Towed Pinger Locator (TPL-25 System)


Q. What is the TPL-25?


The TPL- 25 is the U.S. Navy’s towed pinger locator system. The system meets the Navy’s requirement for locating emergency relocation pingers on downed Navy and commercial aircraft. The emergency relocation pingers are mounted on an aircraft’s flight recorder (often referred to as the “black box”).


The TPL-25 system includes a towed, underwater hydrophone, a tow cable and a shipboard processor suite.


Q.  How does this system work?


Once a search zone is identified, the hydrophone will be lowered into the water and towed behind a ship. The hydrophone will be kept approximately 1,000 feet above the sea floor while the ship transverses a search pattern at slow speeds, generally from 3-5 knots depending on the depth, while the hydrophone listens for the acoustic pulse from a black box.


The hydrophone carries a passive listening device for detecting an acoustic pulse. Most emergency relocation pingers transmit a 37.5 kHz frequency pulse, once a second (The TPL-25 can detect any pinger transmitting in a frequency between 3.5 kHz and 50 kHz, at any repetition rate).


Once a signal from the black box is received, the signal is transmitted up the tow cable and is presented audibly, and can be output to either an Oscilloscope, or Signal Processing Computer. The system operator monitors the greatest signal strength and records the navigation coordinates.


This procedure is repeated on multiple track lines until the relocation pinger/black box position is approximated via triangulation.


 Q. What kind of range in terms of distance would TPL-25 be able to pick up?


The TPL is able to detect a signal from a distance of approximately one mile with a maximum depth of 20,000 feet.  The U.S. Navy can search about 150 square miles per day with a TPL-25 system. 


Q. How many TPL-25 systems does the U.S. Navy own?


The U.S. Navy has two full TPL-25 systems.  The Navy has one additional hydrophone, which was sent to Perth as a spare.


Q. What's the biggest challenge in finding black boxes with TPL-25?


For the TPL to find a black box, its likely general location needs to be determined prior to the system’s deployment. Typically, this involves the identifying surface debris along with an analysis of ocean currents.


Time may also present a challenge. The emergency relocation pinger’s battery power allows for the transmission of a signal for approximately 30-40 days. Once the battery power runs out there will be no signal for the TPL-25 to detect. If the black box’s emergency relocation pinger is damaged during impact, as was the case with Air France 447, that may further limit the hydrophone’s ability to locate it.


Q. Is this TPL-25 the same equipment utilized to search for the Air France 447 aircraft?


The TPL-25 is a newer model that uses the same underlying technology. The system used for the flight 447 search was not able to locate the aircraft’s black box because the box was damaged during the crash.


Q. If the TPL-25 picks up a signal, how do you go about about finding the black box?


Once the pinger signal is detected, the TPL-25 team will approximate the black box location through triangulation.  Next, the sonar and/or still camera of a Bluefin-21 system can be used to determine the black box's exact location. The Bluefin can also be used to search the sea floor if a black box signal isn’t detected.


Q. In what capacity has the U.S. Navy successfully used the TPL in the past?


The Navy has successfully used TPL devices for around 20 years, primarily to find black boxes from military aircraft. The most modern iteration, and the version that was sent to Australia to assist in this incident, first entered service in 2010 and was successfully used to find the black box from a U.S. Marine Corp AV-8B Harrier that crashed in Gulf of Oman in 2011. An earlier system was used to successfully locate TWA Flight 800's black box.


Q. Will any U.S. Navy personnel be on hand to operate the TPL?


Yes. The crew is made up of two naval officers and eight Navy contract personnel from Phoenix International.  The processing suite may be manned by U.S. Navy personnel or Navy contractors.


To learn more about the Towed Pinger Locator, please review the U.S. Navy Fact File:



Q. What is the Bluefin-21?


Bluefin-21 is an autonomous underwater vehicle (AUV) that can be equipped with either a side-scan sonar or a still camera to gather underwater data.  Bluefin-21 is 16.2 feet long, has a diameter of 21 inches and its dry weight is 1,650 lbs. Its depth rating is 14,763 feet and has a maximum speed of 4.5 knots. Its mission endurance is 25 hours with standard payload at a speed of 3 knots.   


Q. How does this system work?


Technicians program the Bluefin AUV to search a specific area before the vehicle goes in the water.  During the search, the system uses a side-scan sonar or a still camera to gather data. Once an area search mission is completed, the AUV is removed from the water and plugged into a processing suite for data download. The data is then analyzed. While data is being analyzed, a subsequent mission may be programmed and the AUV may be deployed to perform its next area search.


Q. How much ocean can be covered by the Bluefin-21?


The effective range of a sonar device and therefore the Bluefin-21 is dependent upon a number of factors. These factors include the water depth within the search area, the contours of the sea floor, and the size of the material being searched for.  Therefore it is not possible to provide a standard maximum range for the side-scan sonar/Bluefin-21. However, as a rough estimate, the Navy expects to be able to search approximately 40 square miles per day using the Bluefin.  This estimate will be affected by the specific environmental factors encountered.


Q. What role will Bluefin-21 play in the search for MH370?


If deployed, Bluefin-21 will be used to search for debris under the water surface and on the ocean floor.   It is expected to initially deploy with the sonar payload. The TPL-25 and Bluefin-21 systems would not likely be deployed until there is a high degree of confidence as to the general area in which the plane may have entered the water.


Q. Has Bluefin-21 been used in other missions?


This is the second time the U.S. Navy has used the Bluefin-21 for salvage operations. It was previously used it to recover an Air Force F-15 off the coast of Japan.


Q. The Bluefin can't get GPS signals underwater, how does it know where it is while it performs its search?


The Bluefin-21 takes a GPS signal as it goes underwater, and then operates within its pre-programmed box. The Bluefin-21’s GPS, coupled with its inertial navigation system provides a high degree of navigation accuracy (the dead reckoning drift is typically less than 0.1% of distance traveled).


Q. Does the Navy own this system?


No. The Bluefin-21 system is owned and operated by Phoenix International.


Bluefin Robotics website: 




Q. Why doesn’t the U.S. Navy fly the TPL and Bluefin out to the search area so they arrive there faster (rather than transporting 1,400 miles by ship)?


Perth offered the best location for the U.S. Navy to load the equipment aboard a suitable ship -- the Royal Australian Navy ship Ocean Shield.


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