NorthStar 2505 Installation Manual page 15

Understanding differential
Standard 2505 features
2505 INSTALLATION MANUAL, Revision A
need to see it, touch it, or send commands to it. By eliminating
virtually all of the attention needed to operate the 2505, you
can devote more time to other aspects of your work.
The 2505 receives differential WAAS corrections from
strategically located ground-based transmitters called beacons,
as well as from satellites transmitting WAAS differential signals.
The 2505's advanced beacon/WAAS engine provides greater
accuracy than GPS alone.
Beacon differential
The Global Positioning System (GPS) uses a system of 24
satellites (with three spares) operating in three orbital planes at
about 11,000 miles elevation. Position is determined by
measuring the time delay of the 1575-mHz signal from at least
three of the satellites, and computing position based on known
satellite locations. Beacon differential signal corrections are
generated by ground-based reference stations, at precisely
surveyed locations, and transmitted to the end user via
minimal shift keyed 300-kHz U.S. Coast Guard beacons
(ground-based).
WAAS differential
As with the Coast Guard radiobeacons, the Wide Area
Augmentation System (WAAS) satellites use surveyed reference
stations to monitor GPS signals from each visible satellite and
determine if errors exist, and then develop corrections.
However, unlike Coast Guard radiobeacons or other land-based
transmitters on other frequencies, WAAS uses a network of
geostationary communication satellite transponders, in fixed
positions at approximately twice the distance from the earth's
surface as the orbiting GPS satellites, on the same frequency as
GPS (1575 mHz). These satellite-based transmissions provide
consistent position corrections over a much greater contiguous
geographic location than the other radiobeacons. WAAS
differential corrections maintain a position accuracy of better
than 3 meters 2dRMS, without the signal interference problems
periodically experienced with low-frequency, shore-based
radiobeacon differential signals.
The 2505's standard features include the following:
•
12-channel, high-performance GPS engine receiver with
integral two-channel WAAS differential reception and
position processing
SECTION ONE Introducing the 2505
Page 5