Community Base Station Links
One of the most important things you can do when populating a GIS with field-collected data, is to insure it is of the highest quality possible. In addition to setting up your datalogger correctly, you should be prepared to differentially correct your data using CBS (Community Base Station) files. Always use the CBS closest to your project site. This will minimize baseline error. Below are just a few CBS links that offer free, public access.
Note: when using the ProXRS, GeoXT, or Pro6 Series, we recommend you be no farther than 155 miles from the base station to maintain the specified accuracy of 50cm, submeter, or 1-3 meters respectively, plus 1ppm horizontal and submeter plus 2ppm vertical.
Important: when using a GeoXH or ProXH, Pro6T, Pro6H the following rules must be followed in order to achieve <30cm (or <10cm horizontal accuracy with Zephyr antenna).
The GPS antenna must be able to "see" the same constellation of satellites anywhere from 30-120 seconds, depending on conditions, with no interruption of signal. In the office, you must post-process the field data with one dual-frequency base station within 80 kilometers (50 miles) of your project site, or three dual-frequency base stations, all within 200 kilometers (124 miles) of your project site. Failure to meet either of these requirements will result in excellent submeter positions, rather than <30cm or <20cm horizontal accuracy.
Note: the majority of US Forest Service and BLM base stations are single frequency stations. Dual-frequency stations are required to process H-Star data from the ProXH or GeoXH. In addition, the base station Integrity Index (as listed in the Pathfinder Office and GPS Analyst differential correction utility) must be "80" or higher. We recommend that you download a new list of base stations from Trimble's Web site at least once per month. Trimble computers update the Integrity Index of all base stations listed once per month.
Examples of dual-frequency stations: COOP_CORS, CORS, SOPAC, UNAVCO, GSI, etc. In the Pathfinder Office or GPS Analyst differential correction wizard, make sure to select "Dual Frequency Carrier," next to the "Show Integrity Index of Type" item. This will insure that you can only pick from dual frequency base stations It's important, however, that you do not mix and match base station types. In other words, make sure you use only CORS - COOP_CORS, or UNAVCO, but do not use a mix of these.
When using multiple stations to post process H-Star data, we recommend you select stations that create strong, physical geometry. In other words, do not select base stations that are all north (or in any one direction) from your project site. Best results are achieved when choosing base stations that are in all directions from your project site (i.e., one northeast, one northwest, one south, etc.).
If you have any questions on these issues, please don't hesitate to call or email Jim Lahm at (503) 624-6133.
Skagit County, Washington- use this TRS (Trimble Reference Station) to download different file types: SSF, DAT, ALM, EPH, RINEX. Click on "GPS Files" in the left margin of the page.
US Forest Service GPS Home Page- this site gives you access to over 90 base stations in the U.S.
Send mail to Jim Lahm with questions or comments about
this web site.