[aprssig] Differential GPS?

Steve Dimse steve at dimse.com
Mon Apr 20 23:19:29 EDT 2009

My recollection is we added this when Steve Bible of TAPR produced his  
differential GPS generator, it cost maybe $300 including the GPS. That  
project never got traction for the channel load issue you mention, and  
then Garmin killed the GPS it depended upon which killed the project.   
It sure was a cool thing though. This was very different from the boat  
store models. Boat store DGPS receivers depend on a site which  
determines the GPS errors and transmits them on subcarriers of  
broadcast FM stations or other transmitters. The TAPR kit essentially  
created your own DGPS station, which even today is very expensive.

So yes, given few people bought these kits, that they are permanently  
unavailable, that no one seems to be using them now, and that the  
shutoff of SA and adoption of WAAS has all but eliminated the need,  
I'd say it is OK to remove the code.

Steve K4HG

On Apr 20, 2009, at 8:08 PM, Robert Bruninga wrote:

> Does anyone do differential GPS in APRS?
> There is a provision in APRS that will allows someone to connect
> a TNC to a GPS differrential receiver and then transmit those
> corrections to a mobile on the APRS channel.  We experimented
> with it in the early days, but I just don't see that it is
> something we need to continue to support.
> Reason being that we found  you needed frequent DGPS
> transmissions to have any value, and the APRS channel could not
> handle a DGPS transmission every 15 seconds just for one or two
> people to use it.  Yet, if you reduced the DGPS rate to say once
> a minute, then the DGPS data was so stagnant, that there was
> little value added.
> Of course it was a benefit back when selective availablility was
> on, but now that the US government leaves it off, we don't need
> DGPS (integrated into APRS).  Also, back then the DGPS receier
> cost maybe $50,000.  Now, I think anyone can buy one at the boat
> store?  And if he needs it, he feeds it directly into his own
> GPS.
> Can we remove the DGPS-on-APRS-channel function it to save code?
> Bob, WB4APR
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