NorthStar 2201 Installation Manual page 16

SECTION ONE Introduction
Acquisition mode
Navigation mode
Page 8
The 2201 enters acquisition mode whenever sufficient
real-time satellite data is available to produce an initial
navigation solution. Acquisition mode can be entered from
initialization, navigation or dead-reckoning mode. The 2201
exits either to navigation mode when satellites have been
acquired or to fault mode if a problem is detected.
After each satellite in view is acquired, the 2201 collects new
ephemeris data by decoding the satellite downlink data
message. When a sufficient number of satellites are being
tracked, position, velocity and time are computed, and the 2201
enters navigation mode. If the 2201 can't acquire signals
normally due to an absence of valid almanac data, it initiates a
"Search the Sky" acquisition, attempting to acquire any and all
satellites in the GPS constellation. After a satellite has been
acquired, ephemeris data is decoded from the satellite's
down-link message. In "Search the Sky," the Time-To-First-Fix
(TTFF) is typically less than two minutes. After a sufficient
number of satellites have been acquired, the 2201 enters
navigation mode. For TTFF details, see Table 1, "2201 and
2201/NMEA technical specifications," on page 31.
The 2201 operates in navigation mode whenever sufficient
satellite information and measurement data are available to
produce a GPS fix. Navigation mode is entered from either
acquisition or dead-reckoning mode, and exits to
dead-reckoning or fault mode.
Navigation mode has three sub-modes (the 2201 automatically
chooses the highest-performance sub-mode available for the
satellite data currently being received, and reports the current
navigation sub-mode to its host port):
•
differential 3-D
•
differential 2-D
•
3-D nav
Differential 3-D mode
The 2201 operates in differential 3-D mode when it receives
data from at least four satellites with adequate geometry and
differential corrections, from either WAAS satellites or
radiobeacons, via SC104 data supplied on the AUX INPUT port.
This is the navigation mode with the highest performance.
2201 INSTALLATION MANUAL, Revision B