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HOT START
If the unit has been turned off for only a short time (less than about an hour), the precise ephemeris
previously found for the currently visible satellites will still be valid, and start-up can take place without
having to spend the 30 seconds required to collect ephemeris data. This is known as a Hot Start, and cuts
the time to first fix down to about 15 seconds under typical conditions.
It is important to note for all start-up modes that TTFF does not always correspond to the time it takes for
Time Valid to occur. This is because in order to declare that time is valid, the offset from UTC must be
known. In the GPS system, this offset value is transmitted only once every 12.5 minutes.
OSCILLATOR MODES
The ovenized oscillator is controlled by sophisticated algorithms in the TM-4. As the unit operates, the
control of the OCXO involves several stages and processes. The various operating modes are described
below:
Mode 1: Warm-up. The oven in the oscillator is being preheated to bring the OCXO to the desired
operating temperature. Until this temperature is reached, the oscillator cannot be tuned or
controlled. This mode can take anywhere from three to six minutes, depending on which oscillator
is installed.
Mode 2: Coarse tuning. The OCXO is being tuned in relatively large steps. This mode may last
anywhere from a few to twenty minutes or more, depending on the initial error of the frequency
when this mode began.
Mode 3 Coarse tuning hold. Tuning in coarse mode is suspended due to an error condition of some
sort (usually due to GPS unavailability). The tuning value is reset.
Mode 4: Fine tuning. The OCXO is being tuned in very fine steps. This is the normal operating
mode of the unit, and is indicated by the illumination of the READY LED. This mode always starts
with the accuracy of the primary frequency better than 1x10-9, and as the unit tunes, the accuracy
becomes greater and greater, approaching that of the cesium clocks on the satellites themselves.
Mode 5: Fine tuning hold. Fine tuning is suspended due to an error condition of some sort (usually
due to GPS unavailability). Intelligent Holdover™ takes control of the unit in this mode and
provides excellent holdover characteristics. This mode is indicated by the ALARM and READY LEDs
flashing, with the POWER LED remaining illuminated.
TIMING MODES
Two timing modes are provided in the TM-4 for maximum accuracy and flexibility. In the Dynamic Timing
Mode, the reference position for the purpose of determining precise time is the current position as
determined by the GPS position solution. This mode is provided to accommodate users who require precise
timing while operating on a moving platform.
When operating in the Static Timing Mode, the TM-4 uses a fixed position as the reference for deriving
time. In this case, the position may be user entered or previously derived from GPS. Information from all
satellites tracked is used for timing, and time remains valid as long as at least one satellite is tracked.
A related function is the Auto Survey mode. If you plan to operate the unit in one location, you may wish to
invoke this function. Auto Survey takes the average of 10,000 position measurements and then automatically
switches the unit to static timing mode. The Auto Survey function takes about 3 hours to complete.
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