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)
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.
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.
ISARA Rideshare Opportunity:
In 2014, The Aerospace Corporation proposed an idea for a secondary payload to fill approximately 1U of unused volume on the ISARA spacecraft. The primary ISARA mission required a 3U bus to accommodate the reflectarray antennae/solar panel structure, but used less than 2U of spacecraft volume. To fill the remaining volume, a combination of two readily available compact uncooled IR cameras was chosen, and a visible camera similar to those flown on prior AeroCube missions, but with greater low-light sensitivity. The Space and Missile System Center's Advanced Development program office (SMC/AD) was interested in gaining experience with flying COTS camera hardware and funded the secondary payload effort. 2)
The sensor capabilities matched specific nighttime remote sensing science goals of The Aerospace Corporation's Space Science Applications Laboratory, including exploring the utility of the near infrared for nighttime cloud detection and combining cloud and nightlight sensors on a single payload. 3)
Payload development was greatly speeded by use of the 5-camera controller card technology for the OCSD/AC-7 satellite. Some added software was necessary for this card to control the new cameras. Power conditioning boards were also necessary for the IR cameras. As mentioned above, two camera cards were necessary, as the main payload had two star sensors, a nadir-pointed fisheye camera, and a reflectarray deployment fisheye camera. Redundant boards also ensured there was less risk to the main reflectarray experiment. The new imaging instruments were as isolated as possible from the primary experiment.
ISARA is a 3U bus to host two payloads, namely the ISARA (Integrated Solar Array and ReflectArray) antenna as primary payload and the CUMULOS (CubeSat Multispectral Observation System), an experimental remote sensing payload of The Aerospace Corporation.
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 1: ISARA spacecraft configuration with 3U CubeSat and Solar Array with Integrated Reflectarray Antenna (image credit: NASA/JPL)
The reflectarray antenna consists of three panels, electrically tied together through hinges, which have an array of printed circuit board patches on them. The size of the patches are 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, leaving plenty of room for payloads such as science instruments or imaging systems. 4)
The ISARA technology will be validated in space during the mission to measure key reflectarray antenna characteristics, which include how much power can actually be obtained over its field of view. ISARA contains a transmitter and an avionics subsystem that features a GPS (Global Positioning System) receiver and a high precision attitude control system designed to orient the CubeSat to enable accurate antenna beam pointing. Once in orbit, ISARA will deploy its solar array and reflectarray antenna. It then will use its attitude determination and control system to stabilize itself. An ultra high frequency (UHF) communications system will be used to make initial contact with the satellite and perform in-orbit checkout procedures.
During the in-orbit test, ISARA's reflectarray antenna will transmit a signal that will be received by a ground station located at NASA/JPL ( Jet Propulsion Laboratory) in Pasadena, California. The spacecraft's location and orientation telemetry data will be analyzed to reconstruct the antenna signal pattern, which will then be compared against pre-flight ground measurements.
ISARA contains a transmitter and an avionics subsystem that features a high precision ADCS (Attitude Determination and Control Subsystem) along with a suite of ADCS sensors and actuators. A high accuracy MAI-400 ADCS is used to achieve the required 0.2° pointing accuracy. The standard bus avionics that are essentially identical to the OCSD (Optical Communication and Sensor Demonstration) 3U CubeSat,include a primary flight computer, two UHF radios and a GPS receiver for navigation. ISARA has a mass of ~5 kg.
Figure 2: ISARA avionics stack (image credit: NASA/JPL, The Aerospace Corporation)
Figure 3: ISARA 3U spacecraft showing the primary reflectarray and the secondary remote sensing payloads (image credit: The Aerospace Corporation, NASA/JPL)
The stowage volume and spacecraft power provided by the ISARA technology also enabled the ISARA mission to carry a secondary payload known as CUMULOS (CubeSat Multispectral Observation System), an experimental Aerospace Corporation remote sensing payload. CUMULOS is composed of a 0.4-0.8 µm visible camera, a 0.9-1.7 µm short-wavelength infrared camera, and an 8.0-13.5 µm long-wavelength infrared, microbolometer camera.
