Geoscience Reference
In-Depth Information
Nationwide DGPS
Operating Sites
Planned Sites
Single Coverage
Multiple Coverage
October 2007
fIGURe 9.13
U.S. differential Global Positioning System (DGPS) coverage. (Courtesy of DGPS System
Management Branch, USCG Navigation Center; http://www.navcen.uscg.gov/dgps/coverage/CurrentCover-
age.htm.)
correction data to a GPS receiver. The actual format is rather complex and lengthy, but it generally con-
tains the following information: (1) the time of the measurement at the reference station, (2) observed
range errors (corrections) for every satellite in view at that reference station, and (3) the range error rate
for every satellite in view. RTCM SC104 version 2.0 (RTCM-2.0) format essentially deals with DGPS
(code-only) corrections, and RTCM-2.1 incorporates several enhancements, especially for PDGPS (car-
rier phase) corrections (www.ccg-gcc.gc.ca/dgps/format_e.htm).
The standard message types within RTCM-2.1 are message types 3, 5, 6, 7, 9, 15, and 16. A type
3 message contains information on the identity and surveyed position of the active reference DGPS
station. Message type 5 notifies the user equipment suite that a satellite deemed unhealthy by its
current navigation message is usable for DGPS navigation. The type 7 message provides informa-
tion of its broadcasting DGPS station and the other two or three adjacent DGPS stations. The type 6
message is a filler message used only when the reference station has no other message to broadcast.
The type 9 message has been selected for broadcasting pseudorange corrections. Two methods of
transmitting the type 9 message are possible. The first method of broadcasting GPS pseudorange
corrections is based upon “Three-Satellite Type 9 Messages,” denoted as “type 9-3” messages. In
this method, corrections for all satellites are assigned to either three satellite type 9 messages or
to a remainder message of either one or two satellites. The transmission rate could be at either 100
or 200 bps. The second method of broadcasting pseudorange corrections is to broadcast individual
type 9 messages for each satellite at a transmission rate of 50 bps. This message is referred to as the
“Single Satellite Type 9 Message,” denoted as the type 9-1 message. The type 15 message includes
atmospheric parameters. The type 16 message can be utilized as a timely supplement to the notice
