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Frequently Asked Questions about GPS

Background
GPS (Global Positioning System) is a satellite navigation system to determine accurate location and time.  It was developed by the U.S. Department of Defense, and is widely used today for both navigation and time synchronization.  The satellites are owned by the Department of Defense, paid for by U.S. tax dollars, and reception of satellite signals is available for public use. More on GPS technology...


ClockWatch Star Sync GPS time synchronization works by using a GPS receiver that is connected to your computer.   Beagle Software has designed a software interface to turn your computer into a network time source.

Here are some frequently asked questions about GPS.  Feel free to   with other questions you have.


How does Star Sync get the time from GPS ?
What type of GPS receiver works best?
How accurate is GPS time synchronization?

How does GPS work?
Where does GPS work?

Understanding GPS technology...
What type of hardware do I need?
Do I need an Internet connection to use GPS?
I already have a GPS receiver; will ClockWatch Star Sync work with it?
Why would I want to use GPS?
Can other computers and devices get their time from Star Sync?


How does Star Sync get the time from the GPS?

The GPS receiver outputs UTC (Coordinated Universal Time) date and time of day in the transmitted data. After the initial position fix, the date and time of day are calculated using GPS satellite information and are synchronized with the one-pulse-per-second output.

The highly accurate one-pulse-per second output is provided to Star Sync from the Garmin GPS35. The signal is generated after the initial position fix has been calculated and continues until power down. This rising edge of the signal is synchronized to the the start of each GPS second. The information transmitted to Star Sync is referenced to the pulse immediately preceding the NMEA 0183 RMC sentence. Beagle Software specially configures the Garmin GPS35 so that this pulse is usable by the Star Sync software.
 


What type of GPS receiver works best?
Some GPS receivers work better for receiving time signals.  This is due to the default port used for time, which is considered by navigational receivers to be a secondary priority.  You will have more accurate time if you use a receiver that has PPS (pulse per second) output available, and that this output is the priority. 

Beagle Software offers the Garmin GPS35 receiver, which is specifically designed for time output.


How Does GPS work?
GPS receivers integrate a radio and a navigation computer and can receive the faint, twenty-watt signals coming from the satellites. The computer uses these signals to calculate the distance between the satellites and the receiver. With this information, the computer can further calculate the position and velocity of the receiver.
The number of satellites visible to a receiver constantly varies between four and eleven according to time and location. Each satellite broadcasts a number of unique spread-spectrum codes, but only one, the Coarse Acquisition (C/A) code, is easily accessible for civilian use. The C/A in orbit 11,000 miles above earth, GPS satellites transmit at twenty watts a number of unique spread-spectrum code. The number of satellites visible to a GPS receiver constantly varies between four and eleven according to time and location. Code is effectively a timing signal synchronized to an international time standard-Universal Coordinated Time (UCT). UCT is kept by a world-wide ensemble of cesium and hydrogen maser frequency standard atomic docks. The highest-quality GPS receivers measure the C/A code to better-than- nanosecond precision. More...
 


How accurate is GPS time synchronization?
GPS time synchronization is highly accurate, but your results will vary depending on your receiver and computer.  The Garmin GPS35 receiver is accurate to within a millisecond of UTC; however, the time that is ultimately synchronized to your computer will not be as accurate.  Why?  Because computer systems are not fast enough nor consistent enough for better accuracy.  Our tests show an accuracy of +/- 0.1 second using the Garmin GPS35. 


Where can GPS work?
GPS reception is available around the globe.  You will need to have a clear view of the skies so the receiver can triangulate at least three satellites.  An office window works well, but you may have problems receiving signals in the inner area of buildings. In general, metal and masonry block GPS signals, while glass, wood, or plastic does not.

Under specific conditions, GPS will not provide the time. For instance, the 1,542 MHz GPS signal does not penetrate buildings, which makes it difficult to receive signals indoors away from windows. Also the signal can be critically weakened by heavy foliage and interfered with by other sources such as poorly maintained television broadcasting equipment.
 


What type of hardware do I need?
You need a GPS receiver, antenna, power supply, data cable and serial port connector.  These come standard with the Garmin GPS35.  You also need access to a power source.  You may also purchase accessories, including mounting units from Beagle Software.

The only other requirement is a functioning Windows PC with an available serial (COM) port. 


Do I need an Internet connection to use GPS?
No.  GPS time synchronization does not require an Internet connection.


I already have a GPS receiver; will ClockWatch Star Sync work with it?
ClockWatch Star Sync will work with most GPS receivers with NMEA 0183 data output connected to an available serial port on the computer running Star Sync software.


Why would I want to use GPS for time synchronization?
Time syncing using GPS is a good alternative for remote or highly secured computers.  ClockWatch Star Sync does not require an Internet or modem connection.

GPS time synchronization may potentially be more accurate than syncing over the Internet or by modem.  The true accuracy depends on the receiver as well as the computer.  Beagle Software's experience is that the Star Sync when used with the supplied Garmin GPS35 receiver is consistently accurate within 0.1 second.


Can other computers and devices get their time from Star Sync?

Yes, computers, routers, printers, PBXs and other devices on your LAN can use Star Sync as a central timeserver. All they need to do run an NTP or SNTP client and point to the computer running Star Sync. Many computers (including Windows) and networked devices have this capability built in.

 

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For more information:
  Advanced Features or Star Sync software
  Star Sync GPS Receivers:
      Installation Information
      Antenna Mounting Options
  Frequently Asked Questions about GPS
  GPS Glossary

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Copyright 2007 Beagle Software. All rights reserved
Last reviewed February 01, 2007