The Global positioning System (GPS) is a US military system primarily intended for global navigation. The system provides highly accurate positioning information for navigation. The GPS system consists of a constellation of 24 orbiting satellites. Each satellite has an on-board atomic clock, which is ideal for providing a highly accurate timing reference.

This article discusses how the GPS clock can be used to provide an accurate timing reference for computers and computer networks. The GPS system is often used for computer timing applications, such as NTP time servers and accurate timing references.

GPS Time

GPS time is continuously broadcast by the GPS system. Time is referenced to UTC (Universal Coordinated Time), which is the same worldwide and does not vary with time zones. GPS timing information can be received with relatively low-cost equipment, such as a GPS receiver and antenna.

The GPS signal is very weak low-power radio signal. The signal has two designated frequencies, L1 and L2. Frequency, L1, is the civilian GPS frequency transmitted at 1575.42 MHz. The signal travels in a straight line and can pass through clouds, glass and plastics but is blocked by objects such as metal and brickwork. The ideal location for a GPS antenna is therefore on rooftop with a good view of the sky. Often, antenna installation on the side of a building or in a window can provide a good enough view of the sky to give adequate results.

GPS Clock

The GPS system provides a free-to-air time synchronisation service; there are no on-going set-up or subscription charges. Many computer systems utilise the GPS clock as an accurate timing reference. Highly accurate NTP server systems use the GPS clock as an external reference to synchronise Network Time Protocol servers. Typically, a GPS receiver can provide timing information to within a few nanoseconds of UTC. The accuracy achieved with a GPS receiver is generally more than required for most computer network timing applications.

GPS Receiver Output

The GPS receiver constantly outputs position and timing information. Generally, information is transmitted to and from the receiver via a RS232 serial interface. Standard GPS interface protocols exist, the most common being NMEA. The NMEA protocol consists of a number of sentences, or character strings, transmitted at 4800 bits per second. Contained within each character string is accurate time and position information. Due to the latencies involved in serial communication, the NMEA sentences are not generally accurate enough to provide a timing reference. Therefore a pulse per second (PPS) output is utilised. A PPS output is an accurate pulse output each second that is aligned to the start of each second. The PPS output can be fed into a control line on the RS232 interface to provide a hardware interrupt input for accurate timing. Often the DCD RS232 line is utilised as a PPS input from the GPS receiver. The PPS output combined with NMEA timing and positioning information provides a highly accurate timing reference for computers.

GPS Antennas

The GPS antenna is a signal amplifier that boosts the GPS signal for transmission along a cable, usually coax, to the receiver. GPS antennas used for timing purposes are generally dome-shaped pole mounting devices for permanent installation in a static location. Typically, the GPS antenna is fairly small in size, measuring less than 900 mm in diameter

GPS Cabling

The cable distance that can be utilised by a GPS antenna and receiver depends on the amplification of the GPS antenna and the quality of coax used in the installation. A typical GPS antenna may have a gain of 35 db. Low-quality coax such as RG58 has an attenuation of 0.64 db/m at 1575 MHz. Therefore, a cable run of 55m can be obtained using RG58 cable. Higher quality coax, such as LMR400, an unaided cable run of 200m can be achieved. However, very high quality coax can be expensive. A good compromise is LMR200 cable, which can be run unaided to 80m. Longer distances can be achieved with the aid of GPS amplifiers, which amplify the GPS signal still further, to increase cable distance.