ashtech DG14, Reference Manual

Page: 23 / 390

General Information 3
General Information
Upon application of power, the DG16 runs a built-in self test of its internal
memory, and thereafter periodically self-tests various functions during normal
operation. Test results are stored for output on command. After self test, the
DG16 initializes its battery-backed RAM. If the battery-backed RAM fails self-test
(due, for example, to a low battery backup condition), the DG16 clears and
reports the loss of stored data, then initializes its channels and begins searching
for all satellites within the field of view of its antenna.
You can view the results of the start-up built-in diagnostics in the response from
the
$PASHQ,HST command. Results from the periodic operational diagnostics
can be viewed in the results of the $PASHQ,BIT command. See BIT: Operational
Diagnostic Tests on page 99, and HST: Start-up Diagnostic Results on page
122 for more information.
The DG16 can track all GPS satellites as specified in the Navstar GPS Space
Segment/Navigation User Interfaces, ICD-GPS-200, Revision B. All 32 satellite
PRN (pseudo-random noise) code numbers are programmed into the DG16’s
firmware. There are active 24 satellites in the GPS constellation. As it acquires
(locks on to) each satellite, the DG16 notes the time and collects almanac and
ephemeris data for each orbiting satellite and stores this information in battery-
backed memory.
• When tracking one satellite, the DG16 gets a time reference from that
satellite’s clock.
• When tracking three satellites, the DG16 computes and time-tags the
horizontal position (2D) and velocity of its antenna. Input of an initial
position estimate is not required. When it receives an appropriate
command message from controller equipment through one of its serial
communication ports, the DG16 sends the results of its computations to
the designated port.
• With four locked satellites, the DG16 determines three-dimensional
position and velocity. Stand-alone position accuracy is 2 meters Circular
Error Probable (CEP) when Position Dilution of Precision (PDOP) is less
than 4; velocity accuracy is 0.1 meter per second. Accuracy levels for
position and velocity are subject to the US Government policy of Selective
Availability (SA). When the DG16 is operating in differential mode, position
accuracy improves to better than 1.0 m CEP.
The DG16 can compute up to 20 independent measurements per cycle (20 Hz),
with no interpolation or extrapolation from previous solutions. Position and
velocity computations are performed simultaneously using all the satellites in
view. The DG16 uses instantaneous doppler values from four satellites to
compute dynamic speed, allowing velocity computations to be made independent