GPS Base Station User Manual

Obtaining Base Station Files

The JMU/GPS Base Station is housed in the Geographic Science Program at James Madison University, Harrisonburg, Va. 22807, (540) 568-6199. Inquiries should be addressed to Chris Rothgeb (rothgecw@jmu.edu).

• Station name: JMU_Geography
• Default file designation: J
• Location coordinates:
  • Lat. 38*26'02.08481" North
  • Lon. 78*51'44.72485" West
  • HAE = 420.195 m
• The base station is set up to collect data from 1100 until 2300 UTC, seven days a week. (If other times are desired contact the Geographic Science Program).
• Data are accessed via the Internet at http://maps.cisat.jmu.edu/gps/.
• All Base Files begin and end at the hour.
• To download base files from the Internet site, simply select the appropriate files from the list and click the left mouse button. A screen will appear that asks for the destination. It is suggested that the base files be saved to the Base Folder in the appropriate Project Folder.
• The standard file naming convention is used.

File Format

File format is: JYYYYMMDDHH.SSF

Where:

• J = the station designation (JMU)
• Y = the year (eg. 2004)
• MM = the Month
• DD = the day
• HH = the hour (UTC)
• .SSF = the file extension for GPS correction data.

*Important*

The system collects data using UTC (Universal Time Coordinates). Therefore, local times must be converted to UTC. The local time offset is negative five (-5) hours during Eastern Standard Time (Nov. - April), and negative four (-4) hours during Daylight Savings Time (April - Nov.). Also, the system uses a 24-hour clock. For example: if you were collecting field data at 10AM EST, the concordant base file time would be 1500 (10:00 + 5:00 = 1500). See the UTC Time Conversion Table below.

UTC Time Conversion Table

The satellite clocks are set to UTC (Universal Time Coordinates), which means that local times are set 4 or 5 hours behind the satellite clocks depending on the time of year. The following table should help when determining the appropriate base files required to match you field data.

Local Time	UTC-4 April-November (Daylight Savings Time)	UTC-5 November - April (Eastern Standard Time)
12:00 (midnight) 1:00 a.m. 2:00 a.m. 3:00 a.m. 4:00 a.m. 5:00 a.m. 6:00 a.m. 7:00 a.m. 8:00 a.m. 9:00 a.m. 10:00 a.m. 11:00 a.m. 12:00 (noon) 1:00 p.m. 2:00 p.m. 3:00 p.m. 4:00 p.m. 5:00 p.m. 6:00 p.m. 7:00 p.m. 8:00 p.m. 9:00 p.m. 10:00 p.m. 11:00 p.m. 12:00 (midnight)	0400 0500 0600 0700 0800 0900 1000 1100 1200 (noon) 1300 1400 1500 1600 1700 1800 1900 2000 2100 2200 2300 0000 (midnight) 0100 0200 0300 0400	0500 0600 0700 0800 0900 1000 1100 1200 (noon) 1300 1400 1500 1600 1700 1800 1900 2000 2100 2200 2300 0000 (midnight) 0100 0200 0300 0400 0500

*Important*

Although base files change names at the hour every hour, rover files that cross hours do not change names at the hour. A Rover file starting at 10am and ending at 11:29am on Feb. 9, 1996 is called R020915A.ssf. The necessary base files to process this data include A1996020915.ssf and A1996020916.ssf.

*Caution*

When downloading base files that cross midnight UTC, the date of the file also changes. You must be certain that if you’re collecting data at 7pm EST or 8pm DST that you get the proper base files. For example, a file collected for 4 hours beginning at 6pm on October 15, 2004 (R101522A) would use the base files: J2004101522.ssf; J2004101523.ssf; J2004101600.ssf; and J2004101601.ssf.