CAI302 altitude in NMEA..

On Oct 22, 11:29*am, Darryl Ramm wrote:
On Oct 22, 5:40*am, "PCool" wrote:

Hello,
* * * * * * I am adding some missing CAI302 sentences to the LK8000 free software I am developing, and from Cambridge specs I can't understand what is the difference between "true altitude" sent from a !w,xxx,xxx,xx etc * sentence and the $PCAID barometer altitude.

I understood that the CAI302 does not send !w sentences, but I guess that the 302A sends both. So if they are the same I am very happy, otherwise I need to know the difference.
*Any help will be appreciated!

Paolo

Paolo

Maybe you swapped the order of what you intended to say, but clearly
the 302 will send both !w and $PCAID.

I don't own a 302A and have never played with one but as others have
mentioned here before the 302A likely does not send !w messages, that
makes most sense to me but again I'd want to see the output myself.

The language in the dataport user's guide would have me believe that
$PCIAD reports pressure altitude (i.e. "barometer altitude") and the
w! "true altitude" is pressure altitude corrected for QNH (not much
else it can be, well they could invoke ellipsoid relative heights etc.
but unlikely). Since !w also reports the QNH you can easily check
this. And hey preto searching the ras archives turns up a post by Paul
Remde with a fragment of output from a 302...

!w,000,000,0000,500,01287,01020,-0668,191,199,191,000,000,100*44
$GPRMC,163805,V,4448.5614,N,09333.0117,W,000.0,000 .0,260304,002.1,E*7D
$GPRMB,V,,,Stanton * * ,,4308.8388,N,09300.9833,W,,,*47
$GPGGA,163805,4448.5614,N,09333.0117,W,0,00,00.0,0 .0,M,0.0,M,,*5F
$PCAIB,00280,00067
$PCAID,N,+00224,000,128*05

So here *$PCAID shows an altitude of 224 meters and !w shows an
altitude of 1,000 meters + 287 meters and a QNH setting of 1020 mB.

Near sea level 1mB ~ 8.6m. So 1013-1020 = -7*8.6 = -60m altimeter
pressure correction. And 286-60 = 226m, close enough to 224m for me.

Of course I'm assuming the sample Paul quoted was real. The $GPRMC
sentence above decodes to a location of 44 48.5614' N, 93 33.0117'W
which is near Paul's home/office but $GPGGA shows the fix is not
valid. And the different reported speeds above are either zero or
invalid so I'm guessing the C302 is sitting inside on a desk at Paul's
for this test. And convineintly his office address altitude according
to Google Earth is 283 meters.

Darryl

Just for comparison: from a 302A

$GPRMC,014040,A,3609.0517,N,11505.0920,W,
000.0,000.0,170909,013.7,E*62
$GPRMB,A,0.0,R,*Jean ,,3546.2200,N,11519.7000,W,
025.7,207.4,V*2D
$GPGGA,014040,3609.0517,N,11505.0920,W,1,00,29.1,5 55.3,M,-25.1,M,,*42
$PCAIB,00863,00067
$PCAID,N,+00544,000,064*0D