The field of wireless vital sign monitoring has a relatively long history, almost as long as radar itself. Doppler radar has been the primary sensor technology for detecting blood flow or fetal heart rate. Over the past decade, there has been an increasing interest and need for creating wireless long-range vital sign monitoring systems. In addition, there has been a strong preference for using non-contact technologies for detection applications, especially for search-and-rescue operations that require the ability to see through debris, walls, and snow. There are also several emerging applications using short-range radar systems for automotive safety systems, remote health monitoring, and heart-based biometric authentication.
Named after the Austrian physicist Christian Doppler, the Doppler effect is a fundamental frequency shift phenomenon that occurs whenever a wave source and an observer are moving with respect to one another. When a vehicle sounding a siren or horn approaches, passes, and recedes, for example, the bystanding observer will hear the sound higher during the approach, identical at the instant of passing by, and lower during the recession. The frequency increase has wellestablished applications in astrophotonics, biological diagnostics, weather and aircraft radar systems, velocimetry, and vibrometry. For instance, ultrasonic pulse probes utilize this Doppler effect to detect the relative motion of blood flow in the human body.
Microwave Doppler radar has the capability to detect vital signs, such as heart and breathing function. Doppler radar achieves the findings by sensing mechanical displacements of the chest cavity in the order of millimeters, resulting from shock waves created by heart and respiration motion. This is known as Radar Seismocardiogram (R-SCG). Using the technology, other cardiac dynamic parameters/features that are unique to each person can be extracted.
In order to enable commercial applications for R-SCG devices, several key problems need to be overcome, including device cost and motion artifacts that distort the signal of interest. The cost and performance have dramatically improved with the advent of RF integrated circuits, contributing to the commercialization of small low-power radar units for many different purposes. It has been shown that high transmit power is not necessary to achieve good results with Doppler radar and some other detection methods, such as Ultra-Wide Band sensing that requires only -41dBm of output power and uses approximately 3 million times lower power than a typical smartphone.
More recently, the increasing computing power of embedded computers and small microcontrollers now enable the implementation of significant computational algorithms to analyze, filter, and clean the data from such small radar systems. With the advent of advanced pattern recognition and machine learning technologies, the screening of unusual heart activity, as well as the monitoring of physiological activity in certain stressful situations, is now feasible.
Despite the advances, however, significant problems remain, including how to find better ways to reduce background motion noise in such systems while maintaining very low cost. To be practical and useful, the devices must acquire, interpret, and provide information in real-world operating environments at competitive costs. There is a tremendous need to demonstrate the efficacy of this system in real-world, naturalistic operating environments.
Enter NASA’s FINDER Technology
The Finding Individuals for Disaster and Emergency Response (FINDER) prototype technology developed at NASA’s Jet Propulsion Laboratory has demonstrated the ability to locate individuals buried under rubble in disaster scenarios. FINDER uses radar technology to sense the heartbeats and breathing of humans hidden behind piles of debris. With the NASA FINDER, four men were rescued in April of 2015 from the wreckage of a collapsed textile factory in the Nepalese village of Chautara. The technology detected the men’s presence even as they were buried under about 10 feet of brick, mud, and wood (see Figure 1).
The FINDER technology was developed in collaboration between NASA's Jet Propulsion Laboratory and the Department of Homeland Security’s Science and Technology Directorate. The radar tool beams microwave signals into piles of debris, and the patterns of the reflected signals are analyzed for changes. The FINDER device senses the tiny motions caused by victims’ breathing and heartbeats, and displays the vital signs along with a reliability score.
The Doppler sensor technology includes a local oscillator which is split into two signals (I and Q) that are offset by 90 degrees -- each going to two different detectors along with the receive signal. I/Q data shows the changes in magnitude (or amplitude) and phase of the signal. If amplitude and phase changes occur in an orderly, predetermined fashion, they can be used to encode information upon the signal, a process known as modulation. I/Q data is highly prevalent in RF communications systems, and more generally in signal modulation, because it is a convenient way to modulate signals.