On a sunny day (Tue, 18 Sep 2012 19:35:15 -0700 (PDT)) it happened mpm
<mpmillard@aol.com> wrote in
<c99847af-e37f-4cfa-9fe3-a47a62d60f3f@googlegroups.com>:
>On Saturday, September 15, 2012 8:03:50 PM UTC-4, Jim Whitby wrote:
>> I'm asking this here in the hope someone knows something about GPS
>> receivers.
>>
>[snip]
>
>> Does anyone have a clue what 100% is relative to?
>>
>
>Well, it's not going to be SINAD since it's not an analog receiver.
>Your choices are most likely Bit Error Rate (BER), though a BER of 20% woul=
>d seem completely unusable to me. The other option is C/I+N but again, tha=
>t's used more for analog modulation schemes (or should be, though some comp=
>anies think SINAD is acceptable for "digital modulation". These people sho=
>uld be tarred and feathered. You could write a book about modulation schem=
>es and their effect on BER, and I'm sure some have. I've never found dBm l=
>ookup tables to be particularly good descriptors for BER. Your mileage may=
> vary.
>
>Bars and SINAD and I+N values aside, if the GPS fades too much, there's pro=
>bably something wrong with the receiver or the placement of its antenna. S=
>ome antennas are powered (i.e., internal active preamps), so you might veri=
>fy the batteries are fresh, or it's otherwise powered-up with the correct v=
>oltage.
>
>Shameless plug: We use a lot of Garmin Montana GPS's (650's and 650t's)for=
> our field work. Expensive, but perfect for what we do. Of course, as wit=
>h any early adopter there were some early software glitches; like "Continue=
> four-thousand one-hundred fifty miles, then make a U-turn", but they squas=
>hed that bug pretty quick. :)
If you use ANY GPS receiver that has a NEMEA output,
the data we are talking about is standarized in the RS232 'GPGSV' message:
This is the offcial NEMEA standard, for that GPGSV message, it shows an example message
and the meaning of the comma separated fields in it.
/* test for SV-GNSS Satellites in View */
/*
GSV-GNSS Satellites in View
Table B-8 contains the values for the following example:
$GPGSV,2,1,07,07,79,048,42,02,51,062,43,26,36,256,42,27,27,138,42*71
$GPGSV,2,2,07,09,23,313,42,04,19,159,41,15,12,041,42*41
Table B-8 GSV Data Format
Name Example Description
Message ID $GPGSV GSV protocol header
Number of Messages1 2 Range 1 to 3
Message Number1 1 Range 1 to 3
Satellites in View 07
Satellite ID 07 Channel 1(Range 1 to 32)
Elevation 79 degrees Channel 1(Maximum90)
Azimuth 048 degrees Channel 1(True, Range 0 to 359)
SNR(C/No) 42 dBHz Range 0 to 99,null when not tracking <-------------------------------------------
....... .......
Satellite ID 27 Channel 4 (Range 1 to 32)
Elevation 27 Degrees Channel 4(Maximum90)
Azimuth 138 Degrees Channel 4(True, Range 0 to 359)
SNR(C/No) 42 dBHz Range 0 to 99,null when not tracking <-------------------------------------------
Checksum *71
<CR><LF> End of message termination
Depending on the number of satellites tracked multiple messages of GSV data may be required.
*/
You will note, that as the OP noticed, his GPS now confirms to this standard,
as his signal to noise is displayed as 0-100% for each satellite, see large arrows.
Linux source code of my gpspc program (that this came from) is availabe upon request,
but be aware that it also interfaces to other modules like for magnetic compass, air pressure, temperature,
and has an interface for auto-pilot, and supports multiple waypoints.
If you want the details for all NEMEA messages look on the web for the SiRF chipset NEMEA reference manual.
I will leave it at that...