Sound ranging

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Sound Ranging (also known as sound location) is a collection of techniques for generating a position estimate of a source of sound. Historically, sound ranging was developed to detect the location of far distant artillery. There are three main approaches to generating a position estimate for an artillery piece.

  • Using the difference in the gunfire sound arrival time at different microphones to generate the position estimate.
  • Using the artillery flash to generate a bearing to the artillery piece and the difference in the arrival time between the gun flash and sound to estimate a range.
  • Using a multiple directional microphones at different locations that each accurately determine a bearing to the target. The different bearings can be used to triangulate on the target.

This article focuses primarily on the methods based on the differences in sound arrival times because they are historically the most important.

Besides locating artillery, other military uses have included locating submarines[1] and aircraft.[2] The civilian uses include locating wildlife[3] and locating the shooting position of a firearm[4]. Using radio signals, the basic methodology of sound ranging is applied to the general navigation problem in the LORAN and GPS navigation systems.

Contents

[edit] Background

[edit] Basic Equipment Setup

A sound ranging system requires the following equipment.

  • An array of microphones
  • A system capable of measuring the sound wave arrival time differences between the microphones.
  • A means of analyzing the time differences to compute the position of the sound source.

Figure 1 illustrates the basic system.

Illustration of the Sound Ranging Operation.
Enlarge
Illustration of the Sound Ranging Operation.

Some systems may not allow arbitrary placement of the microphones. For example, they may require the microphones to be placed on a straight line. These constraints would be imposed to simplify the calculation of the artillery position and are not a characteristic of the general approach.

The microphones also may be designed to pick up only the sound of the gun firing. There are three types of sounds that can be picked up by the microphone.

  • the gun firing (the desired signal)
  • the sound of the shell moving through the air
  • the impact of the shell

During World War I it was discovered that the gun firing makes a low rumbling sound that is best picked up with a microphone that is sensitive to low frequencies and rejects high frequencies.[5]

[edit] Example

Figure 2 shows an example of an artillery location problem. Assume that we position three microphones with the following relative positions (all measurements made relative to Microphone 3).

  • Distance from Microphone 1 to Microphone 3: r5 = 1267.9 meters
  • Distance from Microphone 2 to Microphone 3: r4 = 499.1 meters
  • Angle between Microphone 1 and Microphone 2 measured from Microphone 3: 16.177o

These values would be established during an initial survey of the microphone layout.

Example of a Sound Ranging Operation.

Figure 2: Example of An Artillery Location Problem.

Assume that two time delays are measured (assume speed of sound \approx330 meters per second).

  • Microphone 1 to Microphone 2 time delay: 0.455 s \Rightarrow 150 meters
  • Microphone 1 to Microphone 3 time delay: 0.606 s \Rightarrow 200 meters

There are a number of ways to determine the range to the artillery piece. One way is to apply the law of cosines twice.[6]

\left(r_1+r_2\right)^2=\left(r_1+r_3\right)^2 + r_4^2-2\cdot\left(r_1+r_3\right)\cdot r_4 \cos\theta (\triangleMicrophone 3, Microphone 2, Gun)
r_1^2=\left(r_1+r_3\right)^2 + r_5^2-2\cdot\left(r_1+r_3\right)\cdot r_5 \cos\phi (\triangleMicrophone 1, Microphone 3, Gun)

This is a system of two equations with two unknowns (θ, r1). This system of equations, while nonlinear, can be solved using numerical methods to give a solution for r1of 1621 meters. While this approach would be usable today with computers, it would have been a problem in World War I and II. During these conflicts, the solutions were developed using one of the following methods.

  • graphically using hyperbolas drawn on paper (for a nice discussion of this procedure, see this LORAN example).[7]
  • assuming the artillery is far away and using the asymptotes of the hyperbolas, which are lines, to find an approximate location of the artillery.[8]
  • Approximate solutions can be generated using sets of metal disks whose radii differ by small increments. By selecting three discs that approximate the situation in question, an approximate solution can be generated.[8]

[edit] Advantages and Disadvantages

Sound ranging has a number of advantages over competing methods, such as radar.

  • Sound ranging is a passive method, which means that there are no emissions traceable back to the sound ranging equipment. This is different than radar, which emits energy that can be traced back to the transmitter.
  • Sound ranging equipment tends to be small. It does not require large antennae nor large amounts of power.

Sound ranging also has a number of disadvantages over radar.

