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Since May 2000 the US
Department of Defence removed 'Selective Availability' as a means of corrupting
the signals received by civilian GPS units. This has improved the accuracy
of positioning to less than 10 metres and perhaps in the case of some GPS
units to as much as a few metres, such that there hardly seems any need
to carry out the correction procedures described in this module.
These notes are therefore preserved only for their historic interest!!
It is recommended that the following paper be perused:
Geologic Mapping and Collection of Geologic Structure
Data with a GPS Receiver and a Personal Digital Assistance (PDA)
Computer by Gregory J. Walsh1, James E. Reddy, and Thomas R. Armstrong
at:
http://pubs.usgs.gov/openfile/of99-386/walsh.html
Essential Keywords and Acronyms: Almanac - file that contains orbit information on all satellites. Attribute - Identifiable features (outcrops, trees) have attributes; location or rock type is an attribute; granite, basalt, shale are a domain, and granite is an attribute value. Auto2D/3D - computes 3D positions (i.e. X, Y, and altitude) unless only three satellites are available or the PDOP value (see below) is greater than the PDOP switch value of 6. C/A - coarse acquisition code (civilian code as opposed to the military P-code). Datalogger - handheld data entry computer. Elevation mask - elevation angle (15-20 degrees) above the horizon below which satellites are not tracked. Ephemeris - a list of predicted positions or locations of the 24 satellites as a function of time. HAE - height above ellipsoid. Ellipsoid - 3D figure obtained by rotating an ellipse around its minor axis, the polar axis. Ellipsoid height - normal distance between a point and the surface of the WGS-84 coordinate system ellipsoid. Logging intervals - period of time separating periods of satellite data collection; recommended interval is 5 seconds. PDOP - Position Dilution of Precision (default mask is set to 6; the lower the value the better; the PDOP switch is the value at which the receiver switches from 3D to 2D); vertical accuracy is 2-5 times less than horizontal accuracy. PRN - pseudo random noise; each satellite has its own unique PRN code, e.g. 31. S/A - selective availability; artificial degradation of the satellite signal by the DoD. SNR - signal to noise ratio; signal strength (default mask is set to 6; the higher the better; typically 12 to 20). State Plane Coordinates -NAD-27 (North American Datum 1927) and NAD-83 UTM mapping projections adopted in the USA. URA - user range accuracy; values less than 10 are preferred; a value above 8 (usually 32) is an indication that S/A is operating for that satellite. Software Quick Plan - for mission planning. Installation - install from the Program Manage in Windows (run a:\install). Trimble Pfinder v.3.00HREF="http://www.trimble.com/index.htm"
Installation - place the statements FILES=20 and BUFFERS=20 in CONFIG.SYS, and SET PFPRO=C:\PFPRO\ and PATH=%PATH%;C:\PFPRO\BIN in AUTOEXEC.BAT. The CONFIG.SYS should also contain the Shell statement SHELL=C:\DOS\COMMAND.COM C:\DOS\ /P/E:1024. Install from the DOS command line NOT from Program Manager in Windows (a:\install). To use the Pfinder software the `dongle' must be attached to the parallel port. (The dongle will have already been upgraded and activated from the Geo-PC to Pfinder upgrade activation diskette - P/N 25308-01. Both diskettes A and B have been used to activate the two available dongles.) If a mouse is attached to COM 1, then set communication port in the configuration file as COM 2. Note: do not use commas in character fields GeoExplorer - Pfinder GPS The GEOEXPLORER Operation Manual contains descriptions of all the data logger parameters and GPS procedures, whereas the Trimble General Reference is an introduction to GPS, GIS, and GPS data accuracy. The GEO-PC Software User Guide is replaced by PFINDER Software User Guide and PFINDER Software Reference. Miscellaneous 1) To turn on the backlight hold the on/off button down for 1-2 seconds (when you turn on the data logger, check to see that it is off, since it increases battery consumption); 2) to increase or decrease the screen contrast hold the on/off button and the up or down cursor buttons; 3) the button with the bent arrow is for use with the Way Point WPT functions, and for moving the highlight bar when recording the X, Y, and Elev., Reference Position during configuration of the Base Station logger; 4) to turn the datalogger off, hold the on-off button down for 5 seconds, this will give you time to note the presence of the letter `A' on the screen indicating that an Almanac has been loaded, and the amount of battery usage (e.g. = 1.7 hr); 5) to turn the data logger off without waiting the 5 seconds, quickly press the on/off button while holding down the diamond button; 6) when loading fresh batteries, the battery usage can be set to 0 from within the Configuration/Battery Usage menu. Procedure The following are the steps to follow in organizing a GPS mapping operation. 