Novatel SUPERSTAR II OM-20000077 - Manual (Page 32)

32 SUPERSTAR II User Manual Rev 3
Chapter 4 Operation
The receiver switches between modes automatically. The receiver reports on its host port the current operating
and navigation modes.
1. Self-Test Mode
The receiver enters Self-Test mode upon request from an external source (please refer to Message
ID# 51, Initiated BIT in the L1 GPS Firmware Reference Manual). The time duration spent in the
Self-Test mode is no more than 15 seconds. On self-test completion, the receiver reports the BIT
results on its host port through Message ID# 51. Self-Test mode exits to either Initialization or
Fault mode.
2. Initialization Mode
Upon power-up, the receiver enters Initialization mode. During this mode hardware is initialized
prior to Acquisition mode entry. The Initialization mode is also initiated upon completion of the
Self-Test mode, but always exits to the Acquisition mode.
When the receiver is in OFF mode, it will retrieve data only from NVM (cold start) or from both
NVM and SRAM (warm start). Integrity checking is done on all data retrieved from the non-
operating state. See also Section 4.5.1, Non-Operational State above.
During initialization, the receiver retrieves the last received valid almanac data and last user
position from NVM, the current time from the low-power time source, and predicts which
satellites are currently visible. This list of visible satellites is then used in Acquisition mode to
program the 12 parallel correlator channels.
3. Acquisition Mode
The receiver is in Acquisition mode when insufficient satellite data is available to produce an
initial navigation solution. Acquisition mode is entered from Initialization, or Dead-Reckoning
mode, and exits to Navigation or Fault mode.
To acquire signals from the GPS satellites, the receiver uses:
• Almanac data which describes the satellite orbits
• Time, which in conjunction with almanac data is used to estimate the present
position of satellites in their orbits
• The approximate location of the receiver so a prediction can be made as to which
satellites are visible
The receiver then collects ephemeris data by decoding the satellite down-link data message. After
each satellite in view is acquired, its measurement data set is produced. When a sufficient number
of satellites are being tracked, position, velocity and time can be computed and Navigation mode
entered.
If the receiver cannot perform an acquisition due to an absence of valid almanac data or user
position and/or time, it initiates a "Search the Sky" acquisition. The receiver attempts to acquire all
satellites in the GPS constellation. Once a satellite has been acquired, ephemeris data is decoded
from the satellite down-link message. After sufficient satellites have been acquired, the receiver
enters Navigation mode. In "Search the Sky", the TTFF is typically less than 3 minutes.
4. Navigation Mode
The receiver is in Navigation mode whenever sufficient satellite information and measurement
data is available to produce a GPS fix. Navigation mode is entered from Acquisition or Dead-
Reckoning mode, and exits to Dead-Reckoning or Fault mode.
In Navigation mode, a receiver configured as a roving unit operates in 2 sub-modes: Differential
and Stand-Alone Nav. Sub-mode transition occurs automatically depending on satellite data
availability. A receiver which is configured as a base station unit will operate in Base Station Nav