Chapter 9	FAQ and Tips

Finally, tropospheric error can be significant on long baselines, especially if there is a large height difference. It is suggested that the Utilize tropospheric error state setting be enabled under the Advanced 2 tab of the GNSS settings. This requires that precise ephemeris and clock files be present, which can take as long as 24 hours. See Section , on Page 68 for more information.

For single frequency, it is important that the standard deviations be set properly on the C/A code and L1 carrier phase signals. The L1 phase standard deviation may need to be raised that is, 0.05m to 0.10 m, and a stricter phase rejection tolerance may need to be used as well. Setting proper standard deviations is also helpful for dual frequency processing. In addition, users should download an IONEX ionospheric correction file via the Options tab under Tools

| Download Service Data. You should then add the file to the project via File | Alternate Precise/Ephemeris Files, and then enable the Correct single frequency using IONEX or broadcast model option under Settings | Individual | L2/ Ionosphere.

9.4Integer Ambiguity Determination Tips

This section provides valuable advice for using kinematic ambiguity resolution (KAR) or AdVance RTK (ARTK). Both are very powerful algorithms that can compute integer ambiguities in both static and kinematic modes.

9.4.1How can I detect and fix incorrect integer fixes?

Incorrect intersections (fixes) are caused when KAR/ARTK picks the wrong solution. This error exhibits a near constant offset in the combined forward/reverse separation.

Confirm that the large forward/reverse separation is due to a bad KAR/ARTK fix

1.Open the Combined Separation plot, which will show the times when the forward/reverse separation is poor.

2.Open the Float/Fixed Ambiguity Status plot. This shows when there is zero, one or two fixes available for an epoch. Incorrect KAR/ARTK solutions are most apparent when there are two fixes that is, for both forward and reverse. These areas show up as green. This does not mean that an incorrect fix cannot occur when there is just one fix. In this situation, you are comparing a float solution to a fixed. The large bias is usually due to the float solution, but this is not always the case. In such a situation, plotting the standard deviation of the float solution can be very helpful.

Determine processing direction of bad solution

It is always a good idea to determine which processing direction is the cause since only that direction needs to be reprocessed. This is significant because in the following step, many options will be changed. It is important to only make these trials on the bad direction as you would not want to corrupt the good direction.

1.Load each direction because it is the fastest way to determine which direction is bad.

2.Plot the L1 Phase RMS. The incorrect fix normally exhibits a linear growth to the carrier phase RMS. There may be jumps as the software is trying to correct the problem by removing satellites. After some time, it may stay low if a filter reset has been issued.

Another way to determine the likelihood of a bad KAR fix is to view the Static/KAR Summary file (FSS/RSS). There will be records for each KAR fix. Look for fixes with poor RMS (less than 0.05 cycles), low reliabilities (less than 2 or 3) or large float-fixed separations (greater than 1 m).

Correcting the Problem

For KAR, try using one of the following tips, listed in order of likelihood to help:

•Enable the Stricter RMS tolerance and/or the Stricter reliability tolerance options

•Lengthen the minimum KAR time. As KAR uses more time, it usually does a better job of identifying and rejecting bad fixes. Alternatively, shortening the time can also be helpful.

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•Consider re-engaging KAR manually after the bad fix. Be sure the checkbox Reset Kalman Filter is enabled on the Manual KAR Engage record.

•Try disabling Refine L1/L2 KAR search.

•Use the KAR_SEP_TOL command to force KAR to only accept intersections that are close to the float solution. For instance, use a value of 1 m.

•Lower the maximum engaging distance for KAR.

•If poor geometry is a concern, try lowering the Maximum DD_DOP value

•If all else fails, disable this time period during combination. See Section 2.5, on Page 61 for help. For ARTK, try using one of the following tips:

•Switch to the Engage Only setting under Criteria for accepting new fixes. This causes fixes to only be accepted when GrafNav requires them, thereby minimizing the likelihood of accepting a bad fix.

•Try disabling the setting Rewind back to time of engagement.

•With the Engage Only setting, try adding the user command ARTK_ENGAGE_DIST with a shorter distance value.

•Try raising the quality level. This may, however, result in fewer, if any, computed fixes.

•Try manually engaging after the incorrect fix.

Unless otherwise specified, all of the options discussed above can be found under Settings | Individual | KAR/ARTK. See Section 2.5, on Page 61 for additional information.

9.4.2How can I help KAR/ARTK find a solution?

Resolving the problem of KAR or ARTK not being able to find a solution tends to be more difficult than resolving the problem of picking a wrong solution. This is because the inability to pick a solution is most often related to noisy carrier data on L1 and/or L2. For KAR, playing with the time and search region size is usually all that can be done. Consider disabling the Stricter reliability tolerance and/or Refine L1/L2 KAR search options as well. For dual frequency, switching to other noise models can help if there is a problem with L1 or L2. With single frequency KAR, the L1 phase needs to be very clean, and there has to be a period of at least 10 minutes without a loss of lock. These conditions are not always possible in many environments.

For dual frequency data on short baselines (less than 8 km), try enabling Use fast KAR under Settings | Individual | KAR. Enabling the Use Fast KAR even for 5 SVs option is not suggested.

For dual frequency GPS data with good C/A code, the following tricks can be used:

•Under Settings | Individual | Measurement, set the C/A standard deviation to 2 to 3 meters.

•Under Settings | Individual | KAR, change the size of the KAR dual frequency search region to 1.5 meters.

•Under Settings | Individual | KAR, raise the KAR minimum time for dual frequency to 3 minutes or more. Increasing the maximum time can also be helpful.

•Under Settings | Individual | User Defined, enter the KAR_SEP_TOL=0.75 command.

Using this procedure will help KAR to find a solution, but may increase the chance of a false intersection as well. Therefore, it is important to check that the forward and reverse solutions agree. For ARTK, not much can be done other than lowering the Quality acceptance criteria to Q0.

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