01) Setup a Project and create a Data Dictionary in Pfinder. (Pfinder 4-2; 5-2) 02) Run the Datalogger and Download the Data Dictionary to the Datalogger. (GeoExplorer 4-3) 03) Obtain a current Almanac (GeoExplorer 5-1) and transfer the Almanac data to Pfinder as an .SSF file. (GeoExplorer 6-3, 4; 13-1; QuickPlan 1-9) 04) Download the Almanac SSF to Quick Plan. (QuickPlan 1-9/10) 05) Run Quickplan. 06) Configure critical parameters and calibrate the Base Station and Rover Dataloggers. (GeoExplorer 4-5; 4-19; Pfinder 3-1) 07) Collect data (GeoExplorer 5-12) 08) Transfer data files to the PC Pfinder and examine the .SSF files. (GeoExplorer 6-1; Pfinder Soft. Ref 4-1) 09) Differential correction of position data (.COR files). (Pfinder 9-2) 10) Downloading base station data from a Community Base Station 11) Average the locations for each point file and combine the data as records in a single file (Pfinder 5-22) 12) Convert data to ASCII files. (Pfinder 11-6) Note: Batteries may last for about 4 hours at the most. "Dead" batteries will not operate the receiver but will provide enough power to prevent memory loss. The amount of free memory available at any given time is shown on the File Status screen (Maximum memory = 192 Kb). A point logging rate of 1 sec with log velocity `off' will allow data be continuously recorded for about for 3 hours (15 hours at 5 sec intervals), whereas a base station logging Positions and Raw Measurements at 10 second intervals can last for 2.5 hours. 01) Defining a Project and Setting up the Data Dictionary. From the DOS command line, create a data file `c:\pfpro\data\yourname'. Load Pfinder by typing PFINDER `ENTER' at the C: prompt (Note: Pfinder is run in DOS NOT WINDOWS). (Pfinder 4-2) Initiate a mapping project by selecting `New' and entering yourName, Owner, Root directory (e.g. c:\pfpro\data), Project directory e.g. your name, and any comments in the requisite boxes of the New Menu. Click OK. The default working directory will now have been set to c:\pfpro\data\yourname (Pfinder 5-2)To make a Data Dictionary select Utils from the Pfinder Main Menu, followed by Data Dictionary from the Utils Menu. Select New and type in the name of the dictionary, e.g. `yourname'. Click Ok and on the following menu select Add Feature, where features are, for example, `outcrops' as distinct from `trees' or `roads', and enter the feature name (e.g. outcrop). Leave the User code empty. Click the Point button and then Ok. The attribute menu will appear. [Note: 1) if Point is selected, all GPS data associated with a given feature will be averaged to a single point when using the Display feature option in the Display menu, whereas if line is selected all acquired GPS data will be connected serially to form a line; 2) When collecting data with the data logger, location data is automatically saved to a file e.g. R102518A (Rover Oct 25th time 18.00 hrs UTM feature A) when option 1. `Data Capture' is selected from the Main Menu followed by option 1. `Open Rover' File of the Data Capture menu, but is only associated with a specific feature and attribute when Option 5 `Select Feature' is chosen.] Type a name in the Attribute name field, eg station#, click on the `Numeric' button, and then the `Required' button to force entry of a data value. Click OK and on the resulting `Attr Value - Numeric' menu type in the maximum character length, e.g. 3 for 1 to 100 as a station number; `0' as the Precision value, `1' as Minimum value , `99' as maximum value (only 99 separate files can be created) and `1' as the Default value. Click OK to return to the Attribute Menu, and then click `Done' to terminate the Data Dictionary setup. New attributes can be added by selecting the Edit option of the Data Dictionary Menu. It may be quicker and more convenient however to record only the station number in the data logger, and record all other data in a notebook, to be later entered into Fieldlog after importing the coordinates and station number. 