Take the 2-minute tour ×
Geographic Information Systems Stack Exchange is a question and answer site for cartographers, geographers and GIS professionals. It's 100% free, no registration required.

I was wondering if Fix quality data from GPGGA, can be substituted to Validity field in GPRMC? In my application I have to use only GPRMC, but I don't really know if it sets validity only if Fix quality > 1. Can somebody confirm the logic of Validity field and how it's correlated with Fix quality ?

share|improve this question
    
Could you add some background or context to your question? What are GPGGA and GPRMC? –  R.K. Dec 1 '12 at 4:45
add comment

1 Answer

up vote 1 down vote accepted

I assume that Validity flag in RMC could be set for any "valid" output, where "valid" depends on the opinion of the firmware maker (after all, there is no certification requirements for NMEA outputs or any other GPS receiver functionality).

Edit: From http://gpsd.berlios.de/NMEA.txt

=== RMC - Recommended Minimum Navigation Information ===

------------------------------------------------------------------------------
                                                          12
        1         2 3       4 5        6  7   8   9    10 11|  13
        |         | |       | |        |  |   |   |    |  | |   |
  $--RMC,hhmmss.ss,A,llll.ll,a,yyyyy.yy,a,x.x,x.x,xxxx,x.x,a,m,*hh<CR><LF>
------------------------------------------------------------------------------

 Field Number:

1. UTC Time
2. Status, V=Navigation receiver warning A=Valid
3. Latitude
4. N or S
5. Longitude
6. E or W
7. Speed over ground, knots
8. Track made good, degrees true
9. Date, ddmmyy
10. Magnetic Variation, degrees
11. E or W
12. FAA mode indicator (NMEA 2.3 and later)
13. Checksum

A status of V means the GPS has a valid fix that is below an internal
quality threshold, e.g. because the dilution of precision is too high 
or an elevation mask test failed.

and from earlier in that document:

In NMEA 2.3, several sentences (APB, BWC, BWR, GLL, RMA, RMB, RMC,
VTG, WCV, and XTE) got a new last field carrying the signal integrity
information needed by the FAA.  (The values in the GGA mode field were
extended to carry this information as well.) Here are the values:

FAA Mode Indicator
     A = Autonomous mode
     D = Differential Mode
     E = Estimated (dead-reckoning) mode
     M = Manual Input Mode
     S = Simulated Mode
     N = Data Not Valid

 This field may be empty.  In pre-2.3 versions it is omitted. [NTUM] says
 that according to the NMEA specification, it dominates the Status field --
 the Status field will be set to "A" (data valid) for Mode Indicators A
 and D, and to "V" (data invalid) for all other values of the Mode
 Indicator.  This is confirmed by [IEC].

So if all you can use is RMC, then testing the FAA field (if present) should be enough.

If you have GGA, then I would make use of it. What you consider to be valid data is obviously domain specific, but clearly if GGA gives you 0 satellites, or manual input mode, or a "cannot be happening" altitude, then the data probably isn't good.

share|improve this answer
    
Unfortunately GGA does not provide speed over ground measurement and my device can not afford this calculation so I need to find out what is receiver warning field. –  Pablo Dec 1 '12 at 7:46
    
Are you saying you can only configure the device to output RMC or GGA, but not both? Perhaps you need to ask your GPS device firmware manufacturer about the validity rules? –  BradHards Dec 1 '12 at 8:13
    
Exactly, due to very limited MCU capabilities, which should process the messages. I've tried to ask Chinese manufacturer (gtop-tech) but looks like they don't bother themselves with answering technical questions. So I thought this was my last resort. Hopefully the logic would be similar with other GPS receiver manufacturers. –  Pablo Dec 1 '12 at 8:35
    
I don't think you can safely assume anything for such a device.... –  BradHards Dec 1 '12 at 8:54
    
Would be interesting to see at least for any well known GPS receiver how they describe this bit of information. –  Pablo Dec 1 '12 at 11:37
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.