Aeroacoustics: Acoustic sniper localisation

Acoustic sniper localisation

During various Peace Support Operations (PSO) of the United Nations, for example, in the former Yugoslavia and Somalia, snipers turned out to be a major disruptive factor. In particular, when not locating a sniper quickly (or not quickly enough) makes it difficult to suppress the threat to the PSO. The most common method of detection is by using ear and eye. Snipers often work alone or in teams of two people and usually change positions after each shot. They prefer to use natural shelter where their fire is camouflaged and visual observation becomes difficult. Nevertheless, tracking down and, if possible, making snipers harmless remains an important part of the tasks in the context of monitoring and maintaining PSO agreements with the parties involved.

Research has shown that it is theoretically possible to automatically locate a shooter through the sound of the shot. For that reason, that option offers a relatively inexpensive solution to the sniper localisation problem. TNO-FEL already demonstrated the practical feasibility of this method in the 90s. During measurements carried out on the Infanterie Schietkamp in the Harskamp, it was shown that the theory worked in practice. In these measurements, the sensor part consisted of one array of three microphones, arranged in a triangular formation (triangle leg length approx. 150 cm). The microphone signals from a shot were processed by a PC. The result of this processing was that the direction and distance to the shooter could be calculated relative to the array’s position. In addition, the firing direction could be derived from the recorded signals. The direction to the shooter and the shot direction could be determined with great accuracy; the distance to the shooter was less accurate. A precondition for the correct functioning of the detector is that the array is located in the area where the shock wave of the shot can be received (Mach area) and that a reasonably reliable estimate of the weapon’s ballistic properties is known.

If the array is positioned outside the aforementioned Mach area, the array can only determine the direction in which the sniper is located. If the gun’s ballistic properties are not known, it will have to be estimated. With a reasonable estimate, the error in the calculated position will not be too large. When using a second microphone array, it is not necessary anymore to know or precisely estimate the performance of the weapon. In that case, a larger area can be covered and no prior knowledge of the ballistic properties of the weapon is required. Moreover, the distance to the shooter can also be calculated more accurately.

A device that can passively locate a sniper on an acoustic basis seemed to be a useful technological development. Such a device fits well with the needs of certain military units, especially of units deployed in the framework of PSOs such as an infantry battalion tasked with area security. The NATO Army Armaments Group AC/225, Land Group 6, therefore discussed the operational requirements for such a system.

The principle

The operating principle of the sniper localiser is based on the accurate determination of the arrival times in the microphones of the sound of the shot and of the shock wave of that shot. With a known assumed course of the projectile as a function of time, the location of the gunner and the direction of the projectile’s track can be calculated. The assumed geometry is shown in the figure below. The shock wave of a projectile can also be seen on the inset in the figure below. Provided that the acoustic signals caused by the shot can be properly recognised, the acoustic system will be capable of a good performance. The performance will decrease when the signal-to-noise ratio falls. The signal-to-noise ratio depends on the source strength, the distance to the shooter, possibly disturbing environmental noise in the vicinity of the array, and the local meteorological conditions that determine the acoustic wave propagation to a large extent. The advantage of the system is that it is passive, does not need a ‘line of sight’ and is relatively cheap. A countermeasure by the shooter to reduce the source strength by using silencers will be at the expense of his effectiveness.

Geometry of the sniper and the acoustic array (photo made available by ISL, St. Louis, France) — The geometry of the sniper and the acoustic array (photo made available by ISL, St. Louis, France)

Measurements

In 1996, several signal recordings were made of shots by the Fal and the Diemaco weapons at the military shooting range ‘Infanterie Schietkamp’, the Harskamp (track India and at Oostdorp). The maximum distance during the measurements on track India was about 475 meters, while the measurements at Oostdorp had a maximum distance of approximately 300 meters. Moreover, in Oostdorp there was no optical sight between the shooter and the measuring array as a result of the buildings on the site. For each designated shooting position, ten shots were fired at a target. From these shots, the arrival times of the shot and the shock wave were determined. Then the distance of and direction to the shooter were calculated, as well as the shot direction. For the measurements with the Fal on the India site, the results of these measurements are shown in Figures 2a and 2b below. In these figures, three target points are shown at the top, with several shooter positions below, each indicated by a circled number. At the top right was the microphone array, indicated by a triangle. The results are shown graphically with the corresponding figure in Figure 2a; Figure 2b shows the trajectories found.

Fig. 2a Acoustically determined distance and direction; each digit represents a calculated position (distance and angle)

Fig. 2b Acoustically determined firing direction; the directions are indicated with a dash from the shooter locations. Ten observations were made for each location.. — Fig. 2b Acoustically determined shooting direction; the directions are indicated with a dash from the shooter locations. Ten observations were made for each location.

Figure 2a shows that the direction found along the acoustic path to the shooter is very accurate; the spread in distance was generally greater. For the measurements carried out at Oostdorp where the maximum distance was smaller, both the angle and the distance information were very accurate. In all cases, the calculated shooting direction of the gunner was very reliable (fig. 2b). Under operational conditions, an acoustic detection system as indicated here can initiate a certain reaction to neutralise the sniper or it can be used as information for directing optical targeting equipment (allowing verification).

Demonstrator

In 1997, TNO-FEL developed a sniper localisation demonstrator to demonstrate the successful implementation of the principle with which the performance could be measured under different conditions. The sensor part was equipped with an extra fourth microphone which allowed the determination of the elevation of a sniper. The array was developed for placement on the imperial of a Mercedes-Benz 290GD 5kN of the Royal Netherlands Army. The block diagram of the functionalities is outlined in Figure 3. An important facet of the demonstrator was also the human-machine interface. Figure 4 illustrates the layout of the main screen.

Fig. 3: The array could be placed on the roof of a standard army vehicle while the PC is in the cabin of the vehicle.

Conclusion

Depending on the task to be performed about the threat level of enemy snipers, a passive sniper localisation system can be a useful military tool.
Typical properties of the system are:

no “line of sight” required
mobile and easily transportable
only a short installation time is required
the system is easy to operate
the system could be made compatible with the Ground Sensor Data Processing System (GGS) of the HERMES-2000 Unmanned Ground Sensor System.

Acknowledgement

This page was derived from an earlier article in Dutch by ir. H.A. van Hoof and ing. G.P. van Voorthuijsen.