GPS provides a lot of geographical information for a particular object like its latitude, longitude, direction of travel, GMT etc. This information are assembled in a particular string format which are to be decoded by GPS modems. A GPS modem gives the output data in a following string format called as NMEA Format. A common GPS sentence ($GPGGA) has been explained below.
1. A string always starts with a ‘$’ sign
2. GPGGA : Global Positioning System Fix Data
3. ‘,’ Comma indicates the separation between two values
4. 100156.000 : GMT time as 10(hr):01(min):56(sec):000(ms)
5. 2650.9416,N: Latitude 26(degree) 50(minutes) 9416(sec) North
6. 07547.8441,E: Longitude 075(degree) 47(minutes) 8441(sec) East
7. 1 : Fix Quantity 0= invalid data, 1= valid data, 2=DGPS fix
8. 08 : Number of satellites currently viewed.
9. 1.0: HDOP
10. 442.8,M : Altitude (Height above sea level in meter)
11. -42.5,M : Geoids height
12. __ , DGPS data
13. 0000 : DGPS data
14. *71 : checksum
The main objective here is to find the location of the GPS Receiver in terms of latitude and longitude. The GPS module gives output data in RS232 logic level format. To convert the RS232 logic level into TTL, a line converter MAX232 has been connected between GPS module and PIC18F4550. (Also refer PIC USART) The circuit connection of GPS module with microcontroller is shown in the circuit diagram tab. The latitude and longitude data has been displayed on a 16x2 LCD interfaced to PIC.
2. Enable the SPEN and CREN bits (RCSTA register).
3. Receive the Serial data and compare with the string ‘$GPGGA,’ byte by byte.
4. Wait for comma (,) as string gets matched.
5. Store the data which appears after the above comma into a string which will be the Latitude.
6. After another comma (,), store the data into another string which will be the Longitude.
7. Display both Latitude and Longitude data on LCD.
8. Repeat the steps 3 to 7 to update the GPS module’s positions on LCD.