  • the speed of sound varies with temperature and air pressure. Wind also introduces errors. There are means by which to compensate for these factors.[8]
  • at a distance, the sound of a gun is not a sharp crack but more of a rumble (this makes it difficult to accurately measure the exact arrival time of the wavefront at different sensors)
  • artillery is often fired in large numbers, which makes it difficult to determine which wavefront is associated with which artillery piece)

Military forces have found various ways to mitigate these problems, but nonetheless they do create additional work and reduce the accuracy of the method.

[edit] History

[edit] World War I

World War I was the first conflict that brought together the necessary sensors, measurement technology, and analysis capabiliites required to do effective sound ranging. Like many technology concepts, the idea of using sound to locate enemy artillery pieces came to a number of people at about the same time.

  • A Russian claimed to have invented artillery sound ranging in 1910[9]
  • A German officer patented the method in 1913[9]
  • The French developed the first operational equipment[10]
  • The Americans proposed a scheme early in World War I[11]

World War I provided the ideal environment for the development of sound ranging because:

  • electronic processing of sound was becoming mature because of the development of telephone and recording technology
  • the technology for recording sound on film became available (this facilitated making time difference measurements accurate to hundreths of a second)
  • the need for counter battery artillery fire provided a strong technology driver

While the British were not the first to attempt the sound ranging of artillery, it was the British during WWI who actually fielded the first effective operational system. British sound ranging during WWI began with crews that used both sound and flash detection. The sound ranging operators used equipment that augmented human hearing.[12] The instruments usually consisted of large horns or microphones connected to the operators ears using tubing, much like a very large stethoscope.[13] Using the gun flash, the flash crew would determine a bearing to the gun using a theodolite or transit. The sound detection crew would determine the difference in time between the gun flash and the sound of the gun, which was used to determine the range of the gun. This provided the range and bearing data needed for counter battery fire.[14]

The Germans soon switched to flashless powders that limited the ability of the British to see the gun flash. The British responded by assigning the great Australian scientist Sir William Lawrence Bragg to the problem. When Bragg came on the scene, sound ranging was slow, unreliable, and inaccurate. At that time, sound ranging was being performed using equipment developed by two Frenchman, Bull and Nordmann, which used photograhic film to record the artillery sound. This method was slow because the film took 5 minutes to develop. Also, Bragg found out that the nature of gun sounds was not well understood and that care needed to be taken to separate the sonic boom of the shell from the actual sound of the firing. A low-frequency microphone was invented that separated the low frequency sound made by the firing of the gun from the sonic boom of the shell. With the proposed impovements, enemy artillery could be located accurately to within 25 to 50 meters under normal circumstances.[10] This program was very well developed by the end of World War I. In fact, the method was expanded to determine the gun location, caliber, and the intended target.[5]

Most of the work on anti-aircraft sound ranging was done by the British. They developed an extensive network of sound mirrors that were used from World War I through World War II.[15] Sound mirrors normally work by using moveable microphones to find the angle that maximizes the amplitude of sound received, which is also the bearing angle to the target. Two sound mirrors at different positions will generate two different bearings, which allows the use of triangulation to determine a sound source's position.

[edit] Between the World Wars

Prior to the development of radar, sound ranging was the only technology available for detecting aircraft at a distance. Britain developed a sound ranging system.[16] As World War II neared, radar began to become a credible alternative to the sound ranging of aircraft. However, the sound ranging stations were left in operation as a backup to radar.[17]

[edit] World War II

During World War II, sound ranging was a mature technology. The emphasis was on developing more usable implementations. However, radar as a counter battery weapon proved superior to sound ranging and supplanted both in most situations (however, both methods continued to be used as shown in these sound ranging and flash detection photos). Radar was more accurate and less susceptible to weather conditions. However, radar did have some disadvantages. It was susceptible to jamming. Since radar emits radio signals, turning on a radar set gave away the position of the artillery spotting unit.

Even with the coming of radar, there were many instances of effective use of sound ranging for counter battery operations during World War II. The US Marines includes sound ranging units as standard parts of their defense battalions.[18] These sound ranging units were active in the Marines both before and during World War II. The US Army also used sound locators.[19] During the Okinawa campaign, the US Army used its sound ranging sets to provide effective counter battery fire.[20] The Japanese tried to counter this effective counter-battery fire with the tactic of "shoot and scoot," which means shooting a small number of rounds and leaving the firing position before the counter-battery fire could arrive. WHile an effective tactic against counter-battery fire, this approach tends to reduce the effectiveness of artillery fire.

