ashtech DG16 Reference Manual page 58

Kalman filter behaves as if PMD was set to 2. The Kalman filter does not
reset when it changes from 2-D to 3-D mode.
The Kalman filter automatically resets when either the error mask is exceeded
or if the prediction interval is longer than the set maximum interval value.
After the first set of four or more measurements is received after a total-
satellite-outage period, the DG16:
• continues operation and uses the received measurements to update
the running Kalman filter, if the Prediction Mode was ON
• resets the Kalman filter with the received measurements and resumes
operation similar to a hot start.
• Mode 2: 2D
In 2D mode, the DG16 can only calculate latitude, longitude, and time; the
altitude is held constant. Use the $PASHS,ALT and $PASHS,FIX commands
to set the altitude.
When FIX is set to 1, the 2D altitude is the altitude entered using the
$PASHS,ALT command. When FIX is set to 0, the altitude is the most
recently determined altitude, which can be either that entered using the
$PASHS,ALT command, or the altitude from the last computed height value
computed when VDOP was less than VDOP mask.
The DG16 requires at least three satellites to be locked for the initial 2D
position fix. After the initial fix, there is no required number of locked satellites.
The DG16 continues to operate making use of the locked satellites,
propagating its internal solution and reporting the predicted position until ERM
or any other user set mask exceeds the set value.
You can use the $PASHS,KFP,ON command to enable the Kalman filter for
prediction. Position fixes obtained during the prediction mode are flagged
"predicted" in GGA messages.
If you set PMD to 2 (continuous 2-D mode), the Kalman filter is only initialized
with the reference altitude, for all the next epochs the Kalman filter updates
the position in correspondence with the current dynamic model and the
parameters for this model (maximum velocity and acceleration) for the vertical
direction should be ten times less than nominally.
If you have set the DG16 to Static KF mode and only 1 satellite is available,
the position is still not considered predicted because Kalman filter, in this
case, is nothing but sequential least squares.
In the $PASHS,KFP,PRI command, you can set the maximum allowable
length of time for the DG16 to operate in Prediction Mode (Prediction
Interval). The default for prediction mode is OFF and the default prediction
interval is 10 seconds. After the prediction mode has timed-out, the DG16
needs at least four satellites to reset its Kalman filter and resume positioning.
38 DG14 and DG16 Board & Sensor Reference Manual