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FAQ and Tips
Chapter 9
9.3.10 How do I process long kinematic baselines?
For dual frequency processing on long baselines, it is important to enable L2 ionospheric processing. This option is
set under Settings | Individual | L2/Ionosphere. Once L2 ionospheric processing is enabled, there are two ionospheric
processing methods:
•
The Ionospheric Free model works well when the remote initializes far from the base. This method
creates a carrier signal that is free of ionospheric error. However, a cycle slip on L2 will induce a total
loss of lock for that satellite. In some cases, this method may do better than the relative method even if
initialization is close.
•
The Relative ionospheric model works by only applying an ionospheric correction to the L1 signal. If
initialization is close, then an initial correction of zero is assumed. The error then accumulates as the
remote vehicle travels away from the base. The relative model tends to have a higher L1 phase error than
the iono-free, primarily due to the fact that the model is only an approximation. However, cycle slips on
L2 do not reset the ambiguity like the iono-free solution. Hence, accuracies may be better if L2 cycle
slips are present in large quantities. For days with high ionospheric activity, the Engage distance for
relative ionosphere may need to be lowered. If a static initialization is performed close to the base,
which is always suggested, then it may be beneficial to make this distance tolerance longer than the
distance at which the initialization was performed. Raising the rejection tolerance and/or the PPM value
of the standard deviations under the Measurement tab will stop GrafNav from mixing up large
ionospheric corrections with carrier phase errors. See Section 2.5 on Page 61 for more information.
Although KAR can be used with both ionospheric methods, a KAR fix will have some ionospheric error if the
Ionospheric correction model is not used, under Settings | Individual | KAR . It may also have difficulties on longer
baselines. Therefore, setting the maximum distance for engaging KAR is important. Normally, values between 20
and 60 km are realistic.
On very long baselines, the C/A code can be corrected if P2 code is available. In many cases, this will simply add
more noise, but the noise is very white, and overall improvements can be observed. This is enabled from Settings |
Individual | L2/Ionosphere.
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 (i.e. 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. for additional information. 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.
9.4
9.4.1
KAR Tips
How can I detect and fix incorrect integer fixes?
Incorrect intersections (fixes) are caused when KAR 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 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 intersections are most apparent when there are two fixes
(i.e. 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
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