Early one morning last May, a commercial airliner was approaching El Paso International Airport, in West Texas, when a warning popped up in the cockpit: “GPS Position Lost.” The pilot contacted the airline’s operations center and received a report that the U.S. Army’s White Sands Missile Range, in South Central New Mexico, was disrupting the GPS signal. “We knew then that it was not an aircraft GPS fault,” the pilot wrote later.
The pilot missed an approach on one runway due to high winds, then came around to try again. “We were forced to Runway 04 with a predawn landing with no access to [an instrument landing] with vertical guidance,” the pilot wrote. “Runway 04…has a high CFIT threat due to the climbing terrain in the local area.”
CFIT stands for “controlled flight into terrain,” and it is exactly as serious as it sounds. The pilot considered diverting to Albuquerque, 370 kilometers away, but eventually bit the bullet and tackled Runway 04 using only visual aids. The plane made it safely to the ground, but the pilot later logged the experience on NASA’s Aviation Safety Reporting System, a forum where pilots can anonymously share near misses and safety tips.
This is far from the most worrying ASRS report involving GPS jamming. In August 2018, a passenger aircraft in Idaho, flying in smoky conditions, reportedly suffered GPS interference from military tests and was saved from crashing into a mountain only by the last-minute intervention of an air traffic controller. “Loss of life can happen because air traffic control and a flight crew believe their equipment are working as intended, but are in fact leading them into the side of the mountain,” wrote the controller. “Had [we] not noticed, that flight crew and the passengers would be dead. I have no doubt.”
There are some 90 ASRS reports detailing GPS interference in the United States over the past eight years, the majority of which were filed in 2019 and 2020. Now IEEE Spectrum has new evidence that GPS disruption to commercial aviation is much more common than even the ASRS database suggests. Previously undisclosed Federal Aviation Administration (FAA) data for a few months in 2017 and 2018 detail hundreds of aircraft losing GPS reception in the vicinity of military tests. On a single day in March 2018, 21 aircraft reported GPS problems to air traffic controllers near Los Angeles. These included a medevac helicopter, several private planes, and a dozen commercial passenger jets. Some managed to keep flying normally; others required help from air traffic controllers. Five aircraft reported making unexpected turns or navigating off course. In all likelihood, there are many hundreds, possibly thousands, of such incidents each year nationwide, each one a potential accident. The vast majority of this disruption can be traced back to the U.S. military, which now routinely jams GPS signals over wide areas on an almost daily basis somewhere in the country.
1: To investigate a report, go to the ASRS database: https://asrs.arc.nasa.gov/
2: On the top ribbon, click “Search ASRS Database,” and then choose “Search ASRS Online.” Click on “Start Search.”
3: Follow the steps under “How to Search” at the top. Then, under 7 “Text: Narrative/Synopsis,” click on “[words].” Then click on “Text contains Click Here.”
4: In the pop-up window, enter some of the text that is quoted in this story. In the “Fields to search” field at the bottom, click “Narrative” (but you can also try “Synopsis”).
5: If you’re searching on more than one word, you need to format it inside parentheses, thus: (GPS JAMMING).
6: Click “Save.” The pop-up will disappear.
7: Click “Run Search” at the bottom right.
8: Under “Display your results,” click “View all reports.”
The military is jamming GPS signals to develop its own defenses against GPS jamming. Ironically, though, the Pentagon’s efforts to safeguard its own troops and systems are putting the lives of civilian pilots, passengers, and crew at risk. In 2013, the military essentially admitted as much in a report, saying that “planned EA [electronic attack] testing occasionally causes interference to GPS based flight operations, and impacts the efficiency and economy of some aviation operations.”
In the early days of aviation, pilots would navigate using road maps in daylight and follow bonfires or searchlights after dark. By World War II, radio beacons had become common. From the late 1940s, ground stations began broadcasting omnidirectional VHF signals that planes could lock on to, while shorter-range systems indicated safe glide slopes to help pilots land. At their peak, in 2000, there were more than a thousand very high frequency (VHF) navigation stations in the United States. However, in areas with widely spaced stations, pilots were forced to take zigzag routes from one station to the next, and reception of the VHF signals could be hampered by nearby buildings and hills.
