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How To Use The Gps Clock
By David Evans
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 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 clock can be used to provide an accurate timing reference for computers and computer networks. The 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 system. Time is referenced to UTC (Universal Coordinated Time), which is the same worldwide and does not vary with time zones. timing information can be received with relatively low-cost equipment, such as a receiver and antenna.
The signal is very weak low-power radio signal. The signal has two designated frequencies, L1 and L2. Frequency, L1, is the civilian 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 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 system provides a free-to-air time synchronisation service; there are no on-going set-up or subscription charges. Many computer systems utilise the clock as an accurate timing reference. Highly accurate NTP server systems use the clock as an external reference to synchronise Network Time Protocol servers. Typically, a receiver can provide timing information to within a few nanoseconds of UTC. The accuracy achieved with a receiver is generally more than required for most computer network timing applications.
GPS Receiver Output
The receiver constantly outputs position and timing information. Generally, information is transmitted to and from the receiver via a RS232 serial interface. Standard 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 receiver. The PPS output combined with NMEA timing and positioning information provides a highly accurate timing reference for computers.
GPS Antennas
The antenna is a signal amplifier that boosts the signal for transmission along a cable, usually coax, to the receiver. antennas used for timing purposes are generally dome-shaped pole mounting devices for permanent installation in a static location. Typically, the antenna is fairly small in size, measuring less than 900 mm in diameter
GPS Cabling
The cable distance that can be utilised by a antenna and receiver depends on the amplification of the antenna and the quality of coax used in the installation. A typical 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 amplifiers, which amplify the signal still further, to increase cable distance. Article Source: Article Beam - a service of A1 Web Server Web Hosting D. Evans develops clock and NTP server synchronization solutions that ensure accurate time on computers and computer networks. David has been involved in the development of dedicated time servers, NTP synchronized digital wall clock systems and atomic clock synchronization products. Click here to find out more about NTP Server and GPS clock solutions.
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