Ice Breaker

By Stephen Yan, Assistant Research Professor, University of Kansas, Lawrence, U.S.A.

Systems simulation helps design antennas for the first unmanned aircraft system used to measure polar ice sheets.

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Studying the proximity effect of the avionics system to antenna performance in the field (rearrangement of avionic boxes was needed because of significant RFI from onboard rectifier and alternator)

Vast polar ice sheets hold so much of Earth’s fresh water that if they were to suddenly melt, the global sea level would rise about 216 feet (66 meters). Even a relatively small melt would cause problems in low-lying areas, so scientists are looking for better ways to measure and eventually predict ice sheet behavior. The state-of-the-art method for measuring ice sheets uses ice-penetrating radar mounted on airplanes. However, this approach involves risk to the pilots and onboard radar operators during long flights in desolate and frigid corners of the globe and can be limited by the high cost of transporting fuel to polar regions.

UAS landing after a surveying mission
UAS landing after a surveying mission
UAS in fl ight during surveying mission
UAS in flight during surveying mission
UAS in flight during surveying mission
Stephen Yan in front of Mt. Erebus
Stephen Yan in front of Mt. Erebus
14 MHz and 35 MHz antennas on airframe before avionics integration
14 MHz and 35 MHz antennas on airframe before avionics integration

Researchers at the Center for Remote Sensing of Ice Sheets (CReSIS) at the University of Kansas have made a significant advancement by successfully testing a compact radar system integrated on a small lightweight unmanned aircraft system (UAS) to measure ice thickness and map underlying glaciers. The use of a UAS eliminates pilot risk and consumes only a fraction of the fuel necessary for a manned aircraft. ANSYS HFSS electromagnetic simulation software played a key role by helping to integrate the antennas onto the aircraft in much less time and at a much lower cost than would have been required with physical tests alone.

Importance of measuring ice sheets

In the past, the lack of fine-resolution information about the topography that underlies fast-flowing glaciers — which contain huge amounts of fresh water and govern the flow of the interior ice — made it difficult to model glacier behavior accurately. There is an urgent need to accurately measure the ice thickness of these glaciers to determine the bed topography and basal conditions. This information will then be used to improve ice-sheet models and generate more precise estimates of sea level rise in a warming climate. Without the ability to represent these fast-flowing glaciers correctly, advancements in ice-sheet modeling will remain elusive.

The radar-equipped UAS is an ideal tool to reach some areas that would otherwise be exceptionally difficult to map. Because the aircraft and sensors are lightweight and small, they can be readily transported to remote field locations. The airborne maneuverability of a UAS enables the tight flight patterns required for fine-scale imaging. The UAS can collect data over flight tracks about five meters apart to allow for thorough coverage of a given area. The aerial platform selected by CReSIS is a scaled version of the Yak-54 aircraft with a 5.3-meter custom-designed wing. The Yak-54 is based on a 1990s-era aerobatic aircraft produced by Moscow-based Yakovlev Aircraft Corporation. The scaled version has a takeoff weight of only 85 pounds (38.5 kilograms) and a range of approximately 62 miles (100 kilometers).

Challenge of integrating antennas

Radar operating frequencies of 14 MHz and 35 MHz make it possible to see through areas in the ice with significant volume clutter and temperate ice. The UAS carries two separate dipole-based antennas integrated into its wings. It was extremely challenging to integrate the antennas on such a small platform because of the long antenna wavelength and the near-field coupling effects from onboard metallic mechanical structures, electronics boxes and wiring. Simulation was critical in the design process. The antenna’s relatively long wavelength is influenced by ground effects, so testing the antenna integrated into the aircraft could not be accurately performed on the ground. Airborne physical testing of the antenna, on the other hand, is very expensive and time-consuming, and it is also difficult to get a flight permit and locate a test field for a non-consumer– grade UAS. Therefore, it was critical to get the design right before proceeding to flight testing.

Radar echogram collected by onboard radar showing an ice bed thickness of about 800 meters
Radar echogram collected by onboard radar showing an ice bed thickness of about 800 meters

There is an urgent need to accurately measure the ice thickness of Antarctic glaciers to generate more precise estimates of sea level rise in a warming climate.

Antenna confi gurations before (top) and after (bottom) system integration

Antenna configurations before (top) and after (bottom) system integration

Engineers modeled a single element antenna in ANSYS HFSS and evaluated the tradeoff between antenna size and electromagnetic performance. They iterated to a design that met the size and performance requirements. The team used HFSS to predict the radiation performance and mutual coupling between the 14 MHz and 35 MHz radar antennas to make sure they satisfied project objectives. Next researchers modeled the antennas integrated into the wings of the aircraft. The antenna was designed at the same time as the placement of radar and avionics systems so researchers had the freedom to modify both as a system to optimize antenna performance. Researchers added cables inside the aircraft to the model, evaluated their impact on antenna performance and provided guidelines to the team that was modifying the UAS. For example, researchers were able to specify the length of the servo cables to avoid resonance at the radar frequencies.

Simulation captured integration effects

The HFSS simulation accurately captured platform integration effects. After full system integration, engineers observed a relatively significant frequency shift caused by cabling for the nearby avionics, especially at 35 MHz. The frequency shift caused the antenna’s matching network to fail to operate properly and resulted in significant ringing. Ringing is a common type of noise in ground-penetrating radar signals that appears as periodic horizontal events. Researchers changed the design to address this problem and re-ran the simulation to ensure that the problem had been corrected.

Flight testing confirmed the effectiveness of the design that was developed with simulation. The reflection coefficient (S11) simulation result obtained by HFSS after system integration closely agreed with in-flight radar data. The preflight electromagnetic simulation helped fine-tune both the antennas and matching networks in the field. This would not have been possible with conventional measurement on the ground due to ground loading and scattering effects.

The use of a UAS eliminates pilot and radar operator risk and consumes only a tiny fraction of the fuel necessary for a manned aircraft.

35 MHz antenna implementation on the wing
35 MHz antenna implementation on the wing

With support from the National Science Foundation's Division of Polar Programs and the State of Kansas, the CReSIS team recently successfully tested the UAS at a field camp in West Antarctica. The measurements were the first-ever successful sounding of glacial ice with UAS-based radar. If further tests in the Arctic prove as successful, the UAS could eventually be routinely deployed to measure rapidly changing areas of the Greenland and Antarctic ice sheets. The new airborne tool will allow radar measurements that previously would have been prohibitively expensive or difficult to carry out with manned aircraft.

With the successful test completed in the Antarctic, researchers will begin analyzing the data collected in the field, miniaturizing the radar further and reducing its weight to 1.5 kilograms (3.3 pounds) or less, and increasing the UAS radar transmitting power. They are also performing additional test flights in Kansas to further evaluate the avionics and flight-control systems, and to optimize the radar and transmitting systems. In 2014 or 2015, they plan to deploy the UAS to Greenland to collect data over areas with extremely rough surfaces and fast-flowing glaciers, such as Jakobshavn, which is among the fastest flowing, most challenging glaciers to sound in the world.

National Science Foundation, Press Release 14-041 (April 1, 2014), Unmanned Aircraft Successfully Tested as Tool for Measuring Changes in Polar Ice Sheets

This work was completed at the University of Kansas with funding from the National Science Foundation (NSF) Center for Remote Sensing of Ice Sheet (CReSIS) grant ANT-0424589 and with a matching grant from the State of Kansas.

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