Everything changed with the advent of global navigation satellite systems (GNSS), first devised by the U.S. military in the 1960s. The arrival in the mid-1990s of the civilian version of the technology, called the Global Positioning System, meant that aircraft could navigate by satellite and take direct routes from point to point; GPS location and altitude data was also accurate enough to help them land.
The FAA is about halfway through its NextGen effort, which is intended to make flying safer and more efficient through a wholesale switch from ground-based navigation aids like radio beacons to a primarily satellite-enabled navigation system. Along with that switch, the agency began decommissioning VHF navigation stations a decade ago. The United States is now well on its way to having a minimal backup network of fewer than 600 ground stations.
Meanwhile, the reliance on GPS is changing the practice of flying and the habits of pilots. As GPS receivers have become cheaper, smaller, and more capable, they have become more common and more widely integrated. Most airplanes must now carry Automatic Dependent Surveillance-Broadcast (ADS-B) transponders, which use GPS to calculate and broadcast their altitude, heading, and speed. Private pilots use digital charts on tablet computers, while GPS data underpins autopilot and flight-management computers. Pilots should theoretically still be able to navigate, fly, and land without any GPS assistance at all, using legacy radio systems and visual aids. Commercial airlines, in particular, have a range of backup technologies at their disposal. But because GPS is so widespread and reliable, pilots are in danger of forgetting these manual techniques.
When an Airbus passenger jet suddenly lost GPS near Salt Lake City in June 2019, its pilot suffered “a fair amount of confusion,” according to the pilot’s ASRS report. “To say that my raw data navigation skills were lacking is an understatement! I’ve never done it on the Airbus and can’t remember having done it in 25 years or more.”
“I don’t blame pilots for getting a little addicted to GPS,” says Todd E. Humphreys, director of the Radionavigation Laboratory at the University of Texas at Austin. “When something works well 99.99 percent of the time, humans don’t do well in being vigilant for that 0.01 percent of the time that it doesn’t.”
Losing GPS completely is not the worst that can happen. It is far more dangerous when accurate GPS data is quietly replaced by misleading information. The ASRS database contains many accounts of pilots belatedly realizing that GPS-enabled autopilots had taken them many kilometers in the wrong direction, into forbidden military areas, or dangerously close to other aircraft.
In December 2012, an air traffic controller noticed that a westbound passenger jet near Reno, Nev., had veered 16 kilometers (10 miles) off course. The controller confirmed that military GPS jamming was to blame and gave new directions, but later noted: “If the pilot would have noticed they were off course before I did and corrected the course, it would have caused [the] aircraft to turn right into [an] opposite direction, eastbound [jet].”
So why is the military interfering so regularly with such a safety-critical system? Although most GPS receivers today are found in consumer smartphones, GPS was designed by the U.S. military, for the U.S. military. The Pentagon depends heavily on GPS to locate and navigate its aircraft, ships, tanks, and troops.
For such a vital resource, GPS is exceedingly vulnerable to attack. By the time GPS signals reach the ground, they are so faint they can be easily drowned out by interference, whether accidental or malicious. Building a basic electronic warfare setup to disrupt these weak signals is trivially easy, says Humphreys: “Detune the oscillator in a microwave oven and you’ve got a superpowerful jammer that works over many kilometers.” Illegal GPS jamming devices are widely available on the black market, some of them marketed to professional drivers who may want to avoid being tracked while working.
Other GNSS systems, such as Russia’s GLONASS, China’s BeiDou, and Europe’s Galileo constellations, use slightly different frequencies but have similar vulnerabilities, depending on exactly who is conducting the test or attack. In China, mysterious attacks have successfully “spoofed” ships with GPS receivers toward fake locations, while vessels relying on BeiDou reportedly remain unaffected. Similarly, GPS signals are regularly jammed in the eastern Mediterranean, Norway, and Finland, while the Galileo system is untargeted in the same attacks.
The Pentagon uses its more remote military bases, many in the American West, to test how its forces operate under GPS denial, and presumably to develop its own electronic warfare systems and countermeasures. The United States has carried out experiments in spoofing GPS signals on at least one occasion, during which it was reported to have taken great care not to affect civilian aircraft.
