The GPS system is a satellite navigation system operated by the United States military. The system is also available for civilian use without any subscription requirement. The GPS system is primarily intended as a highly accurate global positioning system. However, each GPS satellite also has onboard a highly accurate atomic clock, ideal for providing a precise time reference for computer network time synchronisation.
This article discusses how GPS time server systems obtain precise timing information to provide a highly accurate timing reference for network time synchronisation.
GPS Time Servers
The GPS system consists of a constellation of 24 orbiting satellites, each with a precise atomic clock timing reference. The system covers the entire globe. Precise time information is continuously broadcast from each of the satellites. The broadcast timing information can be easily received with a relatively low-cost GPS antenna and receiver.
GPS time is broadcast as Universal Coordinated Time (UTC), which is similar to Greenwich Mean Time (GMT). UTC time is the same worldwide; it does not vary with time zone or daylight saving time. UTC is a high precision atomic time standard maintained by atomic clocks located in national standards laboratories. UTC has uniform seconds defined by the International Atomic Time (TAI) institute.
GPS Radio Transmissions
The GPS satellites transmit information as very low-power radio frequency transmissions. Two designated frequencies are used, one for civilian use coded L1, and one for military use coded L2. The civilian L1 frequency is transmitted at 1575 MHz. The weak radio transmission can easily pass through plastics and glass but is blocked by metal and brick.
GPS Antenna Location
The GPS antenna needs to have a good view of the sky in order to receive transmissions from as many satellites as possible. Ideally, a roof-mounted antenna is best with a full 360-degree view of the sky. However, quite often an antenna located on the side of a building with a 180-degree view of the sky is adequate provided the horizon is not too obscured.
GPS Antenna Types
The GPS antenna is essentially a signal amplifier. The antenna boosts the received GPS signals for transmission along a cable to a receiver for decoding. Coax cable is generally used to transfer signal information between the GPS antenna and receiver. GPS antennas are available in a range of shapes and sizes. The most common antennas types being pole-mounting dome shaped antennas and small patch type antennas. The pole-mounting antenna screws onto a threaded pole for mounting. While the patch type antenna is a small flat-bottomed device ideal for mounting on a windowsill.
GPS receivers decode the GPS transmission received from the antenna into a useable format. There are a number of protocols utilised by GPS receivers, the most common being NMEA. The NMEA protocol consists of a number of sentences. Each NMEA sentence provides a packet on information consisting of time, date and positioning information. The protocol also provides information indicating visible satellites and satellite location.
GPS time servers may utilise a specific GPS timing receiver. Timing receivers have additional functions to ensure a highly accurate reference time. They can also perform an automated survey and compare satellite atomic clocks to check for synchronicity. GPS time servers may also utilise an accurate pulse per second (PPS) output generated by the receiver. A PPS output provides a highly accurate reference trigger for GPS timing.
GPS Antenna Installation
The maximum cable distance that can be utilised by a GPS antenna and receiver depends on the gain of the antenna and the coax utilised. A typical antenna used with a GPS time server may have 35dB gain. RG58 coax has an attenuation of 0.64dB/m at 1575MHz. A cable run of 35/0.64 = 55m could therefore be utilised. Higher quality coax has a much lower attenuation value allowing longer cable runs. Also GPS amplifiers and updown converters can be utilised to increase cable length still further. Surgelightening arrestors are also recommended on externally mounted GPS antennas to protect expensive network equipment from potential damage by lightening strikes.
The Global Positioning System provides a highly accurate reference clock ideal for network time server systems and computer time synchronisation. Accuracies of a few nanoseconds can be reasonably achieved with relatively low-cost GPS receivers and antennas.