GPS data validation

Question

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 ?

Answer 1

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.