Rapid Searches for Effective Rescues

A novel sensor that rapidly locates emergency beacons could enhance search and rescue operations.

The instantaneous direction-finding vector sensor can be integrated on an airborne platform (upper right) to enable rapid geolocation of a radio-frequency signal. This novel sensor utilizes an array antenna (left) and a multichannel digital receiver system (lower right).The instantaneous direction-finding vector sensor can be integrated on an airborne platform (upper right) to enable rapid geolocation of a radio-frequency signal. This novel sensor utilizes an array antenna (left) and a multichannel digital receiver system (lower right).

A search for hikers lost in a remote area of a national forest escalates as the weather turns forbidding, with temperatures dropping and cold rain starting. Local emergency responders call for aerial backup to scour the acres of terrain the hikers may have been exploring. The pilots of the three small aircraft assigned to the search are given coordinates for the general location of the hikers' route; however, in this region, reception of GPS satellite signals is patchy at best, and sophisticated, GPS-based rescue systems can fail. Although the hikers had the forethought to carry an emergency beacon, the planes are carrying direction-finding sensors that can only process one-dimensional, line-of-bearing information. The pilots are thus forced to fly multiple passes over the forest, seeking the radio-frequency (RF) signal from the beacon and then trying to triangulate its location from multiple signal detections. This search process is taking a lot of time, the enemy of all search and rescue operations. If the air support crews had a method for rapidly locating the beacon, on-the-ground workers could get aid to the hikers before nighttime hampers or halts the rescue.

Researchers at MIT Lincoln Laboratory may have that rapid geolocation method. Leveraging previous work on novel antennas, they have developed an aircraft-mounted direction-finding vector sensor that can detect and locate an RF beacon in two dimensions (azimuth and elevation) in less than 45 milliseconds. With only a single "look," this sensor synchronizes the azimuth and elevation angles to find the beacon; no time-consuming triangulation is needed to home in on the signal.

"The distinguishing feature of this sensor is that it is very fast," say Beijia Zhang, Associate Technology Officer at Lincoln Laboratory and one of the developers of this technology. "Other airborne techniques require too many aircraft flying around for too long before they can determine the direction from which a signal is coming." In addition, while most commercial direction finders provide 3- to 5-degree angle-of-arrival measurement accuracy, Lincoln Laboratory's direction-finding sensor delivers approximately 1-degree accuracy, enabling the sensor to spend less time on geolocating a beacon.

The direction-finding vector sensor consists of a multipolarized antenna array, a multichannel digital receiver, and a microcomputer. These components work as follows to find the location of a beacon: When the beacon transmits an RF signal, the multiple antennas of the array, calibrated for amplitude and phase receive patterns, receive the signal. The digital receiver channels that are connected to the antennas filter, downconvert, and digitize the signal data. "The computer processes these data, comparing the [signal's] different angles of arrival perceived by the different antennas to determine the direction from which the signal is coming," explains Zhang. Onboard GPS and inertial navigation system data are then applied to the directional data to determine the beacon's location. Essentially, this process happens instantaneously.

The vector sensor's design uses both orthogonal loop and monopole antenna modes, exploiting the radiation pattern shape advantages of each. Both types of antenna can be fabricated small enough to be integrated into a system that can be carried by the types of small helicopters or fixed-wing airplanes flown in search and rescue missions. The orthogonal loop antenna allows the sensor to receive a range of radio frequencies and can lessen the interference from electromagnetic waves other than the ones at the receiving frequencies. "The crossed-wire-loop antenna is an aerodynamic design," says Alan Fenn, a member of the senior staff in the RF Technology Group and of the vector sensor development team. "It is also a straightforward design that is not prone to failure." The monopole antenna is also a simple model that is reliable and low cost. Because monopole antennas are omnidirectional, they are well suited to direction-finding.

The research team is confident in the vector sensor's direction-finding capability. "Extensive field testing for the Army Corps of Engineers indicates that the sensor can rapidly find an emergency beacon," says Peter Hurst, a codeveloper of the sensor and a member of the technical staff in the Advanced Undersea Systems and Technology Group.

The fast geolocation enabled by the direction-finding vector sensor, the reliability of the antennas, and the system's small size and low cost would make the system a practical tool for a variety of response agencies. According to Hurst, the sensor could be used over water, making it a candidate for adoption by the Coast Guard for their search and rescue operations. The Department of Homeland Security is another agency that may have interest in the sensor as a tool in disaster response efforts. For example, firefighters working to suppress major wildfires can become disoriented in areas where heavy smoke and airborne ash make visibility poor; if they carried an RF emergency beacon, then the firefighters could be geolocated by the sensor and given directions by incident commanders in communication with them through wireless devices.

Fenn says that future work on the direction-finding vector sensor could include adding a capability for receiving and displaying real-time GPS data when they are available, thus providing real-time search and rescue coordinates. A patent application for this technology, under the title "Multipolarized Vector Sensor Array Antenna System for Search and Rescue Application," has already been filed with the U.S. Patent and Trademark Office.

Posted November 2015

 

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