1. PCI/PCIe TFP Hardware
Byte
Item
Value
4
request packet ID
0x27
1.7.4. Position Fix Modes
An important aspect of GPS operation is the selection of the position fix mode (GPS packet 0x22.)
The GPS receiver supports the following four GPS position fix modes.
Position Fix Mode 0
This mode uses as many satellites as are available to perform both position fixes and timing func-
tions. Confusioncan arise because this mode selection interacts with the dynamics code selection
(GPS packet 0x2C.) If a non-static dynamics code is selected then only three or four satellites will be
used because the GPS sensor assumes that it is moving. If only three satellites are usable then alti-
tude will be held constant. If a static dynamics code is entered then mode zero will use three or four
satellites for a navigation solution as previously, however, if only one or two satellites are available
the sensor will use the satellite with the highest elevation to continue calculating bias and bias rate
(the timing functions will continue unimpaired). It is good therefore, to enter a static dynamics code if
the sensor is static.
Symmetricom recommends using this static mode if the card is not on a moving installation.
Position Fix Mode 1
In this mode, a user-specified satellite is used for timing functions. If mode 1 is selected, only a single
satellite will be used for timing, and the current position will be assumed accurate and static.
GPS packet 0x34 allows the satellite associated with mode one to be selected. This packet has one
data byte that specifies the PRN of the desired satellite. If a data byte value of 0 is entered, then the
sensor will always track the single satellite that has the highest elevation within the constellation in
view.
It is good to operate in a single-satellite highest elevation mode for timing applications. The greatest
contribution to timing error is the electron content variation in the path between the satellite and the
receiver. Selecting the highest elevation satellite minimizes this variation.
Position Fix Mode 3 and 4
These modes are rarely used for timing applications unless the user operational platform is dynamic.
Mode 3 is particularly useful in a marine environment where the sensor altitude is relatively constant.
1.8.5. GPS Default Parameters
The TFP sends the following GPS packets to the GPS receiver on reset or whenever the Timing
Mode is changed to GPS Timing Mode 6.
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