On Sat, 25 Apr 2009 17:28:51 +0100, soaringtheskies wrote:

I have a Cambridge LNAV and I want to replace my LX400, which is
currently being used purely as a GPS receiver, with something else to
provide a GPS feed to the LNAV, preferably without taking up a space on
my instrument panel.

What NMEA sentences does it need?

Why does it need a GPS? Just for wind calcs or do you want it to do more?

The NMEA sentence information will hopefully be in its manual. Knowing
that will help you select a suitable GPS.

I just went though a similar exercise when I replaced my Garmin GPS II+
with a Binatone PNA running XCSoar. This has no way to output GPS data.

Anyway, I wanted my SDI C4 vario to still calculate wind, so I connected
a blind GPS that I hid behind the panel: I have room on my panel for the
GPS II+ OR the PNA and no spare slots. Like all blind GPS receivers mine
only outputs GPRMC sentences, which is the same as a GPS II+ does without
an active waypoint or task. However, the C4 also needs destination
sentences (GPRMB, GPWPL and GPR00) to calculate wind, so all the blind
GPS is doing right now is draining my battery.

The point being, that, if the LNAV needs destination data to calculate
wind, you'll not be able to use a blind GPS. You'll need one that you can
enter a waypoint or a task into, so it will probably occupy panel space.

Your LNAV may be the same. If you can't find anything in the manual for
the LNAV or the LX400, and nobody else on r.a.s knows what sentences it
needs, there are a couple of other ways to find out:

- connect the LX400 to a laptop with a serial port set to 4800 baud,
1 stop bit and 8 data bits/no parity and run any program that can
display or capture the data coming down the serial line.

Alternatively, borrow a datascope, and use that to show you what
the LX400 outputs.

- Another way is to borrow a Garmin GPS II+ or a 12XL, connect it to
the LNAV. If the LNAV is like my C4, it will not calculate wind until
you put a waypoint into the GPS.

Of course, there is another way: accept that the LNAV will become a
simple TE compensated averaging vario, buy a PNA and run XCSoar on it.
XCSoar is both a moving map navigation system and a glide computer. Its a
cheap solution: XCSoar is free Open Source software and will run on any
PNA with an SD card slot and Windows Mobile 5. PNAs are inexpensive: my
Binatone X.350 cost £50 (about $US 75) plus £5 ($US 7.50) for a 2GB SD
card.

HTH


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
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