Distributed spectrum monitoring networks

Monitoring spectrum usage using a distributed networks of large numbers of intelligent, covert, high performance spectrum monitoring nodes is the only practical way of providing an accurate, and sufficiently detailed, picture of spectrum usage across a wide geographic area with minimum risk to manpower and machinery. Such a network, together with powerful analysis software (automated where possible) to identify the sources of potentially suspect radio activity or jamming (either from hostile sources or inadvertent jamming from coalition forces), will make a significant contribution to the efficiency of any military or security operation. The same concept can be used for border and perimeter monitoring, identifying radio transmissions at the periphery of critical installations or territories which could potentially indicate the presence of intruders.

This article identifies one solution based on small, low cost commercial off the shelf devices originally developed for civilian spectrum monitoring which can be redeployed to an active operational military environment.

System requirements

Any system deployed in a military environment needs to be tough, and built to survive an arduous physical environment.  A wide temperature range, rugged design to cope with bump and vibration, and environmental sealing are absolute requirements which cannot be compromised on.

Low power is also a prime requirement for the spectrum monitoring nodes, both for easy deployment in fixed and mobile locations using batteries or vehicle supplies, and to reduce internal temperature rise for improved reliability.  From a radio point of view, the devices will need a wide frequency range, and fast DSP-based signal acquisition to ensure pulsed or intermittent signals are captured effectively.

Two other requirements must be met to ensure that the monitoring nodes can be deployed in many different environments whilst maximising their usefulness.  Firstly, they must be able to operate covertly, collecting data and storing it locally, where necessary making their own (programmed) decisions on what data is worth saving and which is not.  Secondly, they must be able, when necessary, to download this data (perhaps when mobile units have returned to base) to the central database quickly and efficiently using whatever communication mechanism (cellular network, WLAN or wired connection) is available.

The devices also need to have built-in GPS so that data can be logged against location: the GPS also provides very accurate time synchronisation between nodes, allowing the use of TDOA (Time Difference Of Arrival) techniques to provide signal location estimation (direction finding).  Finally, the nodes must be quickly re-deployable between fixed, vehicular, or man-portable (backpack) applications.

RFeye spectrum monitoring node

The CRFS 'RFeye' spectrum monitoring node is an example of how these requirements can be met in a small, low-cost, highly integrated package.

A frequency range of 10 MHz to 6 GHz, the ability to sweep the entire frequency range in less than 100 ms, and DSP-based measurements with a 20 MHz instantaneous bandwidth allow the capture of a wide range of persistent or intermittent signals. Four switchable antenna inputs allow switching between different antennas, or location estimation based on relative power.  Housed in a rugged, environmentally sealed housing, and capable of operating from -30 to 55°C with a typical power consumption of 12W, the node is designed to survive in severe operational environments.

CRFS 'RFeye'

A built-in Linux PC provides powerful 'if-then-else' programming capability to allow the selective capture of key data, with other data (for instance, data where the power is below a specified mask) being discarded to reduce memory storage or data transfer requirements. The node may also automatically perform statistical analysis on data captured, or send an SMS if a signal is detected which is outside of normal limits. Flexible connectivity using secure network protocols is provided by Ethernet and USB interfaces, or the in-built GPRS/UMTS modem. The integrated high-accuracy GPS receiver ensures all data is accurately location- and time-stamped. Bulk local storage may be connected via the USB interfaces.

The RFeye node can be used in fixed and mobile applications, including a battery-powered backpack system.

Analysis software

Powerful spectrum monitoring systems based on fixed and mobile RFeye nodes can collect significant amounts of data in a short space of time, and it becomes imperative that this data can be both (a) managed effectively, and (b) analysed rapidly and efficiently to extract the key intelligence value - especially in the heat of battle.

The CRFS data analysis system allows the import of data from (if necessary, many thousands of) nodes into a centralised database. The analysis tools provide easy visualisation of spectral power and spatial utilisation at any given frequency, allowing users to formulate a clear view on the general level of spectrum usage at a particular frequency on a theatre-wide basis or at any desired level of resolution. Starting at the highest representation level, users can 'zoom in' to selected areas for in-depth analysis when required.

For example a possible data analysis system can display data recorded in the ISM band .  Each 'tile' on the map is coloured to represent the highest power measured in the 2.4 GHz band by any of the measurements taken in a particular area. High powers are shown in red (i.e. 'hot') and low power areas in blue (i.e. 'cool').

The individual spectrum measurements contributing to the overall result for the 'detailed analysis zone' area can also be shown.

In conclusion, only distributed spectrum monitoring networks using low cost, intelligent monitoring nodes such as the RFeye nodes, deployed on vehicles or in fixed locations, can provide the essential information on battlespace spectrum usage. As the value of 'signals intelligence' escalates, such systems will come to be seen as an essential adjunct to other sources of information, and will be a key factor in minimising risk to coalition forces performing counter-insurgency operations in dangerous locations.

Stirling Essex is Director of Business Development at CRFS

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