See TN199812A for general information on the GPS system.
Some aspects of GPS operation take much longer than people expect. This technical note discusses two of these.
The time used by the GPS system does not include leap seconds. This means that every eighteen months or so, its "time" changes by one second with respect to UTC which does include leap seconds. The difference between GPS time and UTC is 13 seconds in early 1999 and is referred to as the UTC offset. This value is obtained by the GPS receiver from the satellites in a "navigation message".
The navigation message is transmitted by the satellite on the L1 data link at a rate of 50 bits per second. The message structure uses a basic format of a 1500 bit long frame (30 seconds) made up of the five subframes mentioned in TN199812A with each subframe being 30 bits long. Subframes 4 and 5 are repeated 25 times each, so that a complete message will require the transmission of 25 full frames (12 1/2 minutes). The UTC offset is only transmitted once per frame, so it is only transmitted once every 12 1/2 minutes. Depending where you were in the cycle when the GPS receiver is first switched on, it can thus be up to 12 1/2 minutes until the UTC offset value is transmitted from a satellite.
Different models of Trimble GPS receiver contain different operating software. This changes the way they behave. Over the years, the following models have been used in SRC equipment:
In general, there are not many differences apparent between the models. Each newer model uses less power than the one before it and may have better performance in some situations. Current versions of software for the Kelunji Classic and Kelunji D Series support all the listed models.
With the ACE-III, Trimble have changed one aspect of the functionality which does affect Kelunji users. With this model, the pulse-per-second (PPS) output signal is not generated after power up until a position fix has been obtained. This was not the case in earlier models. In the Check Clock screen, the right hand (GPS) box will have the status line "Have obtained GPS time", but no sync value will be displayed in the left hand (Kelunji) box until a couple of seconds after a position fix has been obtained.
The implication of this
is that at sites with poor visibility of the sky (or many antenna extensions)
will now take a long time to obtain a clock sync. This may require adjusting
the time-out setting in the Kelunji. For the Kelunji D Series, there are also
a number of settings in the Settings:Advanced:GPS receiver menu. These can be
adjusted so that more satellites are used (by decreasing the minimum signal
strength setting) or so that a position (albeit inaccurate) can be given quicker
(by increasing the maximum PDOP for 2D and 3D locations).
Note that the ACE-III has been installed in Kelunji D Series recorders delivered since early 2002 and in most (all?) KG1's delivered.