GrafNet	Chapter 3

After processing, baselines will be green if they passed or red if they failed. If an error occurred and only an approximate 1-5 metre solution was extracted, the baseline will show up as purple. If the baseline shows up as blue, a serious error has occurred. See Section 3.2.1, on Page 152 for steps to follow if a baseline fails.

3.1.3Computing Coordinates

Once the processing is complete, there are two methods to produce coordinates for each station.

Traverse Solution

This solution automatically computes during processing. It starts from known stations and transfers positions to neighboring stations one baseline at a time. A tie or closure will be computed for stations that already have coordinates transferred.

Network Adjustment

This method takes all of the baselines into account into a single weighted least-squares adjustment. It spreads the errors out over the entire network and takes advantage of redundant baselines using covariance weighting to produce more accurate station coordinates. The network adjustment flags erroneous measurements in the sense that the residuals (calculated baseline vector minus observed value) will be unacceptably high. This is most obviously displayed by the PPM (parts per million) value.

3.2Start a Project with GrafNet

The information in this section tells you how to start a project and quickly process a static network using GrafNet. The individual menu items are discussed later in this chapter.

Install Software

Verify that the installation was successful by ensuring that you have a Waypoint GPS program group on your computer. If this program group is not there, see Section 1.2.2, on Page 24 for installation instructions.

Create a Project

Follow the instructions in the shaded box.

How to install the software

1.See Section 1.2.2, on Page 24 for installation instructions.

How to create a project:

1.Select GrafNet from the Waypoint GPS program group in your start menu.

2.Select File | New Project.

3.Find the directory where the project files will be located. GrafNet creates many files during processing so using a new directory makes things easier.

4.Give the project a name and click Save.

Entering the name of a project that already exists overwrites the file contents.

GrafNav / GrafNet 8.10 User Guide Rev 4

149

Chapter 3	GrafNet

How to convert data

If Waypoint's logging utilities were used to log directly into GPB format, then skip these steps.

1.Select File | Convert | Raw GPS to GPB.

2.Browse to the directory containing the raw data.

3.Click Auto Add All or select the appropriate GPS receiver and convert the data file.

How to add observation files

1.Select File | Add / Remove Observations.

2.Click Get Folder and select the directory containing the GPB and EPP files.

3.Select the files that you want added.

You only need to select GPB files. The EPP files are assumed to have identical prefix names.

4.Verify the station ID and the antenna height when the dialogue box appears.

5.Click OK.

6.Select File | Save Project.

How to add a ground control point

1.Select File | Add / Remove Control Point.

2.Press the Add button.

3.Select the ID corresponding to the control point to be added.

4.Enter the latitude, longitude, and height for that station.

These coordinates should be in the same datum as the selected under Options | Datum.

5.Click OK.

How to set the processing options

1.Select the desired static processing mode. These modes are described in Section 3.1.2, on Page 148 .

2.Select the desired processing datum.

Convert Data

To be processed, raw GPS data files have to be converted into Waypoint’s GPB format, including raw data from Waypoint’s data-logger program. Instructions on how to convert these files are in the shaded box.

Add Observation Files to the Project

Files can be added one at a time by clicking Add, or all files in the directory can be added by using Select All followed by Add. A dialogue box appears requesting verification of the station ID and antenna height. With some receivers this information will be filled in automatically. Always ensure that the ID and antenna heights are correct. Steps on how to add observation files to a project are in the shaded box.

If a station has been observed more than once, the station ID should be the same for each observation. Otherwise, two separate stations will be formed and solved for.

Add a Ground Control Point

Follow the steps in the shaded box to add a ground control point.

Set the Processing Options

Follow the steps in the shaded box to set the processing options.

150	GrafNav / GrafNet 8.10 User Guide Rev 4

GrafNet	Chapter 3

Process All Sessions

Follow the steps in the shaded box to set the process all sessions. After these steps are completed, error ellipses on each of the single session baselines and the processing window should both appear.

Verify That All Baselines Have Passed

Passed baselines will be plotted in green, failed baselines in red, purple, or blue. Duplicate (yellow) baselines do not show the pass / fail via coloring and must be verified. To verify, follow the steps in the shaded box.

If it is a closed loop network and you feel that the solution is correct despite failing GrafNet’s statistical tests, right click the baseline and select Override Status to override the status. The traverse will transfer coordinates using failed baselines, while the network adjustment will not.

