FEATURE BROADCAST & COMMUNICATIONS
Aiding time & frequency synchronisation
Howard Venning, Managing Director at Aspen Electronics explores some of the best options to consider when selecting a common time & frequency synchronisation system for military applications
O
ver the past few years Military Communications systems of all types
have grown, and continue to grow, at an ever increasing rate. Voice and data communications have taken advantage of advances in “digital” communications to allow more channels and greater data rates to be transmitted. Transmission methods include radio transmission (at a frequency suitable for the location, transmission type and range required) or wired over either copper or fibre cables. In addition, military platforms are being
fitted with an ever increasing array of sensors, all of which transmit data of varying rate and size dependent upon application. This data might travel a few tens or hundreds of metres from sensor to an operator on board the platform. However, transmissions may be accessed remotely and travel hundreds of kilometres over a varying array of transmission systems where the sensor is being operated or where the information received needs to be re-transmitted to others. Lastly transmissions, irrespective of source and type, may be encrypted to maintain security with encryption and de- encryptions systems implemented in different segments of the overall network. These days signals will all be digital and,
irrespective of what type of information is being transmitted (voice, data, images, etc.), to provide an effective end to end communication all these transmission scenarios rely upon a common element - accurate network timing. For the vast majority of military
communication systems network timing is derived from a GNSS signal, such as GPS in
14 SEPTEMBER 2014 | ELECTRONICS
USA/Europe. Other sources will include the European ‘Galileo’ system when it goes into service in a few years’ time; plus the Russian system ‘GLONASS’ and the Chinese system ‘Beiduo’. Today’s users typically rely upon the US GPS system until the other systems are implemented globally. Initially the US GPS system was
exclusively designed as a military positioning system before becoming a dual use system for both civil and military use - an option called Selective Availability (SA) being deployed to introduce intentional errors to reduce accuracy for civil users. Military users could disable this option to gain precise positioning but the US government decided to set SA errors to zero making precise positioning available to all. With such a vital network element,
their reference network timing signal, being delivered by a satellite system whose full technical detail was open to the world at large, military system designers implemented a security measure known as SAASM (Selective Availability Anti-Spoofing Module). This module introduces a level of cryptography to protect authorised users from false GPS signals that might be generated by others in order to undermine a system. With the SAASM option deployed you can be assured that your GPS timing signal is accurate and secure. When considering the variety of
military systems that require precise timing information for system synchronisation, or high speed machine to machine transmission, any system
Figure 1:
The Spectratime SecureSync modular time and frequency synchronisation system from Aspen Electronics
used to provide this vital system component must be capable of providing this in the correct format. Whilst certain systems might require a precise 10MHz or 1PPS signal, computer based systems can use one of many timing protocols such as IRIG, NTP or the more recently introduced PTP. Properly known as IEEE- 1588 Precision Time Protocol, PTP was introduced to meet the need for increased data transmission speeds. An important part of any system that is used to process GNSS signals, to provide precise timing signals, is “what happens when you lose the GNSS signal?” In general GNSS signal reception is not 100 percent reliable and is not as much of a problem for fixed platforms as it is for mobile platforms. The typical solution is to rely upon a built in system clock that, for short term use, will provide a sufficiently accurate timing signal whilst the GNSS signal has been lost. This is referred to as a disciplined oscillator and will typically be a high stability quartz crystal oscillator, such as an OCXO or for more demanding applications a Rubidium oscillator. In both cases these oscillators are locked to the GNSS signal whilst it is being received, and take over when the GNSS signal is lost. In selecting a product to provide all these features and functionality users and system designers would benefit from the Spectratime SecureSync modular time and frequency synchronisation system from Aspen Electronics. With more than 26 output modules and 10 mainframe options this system allows the user to find a configuration to exactly match their application. Modules can be customised to meet exact requirements via a variety of option cards to add to your configuration of timing signals, including additional 1PPS, 10MHz, time code (IRIG, ASCII, HaveQuick), other frequencies (5MHz, 2.048MHz, 1.544MHz), telecom T1/E1 data rates, multi-network NTP, and PTP. Two or more SecureSync systems can be connected as master slave with connection via coax or using fibre optic interfaces. The use of fibre optic interfaces might be useful in a high security system working in a red/ black network configuration. Timing signals can be relayed from the black, unsecure side to the red, secure side, via optical fibre maintaining the separation required. Aspen Electronics
www.aspen-electronics.com 0208 868 1311
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