it !

DGPS (and 'sometimes estimated mode') instead of SPS with my BTQ1000P, firmware Bcore 1.1, V1.38. Surprisingly, I don't find any of the satellites I expected (SID).

I'd like to make some averaging on a fix, to measure vertical dispersion in that mode (at first glance, a couple of meters, with a VDOP around 0,99.

But I think I have a trouble reading the proper values of SPEED, VDOP, HDOP, VDOP  and distance in the csv file : from time to time, randomly, they indicate very high values, with no meaning at all.

Could you do something about that ? Please...

Besides, would it be possible to have commas (,) instead of points (.) for decimal separation : my openoffice can read the numbers as characters, and I've got trouble making averages. 

Hi Foxecho

'Estimated Mode' means that you did not have good reception, but the device does some kind of dead reckoning.

Very high values for VDOP, HDOP, PDOP mean that the position is inaccurate.

A high value for Distance will occur if a long way was done since that previously  logged position or if the previously determined position was invalid.

I'll propagate a '.' or ',' setting for CSV only to the UI. It is surely manageable in openoffice at import time, but I agree that it is easier when configureable in BT747.

You can do something about these high values because BT747 allows you to do several things already:

1. Define maximum values for HDOP, PDOP and/or VDOP in the 'Common filter'.  That will filter out these bad positions;
2. Have BT747 calculate the distance - not using the distance value provided by the logger.  You can select that in output settings.  There is an entry 'When missing, calculate distance', 'Always calculate distance' or 'Use distance in log, do not calculate'.

Message out of topic.

Hi Foxecho

Looking at the table, I can understand the supection of a bug.

Can you send me your bin file so that I have a sample to look at what is going wrong (mail in the application in 'about').

First results :

In DGPS indicated, with 4 to 5 sats used (HDOP = 3,1 / VDOP =1), with more than 600 points on a fixed point, we can calculate (thanks to Mario's modification of csv file)

-  average speed of 2,0 km/h

- an average distance between each fix of ... 0,9 m 

- an average vertical dispersion of 2,65 m. 

Note that we never identified one of the expected EGNOS satellites, and that DAGE was stuck to zero, even when in 'estimated position'. This is maybe due to the way QSARTZ programs the GPS.

In SPS mode on the same point (but on a different day), with 5 to 6 sats (HDOP= 1,6 ; VDOP =2), we have

- average speed of 0,13 km/h

- an average distance of between each fix of ... 0,02 m 

- an average vertical dispersion of 0,56 m.

This aims at giving a reference in both modes, with one good (SPS) constellation, and one medium (DGPS).

I wish I could do the same measurements with two identical GPSs co-located, at the same time, same warm-start, not to depend on sat constellation..

I have an iBlue 747, iBlue 747 A+, Holux M241 and two Skytraq based GPS's (not recording sat information though).

I could do some parallel recording with all fields on - or we have to meet and check it out :-).  The Qstarz is the same as one of the iBlue modules.