RTCM Report
E PI R Bs:
These lifesavers keep
getting better
EPIRBs are smaller, lighter, less expensive, and more effective than they were just a few years ago. RTCM standards
define the performance of the EPIRBs of today, and are preparing for the EPIRBs of tomorrow.
One June 10, Abby Sunder- land activated her Emergency Position Indicating Radio Beacon (EPIRB)
and her Personal Locator
Beacon (PLB) after her quest
to become the youngest person to sail solo around the
world ended when a storm
dismasted her boat and
brought down her communication antennas. The signals
from the properly registered
beacons were received by a
geostationary satellite, and
later by a low-earth orbiting satellite operated through the international Cospas-Sarsat
system. The system provided her identification and location to the relevant search and rescue forces, in this case the Australian Maritime Safety Authority. She was spotted later
(2000 nautical miles) Southwest of Perth by a Qantas A-330 Airbus and eventually rescued by a French fishing vessel.
Abby may not have joined the club of solo circumnavigators, but she did join the definitely non-exclusive club of those rescued by EPIRBs (and the newer PLBs) through the
Cospas-Sarsat system. The system now claims over 28,000 persons rescued since 1982.
RTCM’s Satellite EPIRB and Satellite PLB standards are the basis for Federal Communications Commission (FCC) certification in the US, and have been used as the basis for standards in use in Canada, and elsewhere internationally through the standards of the International Electrotechnical Commission (IEC).
Satellite EPIRBs transmit an identification signal to the satellites on an uplink frequency between 406.0 and 406.1 MHz. The signal is a burst of data about . 5 second long
every 50 seconds or so. For beacons registered in the US, this data includes a unique identification number. When this information is relayed to the US Sarsat office in Suitland, MD,
it is matched with data in the US registration database to identify the vessel that the EPIRB
belongs to. The low-earth orbiting satellites can be used to calculate the location of the
EPIRB to within a few miles.
Before starting a Search and Rescue (SAR) operation, the US Coast Guard tries to contact the vessel by radio to make sure that the signal is not a false alert. If the conclusion
is that the case is a real distress, SAR aircraft and surface vessels are launched and sent
to the scene. Once near the location, SAR forces can use a second continuous signal trans-
The international Cospas-Sarsat system is credited
with rescuing more than 28,000 people since 1982.
The National Transportation Safety Board has
recommended that EPIRBs be required
to include a GPS function.
mitted by the EPIRB on the aeronautical distress frequency of 121.5 MHz to home in on
the EPIRB location.
Operation of Cospas-Sarsat System
As the thousands of rescued mariners can attest, the system has been a terrific success, but there are limitations. The satellites that handle the beacon transmissions are
weather satellites owned by Russia (Cospas) and the United States (Sarsat—an agency of
the National Oceanic and Atmospheric Administration). Two types of satellites are used.
Geostationary satellites orbit high over the equator at a speed that matches the earth’s
rotation. These are usually the first satellites to acquire a beacon signal. The EPIRB message identifies the beacon and therefore the vessel, but the geostationary satellite can not
calculate the position of the beacon. It can be coming from almost anywhere on the hemisphere of the earth “in view” of the satellite. Nevertheless, with the identity of the vessel,
SAR authorities can start an investigation by trying to contact the registered owner to see
if the vessel is underway and actually in distress.
Sometime later, which can be a few minutes to two hours or so, a low-earth orbiting
weather satellite will pass within range of the EPIRB signal. The frequency of the EPIRB
signal will appear to the satellite as being increased as the satellite nears the EPIRB’s position due to the Doppler Effect. At the closest point of approach, the EPIRB frequency will
appear to the satellite as being correct, and then will appear to be decreased as the satellite moves away from the EPIRB. Using the shape of the frequency/position curve, the
probable location of the EPIRB can be determined, although there is some varying ambiguity as to whether the EPIRB lies to the left or right of the satellite’s path. Data from a
second satellite resolves that ambiguity, and the position of the EPIRB is then known to
within a few miles.
(Continued on page 49)
EPIRB design
In the 1990’s, EPIRB manufacturers started offering EPIRBs with GPS processors as an
additional-cost option. The GPS position could be included in the EPIRB data message, so
that the location could be determined as soon as a geostationary satellite acquired the signal. This was a potential lifesaving time-saver for the start of a SAR operation, but early
GPS processors were expensive, adding $300 or more to the price of the EPIRB. Sometimes, the GPS processor could not obtain a position, especially in rough seas. Today, GPS
processors are less expensive, more accurate, and obtain position fixes faster, and RTCM
is now developing standards for their performance when incorporated into EPIRBs. As a
result of a casualty in which the delay in determining an EPIRB’s position may have had
an effect on the saving of lives, the National Transportation Safety Board has recommended that GPS processors become a mandatory EPIRB feature.
