AvMap EKPIV series - Manual (Page 47)

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the reflections of signals that have bounced off of surrounding objects.)
Using DGPS to eliminate the effects of correctable errors requires that the user’s GPS receiver be
connected to a compatible differential beacon receiver and be within range of the broadcasting
beacon. The differential beacon receiver accepts and demodulates the broadcast corrections, which
are then relayed to the GPS receiver. The GPS receiver applies the corrections to the navigation
data it uses to compute a position solution and displays differentially corrected data. Care must be
taken to ensure that the DGPS receiver and the GPS receiver are compatible for this procedure to
be successful.
7.3.4 Monitoring & Controlling the GPS
The Global Positioning System is monitored and controlled by the U.S. Air Force, which is responsible
for updating and maintaining exact satellite position and signal data accuracy.
It is also responsible for performing maintenance on the satellites, which may require the satellite
being taken off-line. Since the system is subject to periodic updates and changes, the almanac data
broadcast by the satellites is current for a limited time — generally about 14 days.
7.3.5 GPS Information Sources
The needs of the worldwide civil GPS user community are served by the Civil GPS Information
Center (GPSIC) in Virginia. The GPSIC is operated and maintained by the U.S. Coast Guard for the
Department of Homeland Security.
Its primary function is to provide information on the Global Positioning System and satellite status
and to serve as a point of contact.
The GPSIC has general GPS literature available free upon request. The Center also maintains up-
to-date almanac data and Operational Advisory Broadcasts containing current constellation status
and planned satellite outages.
There are three ways to quickly obtain current information about the Global Positioning System
from the GPSIC:
1 . Recorded phone message at (703) 313-5907
2 . WebSite: www.navcen.uscg.mil
3 . Live information at phone (703) 313-5900.
There are several GPS bulletin board services outside the United States, including:
South Africa +7 (1) 803 8318
Germany +49 (69) 6333 418 Internet via TELNET: 141.74.40.5
German X.5 Network, Datex-P (from Sprintnet preferred): 45050670
norway +47 314045 or +47 3118369
United kingdom +44 (60) 4111 full access is available only to members of the UK Civil
Satnav Group
There are other sources for GPS information, ranging from free, governmentally produced literature
to purchased professional literature and seminars. The geography department of your local college
or the local office of the National Geodetic Survey may be able to help.