A Practical Guide to Aural Searches


Execution of aural electronic searches for ELT/EPIRB/PLB transmitters is a very straight forward and simple procedure to locate emergency transmitters without special equipment. However the search techniques are based on some fairly complex mathematics and engineering and is often described using technical terms or jargon. Misunderstanding of the meaning and usage of these terms and jargon has often led to search teams misunderstanding how the techniques work. In order to maintain the ability to execute these techniques with accurately and effectively it is important that search teams are aware of, and have a clear understanding of the technologies, mathematics and engineering involved. Do not become overly concerned, however; we have endeavoured to keep this guide aimed at a 'lay' audience and the explanations either non-technical, or where detailed technical information is required, build towards that understanding in easy steps. We have also endeavoured to keep unnecessary mathematical formulae out of the main text. For the curious or sceptical we have included links to pages or external sites where these formulae can be discovered. Do not be concerned if you are unfamiliar with aural search techniques. We will present all the information you need in a logical sequence so that if you proceed as directed you will know all there is to know about these techniques by the time you are done. On the other hand, if you are well acquainted with these techniques, we also recommend that you follow the normal sequence of presentation. If you doubt or disagree with any particular point you can have a look at the common errors and misconceptions section. Whether a novice or experienced hand, if you have difficulty understanding or accepting any material presented here we encourage you to get in touch with the support staff who will assist you and update this guide for the benefit of those who follow.

In this guide when we discuss aural search techniques we are referring to a set of techniques commonly called the Aural Null of which there are two well known sub-types. We will provide a firm theoretical foundation for understanding how the techniques work, what pitfalls exist and how to avoid or compensate, and detailed procedures for not only the two well known sub-types but also a third sub-type that is more robust and effective than the original two.

Why is this guide necessary? Well it is true that most search organization use direction finding equipment as the primary method of locating emergency transmitters, the vast majority of potential search aircraft are not equipped with direction finding equipment. Most potential search aircraft are equipped to perform an aural null. In the event that an aircraft equipped for direction finding is not available, or faces an extended transit from base to the search area, dispatching a crew in a aircraft that is able to perform an aural null may make the difference between a rescue and a recovery. As stated above the technique is simple to perform and master, and if search crews are aware of the information provided in this guide the accuracy of the technique can rival direction finding.

This guide is published on the World Wide Web for three important reasons. First, it is available to the broadest possible audience who can always have access to the latest version of the document. Second, this allows us to link readers to the vast resources, diagrams, applications, and videos available to better explain the concepts important to effective performance of the technique. Third, it allows us to link from technical terms and jargon words to a glossary to help you with the unfamiliar. If you have been provided a static document to study from, you should also have been provided the web address of the on-line version as well. For trainers, if you wish to use information from this document in your own training materials you are welcome to do that provided you also give you students the web address of this guide. We also ask that you show us courtesy by letting us know that you are using the material. A simple email describing your organization and how you are using the material sent to the address below will suffice.

Questions, recommendations or other enquiries should be directed to Support@SARMobile.Ca

The Plan

Aural null searches are described in literature published by the United States Air Force Civil Air Patrol (CAP), and the Canadian Civil Air Search and Rescue Association (CASARA). The procedures work if flown as described, but the explanation of how the procedures work falls short. This has resulted in a number of agencies underestimating the effectiveness of the techniques, or modifying the procedures to the point of introducing failure modes not present in the original. We at SARMobile feel that it is time that an independent repository of technically accurate information on this, and other search and rescue procedures, was created. This repository will remain independent of any single agency or national interest, but we will also be responsive to any requests for enhancement to assist the national search and rescue program of any country.

An aural null is primarily based on geometry. Assume for the moment that an aircraft is equipped with a device that can indicate when it is within some fixed (perhaps unknown) distance from an emergency transmitter. It may provide a visual indication (a light), or an aural indication (a tone), but in each case the crew is able to note the location of the aircraft at the time the indicator is activated. Deducing the location of the transmitter from the aircraft location is then a matter of some simple geometry. So, we will begin with the geometry needed for this deduction.

Unfortunately the aircraft is not so equipped, but there will be equipment on board, the air band communications radio, that will allow the search crew to determine when the aircraft is within this fixed distance from the transmitter. But since the radio is not designed to provide a simple and unambiguous indication like a light or a tone, at the appropriate point the crew will need to know how to interpret the information the radio does provide. So next we will cover the operation, capabilities and limitations of an aircraft air band radio and the human ear. We will also cover the operation of the air band signal transmitted by an ELT/EPIRB/PLB and what can happen to the signal as it moves from the transmitter to the search aircraft. 

Finally we will present a simple set of procedures to execute the three types of aural null techniques along with best practices and advice on how to deal with the possibilities presented at the decision points along the way. 

Throughout this guide we will use screen shots from a SARMobile software application under development to help clarify the procedures. The software is not needed to accurately and effectively execute any of the techniques described in the guide. What the software will do is compute the mathematics involved in the geometry which will offload the chore of plotting and doing the geometry manually on a chart in the cockpit while flying. When the software is available directions for obtaining it will be posted on this page. A beta version of the software is available for downloading to a computer with Microsoft Windows using a BlackBerry simulator supplied by Research In Motion. Instructions for downloading and installing this software may be found here:

Table of Contents

Here are links to the sections of this guide so that you may proceed at your own pace though you should start with Geometry: