NASA’s Small Spacecraft Technology Program is on the countdown clock to advance communications and proximity maneuvering capabilities for CubeSats with the Integrated Solar Array and Reflectarray Antenna (ISARA) mission.
The spacecraft is a 3U CubeSat carrying a Ka-band payload that includes a low-power transmitter, High Gain Antenna (HGA), standard gain reference antenna, and RF antenna select switch. A Ka-band ground station will verify high data rate by signal-to-noise (SNR) measurement, and measure the antenna performance. The HGA gain will be measured by switching between the HGA and an onboard standard gain antenna (SGA), while the spacecraft will be slewed on orbit to measure the antenna patterns. The on-orbit data will be compared to measurements that were taken prior to launch.
The technology benefit of the ISARA mission is to enhance CubeSats with a blend of antenna and solar cells, to allow for higher data-downlink communications. ISARA will use radio frequency Ka-band – the first time Ka-band uses a reflectarray antenna – that will surpass the existing baseline CubeSat transmission rate of 9.6 kilobits per second to more than 100 megabits per second.
“We have a lot of mission firsts with ISARA,” said Richard Hodges, principal investigator of the CubeSat mission at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, CA. As a devoted “antenna guy” with decades of experience, he sees a bright future for the integrated solar array and reflectarray antenna that was perfected by JPL technologists.
“This is a flat antenna style, effectively replacing an antenna such as the curved surface parabolic style. Thanks to a photolithographic etching process, the reflectarray is relatively inexpensive to produce and they are lightweight. Furthermore, this type of antenna makes very efficient use of CubeSat volume. And that means lots of added room for payloads, such as science instruments or imaging systems,” Hodges observed.
To the best of his knowledge, ISARA will be the first in-space demonstration of a reflectarray antenna as well as that of an integrated antenna and solar array. “As far as we know, no reflectarray has ever flown in space. It has been discussed over the years, but now we’re going to demonstrate it does work in the space environment,” said Hodges.
A reflectarray is a relatively new type of antenna fabricated from standard printed circuit boards with an array of square copper patches etched on them. The reflectarray antenna consists of three panels, electrically tied together through hinges, which have the circuit board patches on them. The size of the patches is adjusted so that the phase of the reflected feed illumination collimates the radiation in much the same way a parabolic dish reflector would. Unlike a parabolic dish, however, the reflectarray panels are flat, which enables them to be folded down against the CubeSat. On the opposite side of the printed reflectarray antenna, solar cells have been added. This makes the overall antenna/solar array panel assembly slightly thicker, but the cells are stowed in the “dead space” between the launch rails that would have otherwise been left empty. This combination of antenna and solar cells makes for a very efficient use of CubeSat volume, creating more room for payloads.
Once the three antenna panels are deployed, they narrowly focus the CubeSat’s radio transmission beam to a “sweet spot” in much the same way a parabolic dish reflector would, Hodges explained. “ISARA’s solar array and reflectarray antenna is a very attractive package that enables high-speed data rates of more than 100 megabits per second. That’s our primary goal for this mission.”
Signals from the reflectarray antenna are to be transmitted to a ground station located at NASA’s JPL. Experts there will reconstruct the antenna signal pattern, contrasting that pattern against pre-launch ground tests to appraise overall quality of ISARA’s downlink transmission over months of mission duration.
After deployment, ISARA will deploy its solar array/reflectarray antenna and use the Attitude Determination and Control System (ADACS) to stabilize. The UHF system will be used to establish initial communications with the satellite and perform on-orbit checkout procedures. Once spacecraft health has been established, on-orbit testing of the Ka-band system can begin.
The Ka-band experiments will include the determination of three elements: data rate capability, antenna gain, and antenna pattern. In order to verify the data rate capability, the received signal will be measured and compared against the estimated receiver noise. Antenna gain will be measured by transmitting a signal and switching between the HGA and the standard gain antenna. Characterizing the antenna pattern involves a multi-pass operations procedure. Throughout the duration of a pass, the spacecraft will be held to a commanded attitude. As the ground observation angle changes during the pass, a cut of the antenna pattern is obtained. For subsequent passes, the spacecraft is commanded to new pointing angles, resulting in a sweep of cuts which allows for pattern reconstruction. The figure shows a notional depiction of how attitude would vary throughout a number of passes, with the line-of-sight vector to the ground station in the downward direction.
The ISARA technology will be validated in space during a five-month mission to measure key reflectarray antenna characteristics that include how much power can actually be obtained over its field of view. ISARA contains a transmitter and an avionics subsystem that features a Global Positioning System (GPS) receiver and a high-precision attitude control system designed to orient the CubeSat to enable accurate antenna beam pointing.
At the end of the validation mission, the reflectarray antenna technology will be available for use on other missions that need high-bandwidth telecommunications. The ISARA technology will enable CubeSats and other small satellites to serve as viable platforms for performing missions that were previously only possible on larger and more costly satellites. For a modest increase in mass, volume, and cost, the high data rate this technology enables will pave the way for high-value science missions and formation flying missions that use distributed CubeSats and small satellites.
Visit the ISARA mission page here .