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5. Operation
5.1 1PPS Signal Output
The GPS receiver unit outputs 1PPS signals with each pulse defining the instant of time
described by the preceding frame of serial data. (For the time signal transmission format, refer to
Appendix D.)
Following power-up, the GPS receiver begins outputting 1PPS signals after the acquisition of the
almanac data and the estimation of the receiver location (or the antenna location to be more
exact) are completed.
Almanac data received from the satellites include satellites outline orbit information and UTC
time correction parameters. It takes about 12 to 30 minutes to acquire the data.
The almanac data is backed up temporarily following power-down. If power is removed for 16
hours or more, then the data back-up may be lost and will have to be re-acquired on power-up.
If the almanac data back-up is valid on power-up, acquisition of almanac data is unnecessary.
To estimate the location, signals from four or more satellites are required in the case of normal
mode; and five or more are required in the case of high-reliability mode.
Note: If the antenna is positioned in the shade of buildings, or in other locations where it is difficult to
receive satellite signals, then location estimation may take a longer time or may even be
impossible.
If satellites are closely aligned in relation to the receiver, then it may not be possible to achieve
a location estimate, even when the necessary numbers of signals are received. The reason for
this is that, as the angle between received signals becomes small, the error in location
estimation becomes large.
Accordingly, installing an antenna in a location of narrow visual field such as between tall
buildings may make the location estimation impossible. When using the high-reliability mode
and/or TRAIM function, the alignment of satellites has even stricter limits.
The location estimation may take more than an hour depending on constellation of satellites in
case of the high-reliability mode.
The location estimation is required each time the power is turned on.
After the location estimation is completed, the 1PPS signal is output in normal mode if signals
from one or more satellites are received; and two or more in high-reliability mode.
The initialization time for the 1PPS signal at power-up varies as shown in Table 5.1.1, depending
on the state of the almanac data back-up.
Table 5.1.1 1PPS Output Start Time
Back up of almanac data Initial time needed for 1PPS signal output from power on
Lost 30 seconds to 1 hours (*1)
Available 10 seconds to 1 hours (*1)
(*1) It may take further time depending on constellation of satellites and antenna position.