The ISARA mission will demonstrate a high bandwidth Ka-band CubeSat communications capability that is ready for immediate infusion into commercial, government and military systems. For a modest increase in mass, volume and cost, this technology will increase downlink data rates from a baseline of 9.6 kbps for existing UHF systems to over 100 Mbps - a 105 fold increase in data capacity. The key to this technical advance is a high gain antenna that will be integrated into a commercially available 3U CubeSat solar array with minimal modification of the existing solar panel design. ISARA will fly a nominal 5 month SFV mission that demonstrates the 100 Mbps data rate and elevates the antenna technology from TRL 5 to TRL 7.
This technology development, sponsored by the NASA Small Satellite Technology Program, facilitates fractionated spacecraft sensors and radar/radiometry science missions that need high bandwidth telecom, enabling CubeSats and other small satellites to serve as viable platforms for performing missions previously only possible on larger and more costly satellites.
ISARA is a JPL PM-led mission that will be carried out in close collaboration with Pumpkin, Inc. and Aerospace Corp.
A nominal five month Space Flight Demonstration will be used to confirm a 100 Mbps data rate and verify antenna performance to TRL 7. 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 on-board 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 ISARA CubeSat will nominally fly in Low Earth Orbit with an approximately 90 minute orbital period and 6-7 minute observation time per pass. At least two passes per day will be available to measure telecom data rate, gain and antenna patterns. A high accuracy MAI-400 Attitude Determination and Control System (ADACS) is used to achieve the required 0.2° pointing accuracy. The high data rate allows for "burst" downloads during brief periods in which the antenna is aimed at an Earth station. At other times the solar array can be positioned for optimal solar power generation.
A reflectarray focuses divergent rays from a feed into a beam similar to a parabolic reflector
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 patches collimate divergent rays from a small feed antenna into a focused beam, behaving much like a parabolic reflector. Since a solar panel is made of a compatible printed circuit board material, it is possible to print the reflectarray directly on the back of the solar panel. ISARA will use the Pumpkin Turkey Tail solar array configuration, comprised of a set of 7 solar panels, to make available an area 30cm x 70cm for an antenna aperture. A small patch array feed antenna placed on the side of the bus completes the high gain antenna, which will provide at least 35 dB of gain at 32 GHz. Note that this antenna design can also be implemented in other frequency bands, though only for one frequency band at a time. If multiple frequencies are required, they could be implemented on multiple CubeSats.
Predicted vs. measured SWOT IIP reflectarray antenna patterns
The ISARA reflectarray will be designed using RF design and analysis software that has proven to be very accurate on several previous designs. The figure shows a comparison of calculated and measured patterns obtained at 35.75 GHz on the SWOT IIP panel in the 26cm wide direction of the aperture, similar to the size of the proposed ISARA antenna. The ISARA antenna will be constructed on a 12-mil Rogers RO4003 dielectric substrate, material which was recently characterized and used to demonstrate a TRL 5 reflectarray for the SWOT IIP program. The ISARA reflectarray will require a feed with a 100° x 50° illumination pattern to obtain good taper and spillover efficiency. A simple 2 x 3 element (1 cm x 2 cm) patch array etched on Rogers 4003 will be used to meet this requirement. This thin, flat feed antenna will stow compactly and deploy using a spring loaded hinge with a coaxial service loop to "flip" release the feed into position when the solar panels deploy.
After launch and deployment from the P-POD, ISARA will deploy its solar array/reflectarray antenna and use the ADACS to de-tumble and 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.