- What is GPS and how does it work?
- What is a GPS clock and what is different about GPS time?
- How do we obtain GPS timing information?
The GPS system has gained prominence as a tool to provide highly accurate timing information, and is increasingly used as an accurate time reference for computer timing applications, such as NTP time servers. This article provides an introduction to the GPS system and how it can be utilised to provide precise timing information, along with an overview to the how GPS timing information is obtained.
TimeTools SR Series GPS NTP Time Servers Utilise GPS Technology To Obtain Highly Precise Time.
What is GPS and how does it work?
The Global Positioning System, more commonly known as GPS, is a satellite based navigation and positioning system. Theory behind the GPS system began in the 1970′s and was created by the US Department of Defence with the need to acquire extremely accurate positioning information for global navigation. The GPS system is made up of a constellation of 24 satellites which orbit the Earth, each of which feature on board an atomic clock which acts as a precise time reference.
How is GPS Time Different?
The GPS system offers an uninterrupted broadcast of GPS time. It can be utilised as a global application as GPS time is referenced to Universal Coordinated Time, (UTC), which is the same everywhere in the world, and will not alter with time zones. With the use of comparatively low-cost equipment, such as a GPS receiver and antenna, we are able to reference GPS time relatively easily.
The GPS signal is a very weak low-power radio transmission. Each GPS satellite transmits two carrier signals designated L1 and L2. Transmitted at 1575.42MHz the L1 frequency is utilised by civilian GPS receivers. For best results the GPS antenna is ideally situated on a rooftop which provides it with an optimal view of the sky to acknowledge passing satellites. GPS transmits in a straight line and can transcend clouds, glass and plastic. Satisfactory results can often be achieved by locating the GPS antenna in a window or on the side of a building, however consideration has to be given to metal and brickwork which blocks the signal.
What do we mean by GPS Clock?
The GPS system offers a free-to-air time synchronization service. Completely free of charge, it has no ongoing set-up, or subscription, fees. Numerous computer networks observe a GPS clock as a precise time reference. Highly accurate NTP servers systems synchronize Network Time Protocol (NTP) servers utilising the GPS system as an external reference clock. The high level of accuracy is achieved with a GPS receiver offering timing information to within nanoseconds of UTC time. This level of accuracy is quite often more than necessary for most computer network timing applications.
How do we obtain GPS Time?
The GPS antenna is typically a rounded device in design and often quite small being in the region of 900mm in diameter. GPS timing antennas are typically placed in a permanent, static location, ideally on a rooftop as discussed earlier . Acting as a signal amplifier the GPS antenna intensifies the GPS signal enabling it to transmitted down a cable, typically coax, to the GPS receiver.
A number of points have to be considered with regards to cabling GPS installations:
Cable length from antenna to GPS receiver.
The amplification of the GPS antenna, and the quality of the coax cable that is installed, has a direct effect on the cable distance that can be run between the GPS antenna and receiver. A typical GPS antenna may exhibit a gain of 35db. Low quality coax cable for example RG58 features a attenuation of 0.64db/meter at 1575MHz. As such, utilising RG58 cable a cable run of 55m can be achieved. Longer distances can be reached with higher quality coax cable, LMR400 cable, for instance, has the potential to provide an unassisted cable run of up-to 200m. Such high quality cable can however be expensive and can significantly increase installation costs. An alternative option is provided in utilising LMR200 cable which offers an unassisted cable distance of 80m. With the use of GPS amplifiers, which act to boost the GPS signal even further, it is possible to extend the cable length even further.
GPS Receiver Output
The GPS receiver in a NTP server continually issues position and timing data, typically transmitting the data both to and from the receiver via a RS232 serial interface. Standardised GPS interface protocols are in place of which the most renown is NMEA. The National Marine Electronics Association (NMEA) protocol is defined by a number of sentences, or character strings, which are transmitted at 4800 bits per second. Precise time and positioning data is a feature of each character string. NMEA sentences are generally not recognised as having the high level of accuracy required to act as a time reference, due to latencies involved in serial communication. As a result PPS, or pulse per second, output is brought into operation
Pulse Per Second Output (PPS)
The PPS output refers to a precise pulse output each second and which is aligned to the start of each second. A hardware interrupt input for precise timing is provided by the PPS output being fed into a control line on the RS232 interface. Commonly the DCD RS232 line is adopted as a PPS input from the GPS receiver. In jointly utilising NMEA time and positioning data with the PPS output the result is an extremely precise time reference for computers and computer networks.