Precision timing: a small but critical factor for defence networks.
Timing can cover many aspects across the defence industry. From simple data entry for repair works to secure communications channels.
It is crucial that timing is implemented correctly and is tailored to the specific use cases required. This is particularly important in high-security environments where sensitive information is processed.
There are two primary methods of timing for the defence network;
- NTP – a network protocol that provides timing information from a dedicated NTP time server
- IRIG – a more complex protocol that is only used in specific use cases where NTP is not appropriate
Providing timing information from a dedicated time server to end clients, NTP can synchronise all equipment with a common reference source. These units are usually synchronised to GNSS for continued operation and stability, though in some more secure environments synchronisation is provided via 1PPS and 10MHz signals which do not contain data.
The use cases for NTP within the defence industry are numerous. Anywhere where data is logged, processed or analysed needs a source of time, though not every requirement needs millisecond timing accuracy or below. One of the major areas for NTP usage would be within the personnel management systems, where multiple sites keep notes of entry, exits and other information that could prove critical in identifying security breaches or missing personnel.
There are systems used within the defence industry that require a more accurate implementation of NTP. Frigates, carriers and submarines class among these groups, as the control systems and logging equipment used require millisecond timing for precise processing of instructions and logging of events. While vessels are secure environments, fibre is not generally used for ethernet traffic as the vessels are closed infrastructures with a minimal amount of weak points for malicious activities.
IRIG, on the other hand, is a form of time code that provides timing information through a Binary Coded decimal time of year reference, consisting of seconds, minutes, hours and days. IRIG is a more stable time reference that NTP, though it does need a dedicated infrastructure to support it. This does add additional costs to the initial infrastructure development; however, a properly implemented IRIG infrastructure will provide a more reliable time than a closed NTP infrastructure.
Within closed NTP infrastructures, accuracy and resolution can vary depending on several factors that may be out of the control of the operating personnel such as operating systems, bandwidth usage, path lengths etc. IRIG, on the other hand, has a fixed resolution hardcoded to the time code itself; the most common protocol used, IRIG-B, has a resolution of 1ms or 100us depending on the method of generation. As IRIG has a dedicated line between the generator and the end node, this resolution can be considered stable, providing nothing happens to the system.
For applications where even higher levels of timing resolution are needed, IRIG-G can be used which has a coded resolution of 100us or 10us depending again on the method of generation. A typical application of the more accurate IRIG time code signals would be for the synchronisation of high-speed camera systems for military testing ranges and for specific communication systems which need a higher level of synchronisation than NTP.
Military communication systems
For communications systems specifically, another protocol can be used called PTPv2. PTPv2 is an ethernet based protocol that can achieve accuracies of a microsecond to sub-microsecond by accurately calculating the delays across the network, either through an end to end method or a peer to peer communication system. PTPv2 does, however, requires a dedicated infrastructure, even more so than IRIG solutions. PTPv2 needs to be aware of every delay that may be present within a network environment, through switches, routers and in the end slave system. This can incur high costs for the provision, installation and configuration of PTPv2 aware device and, as such, PTPv2 is only used for the most critical of applications such as secure military communication systems.
For some defence applications, a combined system is required to provide NTP and IRIG for synchronisation of different equipment. One example of this is for air traffic control, where different control systems have different synchronisation requirements. As an example; in a main control tower, the aircraft logging systems will generally use NTP to synchronise, as the flight information and flight times of aircraft do not need to be more accurate than a second or so. NTP will also be used for the logging of any incidents that occur, as each control tower is usually linked to a central database via an ethernet connection.
For the radar systems that detect the position of the aircraft, IRIG will be used above NTP for a few reasons. The first of these is for reliability. In an ATC environment, having the system running 24/7 is vital for smooth operation. Synchronising via NTP can be somewhat variable depending on the load of the network and other factors and can result in a varying time accuracy on the control systems.
IRIG, on the other hand, is only influenced by the attenuation of the cable and the master generator system if the GPS source is lost due to a jamming event. Leading time and synchronisation specialists, Meinberg produces IRIG generation systems equipped with holdover oscillators. The risk of time drift due to the GPS jamming is reduced as a result. As the cable attenuation can also be factored in, and a small adjustment added to the time as a result, it makes IRIG an ideal protocol for continuous, reliable time synchronisation.
Alongside NTP, PTPv2 and IRIG generation, Meinberg offers several industry-tailored solutions including PTPv2 Grandmaster clocks and fibre optic signal generation and distribution solutions. Fibre is essential in most critical applications, being immune from wire sniffing and allowing sites to communicate with each other over large distances without loss of packets or degradation of signal.
A specific application for this would be for transmitting an IRIG signal over a large testing range, using the fibre converters to synchronise the cameras at the other end of the range so that all cameras are tied to the same time source, limiting any variables which may impact the analysis of the footage later on.
While the Meinberg systems can now generate protocols and packets directly over fibre, some infrastructures that may be limited by budget need to make use of the equipment already installed at the site. Meinberg manufacturers a range of converters for TTL and IRIG that convert from coaxial to fibre optic and back again. These products are complemented by the diplexer units that allow for one incoming signal to be split into either twelve or twenty-four outputs depending on the configuration, again allowing for one generator to synchronise dozens of end nodes through a cascading distribution infrastructure.