ISARA (Integrated Solar Array and Reflectarray Antenna)
ISARA is a NASA/JPL (Jet Propulsion Laboratory) nanosatellite (3U CubeSat) demonstration mission with the goal to demonstrate a Ka-band reflectarray antenna that will increase the downlink data rates for small spacecraft from the typical existing rates from a baseline of 9.6 kbit/s to over 100 Mbit/s, a hundredfold increase in data capacity. 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 utilize distributed CubeSats and small satellites. 1) 2)
ISARA is a JPL PM-led mission that will be carried out in close collaboration with Pumpkin, Inc. and Aerospace Corp. In February 2013, ISARA was down-selected for a launch opportunity through the NASA ELaNa program.
Figure 1: Conceptual 3U CubeSat and Solar Array with Integrated Reflectarray Antenna (image credit: NASA/JPL)
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 (Instrument Incubator Program) panel in the 26 cm 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 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. 3)
The ISARA mission is funded through NASA's SSTP (Small Spacecraft Technology Program), a program within the new Space Technology Mission Directorate which was formed as a catalyst for the creation of technologies and innovation needed to maintain NASA leadership in space, while also benefiting America's economy. The SSTP was created specifically to develop and demonstrate new technologies and capabilities for small spacecraft.
A nominal five month Space Flight Demonstration will be used to confirm a 100 Mbit/s data rate and verify antenna performance to TRL 7 (Technology Readiness Level 7). The spacecraft is a 3U CubeSat carrying a Ka-band payload that includes a low power transmitter, HGA (High Gain Antenna), standard gain reference antenna and RF antenna select switch. A Ka-band ground station will verify high data rate by SNR (Signal-to-Noise) measurement and measure the antenna performance. The HGA gain will be measured by switching between the HGA and an on-board SGA (Standard Gain Antenna), 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.
Figure 2: Schematic view of the deployed ISARA CubeSat (image credit: NASA/JPL)
ADCS (Attitude Determination and Control Subsystem): A high accuracy MAI-400 ADCS is used to achieve the required 0.2° pointing accuracy.
Launch: The ISARA mission is scheduled as a secondary payload in Q2 2016 on a Falcon-9 v1.1 vehicle. The primary mission on the flight is FormoSat-5 of NSPO, Taiwan. The launch site is VAFB (Vandenberg Air Force Base), CA. 4)
Orbit of the primary payload: Sun-synchronous near-circular orbit of FormoSat-5, altitude = 720 km, inclination = 98.28º, period = 99.19 minutes, LTDN (Local Time on Descending Node) at ~ 10 hours.
Secondary payloads: 5)
The secondary payloads will by carried on a Spaceflight Services SHERPA tug. SHERPA can accommodate a variety of small satellites from CubeSats up to ESPA [EELV (Evolved Expendable Launch Vehicle) Secondary Payload Adapter] class and beyond.
• eXCITe (eXperiment for Cellular Integration Technologies) of DARPA, built by NovaWurks of Los Alamitos, CA; eXCITe is also known as PTB-1 (Payload Test Bed-1)
• BlackSky Pathfinder-1 and BlackSky Pathfinder-2, microsatellites (~50 kg each) of BlackSky Global, Seattle, WA USA.
• Arkyd-6, a 6U CubeSat technology demonstration mission of Planetary Resources, Redmond, WA, USA.
• EcAMSat (E. coli Anti Microbial Satellite), a 6U CubeSat of NASA/ARC.
• CNUSail-1 (Chungnam National University Sail), a 3U CubeSat (4 kg) solar sail experiment, developed at the CNU (Chungnam National University), Korea.
• ISARA (Integrated Solar Array and Reflectarray Antenna), a NASA/JPL 3U CubeSat (~ 5 kg) and solar array with an integrated deployable reflect antenna and a Ka-band downlink (100 Mbit/s).
• KAUSAT-5 (Korea Aviation University Satellite), a 3U CubeSat (4 kg), developed at the SSRL (Space System Research Laboratory) at the Korea Aviation University, Korea.
• SIGMA (Scientific cubesat with Instruments for Global Magnetic field and rAdiation), or KHUSAT 3 (Kyung Hee University Satellite), a 3U CubeSat developed at the KHU (Kyung Hee University), Korea.
• CANYVAL-X 1,2 (CubeSat Astronomy by NASA and Yonsei using Vision ALignment eXperiment), a mission consisting of a 1U and a 2U CubeSat developed at Yonsei University, Korea in collaboration with NASA.
• STEP Cube Lab, a 1U CubeSat developed at Chosun University, Gwangju, Korea.
• OCSD (Optical Communications and Sensor Demonstration) of The Aerospace Corporation, El Segundo, CA, USA. OCSD-B and OCSD-C are two 1.5U CubeSats.
• Fox-1C, a 1U CubeSat of AMSAT.
• Nayif-1, a 1U CubeSat developed by the Mohammed Bin Rashid Space Center of Dubai, formerly EISAT (Emirates Institution for Advanced Science and Technology) in partnership with AUS (American University of Sharjah).
• skCUBE, a 1U CubeSat, the first Slovak satellite developed by the University of Zilina (UNIZA) in cooperation with the University of Technology (STU) in Bratislava and with SOSA (Slovak Organization for Space Activities).
Orbit of secondary payloads: Sun-synchronous elliptical orbit, 450 km x 720 km, inclination = 98º.
