[aprssig] WAAS and DGPS

Jim Lux jimlux at earthlink.net
Sat Mar 19 13:03:00 CST 2005

Earl wrote:
I keep wondering why not simply turn WAAS on?  WAAS is available
on almost all of today's GPSR's.

         7 3

Earl, you're not thinking like a ham.<grin>  Buy a new <$100 receiver?  No
way... .. you want to use that surplus L-band receiver (vacuum tube, of
course) you bought in 1955 at a hamfest where it was being sold as scrap and
have been keeping in your garage for a project just like this.  Sure, you'll
need a networked cluster of 15 or 20 80286 based PCs running DOS 3.1 to
reduce the observables to a position, but hey, you got the computers for

 But realistically, I don't know whether WAAS has the coverage and precision
comparable to good DGPS using RTCM corrections. I also don't know if we even
need that sort of accuracy in the APRS context.  I can think of situations
where it might be nice, but if APRS is used to provide a "at a glance"
tactical situational awareness, centimeter accuracy is probably not needed
(where is that runner on the Baker to Vegas race?).  If a ham was doing
centimeter accuracy surveying, then perhaps APRS isn't the optimum
distribution link for the data, and they could do it on some other channel.
Likewise, if you were using DGPS for navigating your model airplane, APRS
probably wouldn't be the optimum link.

However, IF you did want to use APRS for the correction messages, and you
wanted to minimize the new equipment required for hams that already own
trackers, etc., the RTCM-104 technique would be useful. It's been around
longer than WAAS, and more GPSes that are "currently fielded"  support it
(although it's unclear whether any TNC, not connected to a PC, supports it
in a useful way)

Jim, W6RMK

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