02) Acquiring an Almanac and transferring the Data Dictionary to the Data Logger. (GeoExplorer 4-3) When the GeoExplorer Rover Data Logger is turned on, the Main Menu is displayed with 7 options: 1) Data Capture (records position and attribute data in files); 2) Position (gives coordinate position); 3) GPS Status; 4) Navigation; 5) Date and Time; 6) Configuration (sets PDOP, SNR, and elevation masks and switches); 7) Data transfer (transfers data collected in files to the PC). Press the ESC key to return to a previous menu; press twice to return to the Main Menu. To determine the viability of the current operation, cursor to option 3 the GPS Status menu of the Main Menu . Option 1 of this menu shows the PRN ID number of the 6 satellites being tracked. An arrow in front of the PRN indicates that the satellite is being used in a position fix. Option 2 displays the location of the satellite (azimuth and elevation) and its SNR value, which should be at least 6. Option 3 (Sat Health and URA) shows the PRN ID number of each satellite, its health status and URA (User Range Accuracy) value. If the URA is 32 it means that S/A is active. Option 4 shows the channels being used by each satellite and the status of the signal (o - old; r - never acquired; e - no ephemeris to use; c - being used to collect an almanac; m - is tracking the satellite). It takes 15 minutes to acquire a new Almanac, and the success of the operation is indicated by the presence of the letter `A' on the screen during the 5 second interval when the Data logger is turned off. To transfer the Data Dictionary to the Datalogger, select `Communications Port' under the Comm Menu in Pfinder. Specify the PC communications port, plug the download cable into the Datalogger and on the Datalogger select `(6) Configuration' and `(7) Communication / Port A'. Set up the parameters for port A as XMODEM, 9600, None, 8, 1, and select `(7) Data transfer' on the Datalogger Main Menu. The data logger should report the comm port to be idle. On Pfinder, select `Data dictionary to Data Logger' on the Comm Menu. Select `Data Dictionary to Transfer' and click on OK. The Data Dictionary will be transferred to the Datalogger. Note: only one Dictionary can be stored on the Datalogger at a time 03) Transferring the Almanac to the PC. (GeoExplorer 6-3) Return to the office, boot your computer selecting the DOS GPS menu, and load Pfinder. Click Comm in the menu bar, select the option Communications Port in the Comm Menu, and specify the comm port to be used in the transfer (Com1 or Com2). Connect the data download cable to the selected comm port of the PC and to the A port of the Datalogger. Turn on the Datalogger and select `Data Transfer' from the Main Menu of the Datalogger. In Pfinder select `Almanac to PC' in the Comm Menu. Click on OK to effect the transfer. The transferred file will have an .SSF extension, and will be named something like 960128.SSF and be found in the directory C:\PFPRO\DATA\yourname. 04) Transferring the Almanac to Quickplan. (QuickPlan 1-9) Quickplan lets you plan a mapping session according to the optimum availability of satellites at any time during the day or night. Load Windows and from within the Windows File Manager copy the almanac `.SSF' file (e.g. 960128.SSF) from c:\pfpro\data\sample\ to c:\qkplan. Start Quickplan (in Windows) and click the highlighted date button in the Select Date dialog box that appears. When you click OK the next `Edit Point' menu will request that you select a known reference point. Select a City (e.g. Cleveland), a point on the World Map, or specify a coordinate in the Keyboard box (e.g. London at 43 00, 81 15) to define the area of interest, and then click on Ok. The Status box appears displaying the selected parameters. Next select `Options', `Time Zone' and set the Time zone to Eastern Std USA : -5.00, and the time difference to GMT to -5. NOTE: coordinates are given only in lat longs. Also need to take into account changes in daylight saving time. 05) Running Quickplan . Quickplan must have a current Almanac available to it. Therefore, select `Options', `Almanac', and load the almanac .SSF file that was downloaded from the GeoExplorer to \PFPRO\data\samples\ and copied to \qkplan. Upon clicking `OK', the status menu will show the Status file being modified, after which the file is ready to be shown by selecting `Show report' in the `Options' menu. The Report indicates what satellites are available at any given time, and their altitude, azimuth and PDOP. Also examine the Satellites option in the Graphs Menu, and the Number SV'S (satellite vehicules)and PDOP graph. 