EPIRBs
Emergency Position Indicating Radio Beacons (EPIRBs) are fundamental to the GMDSS. For vessels which are legally required to conform to GMDSS, they provide one of the two “independent means” of communicating with SAR authorities. For “voluntary fit” vessels, including pleasure craft and small commercial craft operating under MCGA Codes of Practice, they are a very cost-effective means of providing distress alerting with world-wide coverage.
Unfortunately the term EPIRB is used to describe several different quite different types of equipment. In this book, and in the context of GMDSS, the term “EPIRB” refers to a 406MHz EPIRB.
Some EPIRBs are capable of indicating their position, but most are not. Their main purpose is to transmit a distress alert, coded to show the identity of the vessel concerned, which can be detected by satellites of the Cospas-Sarsat system. The satellites use Doppler positioning (see page 58) to calculate where the signal is coming from, and then download the details of the EPIRB’s identity and position to receiving stations ashore.
Direction finding equipment can’t be used on the 406MHz EPIRB signal, so, to allow rescue vessels and aircraft to home in on a casualty, all 406MHz EPIRBs also transmit a homing signal on the lower frequency of 121.5MHz.
For very short range direction finding, they also have a bright flashing light.
The satellites
The Cospas-Sarsat system is an astonishing international venture that was set up in 1979, by the Soviet Union, USA, Canada, and France. It uses at least four (usually more) satellites in low polar orbits, less than 1000km above the Earth’s surface. Because they are so low, they have to fly fast to resist the pull of gravity, so each satellite goes round the earth about 14 times per day. As the Earth revolves inside the satellite orbits, the result is that each satellite passes over a different part of the Earth’s surface each time it goes round.
They are supplemented by three Geosar satellites. These go round the Earth almost 36,000km above the equator, where they take precisely 24 hours to complete one orbit. The result is that each satellite “hovers” over a fixed position on the Earth’s surface.
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92
