For all of you GPS and navigation-noids I
thought I might repost several of the important sites regarding the RTCM
real-time and RINEX post-processing differential corrections systems out
there.
First, you have the Differential corrections as provided by
our US Coast Guard. To access these radio broadcast corrections you'll
need to be under their coverage - essentially the coast lines and central and
navigable river basins of the Mississippi. There is a
National plan to have these type of navigation beacons operational
everywhere. When? Good question.
There are also a number or Army Corps of Engineers
stations operating to the USCG radio standards as well but since the USCG doe
not maintain them they essentially don't know about them (much). For
either system the frequency of update is between once every six to ten seconds
and depending on base leg from the receiver should give accuracy of a meter or
two. Base latency is good although the frequency of update over eight
seconds contributes to degraded accuracy performance. As the broadcasts
are in the AM their clarity are frequently damaged by summer electric
storms. Receivers should be dual channel allowing for automatic switching
if disrupted. Cost ranges from $400 to $900. Someday you'll be able
to buy a $45 receiver form radio Shack but not today.
Next you have the CORS system which is a RINEX
differential recording system. RINEX is a post processing method needing
so call raw data (pusedoranges) to be logged at the remote site and at your
rover. Not all GPS reveivers have this feature and those that do need to have
their raw formats translated to RINEX standards.
The CORS (Continious Operational Recording Site)
network is a grouping of many sites from many sources, both federal and
state. Most are stations put up by State Departments of Transportation and
the like. The CORS RINEX data can be down loaded from the NET. Its
another one of those free(ed) data sites that we all crave.
The receivers are generally very high accuracy
but the CORS network only requires the recording of the puedoranges in RINEX
formats one every ten minutes or so. Individual sites in many cases can be
directly accessed for even higher accuracy if you know what you are doing and
your equipment matches the raw data recorded. Survey grade GPS (under 0.1
meter) from some of the CORS can had without the expensive investment in
kinematic base equipment in this case. It needs the direct connection/download
to and from their local data set. These files can be really large
therefore you will not find them in the CORS network directly.
Depending on several things the CORS accuracy
can be quite good on the order of 0.7 to 2.0 meters. Latency with RINEX is
essentially nill.
And for you real accuracy nuts who constantly are bugged by
maps and photos that never quite line up, you can work through the Goddard
center where they trace out continental drift (when all else fails). I
understand Mt Everest is rising about 20 cm per year and is moving towards the
north east at like 20 meters per year!
For what it is worth,
MidNight Mapper
aka Neil
3.23.99