06) (GeoExplorer 4-5; Pfinder 3-1) Configure critical parameters and calibrate the base station and Rover Dataloggers. There are 10 critical parameters to be set on the Rover Datalogger - Feature logging rate for Points (1 second intervals); the Minimum number of positions to be recorded per station (120); the Not In Feature rate (15 second intervals when not recording a feature); Position mode (3-D); 2D altitude (variable); Elevation mask (15 degrees); SNR Mask (> 6); PDOP Mask (< 6); and PDOP switch (6); Antenna Height (1 metre), Units (degrees, meters, +/- relative to Greenwich, meters/second, decimal accuracy of 3), Geodetic datum (NAD-27), and Coordinate System (UTM). The Base Station is similarly configured but with the Logging and Raw Msmts collection rates set to 15 seconds (= default), and the Reference Position set to the known coordinates (not necessary if this is done in Pfinder). Recording of the PDOP values (Log DOPs) in the Rover or Base Station files is optional. The prefix to Rover files is `R' and to Base Station files `B'. Note that time is recorded as UTM time, which is 5 hours in advance of EST, e.g. UTM 23.00 = 18.00 EST = 6 pm EST. Community base station positions are given with reference to UTM time. 07) Collecting the location data. (GeoExplorer 5-12) Turn on the Base Station datalogger first. Select `Data Capture' and then `Open Base File'; a base file, e.g. B102518A will be automatically opened, where the initial `B' identifies the file as a Base file. Secondly, turn on the Rover Datalogger and in the Main Menu select Data Capture and then Open Rov. File. The Datalogger is now collecting position data to a file its has automatically opened, e.g. R102518A, where `R' identifies the file as a Rover file. Next click Select Feature to display the features in the Data Dictionary. The items displayed are the features in the dictionary you downloaded to the data logger from PFINDER, and when you select a feature it is opened in the current data file. Now the GPS positions are recorded as part of the feature until the feature is closed. Selecting the feature displays the attributes associated with the feature. To enter a letter for a character field, e.g. 1 for station#, move the cursor over the attribute and press the diamond button to enter edit mode. Use the up-down cursor keys to choose a number. Use the horizontal cursor keys to change to the next digit column. Press the diamond button to record the entry. Each station is represented by its own file of position data. Collect at least 120 GPS positions; a count is kept while the Select Feature menu is displayed. The location data is attached to a specific feature and file (e.g. station# and R102518A), and can later (see 10 and 11 below) be reduced to an average value, combined with other stations into a single file, and output as an ASCII file using the Pfinder software. (See 10 below). It is important to note that the Rover file you have created will contain location data collected before and after you have started to collect data to be associated with the selected feature, e.g. the outcrop, and that the Base station record will include data collected prior to and after the data in the Rover Datalogger file. When sufficient data has been collected select the Close Feature option and press the diamond button. Press the diamond button again to refuse collection of a second feautre, scroll to Close File and again press the diamond button. While collecting data it is possible to temporarily return to the main menu by selecting the Main Menu option. It is also possible to pause collecting the data with the Pause option. 08) Data transfer to Pfinder. (GeoExplorer 6-1; Pfinder 1-2) Load Pfinder and select your project as the Current Project. Click on `Communication Port' in the Comm Menu, and specify the PC communication port to which you will connect the Datalogger. Plug in the cable and select `Data Transfer' on the GeoExplorer Main Menu. In Pfinder select `Data Files to PC' under the Comm Menu. Click on the file name to transfer (a tick mark appears). Click on Ok to transfer the files. To display the data graphically (Pfinder 10-1), select `Output' and `Display'. A new menu bar will appear at the head of the screen. Select `Files', `Default' and `File Spec' to call the `Enter SSF File Name' menu. Click OK to see a list of files. Select a file and click OK again. In the following Display options menu, click `Display Positions' and `Display Features', set the symbol and colour in the Display and Features boxes to different values from each other, and then click OK. Decline the option to use a Filter by clicking the No Filter box and then OK. The relative position of the feature recorded each second by the datalogger will now be graphically