Network adjustment

This page describes procedures for doing Network Adjustment using GrafNet, along with ways to improve the results.

1. Go to File > Add/Remove control points and add any available permanent base stations. You must have at least one of these.

2. Go to File > Add/Remove Observations, click Get Folder and select the directory containing your temporary base station observations, click Select All (or select as appropriate) and click Add. Add details of station names and antenna details when prompted. Note: Be careful with station names. GrafNet treats any stations that are named the same as being in the same position, regardless of where they actually are. This can result in very large errors during network adjustment. Any stations in the same place should be named the same (including if they're at control points - they should be given the control point name). Any stations in different places should be named differently, even if they then have to be renamed from the default in their observation files.

3. Select Process > Process Sessions. Leave settings as default and click Process. GrafNet will calculate vector positions from each control point to each observation point. If the standard deviation of the calculated position is below 10cm the line will go green (good solution), if above it will go red (failed). If all lines are green, proceed to step 4, otherwise attempt to improve the results as below.

3.1. Select Process > Process Sessions. Ensure that you're only reprocessing unsuccessful sessions, and on the General tab select Process Direction of Reverse (or Forward if you were already on reverse). Click process - some lines may go green because when GrafNet changes processing direction it may select a different base satellite that gives a better result. If you still have bad lines, try step 3.2 below on both Forward and Reverse processing.

3.2. Select Process > Process Sessions. Ensure that you're only reprocessing unsuccessful sessions, and on the General tab enter an Elevation Mask of 10 degrees (rather than 15). This may improve the number of available satellites.

3.3. For any remaining red lines, click on the line in the map display - you should see the session details in the Data Manager window. For each remaining failed line, right-click it in the Data Manager and select GrafNav to process the line individually. Repeat for each line - it may not be necessary for them all to succeed, but each station *must* have at least one session with status Success to be able to do network adjustment, and the more the better.

3.3.1. If the line is longer than ~15km, select Settings > Load Settings From > Long Baseline Static.

3.3.2. Select Process > Process GNSS (differential) or click the equivalent icon. On the Process tab, ensure Process Direction is set to Both and Kinetic Ambiguity Resolution is set to Automatic and click Process - the line will probably go red (you can click on it to view the StdevTrace, this needs to be 0.1m or less to pass).

3.3.3. Select Output > Plot Results or click the equivalent icon. Under Measurement select "Satellite Lock - Cycle Slips" and click OK. Select "Remote GPB File". If there are long periods with all satellites marked red, find a long period that does not include (m)any red satellites and make a note of the start and end GPS times for the period (move the mouse cursor to the relevant point on the graph and note the X value in the bottom-left). Then select Process > Process GNSS (differential) or click the equivalent icon. On the General tab, uncheck "Process entire time range" and enter the noted begin and end GPS times, then click Process. If the line goes green, save and exit (the line should now show up in GrafNet as green).

3.3.4. If the line is still red, select Process > Process GNSS (differential) or click the equivalent icon. On the General tab, set the Elevation Mask to 10 (rather than 15). Click process. If the line goes green, save and exit (the line should now show up in GrafNet as green).

3.3.5. If the line is still red, select Output > Plot Results or click the equivalent icon. Under Measurement select "RMS - C/A Code". Examine the relative positions of the red line (RMS error) and the green line (standard deviation). The default SD value is 4.0, which should be higher than roughly 90% of the RMS values (red line). If it is significantly above or below that level, make a note of an appropriate value for the SD. Select Process > Process GNSS (differential) or click the equivalent icon. On the Measurement tab under Measurement Standard Deviations, set the Code value to your noted standard deviation value and click Process. If the line goes green, save and exit (the line should now show up in GrafNet as green).

3.3.6. If the line is still red, it may be possible to repeat 3.3.6 for L1 Doppler RMS (but for my test set that didn't work - there was no doppler data available).

3.3.7. If the line is still red, you've had more problems than me - none of the other settings made much difference for me, but I was using a very long baseline (>1000km in places), and many of the settings are designed for baselines of 10-30km length. If you have a baseline that short, play with the Ionospheric and KAR settings to see if you can get an improvement.

3.3.8. At any time you can view the StdevTrace by clicking on the line (note there may be more than one line there - right-click to check). The line will be red as long as this is above 0.1m, but if it's close you might want to just call it good enough - save and exit, and then in GrafNet click on the line to select it, select the appropriate session and select Override Status > Good/Success. If you want to just give up on it then select status Ignored. It may not hurt to ignore a few lines, but each station *must* have at least one session with status Success to be able to do network adjustment, and the more the better.

4. For network adjustment you should only have one ground control point (permanent base station) selected, at least initially. Pick the ground control point that you think will give you the best results, and for all the others right-click on them and select "Toggle between Control/Check".

5. Select Process > Network Adjustment. Leave all settings as default and click Process. If you get an error saying you have a hanging station, you have at least one base station that does not have any green (success) lines connecting it to a control point - go back, find it and make at least one of its connecting lines green as above. Otherwise, a text file with the results will be displayed. Check it as below:

5.1. Scroll down to the "Output Vector Residuals" section. RE, RN, RH, PPM and STD should all be small (these are measures of error). The most useful seems to be PPM, which is the Parts Per Million error relative to the length of the baseline (ie 1PPM equates to an error of 1cm on a baseline of 10km). Any entry marked with a $ symbol should be regarded as unacceptably large - you probably have a setup problem. When I got this, it was caused by the fact that I had three base stations in different places with the same name, and GrafNet had tried to put them all in the same place (causing a 400km positioning error when the distances were checked) - go back and change station names if appropriate, or find whatever else you've done wrong or where your data is bad.

5.2. Scroll down to the bottom of the file. If the Variance Factor isn't close to 1.0, repeat step 5 but use the Variance Factor as the Scale Factor in the dialog in step 5 (rather than leaving it as default). This number should move close to 1.

5.3. Read the Output Station Coordinates section to get your results. The file is saved at the location given at the top of the file header (it has a .net extension for some reason, not a .txt extension).