The receiver cold starts after a power interruption of more than about one minute or after a cold start command sent to it via the serial port. When cold starting the
receiver has no information about the current time, the orbits of the satellites or its own current position. It proceeds to listen for satellites at random until it has acquired one or more. This phase shows nicely on the Satellite
Availability Overview graph below in the form of a pattern of red dots.
Note that the receiver went into this random search mode twice during this initialisation. The Tracking Status graph shows details about that.
Until just after 08:45 there were not enough satellites available for a position fix. For this the receiver
needs at least 4 satellites. Once sufficient satellites are available to calculate position fixes, the receiver starts doing so until it has collected 2000 fixes in order to get a good position to use in the Overdetermined Timing
mode which it enters after this initial phase. In that mode the receiver needs only one satellite to reliably calculate the time; more available satellites make for better timing accuracy. In this example the receiver switched to ODT
mode just after 10:20.
It also very much looks like in this example the initialisation attempt between 08:40 and 09:10 was not
really successful and the receiver firmware decided to start over from scratch.
Looking at the Clock Bias Uncertainty graph for this period we can see that the receiver was getting a
good timing solution already very early on during the second initialisation attempt while still doing the initial position fixes. This is normal behavior. However in position fix mode the timing accuracy is more
prone to bad reception conditions than in ODT mode. In this example the bias uncertainty exceeded the 1 microsecond threshold three times for brief periods causing the PPS signal to the test box to be turned off briefly.
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Obtaining the 2000 initial fixes will normally take 30-40 minutes if sufficient satellites are available. All
this means that an antenna location that has at least one satellite visible during each 24h period is sufficient to keep the clock synchronised; however such a position may not be sufficient to ever let the
receiver initialise itself properly. For this one needs 30-40 minutes of at least four satellites. The Reception Conditions graph can be used to determine these periods.
Note that this graph is for a 48h period. Once can easily see that there are at least two periods of
more than an hour with 4 or more satellites during each 24 hour period. While this is sufficient to initialise the receiver it is still obvious that it may take more than 12 hours to re-initialise the receiver,
should it loose power for more than one minute just after 23:00.
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