displayed, along with the average position of the feature (outcrop). (Should you get an error message, use the edit function as described in the paragraph below, but without actually doing any editing!) Click `Query', `Point Feature', and the `average position' symbol to display in an Attributes box the number of points averaged, and the X and Y coordinates of the average location, OR, click `Position' followed by the average location of the feature displayed on the screen, to display on the command line the X and Y coordinates of the average location. Click the right button of the mouse or puck to permit another Query. Click `Files' on the main menu bar and then `Clear' to remove the graphics screen. Click `Quit' to return to the Main Pfinder menu. Alternatively, click `Add' rather than `Clear' to plot a second .SSF file, eg. the Base Station file or the corrected file (see below). If overlaying data select a different colour for each data set. To View, filter and edit an .SSF file, and in particular to see the actual second-by - second records, use Utils and Edit (Pfinder Ref 6-1). There appears to be no way to edit the fields in an SSF file, since it is a binary file. Edit does however allow the deletion of location records. To see only the text component of an .SSF file, use Utils, View Text Records, enter the File Name, and press OK. (Pfinder Ref 5-34) Carry out this operation before undertaking DIFFERENTIAL CORRECTION 09) Differential Correction. (Pfinder 9-2) Differential correction must be carried out BEFORE averaging or combining files. To change the reference position from that entered in the Base Data Logger, select `Utils' from the `Pfinder Main Menu', and then `Reference Position'. Enter the drive, directory, and base .SSF file name. Select OK. Enter the Latitude, Longitude, and Altitude, of the base station, and a text label in the Label box to identify the base file (name of base station or date of editing). Select OK. At this stage it is also possible to change the base station datum or ellipsoid to that of the collected Rover data (e.g. in the case that the base station data has been downloaded from a Community Base Sation BBS). Files that contain one location/feature (e.g. stationum) per file can be batch differentially corrected using the stand alone program PFCOR (in c:\pfpro\bin). However the basefile must be in reference to the WGS-84 datum, and the corrected data will therefore also be in this format. They can be converted to some other refernce format from within Fieldlog. Single files can also be corrected using MCORR400 and the option -p2 (filtering and smoothing) if the recorded data is suspected to be multipath. To carry out the differential correction of a single file using the PFINDER menu, select `Differential Corrections' from the Pfinder Utils Menu. Enter or change the Base and Rover file names and click the OK box. 10) Downloading base station data from a Community Base Station Instead of using one of the Trimble Geoexplorer GPS units as a base station, it may be more convenient if you have access to a telephone, to use a modem to download base station correction data from a Community Base Station that stores base station location data on a 24 hour basis. This strategy is feasible if you are collecting position data within 500 km of the Base Station. The GPS company CANSEL maintains a Community Base Station in Toronto, that can be accessed through their BBS at 905-856-9305. (CANSEL Survey Equipment, Mike Strutt, Global Positioning Systems Trainer; 2414 Holly Lane, Ottawa K1V 7P1; E-mail: mike_strutt@cansel.ca; GPS support at 1-800-357-0561; Tel: 613 731-4703; Internet: www.cansel.ca) There is also a Community base station in Sudbury, maintained by: Satellite Surveying Corporation, 1984 Regent St. South, Unit 21 Sudbury, ON P3E 5SI; Phone: 705 523-2401; Fax: 705 523-1123 Contact: Peter Bull or Paul Edward The CANSEL facility continuously collects satellite estimates of the location of the base station, and archives the data hourly as compressed files. The files have names in the form T7092614.exe, where `7' stands for 1997, `09' for Sept, `26' for the 26th day, and `14' for 14.00-15.00 hrs UTM (GMT), 9-10 am EDT) The .exe file is self-extracting and expands to a standard .SSF file. If you download several files they can be concatenated using the Pfinder `Combining SSF files' module in UTILS. Check the reference position of the new concatenated base file. To transfer an .SSX file from Cansel using a modem The following description is based on the use of a USROBOTICS modem with QuickLink II communications