Radar Flashlight for Through-the-Wall Detection of Humans

                             Eugene F. Greneker
               Radar Systems Division, Surface Systems Branch
            Sensors and Electromagnetic Applications Laboratory
                      Georgia Tech Research Institute
                      Georgia Institute of Technology
                        Atlanta, Georgia 30332-0856


Prior to the 1996 Olympics held in Atlanta, Georgia, several versions of a radar vital signs monitor (RVSM) were developed by Georgia Tech Research Institute researchers.  The most recent version RVSM was developed to measure the heart rate of Olympic rifle and bow and arrow competitors to determine if their training allowed them to the detect their heartbeats and if so, whether they were capable of using that training to avoid an approximate 5 milliradian movement of the bow or rifle that occurs each time the heart beats.  The RVSM that was developed was tested to detect the shooter's heartbeat at a distance of 10 meters without the requirement of a physical connection to the subject.  It was found that a second channel could be added to the RVSM to detect the shooter's respiration rate from a distance of 20 meters without physical connection between the RVSM and the shooter.

The RADAR Flashlight, a spin-off of these predecessor systems developed at GTRI, is the topic of this paper.  The RADAR Flashlight was designed to detect the respiration of a human subject behind a wall, door or an enclosed space with non-conductive walls.  The use of the system as a foliage penetration radar has also been explored.  It has been determined that the RADAR Flashlight is capable of detecting a human hiding within a tree line behind light foliage.  This paper describes the current status of the RADAR Flashlight and presents typical test data produced when the system is operated in the laboratory environment.

  1. History of System Development
  2. The RVSM Developed for Olympic Application
  3. Radar Vital Signs Heartbeat Signature
  4. Operational Theory and Design
  5. Design Philosophy
  6. Real World Requirements for System Acceptance
  7. Steps Toward Commercialization
  8. Reference

History of System Development

The RADAR Flashlight results from technology developed during several research projects conducted at GTRI over the past 10 years to detect respiration and heartbeat signatures from individuals at a distance and without connections.  The first GTRI RVSM system was developed in the mid-1980s under sponsorship of the United States Department of Defense (DOD).  A patent on the system was issued in 1992.  This frequency modulated (FM) radar was used as a battlefield vital signs monitor.  It was designed to be used during live fire situations to determine if a wounded soldier was alive before risking a corpsman's life to treat him.  The design goal of that system was a capability to detect heartbeat and respiration at distances of 100 meters.  The system was also tested on soldiers wearing a chemical or biological warfare suit to allow vital signs to be monitored without opening the suit and risking contamination of the subject.  The latest RVSM, to be briefly discussed in this paper, was developed by the author for use in the 1996 Olympics held in Atlanta, Georgia.  A variant called the RADAR Flashlight, which is the primary subject of this paper, was developed for use by law enforcement personnel to detect individuals concealed behind a wall or within an enclosed space.

The RVSM Developed for Olympic Application

The operation of the Olympic model RVSM was addressed in a paper that was presented at AeroSense 97[1].  Specifically, the RVSM was developed because it had been proposed that some Olympic archers and rifle competitors shoot between their heartbeats to avoid an approximate 5 milliradian movement of the arms and body.  If this was true, their shooting between heartbeats would provide better accuracy.  A system to detect a heartbeat at a distance was proposed and a prototype RVSM was built to demonstrate the finely honed skills of the Olympic competitors.  It was envisioned that the demonstration RVSM would be of interest to the television networks covering these competitions.  Next, several system requirements were developed.  The operation of the system could not distract the competitors.  To meet this challenge, the radar was designed to be located at least 10 meters from the competitors, under a radome, and mounted on a pan-tilt positioner.  A charged coupled low light level television camera was boresighted with the antenna for aiming the system at the thorax of the shooters under study.  The system also required low sidelobes to avoid detection motion artifacts from the event judges who would observe the shooters during competition.

Radar Vital Signs Heartbeat Signature

Figure 1 shows the a typical heartbeat signature that has been sensed by the RVSM built for the Olympics.  Referring to Figure 1, the subject was seated in the laboratory approximately 3 meters from the RVSM.  The RVSM antenna was boresighted on the thorax region of the subject's chest.

Figure 1: RVSM Heartbeat Signatures

It is thought that the signature that is detected by the RVSM is the shock wave propagating from the beating heart as it spreads across the thorax region of the chest wall rather than the detection of the movement of the beating heart.  Studies have shown that there is little penetration of the chest wall by radio frequency (RF) energy at 24.1 GHz at the low power densities of 0.1 milliwatt/cm2, which is typical of those produced by the RVSM at a range of 3 meters.  It is thought that this shock wave is the same phenomenon that is heard by a health care provider using stethoscope.  The heartbeat signature shown in Figure 1 is relatively complex, indicating that there are numerous frequencies in the signature.  When the digital recording from which the Figure 1 plot was taken is fed into a digital to analog converter and the subsequent output is fed to the input of an audio amplifier with good bass response, the sound that is heard in the speaker is very similar to the heartbeat sounds that are heard with a stethoscope.

