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Dedicated to designing secure navigation systems

April 16, 2015

Professor Enge and Professor Lee, in front of the prototyped UAV displayed at Stanford GPS Lab

Amy Lee

For the second interview of the Office of International Affairs' International Visiting Scholars Series, we met with Professor Per Enge at the Department of Aeronautics and Astronautics and Professor Jiyun Lee, a visiting scholar from Korea Advanced Institute of Science and Technology (KAIST), who is also a Stanford alumna. The two world-leading scholars in the field of navigation system design have been conducting pioneering collaboration at and beyond Stanford on two research projects. They are developing augmentations that toughen the Global Positioning System (GPS) against space weather, un-intentional faults in the human-made systems, and intentional attacks by bad actors that would like to interfere with GPS. Today, these systems are used to navigate piloted airplanes and helicopters. In the future, these augmentations will also be used to navigation unpiloted aerial vehicles (UAV).

Q. With the recent unveiling of Amazon’s Prime Air, a futuristic delivery system using UAVs, commercialization of un-piloted drones are drawing higher public attention and stirring up a controversy. What are the biggest challenges in operating UAV’s and their integration to the existing airspace? What is your focus in enhancing UAV’s performance?

PE: One of the most critical and challenging parts of designing a secure UAV is to have the best possible navigation system on board. For a normal piloted airplane, a pilot and air traffic controllers on the ground work together to ensure that the vehicle is securely performing. However, a UAV lacks at least one of these two parties and the person on the ground doesn’t necessarily have visibility from the vehicle, which makes the device more vulnerable to interruptions.

My focus is to enable UAV’s to work securely even if its GPS is not accurately operating. There are two so-called “bad actors” that prevent an UAV from operating properly or mislead it - jamming and spoofing. Jammers send very powerful signals that wipe out the GPS receivers. Spoofers attempt to deceive a GPS receiver by broadcasting counterfeit GPS signals to mislead the UAV. I am collaborating with Professor Lee and her team at KAIST to develop a secure navigation system that enables an UAV to maintain its performance even with potential interruptions.

Q. Please tell us more about how the newly developed navigation system can improve UAV performance.

PE: The new system is based on GPS and the other Global Navigation Satellite System (GNSS) worldwide. Previous navigation systems were only valid or strong in local areas by providing pathways in the sky that you could fly along, which was a single beam. However, GNSS provides Area Navigation (RNAV) capability. This means that no matter where you are, as long as your antenna is facing up to the sky, you know where you are. The GNSS-based navigation system would bring a huge change in performing both piloted and unpiloted flights by significantly improving a vehicle’s competence to fly, especially for terrain challenged areas such as Juneau, Alaska, shown in the figure below.

Figure 1. Simple drawing of the Ground Based Augmentation System (provided by Per Enge)

Q. Do we have any other solutions to protect UAV from the “bad actors” as we expect more interruptions in the future?

PE: GPS satellite signals have to travel from very far away, usually from 20,000-23,000 kilometers away. Thus, by the time they get to the navigating aircraft, the signal becomes very weak. However, a jammer, due to its proximity to an aircraft, can easily disrupt the vehicle with as little as one watt of electrical power, which is equivalent to what one’s cell-phone radiates. So far, jamming attacks have been rare, but jammers are difficult to locate, because they only need to radiation one Watt. Furthermore, since they will radiate the disrupting signals towards the sky, it’s not easy to identify them from the ground. Thus, having a detective vehicle up in the sky is critical.

In fact, Stanford GPS Lab has recently developed an UAV to better identify the bad actors. The device can hover over a jamming transmitter and take a photograph of it. This way, people on the ground can quickly locate the bad actors. The vehicle that we prototyped can fly up to 40 miles an hour for about 20 minutes on one battery.  Its antennas are both looking up and down so that it gets signals from GPS as well as detects jammers. We successfully tested the device at Camp Roberts in California since the U.S. regulation prohibits an unmanned aircraft from operating on campus.

"Different environments and regulations are what make international collaboration and sharing beneficial."

Q. Please tell us about how Professor Lee’s work contributes to your ongoing research in designing secure navigation system?

PE: Professor Lee is focused on making an aircraft perform well regardless of bad actors such as jammers and also severe space weather. For instance, sunspots can cause turbulent ionosphere, the layer of the Earth's atmosphere that is ionized by solar and cosmic radiation. This is called an “ionospheric storm” and disrupts GPS signals that go through the atmosphere. What Professor Lee focused on during her PhD at Stanford was developing algorithms to detect such abnormalities. With her leadership as one of the world leading scholars in analyzing atmospheric abnormalities, we are currently documenting ionospheric data and their potential threats to GPS performance.

JL: My research in South America requires an engineering approach and scientific observation. As an undergraduate, I studied atmospheric science and astronomy and then I focused on aeronautics during my PhD. When I moved onto KAIST, our team developed software that automatically processes ionospheric data, which was delivered to the Federal Aviation Administration and distributed to the international working groups focused on global navigation satellite systems. We are currently working with governmental institutions and universities in the United States, South Korea, and Brazil. In Brazil, we have several different network stations to collect the data that includes governmental organizations, research institutions, and local universities.

Q. What are some of the challenges working with scholars in three countries and how have you overcome the difficulties?

JL: Atmospheric science is a huge area. To fully understand the nature of different regions’ atmospheric conditions and to develop solutions based on them, we need to collaborate internationally to obtain reasonable data sets and analyze them. Of course there are some challenges.

Acquiring valid data is one challenge because the project requires several years of data. It depends on whether each station has an archive or other types of data repository, but we are trying our best to collect as much valid data as possible. Sharing the huge data among international teams is another challenge. Finally, we need to support and train the participants across the countries. Our analysts run into various issues depending on the platform each data station uses to process the software developed by KAIST. In this case, we resolve the technical issues by writing a new piece of code that works on their platform or upgrading the software, and training the participants.

Q. In general, how do you think your joint research has benefited both universities?

JL: Each nation has different environments and airline regulations that make international collaboration and sharing beneficial. For instance, UAV tests are not allowed at U.S. universities except for very limited authorized areas. In Korea, however, it is legally allowed to test an aircraft that flies below 150 meters from the ground although there are some restrictions in terms of size and weight of the device. Likewise, different environments and regulations are what make international collaboration and sharing beneficial. Collaborating with Professor Enge and his team has been a truly exciting journey. I believe our work will broaden the research scopes of both laboratories and advance our research in designing more secure and innovative navigation system.

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♦ If you would like to know more about Professor Enge and his research, please visit Stanford Profiles.

♦ To learn how to connect with Stanford faculty and invite your visiting scholar to Stanford, please visit: Finding Partners at and Beyond Stanford.

Cover Image: treacherous approach for aircraft going to Juneau Alaska (provided by Per Enge)