Distress radio beacons, also known as emergency beacons, ELT or EPIRB, are tracking transmitters which aid in the detection and location of boats, aircraft, and people in distress. Strictly, they are radio beacons that interface with Cospas-Sarsat, the international satellite system for search and rescue (SAR). When activated, such beacons send out a distress signal that, when detected by non-geostationary satellites, can be located by trilateration.[citation needed] In the case of 406 MHz beacons which transmit digital signals, the beacons can be uniquely identified almost instantly (via GEOSAR), and furthermore, a GPS or GLONASS position can be encoded into the signal, which provides instantaneous identification of the registered user and its location. Frequently, by using the initial position provided via the satellite system, SAR aircraft and ground search parties can home in on the distress signals from the beacons and come to the aid of the concerned boat, aircraft, or people.

There are three types of distress radio beacons compatible with the Cospas-Sarsat system EPIRBs (emergency position-indicating radio beacons) signal maritime distress

ELTs (emergency locator transmitters) signal aircraft distress

PLBs (personal locator beacons) are for personal use and are intended to indicate a person in distress who is away from normal emergency services They are also used for crewsaving applications in shipping and lifeboats at terrestrial system

The basic purpose of distress radiobeacons is to get people rescued within the so-called "golden day" (the first 24 hours following a traumatic event) during which the majority of survivors can usually be saved

Squawk code, (more formally transponder code) a four-digit number sent out by an aircraft's transponder

ELT Emergency locator transmitter - RADIOCROSS OUT - VHF on frequency 121.5 MHz. UHF on frequency 243.0 MHz

SELCAL

In international aviation, SELCAL or SelCal is a selective-calling radio system that can alert an aircraft's crew that a ground radio station wishes to communicate with the aircraft. SELCAL uses a ground-based encoder and radio transmitter to broadcast an audio signal that is picked up by a decoder and radio receiver on an aircraft. The use of SELCAL allows an aircraft crew to be notified of incoming communications even when the aircraft's radio has been muted. Thus, crewmembers need not devote their attention to continuous radio listening Use

SELCAL operates on the high frequency (HF) or very high frequency (VHF) radio frequency bands used for aircraft communications. HF radio often has extremely high levels of background noise and can be difficult or distracting to listen to for long periods of time. As a result, it is common practice for crews to keep the radio volume low unless the radio is immediately needed. A SELCAL notification activates a signal to the crew that they are about to receive a voice transmission, so that the crew has time to raise the volume.

An individual aircraft has its own assigned SELCAL code. To initiate a SELCAL transmission, a ground station radio operator enters an aircraft's

SELCAL code into a SELCAL encoder. The encoder converts the four-letter code into four designated audio tones. The radio operator's transmitter then broadcasts the audio tones on the aircraft's company radio frequency channel in sequence: the first pair of tones, each lasting about half a second; a silence of about 0.2 seconds; followed by the second pair of tones, each lasting about half a second.

The code is received by any aircraft receiver monitoring the radio frequency on which the SELCAL code is broadcast. A SELCAL decoder is connected to each aircraft's radio receiver. When a SELCAL decoder on an aircraft receives a signal containing its own assigned SELCAL code, it alerts the aircraft's crew by sounding a chime, activating a light, or both.

The crew next turns up the volume on the aircraft radio to hear the incoming voice transmission. Using ICAO radio protocol, they must verify with the transmitting operator that they are the intended message recipients. The crew then uses the received information

Pre-flight check

The aircraft station should contact the appropriate aeronautical station and request a pre-flight SELCAL check and, if necessary, give its SELCAL code

When primary and secondary frequencies are assigned, a SELCAL check should normally be made first on the secondary frequency and then on the primary frequency. The aircraft station would then be ready for continued communication on the primary frequency.

SELCAL check and, if necessary, give its SELCAL code

When primary and secondary frequencies are assigned, a SELCAL check should normally be made first on the secondary frequency and then on the primary frequency. The aircraft station would then be ready for continued communication on the primary frequency

בבדיקת Selcal ראשית בודקים התדר המשני ואח"כ את התדר הראשי.

הסיבות:

. כדי להישאר על התדר הראשי בסיום הבדיקה ולהימנע כך מטעות

.אם יש תקלה, רצוי לגלותה על תדר משני שאינו מפריע לתקשורת מטוסים אחרים

transponder

A transponder (short-for Transmitter-responder and sometimes abbreviated to XPDR, XPNDR, is an electronic device that produces a response when it receives a radio-frequency interrogation. In aviation, aircraft have transponders to assist in identifying them on radar and on other aircraft's collision avoidance systems.

Air traffic control units use the term "squawk" when they are assigning an aircraft a transponder code, e.g. "Squawk 7421". Squawk or squawking thus can be said to mean "select transponder code" or "I have selected transponder code xxxx"

Secondary Surveillance Radar

Secondary Surveillance Radar is referred to as "secondary", to distinguish it from the "primary radar" that works by passively bouncing a radio signal off the skin of the aircraft. Primary radar works best with large all-metal aircraft, but not so well on small, composite aircraft. Its range is also limited by terrain and rain or snow and also detects unwanted objects such as automobiles, hills and trees. Furthermore not all primary radars can estimate the altitude of an aircraft. Secondary radar overcomes these limitations but it depends on a transponder in the aircraft to respond to interrogations from the ground station to make the aircraft more visible and to report the aircraft's altitude.

Operation

A pilot may be requested to squawk a given code by the air traffic controller via the radio, using a phrase such as "Cessna 123AB, squawk 0363"

The pilot then selects the 0363 code on their transponder and the track on the radar screen of the air traffic controller will become correctly associated with their identity.

Because primary radar generally gives bearing and range position information, but lacks altitude information, mode C and mode S transponders also report pressure altitude. Without the pressure altitude reporting, the air traffic controller has no display of accurate altitude information, and mustrely on the altitude reported by the pilot via radio

Ident

All mode A, C, and S transponders include an "ident" button, which activates a special "thirteenth" bit on the mode A reply known as Ident, short for

Identify. When radar equipment receives the Ident bit, it results in the aircraft's blip "blossoming" on the radar scope. This is often used by the controller to locate the aircraft amongst others by requesting the ident function from the pilot

Transponder codes

Transponder codes are four digit numbers transmitted by the transponder in an aircraft in response to a secondary surveillance radar interrogationsignal to assist air traffic controllers in traffic separation. A discrete transponder code (often called a squawk code) is assigned by air traffic

controllers to uniquely identify an aircraft. This allows easy identity of the aircraft on radar.

Emergency codes

7500: Unlawful Interference (i.e. Aircraft hijacking)

7600: Lost Communications

7700: General Emergency