Atomic Time And The Gps System

The GPS system is now well known for providing in-car navigation solutions. However, a little know fact is that the system also provides a highly accurate source of time. This article describes how the GPS system provides super-accurate time and how it can be used for precision timing applications.

GPS, or Global Positioning System, is a system primarily intended for worldwide navigation. Its popularity has resulted in an influx of in-car and hand-held navigation systems. However, few people realise that the system was originally developed for, and is still maintained by, the US military. The system consists of a series of 24 orbiting satellites encircling the globe. Using precise triangulation and timing methods, any point anywhere on Earth can be accurately pinpointed. The need for precise timing, for positioning calculations, also provides an ideal resource for timing applications.

Each of the 24 GPS satellites has one or more synchronised highly precise Caesium or Rubidium atomic clocks on board. These atomic clocks can provide time to an accuracy of 1 second in 1 million years. The clocks are monitored and coordinated by the US Naval Observatory (USNO) to provide a reliable and stable source of time.

Accurate UTC time is continuously broadcast by the GPS system. UTC, or Universal Coordinated Time, is the same worldwide and does not vary with time zones or daylight saving changes. Any adjustment for time zone and DST offsets must be performed locally.

The GPS system broadcasts a very weak low-powered radio signal, which has two designated frequencies, L1 and L2. The L1 band is the civilian GPS frequency transmitted at 1575.42 MHz, while L2 is reserved for military operation. As with all radio transmissions, the signal travels in a straight line and can pass through clouds, glass and plastics but is severely weakened by objects such as metal and brickwork. Therefore, the ideal location for a GPS antenna is on a rooftop with a good view of the sky. However, modern, ultra sensitive, receivers can often operate satisfactorily indoors or with a severely limited view of the sky.

The GPS system is a free-to-air service; there are no on-going set-up or subscription fees. Timing and positioning information can be received with relatively low-cost, mass-produced, equipment, such as a GPS receiver and antenna.

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

The receiver receives and decodes the GPS radio signals and continuously outputs position and timing information. Generally, information is transmitted to and from the receiver via a serial interface. The NMEA protocol is a standard GPS protocol, which 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. However, 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. The PPS output is an accurate pulse reference, output each second and is aligned to the start of each second.

To summarise, the atomic clocks located on board each satellite provide a precise source of time for many applications. Many computer timing systems utilise the GPS clock as an accurate time reference. Internet based Network Time Protocol (NTP) server systems use the GPS clock as an external reference to synchronise the Internet. 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.

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About the Author:
David Evans is a technical author in the field of computer timing and NTP server synchronization solutions that ensure accurate time on computers and computer networks. Click here to find out more about timing applications and atomic clock solutions.