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[aprssig] Re: Help for course/speed/altitude etc HF APRS

Stephen H. Smith wa8lmf2 at aol.com
Sun Apr 29 15:19:54 UTC 2007


Curt Mills wrote:
> On Sun, 29 Apr 2007, Jack Chomley wrote:
>
>   
>> I should have looked in the manual further :-) Looks like I can run the TM1+
>> in NMEA mode and simply select a transmit timer value, so it does not continue
>> to broadcast EVERY sentence the TM1+ gets from the GPS.
>>     
>
> Yea but:  Check out the length of a GPRMC or GPGGA sentence compared
> to the length of a normal APRS sentence.  Now check any of those
> against Base-91 Compressed or Mic-E Compressed sentences.
>
> FWIW:  The success rate of your packets getting decoded by other
> stations depends greatly on the length of those packets.  The
> shorter your packets are, the more will be received correctly.
>
> NMEA sentences over the air are nearly the worst packets you can
> send, length-wise.  I think the only ones that are worse are nice
> long weather packets, or any packet with extensive comments at the
> end.
>
> If the TM1+ can do Base-91 Compressed or Mic-E packets, select that
> instead.  If they can do Standard APRS packets instead of GPGGA or
> GPRMC packets, you still win.
>
> Y


The problem is that in raw NMEA mode,  the TigerTrak can only send ONE 
selected NMEA sentence.  So again, you either send the one with 
velocity/heading only  ($GPRMC)    --or-- you send the one with altitude 
only ($GPGGA).  

The TigerTrak can only send standard APRS-format posits parsed from the 
$GPRMC GPS sentence (speed but no altitude data) or raw NMEA (only one 
sentence at a time selectable).    Thus , there is no way to send both 
speed/heading and altitude at the same time with this device.  

I have used the TigerTrak for years for 30M HF APRS because it would 
operate at 300 baud, but was very frustrated that it wouldn't do any 
kind of compressed formats.   When the TinyTrak III came out that could 
do 300 baud and do Mic-E format with altitude by parsing both GPRMC and 
GPGGA, I started using that instead.   

In fact, the two switch-selectable "modes" of the TinyTrack III can each 
be set to different baud rates. One side can be set to run at 1200 baud 
for VHF and the other side set to 300 baud for HF.   This is very 
convenient for "DC-to-light" HF/VHF/UHF rigs like the Yaesu FT-100 or 
FT-817 or Icom 706 that might be used on either 2M or 30 APRS.

The success rate when transmitting the very short Mic-E packets over the 
noisy, interference-prone , fading HF paths is noticeably higher than 
for the longer plain-text standard APRS format.    (I would would 
monitor myself off the internet while triggering HF transmissions from 
the car, and count the number of transmissions that actually got 
successfully igated from HF to the Internet.)



--

Stephen H. Smith    wa8lmf (at) aol.com
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