1 Calculation of GPS Solution (RTKNAVI,RTKPOST,RNX2RTKP)
1.1 Significant Functional Differences
(1) Comments have been added to most of the core positioning algorithm code.
(2) The trace output has been enhanced and modified to provide more relevant information for debugging poor solutions.
(3) The kalman filter state update has been re-written to remove all the zero-element multiplies. This significantly reduces the computation time when receiver dynamics is enabled.
(4) Code has been added to calculate and manage inter-channel biases for the GLONASS and SBAS satellites based on an extension of the fix-and-hold method (direct feedback from the ambiguity resolution results to the kalman filter states) .
(5) Integer ambiguity resolution has been enhanced to include up to three attempts per sample to resolve the ambiguities using different combinations of satellites. Additional attempts may remove GLONASS and SBAS satellites and/or remove newly acquired satellites depending on various conditions.
(6) Additional adjustable constraints for ambiguity resolution have been added to help minimize the chance of false fixes.
(7) "Combined" mode has been modified to avoid resetting the kalman filter states after completing the forward pass and before beginning the backward pass. This enables the backward pass to use the converged position and satellite phase biases from the forward pass instead of having to re-converge from scratch
(8) The common component of the phase-biases has been moved to a separate variable instead of spreading it out among all the satellites. This makes the phase-bias states easier to interpret and also removed some issues with improper adding of cycles from satellites with different carrier wavelengths.
1.2 Additional Input Parameters and Options
(1) pos1-posmode = static-start: Begins solution in “static” mode but switches to “kinematic” mode after first fix. This mode will normally acquire first fix faster than kinematic mode if the rover is stationary but will fail if the rover starts moving before the first fix.
(2) pos2-gloarmode = fix-and-hold: Extends the fix-and-hold method to calibrate the inter-channel biases for the GLONASS satellites and similar errors for the SBAS satellites. Note that fix-and-hold feedback only occurs after the first fix.
(3) pos2-arfilter = on,off: Rejects new or recovered satellites from being used in ambiguity resolution if the AR ratio was significantly degraded by their addition. This is an alternative to using the blind delay of “arlockcnt”
(4) pos2-arthres1 = x: Integer ambiguity resolution is delayed until the variance of the position state has reached this threshold. It is intended to avoid false fixes before the kalman filter has had time to converge. If you see AR ratios of zero extending too far into your solution, you may need to increase this value. The “arthres1” option exists in the release code config file but is not used for anything.
(5) pos2-minfixsats = n: Minimum number of sats necessary to get a fix. Used to avoid false fixes from a very small number of satellites, especially during periods of frequent cycle-slips.
(6) pos2-minholdsats = n: Minimum number of sats necessary to hold an integer ambiguity result. Used to avoid false holds from a very small number of satellites, especially during periods of frequent cycle-slips.
2 Conversion from raw data to RINEX (RTKCONV, CONVBIN, RTKNAVI)
2.1 Significant Functional Differences
(1) The M8T raw output to cycle-slip translation algorithm has been modified to more closely match that of the M8N. This insures that a cycle-slip is flagged after every loss-of-lock and is not flagged until the carrier phase measurement quality is sufficient for resetting the phase-bias estimate.
(2) The half-cycle invalid bit is set for the SBAS satellites based on lock-time since the half-cycle invalid bit from the receiver is not functional for these satellites. This change has been made for both the M8N and M8T in the demo5 code but has only been ported back to the 2.4.3 code for the M8N receiver.
(3) The receiver measurement quality metrics for each sample are recorded in the RINEX observation files. The single character SNR field in the pseudorange and carrier-phase measurements are used for this purpose. The M8T reports quality metrics for both pseudorange and carrier-phase, the M8N reports quality metrics only for the carrier-phase. This is for information purposes only, RTKLIB does not use these fields.
2.2 Receiver Specific Options (U-blox)
(1) -STD_SLIP = x: Carrier phase measurements are flagged as cycle-slips if the standard deviation of the carrier phase measurement (as reported by the receiver) is greater or equal to x (scale=0.04 meters/count). This feature now exists in both versions but is not listed in the 2.4.3 documentation. It is only used for the M8T receiver. If this option is not set, then the same fixed threshold will be used for cycle-slips as for valid carrier-phase measurements, which in the release code can cause cycle-slips to fail to be flagged after the phase-bias estimate is valid.
(2) -TRKM_ADJ=x: Apply corrections to TRK-MEAS pseudorange measurements from a table to remove integer number of meter errors in the GLONASS satellite measurements. With this option set, the TRK-MEAS command gives identical pseudorange measurements as the RXM-RAWX command. Use "x=2" for firmware versions 2.01 and 2.30, and "x=3" for firmware version 3.01.