Search & Rescue Systems

Search and Rescue System By Jim Sparks November 2000 Words that each of us in the aviation maintenance industry dread to hear are "An aircraft went down." However, with the technology and safety measures currently employed, accident...


Search and Rescue System

By Jim Sparks

November 2000

ImagesWords that each of us in the aviation maintenance industry dread to hear are "An aircraft went down." However, with the technology and safety measures currently employed, accident survivability is realistic. The response time of emergency equipment can have a dramatic affect on the ultimate outcome. The ability to pinpoint a crash site is an advantage when dispatching crash and rescue crews because the less time spent searching can often be translated into more lives saved.
Search and Rescue (SAR) systems are devices that most of us hate to think about, yet it is a method to bring together experts from every walk of life whose main mission is to save lives. This not only involves the pilots and passengers in the aircraft concerned, but the people monitoring the distress signals, government, civil, and military forces responding to a crash, and even the Emergency Medical Technicians who are dispatched once an accident confirmation occurs.
The Emergency Locator Transmitter (ELT) is the device tasked with providing the dreaded signal that relays that a crash has occurred. These devices have evolved over the years, but still require periodic maintenance as well as functional and operational testing.
Four types of ELTs exist and are classified as:
1. Automatic Fixed (AF)
2. Automatic Portable (AP)
3. Survival (S)
4. Automatic Deployable (AD)
Operation of the system all revolves around the proper triggering of the ELT unit on board the aircraft. Originally the radio beacon radiated outward with the intent of being received by an orbiting Search And Rescue Satellite (SARSAT). Communication of the signal then goes from the satellite to a ground-based Local User Terminal (LUT). There are just less than 40 of these stations around the world with three providing the coverage across the continental United States. Once a signal is received by the LUT, investigation as to the signal origin begins. A VHF frequency of 121.5 Megahertz (MHz) was the first frequency of choice for ELT communications. Unfortunately, reception of this type of signal is based on line of sight transmission. So, the possibility exists that if an active ELT is not within sight of a SARSAT the emergency transmission will not be heard. In fact, with this type technology, only about 60 percent of the Earth can be physically monitored. Not only this, in order for the satellite to advise the LUT, it must also have "a line of sight." That means the satellite will only relay ELT information to the LUT when it has both the transmitting ELT and receiving LUT within view. There are other drawbacks to the 121.5 MHz units including exact location determination. Tracking this type of signal will generally get within a 15-mile radius of the point of origin. Unfortunately, scanning a 30-mile circular area will absorb significant resources. The 121.5 MHz range is also subject to many false signal transmissions. It is estimated that something less than five percent of all 121.5 MHz distress signals are actual ELT distress broadcasts. In fact, of all the signals received, about 20 percent are coming from an ELT and the majority are false transmissions.

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