Emergency Locator Transmitter Signals Research Articles

ACQUISITION OF SARSAT INFORMATION USING AN AUTO-CORRELATION BASED ADAPTIVE LINE ENHANCER

This paper develops a new approach to the detection of the emergency locator transmitter (ELT) signal using an adaptive line enhancer (ALE). The input signal to the ALE is replaced by its autocorrelation function (ACF) because noise affects the central samples of the autocorrelation function only while it affects all samples of the noisy signal. This gives the ALE the ability to get rid of noise, easily. The output ACF is then used in the spectral estimation and detection of the ELT signal. This approach is related to the signal processing using Higher Order Statistics (HOS) since the ACF is the used input. The paper also compares the results of the new approach to other different previously used methods foe ELT signal detection. The first method uses the signal xk as an input to the ALE, and the second method uses [Formula: see text]. Results illustrate the superiority of the proposed method over the other two methods. A comparison study between the performance of two types of ALE; Fixed pole radius and variable pole radius ALEs in detecting the ELT signal, is introduced.

406 MHz ELT signal spectra for SARSAT

The signal format and spectral properties of the 406-MHz emergency locator transmitter (ELT) used in the search and rescue satellite aided tracking (SARSAT) system are examined. The ELT improves location estimate accuracies and can relay information about the particular aircraft and its problem, by means of the digitally modulated message fields. It is shown that due to the RF signal frequency characteristics and the Doppler shift, processing must be performed over a frequency band of approximately 25 kHz. Through the use of the fast Fourier transformation (FFT), the frequency spectrum of the ELT is analyzed, taking account of effects due to noise, multiple simultaneously received signals, and Doppler shift. It is demonstrated that the FFT provides an effective means for detecting and recognizing the presence of one or more ELT signals over this 25-kHz frequency band. Some recommendations are made to improve the spectral characteristics and the performance of the ELT. >

Spectral Analysis of ELT Signals for SARSAT

Search and rescue satellite-aided tracking (SARSAT) relays the emergency locator transmitter (ELT) signals of distressed aircraft to an earth station for spectral analysis. Of considerable importance are the characteristics of the spectrum of the ELT signal since the probability of locating the downed aircraft is closely related to the quality of the ELT signal spectrum itself. In this paper, it is shown that the spectrum can be adversely affected by a number of factors including the phase and frequency characteristics of the carrier and their interaction with the amplitude modulation. Two new models are proposed which greatly reduce the self-generated interference produced by ELT units presently being used.

Identification Techniques for SARSAT Signals

A process for the identification of emergency locator transmitter (ELT) signals related to search and rescue satellite-aided tracking (SARSAT) is presented. The ELT identification process is particularly important in order to increase the probability of detection and eliminate sources of interference from the data set. A set of ELT signal parameters is introduced and methods for estimating these parameters are developed. A theoretical analysis and performance evaluation of these methods is provided.

Basic Concepts in the Processing of SARSAT Signals

Search and rescue satellite-aided tracking (SARSAT) involves the use of satellites in low-polar orbits which relay the emergency signals of distressed vehicles to an earth station for signal analysis. In this paper, we present some basic concepts and a theoretical analysis of the spectra produced by coherent and noncoherent emergency locator transmitter signals. It is shown that coherent signals can be easily processed using linear spectral analysis. Noncoherent signals, however, require more advanced methods.

Processing of real ELT signals for SARSAT

A significant number of lives have been saved by using a new system called search and rescue satellite aided tracking (SARSAT). The program initially involved Canada, the United States and France with the Soviet Union forming a parallel project called COSPAS. Since then, the United Kingdom, Norway and Sweden have joined the program and it is expected that the system will be used worldwide. The performance of several different signal processors using real emergency locator transmitter (ELT) signals is evaluated. These signals have an unusual modulation characteristic which produces unwanted interference due to sidebands. The periodogram gives good performance for coherent ELT signals especially when a ranking and averaging technique is applied. However, the method is incapable of determining the carrier frequency for noncoherent ELT signals. Use of the maximum entropy method (MEM) in detecting these noncoherent ELT signals is demonstrated and it is shown that an effective carrier frequency can be deduced when the signal-to-noise ratio is high.

Emergency Locator Transmitter (ELT) signal models for SARSAT

Three different models for emergency locator transmitter (ELT) signals are proposed. The first model, the ideal coherent model ELT, produces a highly idealized spectrum which does not exist in practice. The second model, the nonideal coherent model ELT, is capable of producing a wide range of spectra which can be closely related to actual ELT signals. Mathematical analysis identifies a design problem not previously recognized and provides the basis for a new design specification which should become mandatory. The third model, the noncoherent model ELT, represents a class of ELT signals having very poor spectral properties. This type of ELT design should be avoided.

Two Methods of Spectral Estimation for SARSAT Signals

Search and rescue satellite aided tracking (SARSAT) depends on the processing of emergency locator transmitter (ELT) signals which are received by a satellite in low polar orbit. Since the signal from a distressed vehicle is normally immersed in a background of other ELT signals (false alarms), interference, and noise, different methods of spectral estimation can provide advantages in estimating carrier frequency. A comparison between average spectrum and minimum spectrum for several real signals is provided here.

On Finding the Source of a Signal

If an airplane crashes, an emergency locator transmitter (ELT) is activated. The crash site can then be located by taking bearings on the ELT signal. The statistical problem consists of estimating a two-dimensional location parameter, where the data consist of directional bearings observed, with some error, from several known positions. We develop methods for estimating location for several variations of the basic problem: unidirectional and bidirectional observations, biased observations, and finally, techniques that are insensitive to outliers. These methods can easily be implemented on a portable microcomputer, and seem to perform quite well when applied to some actual data.