The capability of the RVSM to provide heart and respiration rate in addition to heart sounds suggests some interesting applications for the technology.  These possible applications include a monitor for telemedicine that does not require the connection of electrodes to the patient.  Physically or mentally challenged patients would only be required to sit in front of a table top monitor to have their heart and respiration rates taken.  Burn wards could use the system to take vital signs of patients without skin for electrode attachment.

Other applications that have been investigated for the RVSM include using it to detect persons hiding in light foliage several feet behind a chain link fence.  The use of the RVSM as a stress measurement system has also been investigated.  It was found that a change in the heartbeat rate of a human as small as 3 heartbeats per minute is measurable.  This capability has law enforcement applications.  It was during the evaluation of law enforcement applications that the concept of the RADAR Flashlight was developed.

The RADAR Flashlight was developed to be a law enforcement tool.  It can detect the respiration signature of an individual standing up to 5 meters behind an 20 centimeter hollow core concrete block wall and wooden doors typical of those found on most homes and which are almost transparent to the system.  Dry plywood, particle board and wall board do not attenuate the signal significantly.

Most system applications for the RADAR Flashlight involve inspection of spaces beyond a door or wall.  For example, the system could be used to determine if a subject is standing behind a door without a requirement that the door be opened.  This technique could be used to detect a subject behind a front door who fails to answer a knock.  It can also be used to inspect a closed space such as an interior closet.  Normally, the closet would have to be opened to determine if someone was hiding inside.

Operational Theory and Design

Figure 2 is a photograph of the current version of the laboratory prototype RADAR Flashlight.  Referring to Figure 2, the system is housed in a flashlight shaped enclosure.  The radar is mounted in the front of the housing, and the system's microwave lens, used to "shape" the antenna beam, is installed in the position of the optical lens normally found on a standard flashlight.  The battery compartment is longer than those found on a normal flashlight.  It is currently planned that the system's signal processor and rechargeable batteries will be housed in the extended battery compartment once the current laboratory prototype is reduced to a field testable prototype.

Figure 2: Laboratory Model of Radar Flashlight with Signal Processor Board

The current external signal processor used with the laboratory prototype is shown in Figure 2 as the printed circuit board to the left of the RADAR Flashlight.  No attempt has been made to miniaturize this signal processor which is currently used to filter the respiration signature from other signals caused by radar self motion, fluorescent lights and other clutter effects.  The laboratory prototype unit shown in Figure 2 operates on a frequency near 10.525 GHz, although an earlier version of the system was operated at 24.1 GHz and demonstrated less sensitivity to motion through a 20 centimeter hollow brick block wall.  The current laboratory prototype is a homodyne radar configuration, although a frequency modulated continuous wave (FM-CW) system could be used for applications where information is required to determine the range to the target.  The current laboratory prototype operates in the near field region of the antenna for most through the wall detection scenarios.

The current laboratory system signal processor (shown in Figure 2) processes the respiration signal and the associated signal in the time domain so that the time domain record is preserved.  The processor essentially acts as a low pass filter with the cut off frequency shoulder just above the highest respiration frequencies that are expected.  This first filter rejects most of the ambient clutter sources such as fluorescent lights.  The analog time domain signal is fed into an analog to digital converter hosted by a laboratory computer where the input signal is converted into a 12 bit analog word and displayed on a computer generated strip chart recording.  Once in digital format, the signal can be subjected to more rigorous processing to retrieve the respiration signal under heavy clutter conditions including those due to body motion and other artifacts.

Figure 3 is a recording of a respiration signature that was taken by the RADAR Flashlight located 24 centimeters from a hollow core 20 centimeter thick concrete building block wall.  The subject was instructed to stand 1.8 meters beyond the brick wall and not to move once in position but to breathe normally.  The RADAR Flashlight's beam projected through the wall and was approximately centered on the thorax region of the subject's chest.

Figure 3: Respiration Signature taken by RADAR Flashlight Through 8-inch Hollow Core Concrete Wall

Referring to Figure 3, time moves from left to right.  The ambient signal level without a subject in the beam is shown as point A.  The point at which the subject enters the beam is shown as point B.  Upon the subject's entry into the beam, there is a large downward shift in signal level.  The shift occurs because the detector is D.C. coupled to the first stage of the signal preamplifier.  As a result, there is a shift in the level of the signal due to a change in phase along the signal path caused by the placement of the subject's body into the beam.  Points C, D, E, F, and G are negative excursions caused by the movement of the chest wall toward the radar during respiration.  The subject was told to breathe once approximately every five seconds and the record shows that this instruction was followed.  The subject steps out of the beam at approximately 52 seconds.  The signal level returns to the ambient level at point H.  There was a D.C. level drift of approximately 230 millivolts over the 60 second period during which the test was conducted.  This signal drift would not normally appear because the output of the detector would be A.C. coupled through a D.C. blocking capacitor between the detector diode and the preamplifier input.

