This section describes how the time stamp is embedded into the I/Q data. Within each 64 bit frame, only the first four extended frames contain time stamp information. Refer to 64 bit Time Stamp frame diagram below.
The GPS seconds is the time in seconds from Jan 1, 1970. The tick counter counts at a rate of 114.375 MHz and it is reset to 0 on every second, triggered by the GPS PPS signal.
The time stamp records the time at the beginning of each extended frame. The elapsed time between each frame is calculated with this formula:
To insert the time stamp without interrupting the I/Q data sequence, the 64 bit time stamp is rotated and inserted into the extended frame by using bit 64 from each frame. To indicate the beginning of an extended frame with a time stamp, a mark bit is set to ‘1’ for the first frame and ‘0’ for the remaining 63 frames. The mark bit uses bit 32 of each frame.
I/Q Frame Structure with Time Stamp
Embedding the time stamp requires using two bits from each frame, which requires modifying the I/Q frame structure.
I/Q Bit Resolution = 24
Each frame contains only 1 I/Q sample pair (one I and one Q). The first column of the IQ vector contains Q and the second column contains I. All the frames will have 24 bits each for I and Q. Each I and Q sample is followed by 7 zeros, then the mark or time stamp bit. Only the first four extended frames will have time stamping.
The remaining extended frames will have zero valued mark and time stamp bit.
I/Q Bit Resolution = 16
Each frame contains two I/Q sample pairs (two I and two Q). The first I and first Q sample in the frame will always have 16 bits. The second I and second Q sample will have 15 bits, followed by the mark and time stamp bit.
I/Q Bit Resolution = 10
Each frame contains three I/Q sample pairs (three I and three Q). All the frames will have 10 bits each for I and Q. Each I and Q sample is followed by one zero, then the mark or time stamp bit. Only the first four extended frames will have time stamping.
The remaining extended frames will have zero valued mark and time stamp bit.
I/Q Bit Resolution = 8
Each frame contains four I/Q sample pairs (four I and four Q). The first three I and first three Q samples in the frame will always have 8 bits. The fourth I and fourth Q sample will have 7 bits if the frame is in the first four extended frames, which uses one bit for mark and one bit for the time stamp.
Having only 7 effective bits instead of 8 bits on every fourth sample will slightly increase the noise floor.
