Taking the search out of search and rescue

Search and rescue (SAR) has come a long way from the rudimentary system depicted in the latest film version of the Titanic disaster. The most recent approach to distress signaling involves multiple satellite systems, and it has the potential to drastically reduce search times and to make rescues quicker and more efficient.

At the time of Titanic, distress calls relied on methods as quaint as signal rockets and as modern as shipboard radio operators sending Morse code using spark gap transmitters. In those days, when radio was something of a free-for-all, getting your signal through meant having the strongest transmitter. Also, individual shipping companies set their own policy for manning the radio shack. Some radio operators were only on duty part-time. An infamous example of the unstructured nature of early distress signaling is the role of the steamer Californian during the Titanic sinking.

In the same ice field as Titanic that April night in 1912, Californian was sitting dead in the water, waiting for daylight to clearly show the ice before proceeding. The deck watch on Californian saw the brightly lit Titanic heave into view and pass them, headed to the west. (There has long been considerable controversy about the distance between the two vessels when Titanic stopped engines at roughly 2330. Anything from 12 to 20 miles separated the two vessels.) The radio operator for Californian stayed on duty until about midnight and then retired to his berth. A mere 15 minutes later, Jack Phillips, radioman on the stricken liner, sent his first distress message. Had the wireless gear on Californian been manned, the steamer would presumably been able to get underway and render assistance.

As a result of the Titanic sinking, the first International Conference for the Safety of Life at Sea (SOLAS) in 1913 required, among other things, that all commercial vessels man their radios 24 hours a day.

Over time the technology of radio steadily improved. Marine radio moved to high frequency (HF) for longer range, frequencies were standardized, single-sideband transceivers were introduced that allowed for more channels in the same bandwidth. A high-seas distress frequency of 2,182 kHz was set.

The next big step in distress signaling came with the introduction of satellites and the emergency position indicating radiobeacon (EPIRB). The first generation of EPIRBs used the twin frequencies of 121.5/243 MHz to transmit their signals. While these frequencies were intended to be monitored by both commercial and military aircraft, the real genius of the system was in making use of NOAA’s polar orbiting television and infrared observational satellites (TIROS). Each of these weather satellites was fitted with a transponder that would receive 121.5 MHz signals and then retransmit them. Thus, these line-of-sight VHF signals, when retransmitted, could be received by earth stations.

The system was designed to take advantage of the Doppler shift of a radiobeacon’s signals as the satellite flew past. Using this frequency shift, land-based rescue coordinators would be able to calculate the position of an EPIRB down to roughly 25 miles. Later, the Soviet Union joined the program and outfitted several of its polar-orbiting weather satellites with SAR packages. The combined program was dubbed COSPAS/SARSAT.

The effectiveness of this first-generation system was tempered by several factors. One was that the SAR transponder on a satellite was only acting as a so-called “bent pipe.” As soon as the transponder received a signal on the 121.5 MHz frequency (or within a frequency band determined by the Doppler shift induced by the motion of the satellite), it would then retransmit the signal. The radio signal was useless, however, without an earth station capable of receiving it. Thus, first-generation EPIRBs were only useful in ocean areas that had earth station coverage. This meant substantial areas of coverage in the Northern Hemisphere, but little coverage in the Southern Hemisphere.

Another problem with first-generation units was a less-than-stringent specification for the transmitter and batteries. This meant that these units occasionally transmitted some of the signal out of band or at low power, making them that much harder to detect. It became standard procedure for rescue coordinators to wait the 90 minutes or so for a second satellite pass over a suspected signal to lock down its position and to verify that an emergency did exist.

In this system, since each EPIRB broadcasts a generic analog signal, there is no way for rescue coordinators to track down a false alarm other than to physically locate the EPIRB in question. The second generation of EPIRBs are the 406 MHz units now widely available. In addition to switching to a higher frequency (from 121.5 MHz to 406 MHz), this generation of EPIRBs also has more stringent specifications for transmitter frequency control and for battery requirements. These technical standards alone make 406 units more effective, since their output signals are more likely to be on frequency and at the right power level.