Design Philosophy

The RADAR Flashlight will detect the body movement of a subject at longer ranges than those at which the respiration signature can be detected when the subject is stationary.  Total body motion presents a much larger Doppler modulated radar cross section than the small respiration induced movement of the chest wall.  Unfortunately, when the RADAR Flashlight is used for law enforcement applications, the subject can not be depended upon to voluntarily move during the search process.  Thus, the detection of the involuntary respiration signature is necessary to ensure that the motionless subject can be detected.

Several system utilization scenarios have been developed for the RADAR Flashlight.  When a fugitive warrant is being executed, interior closets are often the hiding places of choice for individuals who are sometimes armed and dangerous.  It is the duty of those serving the warrant to open each closet door and inspect the interior space.  This requirement puts the law enforcement personnel at a disadvantage.  The RADAR Flashlight can detect fugitives or others hiding in a closet without requiring that the closet door be opened to complete the inspection.

During a hostage situation it may be possible to determine where in a room the hostages are located and it may also be possible to determine where the hostage takers are located at any given time, assuming that the usual hostage scenarios are followed.  Hostages are usually closely controlled and may be physically restrained or under duress to prevent their escape.  Thus, a hostage is generally not moving but will be breathing.  The hostage taker may be highly mobile and may move from room to room to inspect his or her defenses, communicate with police, and continually assess the environment.  There are exceptions, however, but if this scenario is the case even 50 percent of the time, the RADAR Flashlight may be able to help determine the location of the hostage taker(s) and determine the location of the hostages.  It is envisioned that a member of the Special Weapons and Tactics Team (SWATT) would take a position against the outside wall of the room of interest.  The SWATT member would attempt to first detect motion and later detect respiration in a more careful search.  The RADAR Flashlight would be scanned slowly across the room.

Warrant servers are required to go to a home or business to serve warrants on persons who in many cases do not want to accept the warrant or even let the server know that they are present.  This is especially true when the individual will go to jail if they are discovered.  The RADAR Flashlight could help determine if there is an individual behind the door but not answering the door.

Real World Requirements for System Acceptance

The system must be inexpensive to produce in large quantities and in the same price range as a top end weapon carried by a law enforcement officer.  Thus, a target price for the RADAR Flashlight product was set at between $300 and $500.  It is thought that the most expensive part of the system would be the RF section followed by the digital signal processor.  If future marketing studies should determine that high sales volumes can be achieved, the parts count in the system can be reduced significantly by implementing the system in a chip set.  The cost of converting the system to a chip set would be amortized over the high number of systems sold.

There is a requirement that the system should be capable of being operated by a relatively unskilled operator.  This requirement suggested that the packaging of the system was important and that the associated signal processor should be "smart" and make many of the decisions regarding target identification for the operator.  Given this requirement, a flashlight configuration was adopted as a housing.  The final form of the target display has not yet been determined, although a simple display would appear to be an acceptable option.

Steps Toward Commercialization

The RADAR Flashlight is currently a laboratory instrument and, as such, is not designed to be used while in motion.  When the RADAR Flashlight is in motion it receives Doppler shifted signals that are generated from its own motion referenced to fixed objects in front of the sensor.  Depending on the radar cross section of the "radar clutter," the clutter return can be very large compared to the small return from the chest motion generated by respiration. GTRI has developed two approaches to achieve cancellation of the self motion of the RADAR Flashlight.  Research must still be conducted to determine which self motion technique is most effective and to develop the self motion cancellation algorithms.

GTRI has developed a research plan to take the RADAR Flashlight from the laboratory prototype to a field testable prototype.  After field testing, it is anticipated that deficiencies will be found that must be corrected.  After deficiency corrections are undertaken the system will be licensed to a manufacturer to produce as a product.  The next challenge is to find the manufacturer capable of producing a quality product and also capable of funding the research that remains to transition the RADAR Flashlight from a laboratory prototype to a pre-production prototype.


1.  E. F. Greneker, "Radar Sensing of Heartbeat and Respiration at a Distance with Security Applications," Proceedings of SPIE, Radar Sensor Technology II, Volum 3066, Orlando, Florida, pp. 22-27, April, 1997.

For More Information, Contact:

     Eugene F. Greneker
     Radar Systems Division, Surface Systems Branch
     Sensors and Electromagnetic Applications Laboratory
     Georgia Tech Research Institute
     Atlanta, Georgia 30332-0856
     Phone: (770) 528-7744
     E-mail: gene.greneker@gtri.gatech.edu

The "RADAR Flashlight" can Detect the Presence of a Human Through a Wall or Door