Estimating accuracy from raw NMEA data

[fahad]

I am developing an android app for a usb GPS module of UBLOX. Is there any accuracy information in the raw NMEA data which i can use in my GUI? or do I have to use any algorithm to calculate the accuracy?

 

[Nuvi-Nebie]

Although I found some info. stating that Ublox use NMEA sentences, I couldn't find info. about which sentences they output. Some GPS modules supply a single sentence, whereas others output many. Here is a link showing what is contained in a selection of sentences: -

http://www.gpsinformation.org/dale/nmea.htm#PGRME

EDIT
Having examined the link above a little closer, it looks like Ublox sends out the GSA sentence which contains DOP data, so the calculations have already been done for you

 

[fahad]

Although I found some info. stating that Ublox use NMEA sentences, I couldn't find info. about which sentences they output. Some GPS modules supply a single sentence, whereas others output many. Here is a link showing what is contained in a selection of sentences: -

http://www.gpsinformation.org/dale/nmea.htm#PGRME

EDIT
Having examined the link above a little closer, it looks like Ublox sends out the GSA sentence which contains DOP data, so the calculations have already been done for you

View attachment 168

Thanks for the reply, how can I get accuracy in meters/cm from that?

 

[Nuvi-Nebie]

Some GPS manufacturers add their own 'extra' NMEA sentences to their products I.e. Garmin use GPRME, this gives an accuracy figure in metres, I haven't seen a formula detailing how they arrive at these figures, although I would guess that they use the info. in GSA and GGA, here is a link that may help :-
http://edu-observatory.org/gps/gps_accuracy.html

If you have access to a Garmin unit, you could examine the HDOP, VDOP, PDOP, number of satellites in view etc. and compare them to the HPE and VPE figures presented in the PGRME sentence, in that way it may be possible to determine values similar to their's

 

[fahad]

Some GPS manufacturers add their own 'extra' NMEA sentences to their products I.e. Garmin use GPRME, this gives an accuracy figure in metres, I haven't seen a formula detailing how they arrive at these figures, although I would guess that they use the info. in GSA and GGA, here is a link that may help :-
http://edu-observatory.org/gps/gps_accuracy.html

If you have access to a Garmin unit, you could examine the HDOP, VDOP, PDOP, number of satellites in view etc. and compare them to the HPE and VPE figures presented in the PGRME sentence, in that way it may be possible to determine values similar to their's

Yeah in case of Garmin it's already done in GPRME but in Ublox there is no such sentence which gives accuracy in meters, I have gone through this link but I didn't succeed to find a solution. However I read in forum that multiplying 'hdop' with the "best accuracy" of your GPS receiver can give you accuracy in meters, and for an RTK this 'best accuracy' is between 1-5 cm ... I am not sure how accurate is this approach?

 

[semuadmin]

Sorry for very late reply, but if it's a u-blox device you can enable the proprietary NMEA "UBX,00" sentence which includes accuracy in meters (it's the 'hAcc' field). Derivations based on HDOP alone are arbitrary at best. Alternatively you could of course switch the device to UBX protocol instead of (or as well as) NMEA - you then have access to the UBX "NAV-PVT" & "NAV-VELNED" message types, both of which include horizontal accuracy in meters.

By the way, if you're after a free, open source cross-platform utility which supports u-blox device configuration, you might like to check out :-

https://github.com/semuconsulting/PyGPSClient