Precise Time Reference

Applications
Title	Precise Time Reference
Edited by	GMV
Level	Intermediate
Year of Publication	2011

GNSS technologies have a design dependence on accurate timing. The resolution of positioning equations depend on the accurate timestamping of GNSS messages and the four variables resolved by positioning equations are: time plus the 3D position coordinates. Each navigation satellite has atomic clocks that are synchronized from a master clock on the ground and the navigation messages are timestamped with the transmission time of the signal.

This allows GNSS receivers to act as a worldwide synchronized time source with a precision that could only be maintained during long periods by expensive equipments. This enabled a wide set of applications that rely on GNSS synchronized precise time sources. These applications can range from network synchronization and optimization to encryption and digital signature of electronic data.

Application Architecture

Passive Hydrogen Maser Clock

Navigation satellites have extremely precise atomic clocks on board. Atomic clocks use the oscillations of a particular atom as their “metronome”. This form of timing is the most stable and accurate reference that has ever been developed.

The operation of satellite navigations systems is based on the method of triangulation. Knowing the distance from at least three points (i.e. three satellites), the receiver on the ground can calculate its position. The distances are calculated by measuring the time that a certain signal, known to the receiver and transmitted by the satellite, takes to travel the distance between the satellite and the user. Each signal contains information on the time reference of the atomic clock on board the satellite and information on the satellite’s orbit. This allows the user to determine the position of the satellite and his own distance from it with a high degree of accuracy.

For this system of measurement to work, all satellites need to be synchronized so that they can start transmitting their signals at precisely the same time. This is achieved by continuously synchronizing all on-board atomic clocks with a master clock on the ground.

The receiver's clocks, however, are small quartz oscillators like those found in a wristwatch. Quartz oscillators are very accurate when measuring times of less than a few seconds, but rather inaccurate over longer periods. The solution is to re-set the receiver’s time to the satellite’s time continuously. This is done by the receiver’s processor using an approximation method involving signals from at least four satellites.

The accuracy achieved by GNSS-based time synchronization using GPS is <40 ns 95% of time.

Application Characterization

GNSS is being used as a precise time source in several areas, such as:

Network synchronization for power generation and distribution

The growing integration of networks for energy distribution and the emphasis on energy savings and efficiency require increasingly precise and accurate synchronization. GNSS can provide the synchronization to achieve efficient power flow. For example, measurements of perturbations must be time-tagged with errors of less than 0.001 sec. Moreover, the management of high-power generators, such as large turbo gas and large steam turbines, requires strict timing.

Electrical energy is not easily stored and, in the case of malfunctions, current or voltage surges propagate along the lines. There is a tremendous potential for cost savings through the reliable remote reading of meters. Surges are sometimes large enough to damage line equipment and cause long interruptions in service. For tracing the origin of the problem and deciding on what action to take, time-tagging the individual events is mandatory. With time synchronization at the microsecond level, the fault can be located to within 300 m – the distance between power line towers.

Communication networks

New digital technologies and value-added time-sensitive services (real-time video, video conferencing, bank-to-bank encrypted exchange) need reliable network architectures (GSM, UMTS, Internet, ATM). Subscriber growth and consumer demand are driving the operators to emphasize quality, reliability and breadth of services. It is therefore imperative that network timing is addressed and that synchronization problems are solved. GNSS can provide high-precision timing and frequency information without the need to invest in expensive atomic clocks.

Also satellite-navigation techniques could improve the communication capacity of networks. This is especially relevant for the UMTS third-generation using CDMA techniques. A precise time-synchronization of the different base stations (the UMTS emitter-antennas) can significantly increase the traffic capability of the system. GNSS can be a reliable tool not only for positioning but also for timing. It provides the mobile communications operator with a reliable and precise tool for increasing their network performance.

Data encryption and security

The latest technologies for electronic encryption, signature and time-stamping rely on highly precise time references – at performance levels obtainable only from atomic clocks – so they are not affordable to mass-market users. The spread of the use of GNSS as a timing service enables the secure transmission via inexpensive terminals, thus bringing data security within the reach of us all.

These encryption, electronic signatures and time-stamping technologies can be used for securing and authenticate electronic documentation that has become an effective alternative to paper. Many applications for electronic documentation can be enabled by the use of GNSS technology for encryption, electronic signature and time-stamping. Such approach is already being used in the financial sector where security, data integrity, authenticity and confidentiality depend on the accurate timestamps that can be enabled by GNSS

Application Examples

GNSS can be used as a precise time source for:

• Frequency Control - Precise control of transmitter frequency or reference frequencies.
• Clock Synchronization - Synchronized clocks across network, organization or territory.
• Data Encryption and Security - Provide accurate and secure encryption, signature and time-stamping for electronic documents and data.
• Network Synchronization - Precise time across computer networks enhancing application performance, communication protocols performance and security across the network (e.g. acting as a reference for NTP time servers).
• Power Network Synchronization - Troubleshooting in power networks benefit from a synchronous time source in the logging of events.
• Telecom Synchronization - Communication protocols and networks benefit from precise synchronization and timing.
• Test and Measurement - Calibration of equipments often requires precise time sources.
• Time and Frequency Distribution - Distribution of time and/or frequency across networks.