Experiment complement: (FPR)
FPR (Folded Panel Reflectarray)
FPRs are a new type technology that provide a very significant reduction in stowed volume. For modest antenna sizes (~30-35 dB gain), the panels can be stowed entirely within the "dead space" between the bus and the launch canister (e.g. P-POD), so the antenna does not consume any payload volume. These antennas are mechanically simple, depending on a simple spring loaded hinge deployment mechanism, have relatively low mass density, and are expected to have low production cost. Perhaps the biggest limitation of the FPR is scalability. Current technology limits the size to ~3-6 panels due to two factors: (1) tolerance accumulation of multiple hinged panels limits the size of a practical FPR, and (2) there is a practical limit to how thin one can make a panel and still meet the flatness requirements, so stacking a large number of panels will consume CubeSat payload volume. Nonetheless, the inherent advantages of a FPR make this a very useful CubeSat HGA technology for many applications. 6)
Figure 3: ISARA reflectarray antenna optics design (image credit: NASA/JPL)
The ISARA design is particularly unique because it incorporates 24 solar cells on the side of the panels opposite the reflectarray, so it can provide both prime spacecraft power and a high speed datalink. The ISARA design is comprised of three 33.9 cm x 8. 26 cm reflectarray panels designed to achieve 33.5 dB of gain at 26 GHz. As illustrated in Figure 3, the feed is mounted on the bus in an offset configuration with a projected aperture of 33 cm x 27 cm. The reflectarray panels are canted 14º relative to the bus so that the specular direction of the main beam is parallel to the bus axis.
Figure 4: ISARA CAD model and photographs of components (image credit: NASA/JPL)
Figure 4 shows a CAD model of the flight configuration along with photographs of the panels, feed and hinges. The deployment mechanism, illustrated in Figure 5, was adapted from the standard Pumpkin Inc. "Turkey Tail" 3U solar panel design. It is planned to make this a commercially available antenna design.
The requirement to maintain panel flatness with expected heating from the solar cells, Earth albedo, etc. drove the design process. Development of a substrate that provides the stiffness to keep the panels flat with the solar cells attached is challenging due the the CTE (Coefficient of Thermal Expansion) mismatch between the solar cells and the panel substrate material. A unique sandwich substrate material was developed using a co-cure process in which 48 mil graphite composite structural core is sandwiched between a pair of 15 mil Teflon based dielectric circuit boards. The outer circuit boards have a dielectric constant of 3.00 and low loss tangent (tan δ ~ 0.001), which is suitable for high efficiency reflectarray panels.
The electrical design of the reflectarray uses square patches arranged on a square grid with an element spacing of 0.46 wavelengths. The feed is a 4 x 4 element microstrip patch array designed to create approximately -10 dB edge taper in order to minimize spillover loss and minimize power incident on the bus. Circular polarization is formed by the feed patch design, whereas the reflectarray is a dual linearly polarized design.
Figure 6 shows the measured principal plane radiation patterns along with the calculated patterns obtained for the EM design. The -13 dB azimuth pattern sidelobes are caused by the large 1.1 cm gaps between the center panel and the two winglet panels, a result of the hinge design used to accommodate the 3U CubeSat bus. Preliminary measurements indicate that the flight antenna will achieve a >33.0 dB gain. The feed design presents one of the greatest challenges for this antenna. The circular polarization requirement imposes practical constraints on the feed array spacing which makes it difficult to achieve an optimal edge taper. In this design, the spillover + taper loss is ~ 2.0 dB. In addition, the requirement to package the feed into a thin, inexpensive printed circuit board package resulted in about 1.4 dB feed loss. Overall antenna bandwidth easily exceeds the required value of 100 MHz.
Figure 6: Measured ISARA EM (Engineering Model) antenna pattern at 26 GHz (image credit: NASA/JPL)
In summary, the FPR (Folded Panel Reflectarray) has a great practical value for 3U-6U class CubeSats that need a modest size HGA. The advantages of extremely low stowed volume, low production cost and a low mass perfectly match the CubeSat concept. The ISARA design demonstrates the viability of this concept, although it appears that some improvement in efficiency is possible, particularly in the feed. Nevertheless, the FPR design scalability is limited by the mechanical restrictions of panel flatness and hinge tolerance accumulation, so the technology is complementary with PRAs (Parabolic Reflector Antennas). Finally, for some missions the option to add solar cells to the reflectarray panel could be very useful.
After deployment from the spacecraft, ISARA will deploy its solar array/reflectarray antenna and use the ADCS 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 the nanosatellite 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. The 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.
1) "Integrated Solar Array and Reflectarray Antenna (ISARA) for High Bandwidth Cubesats," NASA Facts, 2014, URL: https://www.nasa.gov/sites/default/files/files/ISARA_Fact_Sheet-15Oct14.pdf
2) Richard E. Hodges, Biren Shah, Dhack Muthulingham, Tony Freeman,"ISARA Integrated Solar Array Reflectarray - Mission Overview," Small Satellite Conference Workshop, SSC13, Logan, UT, USA, August 10, 2013, URL:http://digitalcommons.usu.edu/cgi/viewcontent.cgi?filename=0&article=2877&context=smallsat&type=additional
3) "Integrated Solar Array & Reflectarray Antenna (ISARA)," NASA/JPL, URL: http://www.jpl.nasa.gov/cubesat/missions/isara.php
4) "FormoSat-5," NSPO, URL: http://www.nspo.org.tw/2008e/projects/project5/intro.htm
5) United States Commercial ELV Launch Manifest," June 17, 2015, URL: http://www.sworld.com.au/steven/space/uscom-man.txt
6) Richard E. Hodges, Daniel J. Hoppe, Matthew J. Radway, Nacer E. Chahat, "Novel Deployable Reflectarray Antennas for CubeSat Communications," 2015 International Microwave Symposium (IMS), IEEE MTT-S, Phoenix, AZ, USA, May 17-22, 2015, DOI: 10.1109/MWSYM.2015.7167153, URL: http://220.127.116.11/downloads/ims/arftg_imsproceedings/ims_proceedings/PDF/1837-PAQRFTRdEwNs-2.pdf
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).