ashtech DG16 Reference Manual page 264

Example
Enter the following command to enable GSN message on port B.
$PASHS,NME,GSN,B,ON
$PASHQ,GSN,[c1]
This command queries the GSN message, where c1 is the optional port designator
for the output of the response. If a port is not specified, the receiver sends the
response to the current port.
$GPGSN
This message is not output until the receiver has locked on at least one satellite.
The first data field in the GSN message contains the number of satellites currently
being used to compute positions, followed by two fields containing a satellite PRN
number and the measured (dbHz) signal to noise ratio for that satellite. The last
field before the checksum contains the age of correction for differential GPS.
Generally speaking, the receiver will be able to lock more quickly, and will be better
able to maintain lock, on satellites whose signals register higher signal to noise
values. The response is output in the format:
$GPGSN,d1,((d2,d3)*d1),d4*hh
The fields containing the PRN number (d2) and the signal to noise ratio (d3) are
repeated for each locked satellite, with the d1 value used as a multiplier.
Parameter
d1 Number of satellites currently locked
d2 Satellite PRN number
d3 Signal to noise ratio for the corresponding satellite
d4 Age of differential correction. The number 999 is registered in
this field when the DG16 is set in differential rover mode but is
not receiving corrections or when differential GPS is disabled.
*hh Checksum
Typical GSN message:
$GPGSN,07,01,048,29,046,25,048,08,040,31,037,15,048,21,046,
999*4F
244
Table 8.16. $GPGSN Message Format
Description
DG14 and DG16 Board & Sensor Reference Manual
Range
0 to 14
01 to 32 for GPS
33 to 64 for SBAS
000 to 999
1 to 999
2-character hex