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The frequency derivation aspect of global positioning systems is essential for timing digital network elements.

by Austin Lesea

Time is a critical resource. Nowhere is this more apparent than in global positioning systems (GPS). Released by the military for civilian use in 1993, GPS is a series of satellites that provide a time source so precise that it is measured in increments too small to register through most commercial means.

Unlike fixed satellites for television, the non-geosynchronous polar orbit of GPS satellites causes them to move across the sky. As each satellite moves, its location on any given moment can be calculated exactly, because of the precision of its orbit. The GPS orbit is so precise that each orbit is corrected every 24 hours by the U.S. Department of Defense. The combination of the precision time source in a precision orbit has two implications for GPS use: navigation and frequency derivation. Most are aware of the advanced navigational benefits afforded by the GPS satellite network; however, many do not realize GPS's role in timing telecommunications network elements.

Precision Timing

The frequency derivation aspect of GPS is key in providing the precision timing source needed to successfully operate digital telecommunications networks, because of the cesium-based atomic clock located within each GPS satellite. Each atomic clock is synchronized with Universal Coordinated Time (UTC), which is comprised of measurements from atomic clocks in select locations around the world.

The important value of this aspect of GPS is that the highly precise atomic clock output from a GPS satellite can be used to discipline a less expensive oscillator, such as a rubidium-based device, to output a precision frequency. Such precision frequencies can be used in a variety of commercial and laboratory applications, including timing the switching and transmission equipment of a telecommunications network to improve operational efficiency.

Used by Smaller Carriers

Before deregulation, the Bell System network was timed by an ensemble of cesium-based atomic clocks. Smaller telecommunications carriers emerging after deregulation could not afford to duplicate this resource, and thus were forced to use less precise means to time their networks. This resulted in an occasional performance problem called a “frame slip,” which is caused by the loss of synchronization between transmitting and receiving stations. While voice customers may perceive a frame slip as merely a quiet “click” during a phone call, this same frame slip could cause information to be lost during a high-speed data transmission.

The GPS timing reference signal provided the resource that led to the development of a low-cost solution for eliminating frame slip. In the laboratory, a global positioning system can be used as a precision frequency meter. Using frequencies derived from the GPS source, it is possible not only to determine the accuracy of a rubidium oscillator, but to define a pattern of accuracy loss and predict how much drift will occur over a given time period. Knowledge of this error factor can help carriers ensure accurate results when using local oscillators.

By using complimentary products to discipline a less expensive oscillator so it provides the same precision as an atomic clock, telecommunications networks can achieve the precision timing required to provide optimum performance. These added-value products, in essence turn GPS receivers into consummate frequency generators that use a precision time assembly circuit to develop a one-pulse-per-second signal based on the information issued from the GPS satellite. That signal is used to discipline—or tune—a local oscillator to provide the precise frequency necessary for a telecommunications application.

Tuning methods vary among products from different vendors, resulting in a range of performance options. Some products use direct digital frequency synthesis, which can provide better performance than the direct tuning method. Some products achieve the precision of the military GPS frequency by averaging out errors from the civilian frequency over a 24-hour period. Use of Kalman filtering, which determines and disregards signal anomalies, also can increase accuracy.

The degree of precision of an added-value product can be determined by its maximum time interval error (MTIE), which measures the time error on the pulse generated by the time assembly circuit. While several products might claim Stratum 1 or Stratum 2 performance, the MTIE rating positions products in the range between barely meeting and significantly exceeding such specifications.

The precision and cost/performance value of global positioning systems can make GPS a system of choice for frequency-generation applications. Already in the works is an improvement known as the differential GPS (DGPS), which will enhance precision through the addition of ground-based relay stations to the satellite communications network.

Austin Lesea is Vice President of Advanced Product Development and a Director of Larus Corp., San Jose, California.

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