SV P-code Generator. Applied Research Laboratories, The University of Texas at Austin August 2003
SVPCodeGen handles the P-code generation for a particular SV. Each object is initialized based on the particular satellite (identified by PRN code) and the Z-count when generation is to begin. The Z-count must be an even four Z-count (six second) interval. Input values that do not align with four Z-count boundaries are rounded BACK to the next earliest 4 Z-count boundary.
After initializaiton, P-code is generated in 6-second "chunks". The chunks are returned to the caller in CodeBuffer objects. After each call to getNextSixSeconds( ), the application shall call increment4ZCounts( ) to advance the time by four Z-counts. This separation between getting the bits and advancing the time is purposely done to allow for easier coordination when generating code for multiple satellites and performing other operations on the data after generating the P-code and before moving on to the next increment.
A key aspect of this class is the use of the X1Sequence and X2Sequence classes. Analysis of the P-code generation processes reveals that the combined X1A/X1B bit stream repeats every 1.5s (1 Z-count) and is the same for all satellites. At the end of 6s (4 Z-counts), the number of bits is evenly divisable by 32 (which is the assumed wordsize for the applicationn). Therefore, to obtain X1 bits, getNextSixSeconds( ) need only access the indiviual words from the X1Sequence starting at the beginning of the array and progressing to the end. The restriction that the coder must start on an even 6-second (4 Z-count) epoch and generate intervals of six seconds obviates the need to build code to compute bit positions within X1A cycles or any information on the X1A/X1B relationship. See documentation on the class X1Sequence for more information.
The situation with respect to X2A/X2B is somewhat more complicated. Once again, the X2A/X2B sequence is only 1.5s long. However, the X2 sequence is "slipped" wrt the X1 sequence in two ways: (1.) once per week (at the beginning of week), it is slipped by a number of bits corresponding to the PRN ID, (2.) slipped 37 bits for each X2A cycle. This means that it is necessary to keep track of the bit position within the X2 sequence and "chop out" 32 bits at a time. See documentation for the class X2Sequence for more information.
Definition at line 96 of file SVPCodeGen.hpp.
#include <SVPCodeGen.hpp>
Public Member Functions | |
| void | getCurrentSixSeconds (CodeBuffer &pcb) |
| const gnsstk::CommonTime & | getCurrentZCount () const |
| void | increment4ZCounts () |
| void | setCurrentZCount (const gnsstk::GPSZcount &z) |
| SVPCodeGen (const int SVPRNID, const gnsstk::CommonTime &dt) | |
| ~SVPCodeGen () | |
Private Attributes | |
| gnsstk::CommonTime | currentZTime |
| int | PRNID |
| gnsstk::X1Sequence | X1Seq |
| gnsstk::X2Sequence | X2Seq |
| gnsstk::SVPCodeGen::SVPCodeGen | ( | const int | SVPRNID, |
| const gnsstk::CommonTime & | dt | ||
| ) |
SVPCodeGen::SVPCodeGen( const int PRNID, const CommonTime ) - Instantiate and initialize a SVPCodeGen object. Based on the PRNID and the ZCount, determine the appropriate starting location in the X2 sequence and set it up. Set the current time.
Definition at line 49 of file SVPCodeGen.cpp.
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Definition at line 107 of file SVPCodeGen.hpp.
| void gnsstk::SVPCodeGen::getCurrentSixSeconds | ( | CodeBuffer & | pcb | ) |
Starting at the beginning of the X1 sequence and at the appropriate location (as determined by time and PRN), advance through both the X1 sequence and X2 sequences combining the sequences 32 bits at a time until the X1 sequence for this six second period has been exhausted. Sometime within that period, the X2 sequence will be exahusted (including the appropriate delays) and will rollover to the beginning of the sequence.
Definition at line 60 of file SVPCodeGen.cpp.
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inline |
Returns the current time to the calling method.
Definition at line 134 of file SVPCodeGen.hpp.
| void gnsstk::SVPCodeGen::increment4ZCounts | ( | ) |
Generally, the only action is to increment the Z-count by 4 counts. This function COULD be included at the end of getCurrentSixSeconds( ), however, it has been separated to allow the calling application to have a consistent view of all the state conditions (for output and debug) before moving the time forward for the next generation.
Definition at line 113 of file SVPCodeGen.cpp.
| void gnsstk::SVPCodeGen::setCurrentZCount | ( | const gnsstk::GPSZcount & | z | ) |
Allows the user to set the current time. While a any time may be specified, this routine will take the Z % 4 for the actuall time.
Definition at line 118 of file SVPCodeGen.cpp.
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private |
Definition at line 145 of file SVPCodeGen.hpp.
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private |
Definition at line 146 of file SVPCodeGen.hpp.
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private |
Definition at line 143 of file SVPCodeGen.hpp.
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private |
Definition at line 144 of file SVPCodeGen.hpp.