Despite this, many ASRS reports record GPS units delivering incorrect positions rather than failing altogether, but this can also happen when the satellite signals are degraded. Whatever the nature of its tests, the military’s GPS jamming can end up disrupting service for civilian users, particularly high-altitude commercial aircraft, even at a considerable distance.
The military issues Notices to Airmen (NOTAM) to warn pilots of upcoming tests. Many of these notices cover hundreds of thousands of square kilometers. There have been notices that warn of GPS disruption over all of Texas or even the entire American Southwest. Such a notice doesn’t mean that GPS service will be disrupted throughout the area, only that it might be disrupted. And that uncertainty creates its own problems.
In 2017, the FAA commissioned the nonprofit Radio Technical Commission for Aeronautics to look into the effects of intentional GPS interference on civilian aircraft. Its report, issued the following year by the RTCA’s GPS Interference Task Group, found that the number of military GPS tests had almost tripled from 2012 to 2017. Unsurprisingly, ASRS safety reports referencing GPS jamming are also on the rise. There were 38 such ASRS narratives in 2019—nearly a tenfold increase over 2018.
New internal FAA materials obtained by Spectrum from a member of the task group and not previously made public indicate that the ASRS accounts represent only the tip of the iceberg. The FAA data consists of pilots’ reports of GPS interference to the Los Angeles Air Route Traffic Control Center, one of 22 air traffic control centers in the United States. Controllers there oversee air traffic across central and Southern California, southern Nevada, southwestern Utah, western Arizona, and portions of the Pacific Ocean—areas heavily affected by military GPS testing.
This data includes 173 instances of lost or intermittent GPS during a six-month period of 2017 and another 60 over two months in early 2018. These reports are less detailed than those in the ASRS database, but they show aircraft flying off course, accidentally entering military airspace, being unable to maneuver, and losing their ability to navigate when close to other aircraft. Many pilots required the assistance of air traffic control to continue their flights. The affected aircraft included a pet rescue shuttle, a hot-air balloon, multiple medical flights, and many private planes and passenger jets.
In at least a handful of episodes, the loss of GPS was deemed an emergency. Pilots of five aircraft, including a Southwest Airlines flight from Las Vegas to
Chicago, invoked the “stop buzzer,” a request routed through air traffic control for the military to immediately cease jamming. According to the Aircraft Owners and Pilots Association, pilots must use this phrase only when a safety-of-flight issue is encountered.
To be sure, many other instances in the FAA data were benign. In early March 2017, for example, Jim Yoder was flying a Cessna jet owned by entrepreneur and space tourist Dennis Tito between Las Vegas and Palm Springs, Calif., when both onboard GPS devices were jammed. “This is the only time I’ve ever had GPS go out, and it was interesting because I hadn’t thought about it really much,” Yoder told Spectrum. “I asked air traffic control what was going on and they were like, ‘I don’t really know.’ But we didn’t lose our ability to navigate, and I don’t think we ever got off course.”
Indeed, one of the RTCA task group’s conclusions was that the Notice to Airmen system was part of the problem: Most pilots who fly through affected areas experience no ill effects, causing some to simply ignore such warnings in the future.
“We call the NOTAMs ‘Chicken Little,’ ” says Rune Duke, who was cochair of the RTCA’s task group. “They say the sky is falling over large areas…and it’s not realistic. There are mountains and all kinds of things that would prevent GPS interference from making it 500 nautical miles [926 km] from where it is initiated.”
GPS interference can be affected by the terrain, aircraft altitude and attitude, direction of flight, angle to and distance from the center of the interference, equipment aboard the plane, and many other factors, concluded the task group, which included representatives of the FAA, airlines, pilots, aircraft manufacturers, and the U.S. military. One aircraft could lose all GPS reception, even as another one nearby is completely unaffected. One military test might pass unnoticed while another causes chaos in the skies.
This unreliability has consequences. In 2014, a passenger plane approaching El Paso had to abort its landing after losing GPS reception. “This is the first time in my flying career that I have experienced or even heard of GPS signal jamming,” wrote the pilot in an ASRS report. “Although it was in the NOTAMs, it still caught us by surprise as we really did not expect to lose all GPS signals at any point. It was a good thing the weather was good or this could have become a real issue.”
Sometimes air traffic controllers are as much in the dark as pilots. “They are the last line of defense,” Duke told Spectrum. “And in many cases, air traffic control was not even aware of the GPS interference taking place.”
