Post Processing GPS raw observations using data obtained from Continuously Operating Reference Stations (CORS) This guide is written using Leica Geo Office version 4, with base and rover data from a Leica GPS1200 and CORS data in the form of Rinex from the OS website (www.gps.gov.uk). However, the guide will also be relevant for post processing any site GPS data where raw observations have been recorded to CORS data within Leica Ski-pro or LGO. Please Note: To complete this guide it is essential that Rinex import and GPS processing are unlocked on the software licence key (dongle). It is also essential that antenna phase centre offsets are entered for all GPS antenna; downloading and importing the file ski_pro.005 can ensure this is correct. To download this file, navigate to Leica s UK Specific downloads site at: http://www.leica-geosystems.com/uk/en/lgs_25742.htm then select LGO and Ski Pro Antenna PCV s (you will need to be logged in as a registered user to download this). To import the file open LGO (or Ski-Pro) and enter Antenna Management, right-click in the main screen and select Import Antenna, then navigate to the downloaded file (ski_pro.005) make sure include DOME suffix is ticked then press open to import the file. 1 Create A Project: The first task that needs to be completed is to create a project within the software package in which the data can be imported into. To do this open LGO and enter the Project Management window, by selecting the Projects Icon, in the Management list on the left toolbar. (Circled in the image, top right). Once the Project Management window is opened use the mouse s right hand button to access the right-click menu, from here select New (Circled in the image, middle right). Selecting New will open the New Project pop-up box (bottom right), in this box it is possible to select the name of the project, the location it is to be stored in and varying other options regarding the project s properties in this guide the Project is to be named GPS & CORS PP and saved in C:\Data\Processing Projects\LGO-Projects\ all other settings and options are being left as default. Once the project has been setup as desired select OK to return to the Project Management window, in this screen the newly created project is highlighted open the project by double clicking on the project s name.
2 Import GPS Observations: The second step is to import the GPS data from the system that was logging raw data. To do this select Import > Raw Data from the top toolbar (1 st image, top right), this will then produce the Import raw data dialog box, in this window navigate to the location of the raw data from the instrument (for a GPS1200 this data is stored in the DBX folder of the CF-Card, whereas on a GPS500 it is in the GeoDB Folder) make sure that the files of type is set to the appropriate instrument and then the name of the job will be displayed in the main window of the dialog box (2 nd image, right), select the job to be imported and press import. After pressing Import the Assign Data to Project screen is displayed (3 rd image, right) this displays a list of all the projects registered in Project Management (Note: A warning triangle is just to inform the user that the project is in use, i.e. the project is open). Select the project to import the data to by clicking on it, then confirm the import by pressing assign. Press close once the data has been assigned. Note: In the Assign Dialog box there are a number of tabs (4 th image, bottom right) the first tab, General, shows the list of projects which can be chosen to assign the data to, the second tab, Settings, allows for some configuration of the data being assigned a setting to be aware of is the merge interval check box, if ticked this button will merge GPS observations which have had a break in, providing the opportunity to merge a series of observation periods for the same point into one complete observation period. Further Tabs can also be present such as GPS, TPS or Points, these tabs allow you to view the data that is to be imported, change point ids and even decide which points are to be imported and which ones are to be skipped.
3 Import CORS Data: The next stage of the process is to import CORS data in this guide the data will be in the form of Rinex Data from the OS Active Network, downloaded from the OS website (www.gps.gov.uk). To import the CORS data (in Rinex format), select Import > Raw Data from the top toolbar (1 st image, top right), this will then produce the Import raw data dialog box, in this window navigate to the location of the CORS data, make sure that the files of type is set to RINEX files and then highlight all the CORS Stations then press import. Following on from selecting Import, the assign dialog box is displayed (2 nd image, bottom right) as in step 2 assign the data to the correct project. 4 Process - Select References and Rovers: The next step is to actually process the data, to do this you need to define References and Rovers for the process. With the project open switch to the GPS Proc tab of LGO ( ) a screen showing the GPS observations as bars on a time chart will be displayed (1 st image, top right). The CORS data needs to be set as Reference (i.e. highlighted red) and the GPS site data collected as Rover (i.e. highlighted green), this can either be done by right-clicking on each observation bar and choosing Reference/Rover or by using the hammer icons from the top tool bar (2 nd image, bottom right). Once the references and rovers have been selected the processing parameters need to be defined
5 Process - Define The Parameters: In order to control the way the software processes the GPS data the parameters need to be set. To enter the parameters screen from inside the GPS processing tab right-click and then select to Processing Parameters. The Configure GPSprocessing Parameters box will be displayed on the general tab ensure that Show Advanced Parameters is ticked (1 st image, top right) then move to the Strategy tab. On the Strategy tab the settings can be configured to control the way the data is processed the settings used for this guide are shown on the right (2 nd image, bottom right). A setting that can often be changed to improve the GPS processing result is the Tropospheric Model; two of the most commonly used models are Computed and Hopfield. Note 1: The setting of the parameters is a step that is not always required - as default settings can be configured (by selecting: Tools>Options>Default Parameters>GPS Processing>Configure). 6 Note 2: For advanced processing parameters and the use of precise ephemeris, please consult surveying literature, although a guide Quick Guide LGO - GPS Processing Parameters and ephemeris is being made. Process...: With the references and rovers selected and the parameters set the next step for the software to process the data. On the GPS Proc tab (1 st image, top right), with the references and rovers highlighted select process, either by choosing process from the rightclick menu or by pressing the cog icon ( ) on the top toolbar. LGO will process the GPS data and jump to the Results screen (2 nd image, bottom right): On this screen the baselines between each Rinex station and each GPS site station are displayed also shown is information about the baseline processed, including the status of the ambiguity resolution, following processing the ambiguity status will normally be yes, informing that they have been resolved. In order to complete the process the baseline results need to be stored - achieved by selecting store from the right-click menu.
