NorthStar 2505 Installation Manual page 17

2505 INSTALLATION MANUAL, Revision A
EEPROM to ensure accuracy. After initialization, the 2505
enters acquisition mode.
Acquisition mode
The 2505 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 2505
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 2505 collects new
ephemeris data by decoding the satellite downlink data
message. When a sufficient number of satellites are being
tracked, then position, velocity and time are computed, and the
2505 enters navigation mode. If the 2505 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
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 2505 enters
navigation mode. For TTFF details, see Table 4, "2505 technical
specifications," on page 19.
Navigation mode
The 2505 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: differential 3-D,
differential 2-D, and 3-D nav:
• differential 3-D
•
differential 2-D
•
3-D nav
The 2505 automatically chooses the highest-performance
sub-mode available for the satellite data currently being
received, and reports to its host port the current navigation
sub-mode.
SECTION ONE Introducing the 2505
Page 7