GPSy now supports DGPS-IP servers through the Internet. John Sager has been kind enough to license us the code for his RTCM decoder, this allows you to monitor the output of the DGPS-IP server through the DGPS Data Monitor Window.
Some notes about DGPS-IP service:
* Make sure your GPS unit is set to NMEA-Out / RTCM-In serial communications mode
* DGPS corrections are sent out at the same serial BPS as the NMEA-in
* Choose the DGPS server closest to you. There is one on the West Coast (California)
and one on the East Coast (New Jersey). Hopefully more will be added in the future.
If you'd like to host your own DGPS-IP server or for more information, see:
http://www.wsrcc.com/wolfgang/gps/dgps-ip.html
* DGPS-IP clients must report statistical data back to DGPS-IP servers. This includes
a rough approximation of your current location. If you don't like this behavior,
don't use the DGPS-IP service. The reports show up in the DGPS Data Monitor
window as "R 45.0 -72.3 ..." entries where 45.0/-72.3 are your approximate
coordinates.
* Just because you now have DGPS service doesn't mean that your GPS unit
can't be confused by multipath or erroneous signals and give you a false position.
Caution, as always, is warranted and always double-check any navigational aid
against known positions.
* Other DGPS entries are from the RTCM decoder engine, below. We have not
completed testing the code port, so please do not trust the RTCM decoder
output in at-risk situations.
Karen Nakamura
Global Mapping Systems
http://www.gpsy.com
------------
Notes on the Printout Format of the RTCM Decoder.
Written by John Sager john.sager@btinternet.com
Copyright (C) 1999 John C Sager
History:
v0.2 9th February 1999
v0.1 5th February 1999
The RTCM decoder prints a legible representation of the input data.
The RTCM SC-104 specification is copyrighted, so I cannot
quote it - in fact, I have never read it! Most of the information
used to develop the decoder came from publication ITU-R M.823.
This is a specification of the data transmitted from LF DGPS
beacons in the 300kHz band. M.823 contains most of those parts of
RTCM SC-104 directly relevant to the air interface (there
are one or two annoying and vital omissions!). Information
about the serial interface format was gleaned from studying
the output of a beacon receiver test program made available on
Starlink's website.
Air Interface Data Format
-------------------------
This consists of a sequence of 30-bit words. The 24 most significant
bits are data and the six least significant bits are parity. The
parity algorithm used is the same as that used for GPS data
transmitted by the satellites.
Each DGPS message consists of two header words followed by zero or
more data words, depending upon message type.
Printout Format
---------------
In the printout each header is printed, followed by zero or more
lines containing the specific data for that message. The general
format is a line beginning with a capital letter, followed by a tab,
followed by the fields of the message separated by tabs, terminated by
a newline.
As well as data the decoder also prints decoder status messages,
as necessary.
Header (H)
----------
format:
H <message type> <reference station id> <modified z_count> <sequence no>