Network Time Synchronization Through GPS Time Servers

Copyright (c) 2007 Galleon Systems

This article explains how GPS Time server Systems achieves exact timing information to give a highly accurate timing reference for Network Computer time.

GPS System is a navigation system operated by the Military but also can be used by the civilian and does not need any subscription. The GPS System is known as an extremely accurate global positioning system. Each GPS satellite has an onboard a highly accurate atomic clock, ideal for supplying the correct time for computer network time synchronization.

GPS Time Servers: The GPS System exist of a constellation of 24 orbiting satellites, each has a precise atomic clock timing reference and covers the entire globe. The exact time information can be continuously broadcast from each of the satellites. You can receive the broadcast timing information with a relatively low cost GPS antenna and receiver.

GPS time is transmitted as Universal Coordinated Time (UTC), which is similar to Greenwich Mean Time (GMT). The UTC time is the same worldwide and does not vary regarding the time zone or daylight saving time. UTC is a very accurate atomic time standard sustained by atomic clocks located in national standards laboratories. UTC has consistent seconds distinct by the International Atomic Time (TAI) Institute.

GPS Radio Transmission: The GPS satellites broadcast information as very low-power radio frequency transmission. There are two designated frequency that are used, one for civilian use coded L1 and the Military use coded L2.The civilian L1 frequency is transmitted at 1575MHz. It can easiliy pass through plastic and glass but is blocked by metal and brick.

GPS Antenna Location: The precondition to receive transmission from as many satellites as possible is a good clear view to the sky. Ideally a roof mounted antenna is best with a full 360-degree view of the sky. Conversely, an antenna can be located on the side of the building with a 180-degree view of the sky if the horizon is not too obscured.

GPS Antenna Types: The GPS antenna is essentially a signal amplifier. The antenna improves the received GPS signals for transmission along a cable to a receiver for decoding. A Coax Cable is normally used to transfer signal information between the GPS antenna and receiver. These antennas can be provided in different shapes and sizes. The general antennas being pole-mounted are dome shaped antennas and small patch type antennas. The pole-mounting antenna screws onto the threaded pole for mounting. Whereas the patch type antenna is a small flat bottomed device ideal for mounting on a windowsill.

GPS Receivers: GPS Receivers decipher the GPS transmission received from the antenna into a useable format. NMEA is the most common protocol used by the GPS receiver. This protocol insists a number of sentences which provide a packet on information confirm of time, date and positioning information. The NMEA protocol quotes information indicating visible satellites and satellite location.

GPS time servers can use a specific GPS timing receiver. These timing receivers have additional functions to provide a highly accurate reference time. The timing receiver can also perform an automated survey and compare satellite atomic clocks to check for synchronicity. GPS time servers may also use an accurate pulse per second (PPS) output, which provides a highly accurate reference trigger for GPS timing, generated by the receiver.

GPS Antenna Installation: The gain of the antenna and the coax is important for the maximum cable distance that can be used by a GPS antenna and receiver. An antenna used with a GPS time server may have 35dB gain. RG58 coax has an attenuation of 0.64dB/m at 1575MHz. Therefore a cable length of 35/0.64=55m could be used. Higher quality coax has a much lower attentuation value allowing longer cable runs. Also GPS amplifiers and updown converters can be utilized to increase the cable length. Surge lightening arrestors are also recommended on externally mounted GPS antenna to protect expensive network equipment from potential damage by lighting strikes