The RTCA report made many recommendations. The Department of Defense could improve coordination with the FAA, and it could refrain from testing GPS during periods of high air traffic. The FAA could overhaul its data collection and analysis, match anecdotal reports with digital data, and improve documentation of adverse events. The NOTAM system could be made easier to interpret, with warnings that more accurately match the experiences of pilots and controllers.
Remarkably, until the report came out, the FAA had been instructing pilots to report GPS anomalies only when they needed assistance from air traffic control. “The data has been somewhat of a challenge because we’ve somewhat discouraged reporting,” says Duke. “This has led the FAA to believe it’s not been such a problem.”
NOTAMs now encourage pilots to report all GPS interference, but many of the RTCA’s other recommendations are languishing within the Office of Accident Investigation and Prevention at the FAA.
New developments are making the problem worse. The NextGen project is accelerating the move of commercial aviation to satellite-enabled navigation. Emerging autonomous air systems, such as drones and air taxis, will put even more weight on GPS’s shaky shoulders.
When any new aircraft is adopted, it risks posing new challenges to the system. The Embraer EMB-505 Phenom 300, for instance, entered service in 2009 and has since become the world’s best-selling light jet. In 2016, the FAA warned that if the Phenom 300 encountered an unreliable or unavailable GPS signal, it could enter a Dutch roll (named for a Dutch skating technique), a dangerous combination of wagging and rocking that could cause pilots to lose control. The FAA instructed Phenom 300 owners to avoid all areas of GPS interference. Embraer said that it fixed the issue in 2017.
As GPS assumes an ever more prominent role, the military is naturally taking a stronger interest in it. “Year over year, the military’s need for GPS interference-event testing has increased,” says Duke. “There was an increase again in 2019, partly because of counter-UAS [drone] activity. And they’re now doing GPS interference where they previously had not, like Michigan, Wisconsin, and the Dakotas, because it adds to the realism of any type of military training.”
So there are ever more GPS-jamming tests, more aircraft navigating by satellite, and more pilots utterly reliant on GPS. It is a feedback loop, and it constantly raises the chances that one of these near misses and stop buzzers will end in catastrophe.
When asked to comment, the FAA said it has established a resilient navigation and surveillance infrastructure to enable aircraft to continue safe operations during a GPS outage, including radio beacons and radars. It also noted that it and other agencies are working to create a long-term GPS backup solution that will provide position, navigation, and timing—again, to minimize the effects of a loss of GPS.
However, in a report to Congress in April 2020, the agency coordinating this effort, the U.S. Department of Homeland Security, wrote: “DHS recommends that responsibility for mitigating temporary GPS outages be the responsibility of the individual user and not the responsibility of the Federal Government.” In short, the problem of GPS interference is not going away.
In September 2019, the pilot of a small business jet reported experienced jamming on a flight into New Mexico. He could hear that aircraft all around him were also affected, with some being forced to descend for safety. “Since the FAA is deprecating [ground-based radio aids], we are becoming dependent upon an unreliable navigation system,” wrote the pilot upon landing. “This extremely frequent [interference with] critical GPS navigation is a significant threat to aviation safety. This jamming has to end.”
The same pilot was jammed again on his way home.
This article appears in the February 2021 print issue as “Lost in Airspace.”
This article was updated on 26 January 2021.
Mark Harris is an investigative science and technology reporter based in Seattle, with a particular interest in robotics, transportation, green technologies, and medical devices.
Sanctions on the once-mighty Chinese telecom giant have plunged it into survival mode
Craig S. Smith is a former New York Times correspondent and host of the podcast Eye on AI.
Huawei’s P50 series was released without 5G capability.
US sanctions targeting China’s telecommunications giant Huawei Technologies have crippled the company, effectively forcing it out of the global smartphone market and now threatening its domestic phone business as well. They have also shrunk Huawei’s market for fifth-generation wireless network infrastructure around the world.
Huawei chairman Eric Xu said last week that the company’s smartphone revenue will drop by $30 to $40 billion in 2021 from the $136.7 billion reported last year, adding that there are no prospects for recovering that money in the next few years. Xu had said earlier that the company’s goal now is to simply survive.
