Search and rescue without an emergency beacon

When it comes to search and rescue (SAR) capability, present-day mariners enjoy the tremendous benefits of improved communications technology. Never have there been such good SAR tools for finding and rescuing mariners in distress. Exciting cutting-edge research in the esoteric field of synthetic aperture radar and lasers could make SAR efforts even more effective.BR>

A key tool in the current SAR environment is the 406 MHz EPIRBa pronounced advance over the first-generation 121.5/243 EPIRBs. The 406’s high-power, frequency-stable, digitally encoded signal means more accurate positions that SAR teams can use as the starting point for their search. And with the addition of a built-in GPS receiver, such as in Northern Airborne Technology’s GPIRB unit, 406 EPIRBs become even more effective, greatly shrinking the potential search area. A mariner in extremis who is equipped with a 406 GPS-EPIRB can be fairly confident that help is on the way.

However, what if a GPS-EPIRB malfunctions? What if the mariner doesn’t replace a dead EPIRB battery? What if the vessel isn’t equipped with an EPIRB? A mariner who is only able to send a distress message via HF SSB prior to his boat sinking out from underneath him finds himself thrown back to a world where satellites and computers don’t matter; his future rests on searchers using their eyeballs as they criss-cross a large swath of ocean doing a sector search.

What if there were a way to search many square miles of ocean electronically without having to rely on the visual recognition skills of tired searchers? A group of researchers from NASA are doing preliminary work on such a system. Called beaconless search and rescue, this technique makes use of a special type of radar called synthetic aperture radar (for a description of synthetic aperture radar see "Shuttle mapping system may mean better charts," Issue No. 95, Jan./Feb. 1999) to bounce radar signals off the Earth’s surface. A computer then processes the returns and builds a precise, detailed picture. Analysis software searches through the data and flags the distinctive echoes returning from man-made objects, noting their position. These high-probability areas can then be investigated by SAR aircraft or vessels.

The work on this technique was prompted by the need to find crashed aircraft whose emergency beacons (called emergency locator transmitters, or ELTs, in the aviation world) failed to transmit. One of this technique’s big advantages in looking for a downed aircraft is the capability to use low radar frequencies and "see" through foliage. Luckily for mariners, there is no foliage at sea. Still, smaller vessels and life rafts need to have some sort of radar reflector to bounce radar energy back to the synthetic aperture unit. "For water searches, you can stay at higher frequencies," said Houra Rais, scientist/engineer at NASA’s Goddard Space Flight Center, "and reasonable-sized reflectors are okay. But just a life raft bobbing up and down wouldn’t show upyou have to have a reflector."

The advantages of a synthetic radar approach when used for maritime searches include the fact that: 1) It relies on an automated radar system to do the mind-numbing, wide-area search, saving the SAR team for those tasks that people do better than computers, like diving out of helicopters and getting survivors into a hoist basket. 2) Since the radar sees equally well in the dark and is still effective in bad weather, this radar can search 24 hours a day and in most weather conditions.BR>

The "synthetic aperture" part of this radar type derives from the fact that computer-driven signal processing is used to make a radar antenna seem much biggera virtual antenna of sorts. This large virtual antenna gives synthetic aperture radars impressive resolution capability. In fact, the images from synthetic radars can be so detailed as to look like photographs. The uncanny, photo-realistic quality of these images is one of their most striking features.

The key to the potential effectiveness of the synthetic aperture radar approach is something called automatic crash-site detection (ACSDthe term "crash site" shows the system’s aviation origins). With ACSD, software is used to analyze the high-resolution radar data and look for radar returns that contrast with the natural surroundings. To put it in marine terms, ocean waves tend to produce radar echoes that fit within a certain envelope. The analysis software looks at a radar echo and compares it to the "ocean wave" profile. If it fits the profile for an ocean wave, the echo is ignored. However, a radar return from a metal mast or a radar reflector will produce a distinctive echo that does not fit the wave profile. When the software discovers such a return, the crew is alerted and provided with a lat/long position. They can proceed to that position and do a search. This approach saves fuel, plus wear and tear on the crew and the equipment, be it an aircraft or vessel.

Of course, existing non-synthetic aperture radar units on Coast Guard vessels and aircraft can also be used to search for vessels in distress, but they have limitations that reduce their effectiveness. These standard search radars don’t have the precise resolution of synthetic aperture radar. Plus, the radar returns are degraded in periods of bad weather or large waves. Probably the biggest advantage of a synthetic aperture system is that it can be mounted on a high-altitude aircraft or a satellite, both of which can usually operate above bad weather. While a space-based system would be able to cover huge areas, a SAR satellite would have the disadvantage of its having to wait hours until it could pass over an area where a vessel was in distress. A synthetic aperture radar system mounted on a search airplane, a Coast Guard HC-130 Hercules, could theoretically be quickly deployed to an area of interest.

One synthetic aperture radar satellite already in orbit is a Canadian spacecraft called RadarSat. Launched in 1995 and operated by the Canadian Space Agency, RadarSat’s main mission is to provide high-resolution synthetic aperture radar images for environmental monitoring and natural resource management. However, RadarSat has also been used for some search-and-rescue missions, such as searching for the Canadian ocean racer Gerry Roufs, who disappeared in the Southern Ocean in January 1997 during the Vendee Globe Race. Unfortunately, RadarSat was not able to detect Roufs’s boat Groupe LG2.

Another "beaconless" search-and-rescue approach that NASA has investigated involves using lasers. A search aircraft points a laser downward and scans it back and forth as it flies over the search area. In addition to the laser, the plane is also equipped with a sensor for picking up reflections. The object being searched for must have a patch of a special retroreflective material designed to bounce back light. Searchers know they have found the object in question when the sensor receives the specific type of reflection that only comes from the retroreflective material.

The drawback to this method is that mariners would have to apply the specially reflective tape before the laser search plane passed over on a search. Of course, SOLAS convention rules already require that life rafts and jackets be marked with retroreflective material designed to bounce back the light from searchlights.

Both of these methodssynthetic aperture radar and laser SARare still in the early stages of development and may never be actually deployed. Still, all oceangoing mariners can take some reassurance that, as good as today’s search and rescue efforts are, SAR capabilities should only get better in the future. Let’s just hope you never have to put them to the test.

By Ocean Navigator