GPS data (NMEA sentences and PPS sync pulse) over optic fiber

DECODER MODULE

In seismic stations we usually install digitizers taking time from internal GPS receiver through external GPS antenna.

GPS antenna is usually placed some meters away from digitizer, cable could be 3 to 5 meters (usually) but sometimes it can be longer when using low-loss RF cable.

When installing free-field seismic station we have not much problems due to GPS cable length but what about installing seismic station deep into mining compound?

In this case we can use digitizers with external GPS function, where GPS data is sent usually through RS-232 serial line ant there are two separate lines (one for NMEA strings and one for PPS sync pulse).

One solution could be GPS external GPS with RS-232 output and a pair RS232-RS-485 converter:

but this solution is not optimal when using long copper cables during summer rainstorms.

Seismic digitizers usually work with following NMEA sentences (4800-8-N-1):

$GPGGA,,4819.7681,N,00819.3761,E,0,02,,665.9,M,,M,,*72

$GPGSA,A,1,,,,11,,,28,,,,,,,,*14

$GPGSV,3,1,10,01,31,128,,03,33,158,,09,00,029,,11,42,284,45*76

$GPGSV,3,2,10,14,38,092,,19,66,168,,20,12,225,,22,34,054,*79

$GPGSV,3,3,10,28,19,314,38,32,65,069,*7A

$GPZDA,110959.00,27,02,2008,00,00*6E

and we can take a look to signals timing:

NMEA strings and PPS sync pulse should not overlap, so we can encode both signals into a single output signal:

We can send this encoded signal over optic fiber using single core fiber, then demux encoded signals to supply correct data to seismic digitizer.

Encoder block diagram

Decoder block diagram

http://crsengineering.altervista.org/eng/OGS/GPS/index.1.jpg