01 Oct 1998
ADDITIONAL INFORMATION ON USCG DGPS RADIOBEACON SYSTEM
A-1. Site Set-up and Configuration.
a. Equipment. Each USCG radiobeacon site consists of two
GPS L1/L2 geodetic receivers (as reference station receivers)
with independent geodetic antennas to provide redundancy and a
Marine Radiobeacon transmitter with transmitting antenna. The
site is also equipped with two combined L1 GPS / Modulation Shift
Key (MSK) receivers which are used as integrity monitors. Each
combined receiver utilizes an independent GPS antenna and a MSK
near-field passive loop antenna.
b. Site Location. The location of the reference station
GPS antennas are known control points within the North American
Datum of 1983 (NAD83) and International Terrestrial Reference
Frame (ITRF). The geodetic coordinates for these positions were
determined by NGS. DGPS corrections are based on measurements
made by the reference receiver and the NAD83 known antenna
coordinates. These corrections are then transmitted via a marine
radiobeacon to all users having the necessary equipment.
c. Data Transmission (data types). The corrections are
transmitted using the Type 9-3 (three satellite corrections)
message of the Radio Technical Commission for Maritime Services
Special Committee 104 (RTCM SC-104) version 2.1 data format.
Other RTCM SC-104 message types transmitted to the user include
Type 3 (contains the NAD83 coordinates for the broadcast site),
Type 5 (provides information if a GPS satellite is deemed
unhealthy), Type 7 (information on adjacent radiobeacons), and
Type 16 (alerts the user of any outages). More detailed
descriptions of these message types are explained in the
Broadcast Standard for the USCG DGPS Navigation Service,
COMDTINST M16577.1, April 1993 which can be downloaded from the
USCG Navigation Center (NAVCEN) web site (www.navcen.uscg.mil).
(1) Corrections are generated for a maximum of nine
satellites tracked by the reference station GPS receiver at an
elevation angle of 7.5 degrees or higher above the horizon.
Satellites below a 7.5 degree elevation mask are highly
susceptible to multipath and spatial decorrelation. If there are
more than nine satellites observed at the reference station above
7.5 degrees, than the corrections broadcast are based on the nine
satellites with the highest elevation angle.
(2) The sites transmit these corrections at a 100 or 200
baud rate. Since a type 9-3 message is 210 bits (includes header
information and corrections for three satellites), the latency of