Xu’s latest comments came as the US dropped its extradition battle over Meng Wanzhou, Huawei’s chief financial officer and daughter of the company’s founder, who had been trapped in Canada for three years. That led to the release two Canadian citizens who had been held hostage in China as a result. But it may also signal a de-escalation of the pressure Washington has brought to bear on the Chinese company now that Huawei is on its heels.
What’s at stake is control of international 5G networks, which are expected to transform global communications.
Three years ago, Huawei was on the cusp of dominating the world’s 5G infrastructure with equipment priced far below competitors. That alarmed the US, which equated Huawei’s dominance with Chinese-control of global telecommunications—under a 2017 Chinese law all domestic companies are compelled to help the Chinese intelligence services on demand. Huawei has said it would not comply with such a request and believes that it cannot be legally forced to do so.
Washington embarked on an intense campaign to block Huawei, and Ms. Meng’s arrest on fraud charges was widely seen as part of that campaign.
Huawei has long been regarded as a rogue player in the international telecoms market with deep ties to the Chinese Communist Party. It was founded in 1987 by a former People’s Liberation Army officer and Party member and got its start reverse engineering telephone switching equipment from Hong Kong. By the mid-1990s, China was promoting it as a ‘national champion’ in the country’s effort to build up industrial giants that could compete on the world stage.
In subsequent decades, the company was accused of stealing Western intellectual property, supplying sensitive telecoms equipment to North Korea and Iran, and expanding its global market share by undercutting Western telecom equipment prices by as much as a third. Huawei benefits from various Chinese government policies that act as subsidies to its operations.
“Huawei is slow in fixing their vulnerabilities. It’s very difficult to tell the difference between sloppy programming and deliberate backdoors.”
—Roger Entner, founder of Recon Analytics
Most explosively, Huawei is accused of installing backdoors in its software that could allow China to monitor data flowing through its international networks or even shut networks down in the event of a war. Huawei denies doing so, but as a result, the U.S. has pressured allies to exclude Huawei equipment from their 5G networks.
“Huawei is slow in fixing their vulnerabilities,” said Roger Entner, founder of Recon Analytics, a telecommunications research company. “It’s very difficult to tell the difference between sloppy programming and deliberate backdoors.”
The global adoption of fast, high capacity 5G is expected to usher in a new era of smart devices, extending AI deep into the Internet of things, including autonomous vehicles. AI models in the cloud that are too large to reside on an edge device, such as a phone or security camera, will be able to receive and process data from those devices through the network in near real time. 5G devices, meanwhile, can connect directly to each other through radio networks forming an “edge cloud” of their own.
“Data is going to travel along that edge at the speed of light,” said a former US official involved with the issue. With 5G, he said, “the edge is not distinct from the core” in the way that it has been for previous generations of wireless technology.
That’s a problem for all governments connected to the network.
“If you can’t trust the company that’s providing you that infrastructure, even if you tried to push it out to the edge, you’re still attaching it to the network,” said Gilman Louie, a commissioner on the National Security Commission on AI and a former chief executive of the intelligence community’s venture arm, In-Q-Tel, speaking on a podcast.
The National Security Agency, the CIA and other US intelligence agencies have determined that if Huawei equipment is embedded even at the edge of 5G networks, China could write in code, siphon data and cover their tracks minutes later by writing out that code. There would be no way to track it. As a result, the US banned American companies from using Huawei equipment, but it had trouble convincing allies to do the same.
Part of the problem was a lack of alternatives. Few companies could supply 5G network equipment for the middle band of the radio spectrum, between 3 gigahertz and 4 gigahertz—or between three billion and four billion electromagnetic waves per second. That frequency has greater reach and better penetration through solid objects than higher frequencies.
“If you can’t trust the company that’s providing you [with] infrastructure, even if you tried to push it out to the edge, you’re still attaching it to the network.”
—Gilman Louie, commissioner on the National Security Commission on AI
In most of the world, the frequencies between 3 GHz and 4 GHz, called mid-band, were not being used extensively, while in the US they were used by satellite communications providers and the military. This allowed Chinese manufacturers to build 5G equipment for their vast domestic use and export it internationally.