There are two other ways that 406 EPIRBs are improved over their first-generation counterparts. One is an added capability of the 406 MHz transponders on board the satellites. Unlike the simple bent pipe approach of the 121.5 transponders, these units have the ability to receive signals from the surface and then check their on-board memory to see if an earth station is in view. If there is an earth station available, then the 406 transponder broadcasts the distress message. The earth station picks it up and a SAR effort can be started. If, on the other hand, the 406 EPIRB distress signal is received by the satellite while over the Southern Ocean, and there are no earth stations in view, then the satellite stores the signal in memory until a ground station is available. The message then gets broadcast to the nearest earth station and SAR work begins. This store-and-forward capability gives the 406 system true worldwide coverage.

Yet another way in which 406s are superior is the digital encoding of a unique identity code to every EPIRB. When you buy a 406 unit in the U.S., you fill out a registration card and mail it in to the Coast Guard. Your name, your contact information, your vessel name and a few basic characteristics like length, hull color, etc., are entered into a database that is available to Rescue Coordination Centers worldwide. Thus, when you turn on your EPIRB and it sends its unique code, SAR controllers can identify who is sending the signal. They can call you on the phone and determine if the signal is a false alarm.

Finally, 406 EPRIBs, with their improved signal specs, can be more accurately located using the Doppler shift technique. The position of a 406 MHz unit can be nailed down to within two kilometers on the first satellite pass.

It’s in the area of fast and accurate location of an EPIRB’s position that the newest EPIRB technology really shines. Labeled a GPIRB, this version of a 406 EPIRB is the latest device to ingest a GPS receiver. A GPIRB distress beacon can determine its own position and include that position in the 406 MHz digital distress signal. This improves the position accuracy to the level of the standard GPS civilian signal: 100 meters.

To grasp the full significance of this, compare the search area required for these accuracy levels. A standard 406 emergency beacon gives a two-kilometer accuracy on the first pass. This means that the EPIRB can be anywhere within a two-kilometer-diameter circle, presenting SAR elements an area of more than 3.14 square kilometers (1.7 square nautical miles) to search. A 100-meter accuracy, however, means a much smaller area of 7,800 square meters. And the 406 units also still broadcast a 121.5 MHz signal that SAR forces can use as a homing beacon.

This new GPIRB system will not only use the polar-orbiting weather satellites, but, in October 1998, the 406 MHz transponders riding on four geostationary satellites (two U.S. GOES and two Indian Insats) are scheduled to become operational using this GPIRB technology. These satellites can also relay 406 MHz distress messages to earth stations, especially helpful near the equator where polar satellites are the most widely spaced, producing the longest wait between satellite passes. “A geostationary satellite looks at the whole hemisphere,” said Don Hall, sales engineer at Northern Airborne Technology, a manufacturer of EPIRBs. “So it picks up the signal right away.”

With more precise positions of distress signals, it will be possible to launch range-limited SAR helicopters in more cases since the helicopters won’t expend as much fuel for searching.

Probably the only drawbacks to this system are cost and availability. Current 406 EPIRB units cost roughly $900. This compares to the $2,495 suggested retail price for a new GPIRB. And while the COSPAS/SARSAT organization has okayed these GPRIB units, in the U.S. the FCC has not yet approved them (a process called type acceptance). That approval is expected soon, however.

On top of all this, GPIRBs are not scheduled to go into production until May of this year. As of early March, Northern Airborne Technology is the only company with firm plans to manufacture GPIRBs.

Of course, new technology often involves high price and limited availability. After a few years, prices drop and availability increases.

Distress signaling has come far in this century, from a disastrous sinking that took place within sight of another vessel to the capability of reliably receiving distress signals from any stretch of water anywhere on the globe.

By Ocean Navigator