The CUMULOS sensors provide a small-aperture, large field-of-view, remote sensing payload suitable for testing the performance of passively-cooled commercial sensors for weather and environmental monitoring missions. CUMULOS is designed for point-and-stare imaging and will allow almost simultaneous 3-band coverage of regions 230 x 180 km in size, at ground sample distances from 180 to 600 m from an orbital altitude of 600 km. Remote sensing applications to be investigated include: cloud cover detection, surface temperature measurement, hotspot detection (including fires, gas flares, and volcanic activity), and detection of nighttime lights.
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.
KBE (Ka-Band Exciter): The KBE is a Ka-band (26 GHz) RF power amplifier designed specifically for ISARA. As the illustration in Figure 4 suggests, the KBE is a single circuit board that occupies 97mm x 97mm x 20mm of payload volume and wwith a mass of 200 gram. It is located adjacent to the Feed and the SGA in order to minimize transmission line losses. This amplifier produces 200 mW of RF power to meet the ISARA link budget requirements. This power amplifier is compatible with the recently developed IRIS software defined radio, 5) so a fully functional Ka-band telecom system could be developed in a short time.
A key component of the KBE is the RF single pole, double throw switch that toggles output power between the reflectarray and the SGA. As explained below, this is the mechanism used to calibrate gain measurements in the on-orbit test.
Figure 4: CAD model of the ISARA spacecraft showing the payload components. The red lines illustrate the reflectarray beam focusing concept (image credit: NASA/JPL, The Aerospace Corporation)
Launch: The ISARA mission was launched on November 12, 2017 on the Cygnus Orbital ATK CRS OA-8 Mission to the ISS on the upgraded Orbital ATK Antares 230 vehicle. The launch site was MARS (Mid-Atlantic Regional Spaceport) at NASA's Wallops Flight Facility in Virginia. 6) 7)
Orbit : Near-circular orbit, altitude of ~400 km to ISS, inclination =51.6°.
For the OA-8 mission, Orbital ATK is using the Enhanced Cygnus PCM (Pressurized Cargo Module) to deliver cargo to the space station. The cargo capability of the Enhanced Cygnus, developed by Thales Alenia Space, is more than 3500 kg with a total volumetric capacity of 27 m3 (the ascent cargo mass is 3,350 kg). The supply shipment includes:
- 1240 kg of crew supplies. Fresh fruit and vegetables are among the goodies riding inside a refrigerator on Cygnus spacecraft.
- 740 kg of science investigations
- 132 kg of spacewalk equipment
- 851 kg of vehicle hardware
- 34 kg of computer resources
The Orbital ATK CRS-8 (OA-8) launch carried another historic NanoRacks mission to the ISS. With a completely full ENRCSD (External NanoRacks Cygnus Deployer), a virtual reality camera, and educational research, this mission marks over 600 NanoRacks payloads delivered to the ISS since 2009. 8)
This mission is enabling a unique virtual reality opportunity with National Geographic's VUZE camera. Integrated and launched via NanoRacks, VUZE will allow for the recording of the new National Geographic series "One Strange Rock," in which the astronaut crew will record a series of virtual reality pieces for incorporation into a larger documentary about natural history and the solar system. This is National Geographic's first time launching with NanoRacks.
Additionally, OA-8 is the fourth mission in which NanoRacks is providing opportunities for CubeSat deployment from Cygnus after the vehicle departs from the station. The NanoRacks ENRCSD is installed on the exterior of the Cygnus service module with the capability to deploy satellites after Cygnus' completion of its primary ISS resupply mission.
On this ENRCSD mission, NanoRacks has 14 satellites ready to be deployed with customers including the NRO Office of Space Launch, Asgardia, Spire, Tyvak, NASA/JPL, and NRL (Naval Research Laboratory) in Washington, D.C. Included in this External Cygnus manifest are also The Aerospace Corporation's AeroCube B/C satellites, water-based propulsion CubeSats.
The External Cygnus Deployment Program was developed with the customer in mind. The lifespan of CubeSat deployed from the Cygnus vehicle at 500 km adds approximately two-years additional lifetime compared to our ISS NRCSD deployment program.
