NASA Tests Featherweight Radar Antenna for SkyFall Mars Helicopters

When the trio of NASA’s SkyFall helicopters takes to the Martian skies, one of their tasks will be to hunt for frozen water — a critical resource for future astronauts — using ground-penetrating radar. For that radar to work, each rotorcraft will carry a flexible, fabric-based antenna that will need to extend below the aircraft without interfering with landings, or breaking upon impact. It is a specialized design for the mission, and it recently cleared a round of testing at the agency’s Jet Propulsion Laboratory in Southern California.

Although orbiting spacecraft can map the thick Martian ice deposits tens of yards below the surface, they’re effectively blind to the top several yards of regolith, or broken rock and dust. This shallow zone is what will matter most for future astronauts, who will need to easily reach and process ice for water, oxygen, and fuel.

“The only way to detect shallow subsurface ice remotely is to fly close to the ground,” said Adrian Tang, SkyFall’s ground-penetrating radar lead instrument scientist at JPL. “By flying low and slow, a SkyFall helicopter could capture radar images that resolve the fine layering where dry soil gives way to ice, detecting its presence and mapping its extent.”

Composed for Mars

To peer beneath the Martian surface, SkyFall’s ground-penetrating radar will use an ultra-wide frequency range — from 500 to 2,500 megahertz — corresponding to wavelengths of about 24 to 5 inches (60 to 12 centimeters). The longest wavelengths can penetrate several yards below the surface. The shorter wavelengths provide finer detail about the uppermost layers and surface texture.

This is where the unique antenna design comes in. A traditional antenna operating in that part of the electromagnetic spectrum would need to be 19 inches (48.3 centimeters) long and have a clear view of the ground, but the minimum clearance between Mars’ surface and the base of the helicopter’s fuselage is only about 6 inches (15.2 centimeters). The team needed an antenna that wouldn’t interfere with or be damaged by landing.

After an extensive search for the right antenna, the SkyFall team zeroed in on the Vivaldi because it can transmit and receive signals across a large, continuous range of radio frequencies, and because its flat, lightweight profile can be easily cut into flexible, metalized fabrics. The curvilinear antenna was named by its inventor, Peter Gibson, who felt its sweeping lines resembled a violin, an instrument associated with composer Antonio Vivaldi.

While the standard Vivaldi footprint was efficient and robust, it was still too large to work with SkyFall’s ground clearance. Fortunately, SkyFall’s ground-penetrating radar can be further miniaturized because it is designed specifically for shallow surveying (under 16 feet, or 5 meters) in the dry Martian regolith, which blocks radio waves far less than Earth’s soil. The team developed techniques to reduce the antenna’s size even more without sacrificing sensitivity.

“Although we managed to shrink the antenna quite a bit, it is about 1½ times longer than the helicopter’s legs,” said Christine Gebara, SkyFall ground-penetrating radar mechanical lead at JPL. “That means during landing, the Vivaldi has to bend out of the way — and if it lands on a rock, it bends even further. But when the helicopter takes off again, the antenna must spring back into place for data collection. Because SkyFall is expected to make dozens of flights exploring Mars, we needed an antenna that could repeatedly handle those pressures without losing its shape in flight.”

To ensure their downsized Vivaldi could withstand flight operations, the team sheathed it in a polyester and then layers of Vectran — the same flexible, superstrong material used for the landing airbags containing NASA’s Spirit and Opportunity Mars rovers. To help the antenna maintain shape while airborne, engineers added flexible fiberglass tape springs and a lightweight magnesium mounting structure. The entire setup weighs about 5 ounces (150 grams), slightly more than two violin bows.

Testing Vivaldi

Testing space hardware concepts with paper designs and mathematical models can get a mission only so far. The team needed to prove their antenna prototype could survive a full mission’s worth of intense temperature swings and space radiation.

Working in JPL’s Environmental Test Laboratory, engineers bent the antenna to simulate one possible orientation after a SkyFall flight at Mars. Then they ran it through dramatic thermal shifts to simulate the Martian day-night cycle, during which temperatures can swing by as much as 170 degrees Fahrenheit (94 degrees Celsius). Next, they repeatedly flexed the antenna as if it had gone through dozens of landings. Along the way, they paused six times to carry the hardware to an electromagnetic test chamber to verify that its ability to beam and receive radar signals hadn’t diminished.

During radiofrequency testing, the team inverted the antenna so that the bottom pointed up — a setup that stresses the structure more than Mars’ one-third gravity ever would — and tested signal performance. By the end of the test campaign, the antenna had withstood 200 Mars landings, more than double what would be required for a successful prime mission, with no loss of performance.

“This test checked every box it was supposed to and answered our biggest technical questions,” said Tang. “While we still have work ahead of us before the antenna is fully flight-qualified, this was a major milestone, and the hardware performed exactly as expected.”

With the antenna’s first big test campaign behind it, the ground-penetrating radar team is building an engineering model that will endure vibration testing, deployment in a simulated Martian environment, signal testing, and outdoor trials at JPL’s Mars Yard.

Equipped with four instruments each, the three SkyFall aircraft follow in the footsteps of NASA’s Ingenuity Mars Helicopter, which flew 72 times over nearly three years, proving that powered, controlled flight is possible in the rarefied Martian atmosphere. It also demonstrated how an aerial perspective can generate valuable data by helping NASA’s Perseverance Mars rover team plan time-saving routes and choose locations for science-gathering.

SkyFall is expected to launch aboard NASA’s Space Reactor-1 Freedom in late 2028. For more information on SkyFall:

https://science.nasa.gov/mission/skyfall