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Motorola Clock Output Format

Lines starting with two numbers and an IP-address (usually 127.127.20.0) are produced by the GPS receiver. They give the current status of the GPS receiver and are produced about once every 64 seconds. The numbers and address are followed by a keyword ($GPxyz) and several data fields separated by commas. The most important keywords are:

  • $GPGSA,A,B,xx,xx,xx,xx,xx,xx,xx,xx,xx,xx,xx,xx,x.x,x.x,x.x
    Active Satellite info:
    1. If B>1, then the receiver can see enough satellites in order to determine its position.
    2. xx are the identifiers of the satellites that can be seen by the receiver, blank if the receiver does not see a satellite.
  • $GPGSV,N,n,M,a1,b1,c1,d1,a2,b2,c2,d2,a3,b3,c3,d3,a4,b4,c4,d4
    Satellite information:
    1. N: number of messages (1 to 3)
    2. n: message id (1 to 3)
    3. M: Number of satellites in view (0..24)
    4. ai: Satellite id
    5. bi: Elevation
    6. ci: Azimuth
    7. di: Signal to Noise Ratio
  • $GPZDA,HHMMSS.00,DD,MM,YYYY,0,0
    1. HHMMDD is the current (UTC) time
    2. DD, MM, YYYY the date
  • $GPGGA,HHMMSS.00,Lat,N,Long,E,..
    1. HHMMDD is the current (UTC) time
    2. Lat is the lattitude of the receiver, followed by an N or S.
    3. Long is the longitude, followed by an E or W.

This information is primairly intended for optimizing the antenna position and debugging GPS problems.

The most important string is $GPGSA. If you change the antenna position, the number of satellites should go up. For example, with an antenna inside our office, we saw:

  • 50966 46214.117 127.127.20.0 $GPGSA,A,1,,03,,,21,,,,,,,,,,*1E

and the receiver wasn't able to determine its position. Moving the antenna outside changed this into:

  • 50966 47110.148 127.127.20.0 $GPGSA,A,3,15,03,,,21,,,31,,,,,6.0,4.0,4.4*36

e.g. two more satellites.

The $GPGSV string can be used to optimize the antenna position even further: if the number of satellites does not change, but the signal to noise ratio goes up, then the position has been improved.

From the last two strings ($GPZDA and $GPGGA), you can determine your local position and time.