Cygnus carried the following CubeSats:
• ISARA, a NASA/JPL 3U CubeSat demonstration mission of a Ka-band reflect array antenna.
• EcAMSat (E. coli AntiMicrobial Satellite), a 6U CubeSat of NASA/ARC to investigate space microgravity effects on the antibiotic resistance of E. coli.
• Lemur-2 x 8, 3U CubeSats of Spire Global (commercial global ship tracking). Each of these 3U CubeSats carries two instruments: SENSE, which relays Automatic Identification System (AIS) signals from ships, and STRATOS, which monitors the occultation of GPS satellites as they pass through the atmosphere.
• CHEFsat (Cost-effective High E-Frequency Satellite), a 3U CubeSat of NRL (Naval Research Laboratory) to test COTS technologies.
• Asgardia-1, a 2U CubeSat of Asgardia Space to demonstrate long-term data storage in orbit (500 GB). After leaving the ISS, the robotic Cygnus will boost itself into a 500 km orbit and deploy Asgardia-1.
• OSCD x 2 (Optical Communication and Sensor Demonstration), 1.5U CubeSats of The Aerospace Corporation.
• PropCube-2 (Fauna), a 1U CubeSat of NPS (Naval Postgraduate School) for ionospheric calibration measurements.
• TechEdSat-6, a 3U CubeSat, developed by students of SJSU (San Jose State University), the University of Idaho, and NASA/ARC (Ames Research Center). TechEdSat 6 is the latest in a series of CubeSats testing an "exo-brake" deorbit system, a drag device that uses aerodynamic forces — and not propulsion — to re-enter the atmosphere. The technology could eventually allow some space station research samples to return to Earth sooner, and at less expense.
Cygnus arrived at the station on November 14. Expedition 53 Flight Engineers Paolo Nespoli of ESA and Randy Bresnik of NASA used the space station's robotic arm, SSRMS (Space Station Remote Manipulator System), to capture Cygnus. The crew then handed robotics off to controllers on the ground for the berthing operation. Eventually, Cygnus OA-8 was berthed to Node-1 (Unity, Figure 5). 9) 10)
Two newly arrived CubeSats will be transferred inside the space station to join three others for release into orbit through the Japanese lab module's airlock. These are: EcAMSat and TechEdSat-6.
Cygnus will remain at the space station until December 4, when the spacecraft will depart the station and deploy a total of 14 CubeSats from the NanoRacks CubeSat deployer, mounted on the outside of the spacecraft. Among them are: ISARA, Lemur (8), CHEFSat, Asgardia-1, OCSD (2) and ProCube-2 into a higher orbit of ~500 km. — After these deployments, the Cygnus spacecraft will reenter into Earth's atmosphere and disintegrate over the South Pacific Ocean, as it disposes of several tons of ISS trash.
Figure 5: Nov. 14, 2017: International Space Station Configuration. Five spaceships are parked at the space station including the Orbital ATK Cygnus-8, the Progress 67 and 68 resupply ships and the Soyuz MS-05 and MS-06 crew ships (image credit: NASA, Ref. 10)
ISARA mission status
• August 2018: The ISARA mission has successfully demonstrated a high bandwidth Ka-band CubeSat communications capability that is applicable to commercial, government and military systems. For a modest increase in mass, volume and cost, this technology increases downlink data rates from a baseline of 9.6 kbit/s for existing UHF systems to over 100 Mbit/s – a 105 fold increase in data capacity. The key to this technical advance is a HGA (High Gain Antenna) that is integrated into a commercially available 3U CubeSat solar array with minimal modification of the existing solar panel design. To validate this approach, ISARA has flown a 5 month technology demonstration mission that elevates the antenna technology from TRL 5 to TRL 7 and demonstrates the capability for 100 Mbit/s data rate. 11)
- This system demonstrated 100 Mbit/s downlink data rate capability using a relatively simple ground station with a 70 cm parabolic reflector antenna. The spacecraft also verified ISARA solar panel output power and demonstrated the operational capability to accurately point the antenna to a Ka-band ground station, a key requirement for a telecom system.