software. Procedure: Click QuickLink II Fax icon in Windows to initialise the communications software. (If attempting a transfer for the first time, CANSEL will require you to register, giving you first and last names, a password, a data transfer phone number, a voice phone number, and the transfer protocl (use Ymodem/G BATCH). Click the 'Data phone list' button, add the CANSEL phone number to the list, and click the 'Dial' button. When CANSEL responds, give your firstname, last name, and password. In response to the prompt 'Scan Message...', answer 'no'. On the Bulletin Board table, select 'D' for 'Download'. Type in the name of the file as: T7120422.exe for e.g. 1997 Dec 04 10 pm Greenwich Mean Time (5 pm local EST) (Several consecutive files can be transferred in batch mode; the transfer takes about 1.2 minutes per file). Press ENTER and either enter another file name, or press enter to terminate the download selection. In response to the prompt 'Ready to send in Batch mode', press ENTER twice. The prompt 'Sending files - start your download' will appear. In the QuicklinkII File menu, select 'Receive file, Ymodem/G Batch, the file directory into which to transfer the file, and 'OK' '. The file transfer will commence. When the file transfer is finished, click `OK'. This will return attention to the Cansel BBS. Select G on the Bulletin board to logoff, and click Hangup in QuickLinkII to terminate the session. The Toronto Community Base Station location is: Degrees,Min, Sec Lat 43 46 43.269115 N Long 79 33 40.127266 W Alt 140.292 meters Decimal 43.7786859 79.5611465 140.292 UTM NAD27 4848076.603 Northing 615773.816 Easting 171.729 UTM WGS84 4848298.448 615787.282 140.292 UTM NAD83 4848298.670 615786.882 142.21 Accuracy 3 cm, degrading 2mm per kilometre (200 km = 40 cm) Expand the base station .EXE file and place the expanded .SSF file in your subdirectory in PFPRO. Run PFINDER and check (and change if necessary) the reference position of the base data. Correct your position data as described in section 09) above. 11) Calculation of Mean values. (Pfinder Ref 5-19) Select `Utils', `Calculate Statistics', enter the file name (.SSF or COR), click `Yes', and OK. A screen report will appear showing the number of records, and the min, max, mean, and PDOP, for the data set. To average the location data in each .SSF or .COR file and assemble (combine) the averaged location and feature data as a set of text records in a single file, select `Utils', `Averaging Functions', and `Point Features'. Click the `Sort Chronologically' button and OK. 12) To display or output the corrected data as a GIS or ASCII file (Pfinder 11-6; Pfinder Ref 4-1) To convert an .SSF or .COR file to an ASCII file for import into Fieldlog, select Output and then ASCII. Set the delimiters in the Delimiters menu and choose the parameters to output in the Parameters menu. (In particular be careful to correctly set the columns in which the parameters are to lie to correspond to the order of the same parameters in Fieldlog.) Select `Easting', `Northing', `Ellipsoid', `Date', and `Time'. Click on file and select the SSF file to be converted in the input box. The output ASCII file will be the name in the output box. Click the `Output Point Features' box. Click Ok and then Run. The ASCII file will be displayed as formatted in the Parameters menu. The location associated with the outcrop feature will be the average of the locations collected while the file was open. Import the Ascii file into Excel and add a column of station numbers to correspond with the sorted time field. The `convert .SSF to ASCII module' will not output the value of the attribute station #. To output as a GIS file, select the required GIS format and then click Ok. Select Options and press ESC to the GIS Module Menu and select the files to be converted. Click Ok to carry out the conversion. Select Output Path from the File Menu and enter the name of the output file. Click Ok. Select Info from the GIS Module Menu to see the current settings; if parameters are incorrect they can be interactively changed. Click Ok. Select Run from the GIS Module Menu and then Preview Graphics. Click Ok. Select Execute Conversion from the Run Menu. Click Ok. Select View GIS Output from the Run Menu and click Ok. The Main menu option Utils allows the possibility to generate a file containing the average position of a set of locations associated with a given SSF file or several SSF files. In this case however the attribute data is not preserved. It allows several SSF files to be combined as a single file.
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