Huawei—like Ericsson, Nokia, and the rest of the big equipment vendors—highly customizes its networks for each operator. So once a country or company starts with Huawei, it’s very difficult to change. To switch vendors, they have to rip out the entire network and start again, which is a very expensive proposition. This fact has allowed Huawei to expand its already formidable 4G market share into 5G, as it had many carriers already locked into the Huawei equipment universe.
In addition, Huawei benefitted from the economies of scale that came with a massive home market. As a result, many companies around the world began building 5G networks using Huawei equipment.
But holes found in Huawei’s hardware and software could allow the company—or the Chinese government—access to the data of users on the network. A Huawei spokesperson, Glenn Schloss, says that its employees would only have access under strict supervision by the network operators, and that Huawei employee keystrokes are recorded for verification.
In 2018, a U.K. center set up to evaluate the Huawei’s 5G technology reported that it could give “only limited assurance” that Huawei’s infrastructure equipment didn’t pose a threat to national security. The next year, the center found “critical, user-facing vulnerabilities” that it asked Huawei to fix. Last year, it reported that it had found a vulnerability “of national significance” and said Huawei had failed to instill confidence that such vulnerabilities would be addressed.
Also last year, a decade-old confidential report from Dutch telecommunications company KPN, leaked to the press, claimed that China could eavesdrop on the conversations of anyone using its Huawei-built network. Both KPN and Huawei deny that any data has been stolen, but the disclosure rattled the country. Huawei has since been blocked from 5G networks in the Netherlands.
Meanwhile, the US had put Huawei on a blacklist, forbidding US companies and citizens from doing business with the company, and in 2020 tightened those sanctions, barring vendors worldwide from using US technology to produce components for Huawei.
The U.K. had been resisting US pressure to remove Huawei from its networks, but the tightened sanctions were the final straw. The country banned new 5G equipment purchases from the company beginning this month and ordered that existing equipment be removed by 2027.
With the UK’s ban, four of the countries in the Five Eyes intelligence-sharing network—Australia, New Zealand, the UK, and the US—have now formally blocked Huawei from 5G networks. The fifth member, Canada, has effectively done so, forcing its telecom companies there to seek other vendors. Other countries are following suit, though Huawei still has a stronghold in Africa and Southeast Asia.
The troubles quickly spread to Huawei’s smartphone business. With US companies barred from doing business with Huawei, Google could no longer license its Android operating system—the operating system that runs most of the world’s phones—to Huawei. So the U.S. search giant stopped Huawei from offering Google-run services, such as Gmail, YouTube and carrying Google’s Android app store, Google Play.
As a result, Huawei, which was briefly the largest smartphone supplier in the world, has dropped out of the top five.
Worse for Huawei, the tightened sanctions cut Huawei off from TSMC, the Taiwanese semiconductor foundry, which manufactured Huawei’s 5G chips. In the run-up to the April 1, 2020 ban—the US gave Huawei several months notice—the Chinese company furiously stockpiled 5G chips.
How many chips Huawei was able to collect has been a matter of debate, but a series of statements and actions by the company indicate that it is running out. It sold its low-end Honor phone business last year, and in July this year it released the P50 series without 5G capability—using instead 4G chips that the US has allowed Qualcomm to sell to the company. This month, Huawei released two more smartphones without 5G, the Nova 9 and Nova 9 Pro.
Meanwhile, the outlook for Huawei’s fabless semiconductor company, HiSilicon, which designs and packages Huawei’s 5G chips is increasingly dim. Without a manufacturing partner it cannot produce its flagship 5nm, 5G-enabled Kirin 9000.
Huawei has delayed the release of its high-end Mate 50 smartphones. They are supposed to be powered by the Kirin 9000.
Track your activity with inexpensive hardware and our software
Inexpensive components will let you monitor your aerobic workout much better than a smartphone would.
Physical activity is essential to both physical and mental health, something brought home to many people following sedentary pandemic lockdowns. Even without the lockdowns, many parts of the world have been facing an obesity epidemic, which has created a need to help people manage their weight. For such people there are a wealth of fitness and diet apps that rely on smartphone and smartwatch sensors to monitor activity levels and track the calories they have burned. The problem is that smartphones and smartwatches do a terrible job at calorie counting.