During World War II, the British made extensive use of sound ranging for artillery spotting. There a number of excellent memoirs that address their use of sound ranging for artillery spotting available on the web.[21][22] These memoirs include excellent details on the electronic equipment involved with these operations.[22]

While sound ranging for artillery spotting had a secure place during the war, aircraft spotting proved less useful. During the Battle of Britain, sound ranging was used as a backup for radar in locating inbound aircraft.[17] Since aircraft in World War II were much faster than in World War I, aircraft sound ranging only gave a few minutes of warning.[2] This made it a much less attractive approach than radar. Today, the abandoned sites are still in existence and are readily accessible.[15]

After World War II, sound ranging played no further role in anti-aircraft operations.

[edit] Korean War

Sound ranging of artillery was done in Korea, but mainly was supplanted by radar and aircraft-based artillery spotters. Since anti-radar countermeasures were limited at this time and the UN had air superiority throughout the war, these approaches was simpler and more accurate.[23]

[edit] Vietnam

Most counter battery work in Vietnam was with artillery spotting done using radar or aircraft. Australia did deploy some sound ranging units in Vietnam with some successs.[24]

[edit] Present-Day

Sound ranging is undergoing a renaissance. The US government has been showing interest in using sound ranging to determine the location of gunfire in large cities.[4]

A number of military units are moving back to using sound ranging because it is a passive technology and cannot be detected by enemy forces, which is a failing of radar. The British army and US Marines are deploying new systems.[25]

Because the cost of the associated sensors and electronics is dropping, the use of sound ranging technology is becoming accessible for other uses, such as for locating wildlife.[26]

[edit] Notes

  1. ^ Kristian Johanssan et al. Submarine tracking using multi-sensor fusion and reactive planning for the positioning of passive sonobuoys. Retrieved on 2006-05-16.
  2. ^ a b W.Richmond. "Before RADAR - Acoustic Detection of Aircraft", 2003.
  3. ^ Selected Projects. Greenridge Sciences Inc. Retrieved on 2006-05-16.
  4. ^ a b Lorraine Green Mazerolle et al (December 1999). "Random Gunfire Problems and Gunshot Detection Systems". National Institute of Justice Research Brief.
  5. ^ a b Bragg, William Lawrence. "Personal Reminiscences". Retrieved on 2006-05-14.
  6. ^ J.B.Calvert. Ranging. Retrieved on 2006-05-15.
  7. ^ Bowditch, Nathanial. “Hyperbolic Systems”, The American Practical Navigator, 1995. Retrieved on 2006-05-29.
  8. ^ a b c Harry Bateman (January 1918). "Mathematical Theory of Sound Ranging". Monthly Weather Review: 4-11;.
  9. ^ a b Nigel F Evans. "British Artillery in World War II: Target Acquisition & Counter Battery", 3 December 2005.
  10. ^ a b Mallet, Ross (27 November 1998). "The Interplay between Technology, Organization, and Tactics in the First AIF". University of New South Wales. Retrieved on 2006-05-13.
  11. ^ Ray Brown. Historical Tidbit:The Birth of The Seismic Reflection Method in Oklahoma. Retrieved on 2006-05-14.
  12. ^ Jim Mulligan. Photo of Sound Locator. Retrieved on 2006-05-15.
  13. ^ Douglas Self. Acoustic Location and Sound Mirrors. Retrieved on 2006-06-01.
  14. ^ Fraser Scott. "Artillery Survey in World War One"". Retrieved on 2006-05-14.
  15. ^ a b Phil Hide (January 2002). Sound Mirrors on the South Coast. Retrieved on 2006-05-13.
  16. ^ Andrew Grantham. "Early warning sound mirrors", November 8, 2005.
  17. ^ a b Lee Brimmicombe Woods. "The Burning Blue: The Battle of Britain 1940", GMT Games LLC, 7 December 2005.
  18. ^ Major Charles D. Melson. "Organization and Equipment for the Defense Battalion", Marine Corps History and Museums Division.
  19. ^ Appleman et al. "Tactics and Tactical Decisions", Center of Military History - US Army.
  20. ^ "Japanese Artillery", Combined Arms Research Laboratory.
  21. ^ The 4th Durham Survey Regiment. Sounds like the Enemy. Retrieved on 2006-05-14.
  22. ^ a b Artillery Sound Ranging. The Wireless-Set-No19 Group. Retrieved on 2006-05-14.
  23. ^ N. L. Volkovskiy: The War in Korea 1950-1953:The Use of Artillery. Military Historical Library. ISBN 5-89173-113-4.
  24. ^ Locating Artillery Overview. Locating Artillery Association. Retrieved on 2006-05-14.
  25. ^ SELEX Sensors and Airborne Systems. HALO: Hostile Artillery Locating System. Press release. Retrieved on 2006-05-14.
  26. ^ John L. Spiesberger (June 2001). "Hyperbolic location errors due to insufficient numbers of receivers". The Journal of the Acoustical Society of America 109 (6).
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