How to use the GPS clock

The Global Positioning System (GPS) is an American military system primarily intended for global navigation. The system provides accurate positioning for navigation. The GPS system consists of a constellation of 24 satellites in orbit. Each satellite has an atomic clock on board, which is ideal for providing a reference point of time of high precision.

This article explains how the GPS clock can be used to provide a precise 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 transmitted continuously from the GPS system. The time refers to UTC (Coordinated Universal Time), which is the same all over the world 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 radio signal from low power. The signal has designated two frequencies, L1 and L2. Frequency, L1, is the civilian GPS frequency transmitted at 1575.42 MHz signal travels in a straight line and can pass through clouds, glass and plastic, but is blocked by objects such as metal and masonry. The ideal location for a GPS antenna on the roof with a good view of the sky. Often, the installation of the antenna on the side of a building or in a window can make a heaven good enough to give adequate results.

GPS Clock

The GPS system provides a self-service in time for the synchronization of air, there is no current registration or license fee. Many computer systems use the GPS clock as time reference specified. Highly accurate NTP server systems use GPS as an external reference clock to synchronize the server Network Time Protocol. Typically, a GPS receiver can provide timing information to within a few nanoseconds of UTC. The accuracy obtained with a GPS receiver is usually more than necessary for most applications, the timing network of computers.

GPS Output

The GPS receiver outputs a constant position and dating. Generally, information is transmitted to and from the receiver via a serial interface RS232. GPS interface standard protocols exist, the most common NMEA. NMEA protocol consists of a series of phrases or strings, transmitted at 4800 bits per second. Position Information in any string of characters is correct and timely. Because of the latency involved in the communication standard, the NMEA sentences are usually not precise enough to provide a reference for synchronization. Therefore, a pulse per second (pps) output is used. A production of PPS is an accurate output pulse every second, which is aligned with the beginning of every second. The SPA can be injected into a line of control on RS232 interface to provide a hardware interrupt input for precise timing. Often, the RS232 DCD line is used as input from the PPS GPS receiver. The MAP combined with the timing and positioning information NMEA provides a very precise timing reference for the computer.

GPS Antennas

The GPS antenna is a signal amplifier that amplifies the GPS signal for transmission over a cable, typically coaxial receiver. GPS antennas are usually used for timing a dome-shaped pole mounting devices for permanent installation in a static. Typically, the GPS antenna is quite small, measuring less than 900 mm in diameter

GPS Cabling

The distance of the cable can be used by a GPS antenna and receiver depends on the amplification of the GPS antenna and the quality of the coaxial cable used for installation. A typical GPS antenna can be a gain of 35 dB. From as low quality RG58 coaxial cable has an attenuation of 0.64 dB / m at 1575 MHz Thus, a cable run of 55m can be obtained using the RG58. Best quality coax, such as LMR400, without using a serial cable to 200m can be achieved. However, high-quality coax can be expensive. A good compromise is the LMR200 cable, which can be performed without the aid 80m. Greater distances can be achieved with the aid of GPS amplifiers, which amplify a GPS signal even more, to increase the distance by cable.