X
X
L
L
D
D
C
C
M
M
A
A
N
N
.
.
d
d
o
o
c
c
3
3
-
-
4
4
R
R
E
E
V
V
E
E
When a third satellite is acquired, a unique position solution exists given an assumed ellipsoid height
near 0 meters. At this time, the Serial Function F53 command returns "F53 AUTO: 3 SATS" and the
first line of the display indicates "MODE: AUTO 2-D", giving the positioning mode of the GPS core unit.
The second line indicates "sats ## ## ##", giving the PRN numbers of the three satellites being used in
the two-dimensional (2-D) fix. With this position, the remaining visible SV's are determined based on
the almanac and the time and are acquired rapidly. Once a 3-D position fix has been determined,
synchronization to UTC begins and the first stage of AUTO Mode has almost ended. At this time the
Serial Function F53 command returns "F53 AUTO: # SATS", where # is the number of SV's being
tracked, which may be as many as 6. The first line of the display indicates "MODE: AUTO 3-D" while
the second line indicates "## ## ## ## ## ##", giving the PRN numbers for up to six satellites being
tracked.
During the position refinement stage of AUTO mode, the constellation may change such that 3-D fixes
are not available. This will be indicated on the status display. Though these 0-D and 2-D fixes will not
go into the position average, they will be used to control the time and frequency outputs of the XL-DC.
When the XL-DC is synchronized to UTC using the approximate position found, the unlock and time
error annunciators are extinguished, thus indicating the end of the Current Position Search phase of
AUTO mode.
Current Position Averaging and Refinement: After completing the first Current Position Search phase of
AUTO mode, AUTO mode automatically begins averaging position fixes, providing an increasingly
more accurate and stable time and frequency reference position. The quality of the timing and
frequency outputs will improve until a terminal average of approximately twenty-four hours duration has
been obtained. At this time, the XL-DC returns "F53 TIME: # SATS" in response to the Serial Function
F53 command and the first line of the display indicates "MODE: TIME X-D", where X is either 2 or 3
depending upon the satellite visibility. As in AUTO mode, the second line of the display indicates the
PRN numbers of the current satellites being used in the TIME solutions.
Invocation of TIME Mode: After the position average is complete, the AUTO mode switches the XL-DC
to TIME mode and the averaged position will be used for all future timing solutions. TIME mode inhibits
further surveying. The auto installation process is concluded.
The XL-DC will remain in TIME mode and will power-up in TIME mode using the averaged position
after a power outage. However, after powering up in TIME mode, if the computed positions
consistently differ from the previously stored average position by more than 1 km for a significant period
of time, the XL-DC will automatically invoke AUTO mode to re-establish the position. Otherwise,
operator intervention would be necessary to re-invoke AUTO mode.
3-12 SURVEY MODE
When in the SURVEY operational mode, the XL-DC will repeatedly calculate position and time based
on the unaveraged position. The position solutions are not averaged, and multiple satellite averaging
techniques for reducing the effects of SA on the time solution are not employed. This mode of
operation is appropriate in dynamic or pseudo-static
platform applications. Strictly stationary users
should use the far more accurate and stable TIME mode. The specified time and frequency
performance levels may not be met when the XL-DC is operating in SURVEY mode.
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