EPIRB or Emergency Position Indicating Radio Beacon Station is an emergency transmitting device that blue water sailors or explorers can use in case of distress. They have proven their worth over the years and are absolutely essential for anyone wanting to wander off the grid (Ref.1). At least 2000 people have been saved every year since 2017, and over 50,000 since 1982 (Ref.2). Survival radios date from the Titanic disaster in 1912, to WWII with the appearance of units like the famous “Gibson Girl”. Transmission was on 500KHz at approx. 5W using a box kite for an aerial and having a hand crank generator. The famous Canadian Atlantic rafting expedition L’Égaré had one on board (Ref.3/Ref.4).
ELT, EPIRB, GPIRB
After WWII, ELT or Emergency Locator Transmitters were used as survival radios. They operated on 121.5MHz and had a “Siren” type of AM modulation. This consisted of a swept AM modulation tone of 1600Hz to 300Hz with a frequency of 2 to 4 sweeps per second. These transmissions were received by land based receivers or by aircraft. The COSPAS-SARSAT satellite system started in 1979 used beacons at 406MHz which allowed for dopler triangulation. These devices were known as EPIRBs. They sent out a special digital code with registration information. Later improvments led to GPIRBs which are EPIRBs with built in GPS receivers that send the GPS coordinates along with the registration information. COSPAS-SARSAT LEO Low Earth Orbiting satellites can triangulate the GPIRB signal, and GEO Geosynchronous Earth Orbiting satellites (GOES/MSG/INSAT etc.) can detect and decode the GPS coordinates. All this information is passed on to the appropriate regional control centres.
|Survival Radio/ELT||1920s:Spark Gap/Morse_sos/500KHz|
+ 121.5MHz ELT Transmitter
ELT Simulation Siren Signal
Figure 3 shows a Scicos simulation used to create either a downward “Siren” signal from 1600 to 300Hz or an upward signal from 300 to 1600Hz. ELTs use the downward signal, whereas PLB Personal Locator Beacons use the upward signal. Figure 4 shows the downward sweep.
Figure 5 shows a summary of the packet structure of modern EPIRBs. A long packet is used to send embedded GPS location information. The packet is preceded by 160msec of 460MHz carrier, followed by 360msec of data at 400Baud using Manchester coding. Figure 7 shows a Scicos simulation with Figure 8 showing the various waveforms and Figure 9 showing the spectrum. For convenience, a carrier of 4KHz is used. The spectrum shows the random Manchester data null at Fc +/- 800Hz and 400Hz harmonics because of the 15 x 1’s for data sync.
RTL-SDR can be used to receive ELT & EPIRB transmissions since it covers the frequency range of 25 – 1725MHz. In the YouTube video below I discuss using SDR# for local reception and Multipsk for Satellite Reception.
#1. “EPIRB” Wikiepdia
#2. “International Cospas-Sarsat Programme”, Wikipedia
#3. “The Raftsmen”, Ryan Barnett, Firefly Books, ISBN-13 : 978-1770859784
#4. “Gibson Girl SCR 578 Life Boat Radio”, VK3ASE
#5. “Specification for Cospas-Sarsa 406 MHz Distress Beacons”, C/S T.001, Cospas-Sarsat