International borders are prime targets for criminal activity, with weapons, drugs, and even humans being smuggled between countries by land, sea, and air. Border protection agencies are tasked with identifying and preventing illegal trafficking while also securing and facilitating legal travel and commerce, which can be challenging in remote and mountainous areas.
Rugged border regions, which may cover hundreds of miles of rough terrain, provide cover for illegal activity and make it difficult for security services to control access into and out of a country. To help compensate for these challenges, border security relies on monitoring technology, which includes radio frequency (RF) surveillance solutions and geolocation techniques.
Optical surveillance may be useful in daylight, fair weather, or open terrain, but mountainous regions are often covered in fog or foliage below the treeline, rendering line-of-sight (LOS) surveillance ineffective. In contrast, RF emissions are detectable any time of day in any weather condition, and many emissions will pass through foliage.
Because cellular service is often unreliable in remote border areas, most communication is conducted using peer-to-peer push-to-talk radios. In mountainous regions, low frequencies below 500MHz are required to maintain non-LOS communications between handsets.
In fact, all of the most commonly used wireless communication signals in remote and mountainous regions are relatively low frequencies and narrow bandwidths, with a carrier frequency in the 30-500 MHz range and signal bandwidths of 20 kHz or less. Satellite communication terminals may also be used in these areas, but these operate at slightly higher frequencies, typically L-band.
When conducting border control missions, RF signal detection also needs to include geolocation or line-of-bearing (LOB) capabilities to establish which side of the border a signal is coming from, locate the transmitter, and reject false positives. The most effective RF geolocation techniques for remote, wide-area missions are angle of arrival (AOA), which uses direction finding (DF) arrays, and time difference of arrival (TDOA), which uses a group of at least three receivers that are precisely synchronized in time.
The position of the RF receivers is critical for geolocation accuracy, especially when using TDOA. For example, when monitoring long, uneven borders, using a staggered formation helps create enough space between receivers to more accurately triangulate a signal and isolate its source.
Although using DF arrays and AOA is generally an effective means to obtain a LOB from an incoming RF signal, there are some disadvantages to relying on AOA to monitor borders in mountainous areas.
AOA is conceptually simple to understand for geolocation.
AOA only requires a single array to generate a LOB and two arrays for AOA geolocation.
AOA is capable of direction finding very narrow-band signals (e.g., signal bandwidth of less than 20 kHz).
AOA arrays require direct LOS to the emitter for accurate bearing.
AOA accuracy is degraded by multipath. (See this in action here.)
AOA array directional beam patterns are less suited to mountainous terrain than omni antennas.
AOA arrays may have a reduced probability of intercept (POI) due to commutation around sectors of the array.
Some Arrays can selectively blank out sectors of the array to minimize multipath reflecting back and degrading the bearing accuracy: for example, when a sector is in the shadow of a mountain peak. Real-time RF software can process the DF bearing results and produce polar plots and bearing quality measures so that the effect of multipath on the bearing can be minimized.
Arrays that use pairs of omni antennas have a significant advantage in mountainous terrain as they are less directional in elevation than standard sector antennas. Additionally, Arrays with spiral directional antennas are sensitive to most incoming signal polarizations, including linear, which allows reliable detection of signals that are invisible to most DF systems.
TDOA requires at least three RF receivers, typically physically separated by 1 km or more, and each precisely time synchronized using global navigation satellite systems (GNSS). The advantages and disadvantages of TDOA include:
TDOA is less affected by multipath than DF arrays.
TDOA systems typically use omni antennas with less restrictive elevation beamwidths than AOA sector antennas.
TDOA systems do not need to commutate, so POI can be higher.
Geolocation accuracy can be very high.
Small size, weight, and power requirements make receivers ideal for unmanned air platforms when extended coverage is required.
All receivers in the TDOA group must be kept in precise time sync, which makes them vulnerable to GPS jamming.
Narrow band signals (10 kHz or lower) give degraded, but still operationally valuable, geolocation accuracy with TDOA.
At least three receivers need to be illuminated by the signal for TDOA; however, in mountainous regions, receivers may be blocked from the signal. Widening the TDOA receiver baseline is difficult due to terrain blockage increasing as the baseline increases.
The topology of the receiver network is important for TDOA. The best results are achieved using an open shape with the target ideally inside the baseline. However, this is not typically possible along a borderline, and so a zig-zag pattern is preferable to a straight-line arrangement.
Precise time sync is essential for TDOA geolocation. If nodes and arrays can be fitted with GNSS holdover modules, time sync can be maintained even if the GNSS signals are jammed or unavailable.
Extremely sensitive nodes can detect low-level signals in scenarios where other receivers would only see noise. Nodes that run pulse detectors can also give an improved signal-to-noise ratio (SNR) on specific pulsed signal types. This increases the number of nodes that receive the signal with sufficient SNR to take part in the TDOA geolocation. Additionally, compatible software can create a heatmap based on TDOA isochrones that can be used even when the target emitter is located far outside the baseline of the nodes.
When monitoring borders in remote regions, security services need every available tool to detect and geolocate RF signals. The signals may be faint, buried in noise, obscured by mountainous terrain or short-pulsed and only active in brief bursts.
There are unique ways to combine the strengths of AOA and TDOA systems to optimize RF border surveillance capabilities:
Multi-heatmap geolocation: software can combine heatmaps from AOA and TDOA missions to produce more accurate geolocation that wouldn’t be possible from a single technique.
Pulse detectors: These detectors can be used on a single node to deploy rapid alerts even before a geolocation mission is triggered.
Hand-off receiver functionality: Certain array configurations can simultaneously monitor the spectrum and DF a specific signal. This allows a monitoring receiver to automatically hand off to the DF part of the array with a very high POI.
A mixture of arrays and nodes provides the most flexibility for deployment along a border. Arrays can be positioned at larger intervals, with coverage infill using smaller nodes in a zig-zag pattern along the border to avoid a straight-line geometry. The arrays provide DF bearings to augment the TDOA measurements. The nodes provide more cost-effective local coverage where the terrain is blocking DF arrays.
Border monitoring in mountainous regions can be extremely challenging, but the tremendous amount of intelligence that can be gained from geolocating RF signals in border areas makes it well worth the effort. By utilizing a mixture of nodes and arrays with the best AOA and TDOA techniques, it’s possible to monitor and geolocate a range of signals emanating from either side of a border with a high level of accuracy.
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