MAINTENANCE/TROUBLESHOOTING
B. DYNAMIC MODE
The Dynamic mode of operation is a mode in that it is solving for all four variables (X, Y, Z,
and T) at the same time. This mode is used either by the system to determine its location or
used in a three-dimensional moving platform (i.e. aircraft). The GPS receiver used in the
ExacTime is a OnCore Module. This receiver has eight channels in which to decode satellite
information (what is referred to as tracking a satellite). Four of the channels will be
collecting data from four satellites while the remaining four channels will be collecting data
from as many as four more satellites in a multiplex scheme. In this way the receivers
processor can be solving for the four unknowns from as many as four sets of equations at the
same time. This is referred to as an “over-determined solution.” Some of the effects of SA
can be smoothed out using this process. However, in this mode the time information is
moving as well as a function of newly calculated positions each second. This is not the most
stable timing mode to operate in, but for moving platforms it is the best mode of operation.
C. STATIONARY MODE
It is better to refer the Stationary mode as a one-dimensional mode. The receiver will use the
stored position in memory and solve for the time information from as many as eight satellite
at the same time. The receiver will then provide an average solution of the time information
to the ExacTime, thus providing a more stable time in which to operate under the effects of
SA. This is the best timing mode of operation as long as the equipment is in a static
environment and a good position has been entered into memory.
D. AUTO MODE
The Auto mode is a preprogrammed mode in the ExacTime and is the default mode that the
unit is shipped to the customer in. At power-on the unit does not know where it is nor what
time it is, thus it must determine all of this. At power-on the Auto mode commands the
receiver into the Dynamic mode of operation so that it can determine the location as well as
time. The ExacTime will first set an approximate time and position as soon as it tracks four
satellites. It will then begin stabilizing the internal oscillator in order to get it close to the
correct frequency. After completion of the oscillator stabilization the unit will begin
collecting position information. It will average the default selection of 200 positions, the
unit can be set to collect as many as 99,999 positions. After the 200 positions have been
collected it will command the receiver into a Stationary mode, load the average position into
the receiver, set time and begin normal Stationary mode of operation. We will discuss this
normal mode of operation in more detail later.
Symmetricom Inc ET6xxx ExacTime GPS TC & FG (Rev C) 5-7
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