Generate X Sequence for GPS Applied Research Laboratories, The University of Texas at Austin August 2003
GenXSequence holds a bit buffer containing one of the GPS code sequences: X1A, X1B, X2A, X2B.
The first 4092 or 4093 bits are constructed using the algorithms from ICD-GPS-200, section 3.3.2. The process of generating the bits is identical for each of the four sequences. The process consists of a 12-bit shift register which is initialized to a specific value and a specification of the "taps" on the shift register. As a given bit is read off the high end of the register, the sum of the bits specified by the taps is used to determine is 0 or 1 is used as the next input to the shift register. This process is defined in more detail in ICD-GPS-200 section 3.3.2.2.
Once the 4092 or 4093 bits are constructed, in the case of X1B, X2A, and X2B it is necesary to add copies of the last bit to account for delay states in which the register is "held" in it's final position for many counts while X1A completes a cycle.
Once the sequences are created, the array subscript operator is used to access the sequences 32 bits at a time. The index passed to the array subscript operator is the bit position within the sequence (first bit is equal to bit 0). When the process reaches the end of the sequence, the operator will "wrap around" and restart at the front of the array in order to fill the 32-bit return word.
For X1A, the length of the sequence will always be 4092 bits. For all other registers, the length of the sequence is a variable, dependent on the epoch and the time of week. That information is not available to GenXSequence; therefore, the method GenXSequence::setEndOfSequence( los ) is provided to allow the calling program to adjust the end of sequence accordingly. NOTE: this means the calling program also needs to specify a maximum sequence length (see constructor) that is equal or greater than the maximum length the calling program will use.
Definition at line 96 of file GenXSequence.hpp.
#include <GenXSequence.hpp>
Public Member Functions | |
| GenXSequence (const unsigned int initialState, const unsigned int taps, const int initialEndOfSequence, const int max_delay) | |
| uint32_t | operator[] (int i) |
| void | setLengthOfSequence (int i) |
| Set the end of sequence for the current cycle. More... | |
| ~GenXSequence () | |
Private Member Functions | |
| void | addBitToSequence (uint32_t newBit) |
Private Attributes | |
| int | bit_num |
| uint32_t | bits [MAX_WORD] |
| bool | debugPrint |
| int | lengthOfSequence |
| int | maxOfSequence |
| int | word_num |
| gnsstk::GenXSequence::GenXSequence | ( | const unsigned int | initialState, |
| const unsigned int | taps, | ||
| const int | initialEndOfSequence, | ||
| const int | max_delay | ||
| ) |
The initial state of the register and the defintion of the taps are provided to the constructor. In each case, the 12-bits of interest are right justified in the argument. The lsb of the variables corresponds to stage 1 in figures 3-3 through 3-6 of ICD-GPS-200. The constructor runs the X generator process, creates 4096 bits of X-code, and stores the results internally in an array of 128 unsigned long (32-bit) variables as packed integers. Within the storage words the bits are ordered from word[0] to word[xxx] and from msb to lsb within the word.
initialEndOfSequence is the number of time the X-register process should be incremented and should be either 4092 (X1A, X2A) or 4093 (X1B, X2B).
maxDelay represents the number of times the final bit of the sequence should be copied after initialEndOfSequence is reached. This should be equivalent (or greater than) the maximum number of delay states that this variable can experience. For X1B, that's the end-of-X1A-epoch delay (349 counts). For X2A and X2B the 37 chip end-of-X2-epoch and the end-of-week delays must be considered.
Definition at line 51 of file GenXSequence.cpp.
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Definition at line 127 of file GenXSequence.hpp.
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Definition at line 112 of file GenXSequence.cpp.
| uint32_t gnsstk::GenXSequence::operator[] | ( | int | i | ) |
Fetch 32-bits from the sequence starting at the specified bit location. Bits are counted started at 0 and run to initialEndOfSequence+maxDelay. Attempts to reference bits outside this range will generate exceptions and halt the program. These conditions shouldn't occur during operation, only during programming. If there are not 32 bits remaining prior to the current value for lengthOfSequence, the function will wrap around to the beginning of sequence.
Definition at line 139 of file GenXSequence.cpp.
| void gnsstk::GenXSequence::setLengthOfSequence | ( | int | i | ) |
Set the end of sequence for the current cycle.
Definition at line 206 of file GenXSequence.cpp.
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Definition at line 151 of file GenXSequence.hpp.
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Definition at line 146 of file GenXSequence.hpp.
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Definition at line 149 of file GenXSequence.hpp.
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Definition at line 147 of file GenXSequence.hpp.
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Definition at line 148 of file GenXSequence.hpp.
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Definition at line 150 of file GenXSequence.hpp.