** Do the research to figure out just what the heck is going on with status bits

NMEA actually has *four* kinds of validity bits: Mode, Status, the
Active/Void bit (some sources interpret 'V' as 'Navigation receiver
warning'), and in later versions the FAA indicator mode.  Sentences
that have an Active/Void send V when there is no fix, so the position
data is no good.

Let's look at which sentences send what:

                GPRMC     GPGLL     GPGGA     GPGSA
Returns fix      Yes       Yes       Yes        No
Returns status   No        Yes       Yes        No
Returns mode     No        No        No         Yes
Returns A/V      Yes       Yes       No         No

In addition, some sentences use empty fields to signify invalid data.

My first conclusion from looking at this table is that the designers
of NMEA 0183 should be hung for galloping incompetence.  But never mind that.
What are we to make of this mess?

The fact that the FV18 sends GPMRC/GPGLL/GPGGA but not GPGSA
argues that GPGSA is optional.  I don't see how it can be, since it
seems to be the only status bit that applies to altitude.  Just how are
we supposed to know when altitude is valid if it doesn't ship GSA?  
Can a receiver ever ship a non-empty but invalid altitude?

Which of these override which other bits?  I don't think status is ever
nonzero when mode is zero. So status overrides mode.  What other such
relationships are there?

* Things not to do:

** Try to crank the update rate up past 1 per second

NMEA doesn't give us control of the update rate, and SiRF/Zodiac chips
don't seem to be able to set cycle times below once per second even in
binary mode.  (Turns out the reason is that a full NMEA fix report
takes about a second at 4800bps.) Even on a chipset that permitted it,
at 50km/h (31mi/h) that's only 13.8 meters change in position between
updates.  Faster refresh might make sense for aviation applications,
but not on foot or in a car.