• Alt. Control:

  Set this to "No altitude control" most of the time. The other options apply only when you are getting poor reception. The other options give you an opportunity to correct 2D points by supplying the altitude.

  Altitude Always Fixed: If you know the altitude of the job (assuming all points in the job are at about the same altitude) you choose "altitude always fixed." All points will then be fixed to the HAE altitude in "Enter Altitude for above (if needed)."

  Fixed When # of Sats = 3, GDOP >5: This option works the same as "Altitude Always Fixed", except it applies the fixed altitude only to those positions where there is data from only 3 satellites or where the GDOP is greater than 5.

  * Example: You collect points on the streets of a city. You get poor reception (because of the buildings, for example.) Most of your points are 2D, so they can not be differentially corrected by normal means. However, you know that the elevation was about 250 meters HAE, so you set ALT Always fixed to 250. All points in the job will be fixed to 250 meters HAE, and 2D points will differentially correct.

  Warning: You must be careful to enter the correct height, though. For example, if one point in the city was really at 280 meters HAE about 30% of the vertical error (30 meter vertical error) will carry over to Horizontal error (so the point would have a 10 meter horizontal error).

  Use the Mean of the Last Five Readings: This option supplies the altitude by taking the mean of the last five altitude readings which did differentially correct and supplying this to positions which have data from only 3 satellites or where the GDOP is greater than 5.

• PC-GPS will not correct any points for which the DOPS are greater than the values here. For instance, If a point feature has VDOP of 12, and VDOP limit is set to 10, this point will not correct.

  • VDOP - Vertical Dilution of precision
  • HDOP - Horizontal Dilution of precision
  • GDOP - Geometric Dilution of precision

• Elevation Mask:

  PC-GPS will not use satellites below this angle (degrees above the horizon). This number should be the same as the elevation mask of the base station. If you are not sure, enter "0"°.

• Base Position:

  Enter the coordinates of the base station here (LLA, WGS-84, HAE). Any error in the position of the base station will carry directly over to all corrected points.

• Residual Limit:

  Residual is a measure of error that can not be corrected by differential correction. This number can be set to 20 or lower. The default is 10.

• The BASE button allows you to access and maintain a library of base station coordinates. This is very useful if you routinely use files from different base stations to do differential corrections. When you select a base station from the library, the Base Position field is automatically updated.
  

• Missing PC-GPS files: make sure all relevant files are installed in the PCGPS directory. Verify that your software is installed correctly by performing differential correction on the tutorial Job included with the software.
  
• Incorrect Base Coordinates: incorrect entry of your base station coordinates will give you a error message. Make sure the right Base Position coordinates are entered in the Differential Setup dialog box.
  
• DOPs and Residuals are too high: DOP and Residual limits restrict the correction of marginal positions. For best results, DOP and Residual limits should be set to 10 or higher. If they are set too low (10 or less) some of your points may not correct.
  
• Lack of space on your PC's hard drive: you need at least 100 Mb of free disk space on your drive to perform differential corrections.
• Time Ranges don't match: make sure the times of the rover file(s) and the base station file(s) match correctly.
  
• No Ephemeris Data: If differentially correcting with a base type of Ashtech M-XII, Ashtech Z-12, Corrections, RINEX or Trimble 4000, be sure to put the matching .EPH file in your Base Directory.
  
• Not enough satellites available to get a 3D fix: remember, points taken in 2D will not correct without additional user-supplied information. The more satellites your GPS receiver receives while you collect data, the more likely the data will correct. You can use the various options in the Altitude control box to assist with this problem.

You can also use a Leica Engine, Leica MX9000, Motorola, Ashtech M-XII and Dimension, Trimble 4000, RINEX and Winbase^® base data to correct your rover file. PC-GPS can use all these other base types as-is. No additional files are required.

BasePC mode uses Kermit as a transfer protocol to store the differential correction file onto a PC. Connect to your PC just as you would for standard file transfers.

  The National Geodetic Survey (NGS) has a network of base stations across the United States. They provide their base station data free of charge to the public.

1. In PC-GPS the Batch Differential Processor can automatically download from the National Geodetic Survey CORS base station network. 

2. You can also obtain the CORS Rinex data via non-ftp site through the world wide web. Please click the above "base station data" link, or use the following address (case-sensitive):

http://www.ngs.noaa.gov/CORS/Data.html

You will need to download the "RINEX 2 data" and also the "Global/Broadcast Ephemeris data" (under the non-site specific section) for the day and base station you need. After downloading and unzipping the data, you will see the file extensions: *.04o and *.04n. The file prefixes also need to match.

The global ephemeris file has a generic name like: brdcxxx.04n.

Please rename the file prefix to match the *.04o file. For example, if the *.04o file is named: fts11230.04o, then rename the brdc1230.02n file to be: fts11230.04n.

Now you are ready to differentially correct the data in PC-GPS.

Please note: When using CORS base station data, you do not have to enter base station coordinates (they are automatically entered by PC-GPS). If using SSF files, you will need to enter the base coordinates and select the proper base station to use under the C/A Code differential.

• Coast Guard Beacons (DGPS) (See CMT's Beacon Receiver Package.)
• Subscription Service (L-Band, FM radio)
• Broadcast from your own base station