ashtech GG Surveyor, Reference Manual

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30 GG Surveyor GPS+GLONASS Reference Manual
5), or 2 (time shift held fixed), four satellites are required to compute a 3D position;
2D position is not computed in this mode.
In mode 1 depending upon the setting of GTM and GTP, five or four satellites are
required to compute a 3D position, and four or three satellites to compute a 2D
position.
In mode 2 depending upon the setting of GTM, three or four satellites are required to
compute a position. In this mode, altitude is always held fixed and only 2D position is
computed.
In mode 3 depending upon the setting of GTM and GTP, three or four satellites are
required to compute a 2D position. To compute a 3D position, four or five satellites
are required, and the computed HDOP must be less than the HDOP mask. If HDOP is
higher that the mask, a 2D position is computed.
Altitude Definition
Two modes define the altitude selected when the GG Surveyor is in altitude hold
mode. Use the $PASHS,FIX set command can be used to select between these modes.
See “$PASHS,FIX,x” on page 83 for more information.
In mode 0 the most recent altitude is used. This is either the one entered by using the
$PASHS,ALT set command or the one computed when four or more satellites are
used in the solution, whichever is most recent. If the last altitude is the one computed
with four or more satellites, it is used only if VDOP is less than the VDOP mask.
In mode 1 only the last altitude entered is used in the position fix solution.
On initial power-up or after use of the $PASHS,INI memory reset command, or
$PASHS,RST default parameter reset command, the most recent antenna altitude is
set to 0.
Time Shift Hold Definition
Two modes determine what time shift is selected when the GG Surveyor is in time
shift hold mode. The $PASHS,GTF command selects the mode, 0 or 1. See
“$PASHS,GTF,d” on page 85 for more information.
In mode 0, the GG Surveyor uses most recent computed time shift is used: the time
shift entered with the $PASHS,DTG command, or the time shift computed in the
position solution. If the most recent time shift is from the position solution, it is used
only if TDOP is less than the TDOP mask.
In mode 1, the GG Surveyor uses only the last time shift entered using $PASHS,DTG
in the position solution. The GG Surveyor does not compute a position when the time
shift entered using the $PASHS,DTG, if this command is not close to the real time
shift (varies slightly, current value -1.3 µsec).