A 2017 study looked at seven such typical devices, and found their counts were off between 27 and 93 percent, depending on the device. We decided to see if a better calorie counter could be made, at least for some forms of activity. The answer is yes—and it’s one you can build yourself with parts any maker can easily obtain.
The road to the calorie counter began in our lab (the Human Performance Laboratory in the Stanford University School of Engineering), where we study things like the metabolic cost of walking. We broke out every possible sensor we had in the lab and attached them to participants. This included sensors that monitor muscle activity, inertial measurement units (IMUs) to monitor movement on different parts of the body, and instrumented insoles in shoes to monitor forces produced by walking and running. We used respirometry, a lab-based method of measuring energy expenditure by monitoring the oxygen intake and carbon dioxide expelled with each breath, so as to get a ground-truth measure of the calories burned as participants moved. With all of this data we looked at building a fundamental relationship between the movement of the body (and thus the activity of the muscles burning calories) and the actual energy expended by the whole body.
We found that by looking at the motion of the thigh and the shank (lower leg) we could estimate caloric expenditure during aerobic activities with an accuracy of about 13 percent. (For the full details of our analysis, see our recent paper in Nature Communications.) What’s more, we could do it using inexpensive IMUs.
Two small inertial measurement units [top right] are strapped to the user’s thigh and shank. Using a I2C expansion board [top middle], the IMUs feed data into a Raspberry Pi [bottom middle] powered by a USB battery pack [left].James Provost
We used Adafruit Precision NXP 9-DOF breakout board IMUs. This board combines two sensor chips, a six-degrees-of-freedom accelerometer/magnetometer, and a three-degrees-of-freedom gyroscope. The counter uses two of them, one attached at midthigh, the other at midshank. In our tests, we sometimes used toupee adhesive to hold various IMUs in place, but a Velcro strap works great, too!
The IMUs are connected to a Raspberry Pi using the I2C protocol. Although the Pi is bigger and has a higher power draw than, say, a Teensy board, we chose it because it was easy to stream data wirelessly from the Pi and monitor it during testing and calibration.
About 20 to 40 percent of the steps we take each day occur in bouts of walking that are 20 seconds or less.
The Pi, running a stock version of the standard operating system, also allowed us to use established Python libraries to do onboard data processing. We used the NumPy scientific computing library for storing data in a convenient format, and the Scikit-learn machine-learning library for analyzing motion data from the IMUs.
We trained a linear regression model to perform gait detection to segment each step and make an estimate of the calories burned. (Our Python scripts are available for download from a public repository.) We pull in sensor data at a fixed rate of 100 hertz. We determine when a step occurs by detecting when the leg stops rotating in one direction and starts rotating in the other direction, occurring around the time the heel touches the ground. Then we pass the leg motion from that step into our model that estimates energy expenditure.
This kind of direct analysis provides a much better way of tracking instantaneous energy expenditure over other measures such as heart rate or respiratory rate. These latter indicators take up to a minute to reflect changes in activity, so if you got up from the couch and walked 10 steps they wouldn’t detect that movement. About 20 to 40 percent of the steps we take each day occur in bouts of walking that are 20 seconds or less, which contributes significantly to the total energy expended every day.
The red line shows how quickly the wearable system responds to the user starting and stopping different aerobic activities (from left to right: walking, running, stair climbing, bicycling). The stop is indicated by the dotted line in the middle of each activity.James Provost
We are currently working to create a more compact version of our calorie counter. We’re interested in tracking energy expenditure over longer periods of time to hopefully improve weight management and sports training. What happens if you wear this device for a week or two at a time? We are also thinking about how we can integrate tracking upper-body activity by, say, looking at the motion of a paired smartwatch.
We plan to validate the data using the gold standard technique of so-called doubly labeled water, which lets us track energy expenditures on this kind of timescale very accurately. A subject drinks water containing deuterium and oxygen-18. Tracking how these are eliminated from the body in urine gives us the rate of carbon dioxide production, which is directly related to energy expenditure. In the even longer term, we hope to replace the current IMU boards with technologies being developed for flexible electronics that can be pressed on like a sticker or even possibly for direct printing on the skin.
Because suppliers test adhesives so differently, temperature resistance values on data sheets are notoriously inconsistent–Master Bond’s latest white paper takes a closer look at some of these crucial issues
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