Actually the SV6 does support almanac and ephemeris exchange with the attached device but not via NMEA. The SV6 from BGMicro was an OEM product that has a limited subset of NMEA implemented with lots of fail-safe provisions, probably used for vehicle tracking or ?? The NMEA mode in these SV6s doesn't allow much, if any configuration.

If you reflash it to TSIP you get a much richer interface that allows upload and downlod of ephemeris, almanac, initial position and time along with customizing many other of its modes of operation. For example, you can set it to provide an overdetermined solution where it will use more than 4 SV to improve the precision of the nav solution. You can also set it to track the highest 8 SV or the highest 6, change the mask parameters, dynamics filters, operate in MSL or geoidal height modes, etc. It is a real luxury for Trimble GPS users that Jan has included the TSIP parser in GPSapp.

NMEA, per se, doesn't allow almanac or ephemeris interchange. Individual manufacturers have defined proprietary NMEA-like sentences that are then useable for expanded functions. Unfortunately, these are different for each manufacturer's GPS so the interface to use them gets almost as complex as providing for the various binary protocols.

NMEA was originally intended to interconnect marine electronics such as speed logs, compasses, auto pilots, LORAN etc. GPS came later and was added as the $GPxxx series along with a provision for manufacturers proprietary messages in the $Pyyyx series, where YYY denotes the manufacturer (such as GRM for Garmin).

When all is said and done the almanac/ephemeris functions really belong in the GPS. If a GPS receiver (designed for navigation use) doesn't store these then it is most likely a hardware fault (bad battery or "Gold Cap" or BBRAM). I don't know of any decent nav receiver that doesn't store this data by design. Dependency on an external device is devistating to performance.