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DGPS

the project developed in the 2002 with good success

NMEA data elaboration

The NMEA data format is copyrithed of NMEA Organization
lot of parsers are available on Internet, Both the dgpsip and dgpsnavigator have good NMEA parser

GPSD Parser output

NMEA strings examples 1)$GPRMC,225446,A,4916.45,N,12311.12,W,000.5,054.7,191194,020.3,E*68 2)$GPRMB,A,0.00,R,010,ASCEND,3743.000,N,12214.000,W,000.0,000.0,,A*28 3)$GPGGA, 123519,4807.038,N,01131.000,E,1,08,0.9,545.4,M,46.9,M, , *42 4)$GPGSA,A,3,01,03,,,15,21,23,25,31,,,,,,,,0.7,0.4,0.6*36 5)$GPGSV,3,1,09,01,70,041,49,03,17,209,46,09,01,031,00,14,05,281,34*76 6)$GPGSV,3,2,09,15,30,315,50,21,46,069,50,23,11,086,39,25,31,159,49*7F 7)$GPGSV,3,3,09,31,28,249,48,,,,,,,,,,,,*4B 8)$PGRME,5.9,M,10.1,M,11.7,M*25 9)$GPGLL,3743.000,N,12214.000,W,011243,A*3B

String 1 Parsing
RMC - Recommended minimum specific GPS/Transit data
* 225446 Time of fix 22:54:46 UTC
* A Navigation receiver warning A = OK, V = warning
* 4916.45,N Latitude 49 deg. 16.45 min North
* 12311.12,W Longitude 123 deg. 11.12 min West
* 000.5 Speed over ground, Knots
* 054.7 Course Made Good, True
* 191194 Date of fix 19 November 1994
* 020.3,E Magnetic variation 20.3 deg East
* *68 mandatory checksum

GPS NMEA data format
Normally the GPS has the possibilty to send out the data in different format
One of the most used protocol is NMEA out/ RTCM in, this means that every
kind of parser can get the NMEA data, and give you a every kind of informations.
The DGPSNavigator      is a special RTCM client whit satellite signal quality elaboration.

RTCM data elaboration

The RTCM data format is a copyrigth of RTCM Organization The best parser is rtcm.c by J.Sager
An RTCM station can send out different data trama, for example one for station position one for corrections,
one or more for phase corrections...etc.
Tipically the message number one is for RTCM correction, the message number nine is for partial correction.
Here below (but better is if you look at J.Sager site ) you can see an
example of rtcm data format.

Word 1 (Common word messages)

Preamble | Message Type | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 bit Station I.D | Parity 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

This is the RTCM common word, every message start whit this word The first flag is the special header, 01100110, ( 0x66).

M	state change: WORD_SYNCING -> WORD_SYNC
M	state change: WORD_SYNC -> FRAME_SYNCING
M	state change: FRAME_SYNCING -> FULL_SYNC
H	18	330	2631.6	2	19	0
?	18
H	19	330	2631.6	3	19	0
?	19
H	18	330	2631.6	4	19	0
?	18
H	19	330	2631.6	5	19	0
?	19
H	1	330	2632.8	6	15	0
S	29	0	155	2632.8	-5.280	0.000
S	10	0	67	2632.8	-23.780	0.000
S	26	0	213	2632.8	-4.260	0.000
S	5	0	131	2632.8	-23.700	0.002
S	18	0	227	2632.8	-10.080	0.000
S	9	0	72	2632.8	-8.840	0.000
S	28	0	74	2632.8	-7.540	0.000
S	7	0	90	2632.8	-5.780	0.002
S	8	0	211	2632.8	-18.380	-0.002
H	18	330	2632.8	7	19	0
?	18
H	19	330	2632.8	0	19	0
?	19
H	18	330	2632.8	1	19	0
?	18
H	19	330	2632.8	2	19	0
?	19
H	1	330	2634.0	3	15	0

dgpsip -d output
Using the rtcm decoder you can see the station messages,the frequency, and postion.

H) Type of message (Ex. H1 ) S) Satellite number (1,2,....)

H 1 330 2632.8 6 15 0

H = Haeder message 1 = Message type 330= Station ID 2632.8 Modified Z-Count ( time reference in seconds) 6= Sequence packet number 15= Length 0=Option