Leica GPS1200 Series Technical Reference Manual page 1351

Null fields
NMEA Message Formats
Symbol Field
No symbol Information
unavailable for
output
Fields are always separated by a comma. Before the Checksum field there is never a
comma.
When information for a field is not available, the position in the data string is empty.
Description
Null fields do not contain any information at
all.
GPS1200
Example
,,
1351