7 Finished?: With the Processing (and Storing) of GPS baselines between the CORS points and the GPS Site Data completed the position of the GPS site data point(s) will have been updated if the GPS site point which had raw data logged on it was also used as a real time base station (as is often the case) then the reference coordinates of this point need to be updated for all the Site Rover points to have their positions corrected. If the GPS site point(s) were not used as real-time references and just had raw data logged then the GPS Processing is finished and the points will have their updated positions as their average triplet (provided more than one CORS point was used in the processing) which is then viewable under the mean tab in point properties (1 st image, top right) accessed in the right-click menu of the point in either the View/Edit or the Points tabs. The Mean tab shows a summary of all the measurements to a particular point (in this example it shows all the GPS baselines processed and stored in Step 6-2 nd image, bottom right). The mean tab does allow the user to select/deselect measurements in order to influence the calculation of the mean coordinates of the point, it is not recommended that users alter the automatic selection without evidence of problems/inaccurate GPS measurements. 8 Real-Time Rover Points: If Real-Time Rover points were measured with one or more post processed point used as a Real-Time Reference, then the Rover points will need to have their positions updated. The first stage is to import the Rover data to the project (if this has not been done already). This is done by following the routine outlined in Step 2, however this time navigate to and select the data from the GPS kit used as the rover not the one used as the reference.
9 Update the Reference: Once the Real-Time Rover points have been assigned to the project they will be displayed on the View/Edit tab (1 st image, top right), it may be required to zoom in towards the reference point to be able to see the rover points as they will be much closer to the GPS processed point than the CORS points are. At present the Real-Time Rover points are using their Reference s navigated position as their base point the Reference Triplet), so the points are correct relative to one another and relative to the navigated position of the base, but that base position is only a navigated solution. As during the post processing of the GPS Site Point the raw GPS observation data has been processed to produce an accurate position (the average triplet) the Real-Time Rover data needs to use this accurate position rather than a navigated position as its base for relative positioning. In order to replace the Navigated position of the Reference Triplet with the Averaged post-processed position, LGO has a feature called Update Reference Triplet, this is accessed in the right-click menu of the point in either the View/Edit or the Points tabs (2 nd image, right centre). A new dialog box is displayed (3 rd image, right cente), here LGO allows the Reference triplet of a point to be updated with the Averaged coordinates in effect this is updating the coordinates that the base station transmitted to the rover with the coordinates for the base station that have been calculated from the CORS points to do this highlight the Averaged Point and then select OK. A warning box is then displayed (4 th image, bottom right) informing the user that a Reference Point has been moved and as a result Rover points will also be moved, press OK to Confirm this and return to the now fully processed GPS Project. Note: earlier versions of the software do not have Update Reference Triplet as a selectable option, if using older software the user has to copy the Average Triplet s coordinates and paste them into the Reference Triplet, using the copy/paste buttons in the Point Properties box.
10 Finished? (2): The data in the project is now fully processed to the CORS points; however, the points in this project will only have WGS84 coordinates unless a coordinate system is attached to the project. 11 If a Coordinate System is to be attached continue to Step 11, if not then the process is finished. Attach a Coordinate System: To attach a coordinate system to the project select Project Properties, from the File drop down menu on the top toolbar; Then select the coordinates tab and select a Coordinate System (image, right). Often the coordinate system used will OSGB36 (O2), if so then continue to Step 12, if not then select the desired coordinate system, press OK and the process is finished. 12 OSGB Coordinate System: In the Project Properties dialog box select the coordinate system as OSGB and check that settings are as shown (1 st image, top right) then select OK. If the settings do not match, or there is no OSGB coordinate system available then the coordinate system needs to be downloaded and (re)imported into the software. To do this navigate to, Leica s UK specific downloads area at: http://www.leicageosystems.com/uk/en/lgs_25742.htm and select to download OSGB Coordinate Systems for LGO and Ski then use the guide in the downloaded zip file for instructions on installing the Coordinate System. 13 Once the OS Coordinate System is attached the final step is to check that Orthometric heights are available for all points (this can be seen in point properties or the on the points tab. If the Orthometric heights are not visible then the Geoid Separations need to be computed. Complete this task by selecting Compute Geoid Separations from the Tools drop down menu of the top toolbar (2 nd image, right centre). The Separation will be computed and a confirmation dialog box will be displayed (3 rd image, bottom right). Finished? (3): With the OS Coordinate System attached it is now possible to view the data in WGS84 or OSGB coordinates by toggling the 84, Local, Cartesian, Geodetic and Grid icons on the top toolbar, in either the View/Edit or the Points tab.