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The receiver switches between modes automatically. The receiver reports on its host port the current operating 
and navigation modes.

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.

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. 

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.

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