** 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:

Here are several things that look like good ideas, but that turn out
on closer inspection to be not possible or not worth the effort.

** 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.  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.

** Speak SiRF binary protocol

It can't do anything interesting, for our purposes.  There's 
little point in upping the baud rate when we can't get updates above
1Hz -- even transmitting at infinite speed would only cut a half second
off the expected latency,  (OK, push-to-fix mode might be interesting
if they actually documented how to set it.)

** Try to autodetect USB devices

Sigh.  USB scanning won't work.

The fundamental problem is that there is no GPS device class in the
USB standard.  When you get device information about a GPS from the 
USB subsystem, all you get is info on the USB-to-serial chipset it's
using.

This means that GPSes are not distinguishable from other USB-to-serial
devices, in particular USB-to-RS232C adaptors.  This is a crash
landing.  We could live with the scanning code failing to detect a
GPS that's there, but we can't live with having it mis-identify
another USB device as a GPS.  The least bad result from that would 
be that the daemon opens and locks up the other device.

This cannot be fixed, short of USB 1.1 growing a USB device type
and vendor firmare advertising it.