Run Network Adjustment

Follow the steps in the shaded box to run a network adjustment. After these steps are completed, The

Network Adjustment Results will open, while error ellipses will be plotted for each station on the Map Window.

Export Station Coordinates

Follow the steps in the shaded box to export station coordinates. See Section 2.7.4, on Page 111 for help.

Clean Up processing Files (optional)

Follow the steps in the shaded box to remove some of the intermediate processing files.

How to process all sessions

1.Select Process | Process Sessions.

2.Select All unprocessed as the Sessions to Process under the Process tab.

3.Click the Process button.

How to verify that all baselines have passed

1.Right-click the failed baseline.

2.Select View Results or View Information to find out where the problem lies.

See Section 9.5, on Page 268 for tips on dealing with these baselines.

How to run a network adjustment

1.Select Process | Network Adjustment.

2.Press Process.

How to export a station coordinate

1.Select Output | Export Wizard.

2.Enter an output file name.

3.Select the source for the coordinates (usually Network).

4.Select a profile containing the desired output variables.

How to clean up processing files

1. Select File | Remove Processing Files.

GrafNav / GrafNet 8.10 User Guide Rev 4

151

Chapter 3	GrafNet

How to fix bad baselines

Fixed Static Solutions

If fixed solution fails, then try to switch to a float solution. Remember that float solutions are not very good at achieving centimeter accuracies unless there are many hours of data.

Forward / Reverse Processing

Switch from Forward to Reverse processing. The reverse solution might pick a different base satellite and have a different solution that passes. GrafNet will pick the forward or reverse solution with the latest date so reprocess the forward solution if the reverse is unacceptable.

Changing the Elevation Mask

GrafNet by default uses a 15° elevation mask. This is because tropospheric and ionospheric errors increase significantly on low satellites. Lowering the mask to 10° allows more satellites into the solution, strengthening the geometry. This improvement might offset the atmospheric errors.

Use KAR

Use KAR in GrafNav for an individual baseline. Edit with GrafNav and then reprocess with GrafNav and then reprocess by enabling KAR and engaging KAR in static mode. See GLONASS Options on Page 83.

Start and End Times

The start / end times can be modified from within the General tab. Sometimes a data set can have parts that are unusable. A good indicator is the L1 Phase RMS plot. Entering the start and end times eliminates these parts by selecting the appropriate times. Another way to find good sections of data is to view the FSS or RSS files. There is a section at the start showing numerous periods of cycle slip free data for various satellites, and it also shows which period was used. You can also try entering the start / end times from another period exclusive to the one used.

Continued in the shaded box, on the following page.

3.2.1Fix Bad Baselines

Some baselines appear red when running GrafNet and duplicated baselines will stay yellow. When examining the baseline sessions from within the Sessions window of the Data Manager, one or more sessions will have Bad indicators. This means that one or more tests have failed in the static solution. If a baseline is flagged as Approximate, then the baseline was only able to achieve an approximate (1-5 m) solution and it has failed. These baselines can also be considered Bad.

The purpose of this section is to help you re-process these baselines to achieve better results. There are many causes of a failed baseline, and not every solution is given. In some cases, the correct solution has been found, but the statistics just fail. This indicates that GrafNet cannot label this baseline as correct. If there are loops that are closed in this solution, then you can change the status from Bad to

Good using Override Status from within the Sessions window of the Data Manager.

The shaded boxes on this page and the following page contain optional approaches to fix bad data.

152	GrafNav / GrafNet 8.10 User Guide Rev 4

GrafNet	Chapter 3

How to fix bad baselines continued...

Doppler Usage

Bad Doppler data sometimes makes data process poorly. Detect this by plotting the L1 Doppler RMS. If there are any large spikes (5 or more times the size of the Doppler standard deviation), then this data may have problems. Try to shut the Doppler OFF for carrier phase processing from within the Advanced Options.

Satellite Omission

A bad satellite has many bad data warnings in the message log file (FML/RML). Omit this satellite with the Advanced tab options.

Measurement Standard Deviations

For a float or iono-free solution, changing the standard deviations for both the L1 carrier phase and the C/A code improves a solution. Obtain suitable standard deviations by viewing the plots for both the C/A code RMS and the L1 phase RMS. A good standard deviation value is one which about 90% of the RMS falls below. Realistic values allow the Kalman filter to perform better.

GrafNav / GrafNet 8.10 User Guide Rev 4

153