Installation of a New Antenna Array on a P-3 Aircraft

News

By Nick Mott
Spring 2011

Anyone that owned a TV before cable knows that fiddling with an antenna is no easy task. Supersize that antenna, multiply it by 15 and mount it on an airplane, and even an expert at getting television reception would be lost. When working with high-tech antennas, though, CReSIS engineers feel right at home.

In preparation for the 2011 Greenland field season, the CReSIS team made major structural and electrical changes to the antenna array on the P-3 aircraft.

The main changes took place on the MCoRDS radar system. The MCoRDS, or Multi-Channel Coherent Depth Sounder, penetrates kilometers into polar ice to chart the topography of the bed below. Rick Hale, Associate Professor of Engineering, describes the radar as an energy incident technique. Radar operators send out a chirped signal and record what comes back. The radar signal penetrates the ice surface, the intermediate layers, and the bed of the ice. “It’s wizardry in signal processing,” Hale said.

Nick Roberts

Photo 1: Graduate student Nick Roberts and lab technician John Hunter work on the installation of the antenna array. Photo courtesy of Rick Hale.

Preliminary changes focused on the structure and materials of the antennas and mounting systems. After observing the successes and downfalls of a radar system, Hale said, “you sharpen your pencils, and you do a little better job.” The new design, for example, limited metallic features that fasten the antennas in place. Metallic features interfere with the radar and skew the center frequency. The new alterations, which were made in a joint effort between the electrical and aerospace engineering departments, included changing bolts, fasteners, and other critical structural elements.

“I think we bought back about 10 db of performance just in changing the structural parameters,” Hale said.

The biggest changes, though, had to do with the antennas themselves. The CReSIS team, headed by Hale and Carl Leuschen, Assistant Professor of Electrical Engineering, designed and installed an array of fifteen antennas, almost double the size of last year’s array. When the P-3 reached the field, Leuschen said NASA members were impressed by the size of the array. “They said as far as they know this is the largest antenna array they’d seen on a P-3 in civilian applications.”

Leuschen said that the hard points to which the antennas were mounted on the outside of the airplane were a crucial factor in the decision to install new antennas. The existing hard points limited expansion of an antenna array. “At that point we saw an opportunity to do something that would allow us to put a much larger antenna array on the P-3, so we went ahead and proposed the idea of installing three different subarrays, which would allow us to put 15 elements on the P-3, whereas we only had 8 before,” Leuschen said.

After the installation, the aircraft had weeks of additional testing to go through. Once installed, the antenna subarrays needed force tests, vibration tests, and aerodynamic simulations. When the simulations gave satisfactory results, the plane ran its propellers to test the effects of airflow on the structures. After several in-air tests, the plane was flown over the Atlantic ocean, just off the coast of Virginia, to test the initial performance of the radar systems with the new antennas.

P-3 Aircraft

Photo 2: The P-3 aircraft tests the finished product. Photo courtesy of Rick Hale.

Increasing the number of antennas on an airborne radar is like increasing the megapixels on a camera. The more antennas, the higher resolution the final product. By increasing the number of antennas in the array, the resolution of the radar was also increased. The new array is co-linear, which means that rather than having the antennas oriented in the direction of the flight, the new arrangement is perpendicular, which helps to reduce clutter problems caused by rough ice surfaces.

Overall, Leuschen said that the new antenna arrays will help produce data with more detail in the layering, better resolution, and better accuracy. CReSIS projects focus on some of the most difficult terrain to sound. The P-3 will fly over temperate glaciers, glaciers with surface melt, and glaciers with liquid water at the bed. In such terrain, as in mountainous areas and areas with crevassing, the radar must be able to reduce clutter, or unwanted signals. The new array on the P-3 is fine-tuned to do exactly that.

“These antennas will be able to focus the beam where we’re looking straight down, and the amount of energy that’s actually going off to the side is much less, so we won’t see much clutter, which masks the bedrock, so we’ll be able to filter that out better,” Leuschen said.

Emily Arnold, a graduate student in Aerospace Engineering, spent weeks helping with the installation on the P-3. “They recently sent us a photo that they generated from some of the data they gathered from the P-3 and it’s probably the best image I’ve ever seen from data we’ve gathered,” she said.

The new array on the P-3 has already been put to hard, subzero work in Greenland, contributing to NASA’s Operation IceBridge program. The IceBridge program is a six-year endeavor, the largest survey of polar ice ever flown. The P-3’s improved radar will help produce quality data about the changing behavior and features of the Greenland and Antarctic Ice Sheets.

For Rick Hale, the technological advances complement another innovation this field season: the speed of data transfer. From his desk in Lawrence, KS, Hale was able to receive data collected only 24-hours before from thousands of miles away.

“We are now getting very quick turnaround on pretty high fidelity data,” Hale said. “That’s the most interesting story this year. Usually we collected significant amounts of raw data, got some field processing to some level of fidelity, but it was well after we got home and processed the data that we would have learned what we might have needed to perhaps improve the mission.”

This summer, Arnold said, the plan is to work with CReSIS REU students and to conduct simulations and experiments to help characterize any issues with the new antenna array.

After this field season, the pencil-sharpening will continue. CReSIS engineers will continue to fine-tune the MCoRDS and other radar systems to produce even more detailed data that will advance our understanding of the polar ice sheets.