Hmmm... the specs don't say. but anything else that I've seen have suggested that NULL is used when there is no data. That could throw a spanner in the works though... skip() would keep going through the following parameters. (geoidal separation, differntial data age and differential station id), but all of those should also be null.

Could be related to casting somehow? I noticed that gpsapp uses strtod for the conversion, and that gps_state.alt is a double, but draw_sat off-loads the display to formatdist that expects an unsigned int