- The spacecraft also verified ISARA solar panel output power and demonstrated the operational capability to accurately point the antenna to a Ka-band ground station, a key requirement for a telecom system.
• July 3, 2018: The two images of Figure 6 were acquired with the CUMULOS cameras of the ISARA 3U CubeSat mission. The image on the left, taken by the SWIR (Short-Wavelength Infrared) camera, captures a larger area of the lake and shows strong contrast between land and water features. The narrower field of view image on the right taken by the payload's LWIR (Long-Wavelength Infrared) camera indicates a difference in water temperature between the lake's center and the water in the bays and inlets. 12)
- CUMULOS is testing the performance of commercial sensors for weather and environmental monitoring missions. The cameras are designed for point-and-stare imaging and allow nearly simultaneous coverage of Earth regions from an orbital altitude of 452 km.
- CUMULOS is hosted as a demonstration of an experimental payload on NASA's ISARA (Integrated Solar Array and Reflectarray Antenna), which is managed by NASA's Jet Propulsion Laboratory in Pasadena, California, and operated by The Aerospace Corporation.
Figure 6: These two images of Lake Superior and the surrounding area are the first data downlinked from the CubeSat CUMULOS (Multispectral Observation System) cameras (image credit: NASA / The Aerospace Corporation)
• March 29, 2018: NASA's ISARA and OCSD (Optical Communications and Sensor Demonstration) nanosatellite recently completed systems checkout and have moved into the operational phase to demonstrate a number of technology firsts. 13)
- The ISARA mission is the first in-space demonstration of a reflectarray antenna, as well as that of an integrated antenna and solar array. ISARA is also the first demonstration of the radio frequency Ka-band from a reflectarray antenna. A relatively new type of antenna, the reflectarray consists of flat panels with an array of printed circuit board patches arranged to focus the radio signal in a similar manner as a parabolic dish.
- ISARA initiated demonstration of its radio frequency communications technology by successfully generating a signal tone through its reflectarray antenna to the ground station at NASA's Jet Propulsion Laboratory in Pasadena, California. This demonstration enables high-bandwidth radio downlink of data from a CubeSat-scale spacecraft. The ISARA team will continue to characterize the reflectarray antenna to include measurements related to signal strength and solar array power attainment.
- OCSD consists of a pair of spacecraft each equipped with lower power laser communication systems. Each spacecraft also has a limited water-based propulsion system. OCSD will demonstrate the first-ever high-speed laser communication from a CubeSat to a ground station. OCSD will also demonstrate an optical communications uplink to a CubeSat for the first time.
- Demonstration of OCSD's optical communications payload requires nighttime operations and clear weather, due to the limited power of the laser. During the initial part of this technology demonstration phase, the mission team is working to align each spacecraft's laser with a ground station in preparation for the final demonstration of high-speed downlink optical communications. An optical telescope on Mount Wilson in Southern California will be used for the final demonstration.
- An additional demonstration will involve proximity operations by maneuvering the pair of OCSD spacecraft to within 650 feet of each other. OCSD's proximity operations demonstration requires that the two spacecraft decrease their distance to three miles to enable the laser rangefinders mounted on each spacecraft to locate each other. Currently 100 miles apart, the OCSD spacecraft have fired their water-based propulsion systems to initiate maneuvers to close their distance. Over the coming days, the two spacecraft will approach to a final distance of 650 feet to begin proximity maneuvers.
- The technology demonstrations for both ISARA and OCSD will continue into the summer until completion.
- Managed by NASA's Ames Research Center in California's Silicon Valley, the ISARA and OCSD missions are funded by the Small Spacecraft Technology Program within the agency's Space Technology Mission Directorate. The ISARA payload is being developed by NASA's Jet Propulsion Laboratory in Pasadena, California, and will be demonstrated on a CubeSat developed by The Aerospace Corporation in El Segundo, California. JPL partnered with Pumpkin Inc. in San Francisco, California, to develop the solar array. The OCSD satellites are developed and operated by The Aerospace Corporation of El Segundo, California.
• December 7, 2017: Last night, NanoRacks successfully completed the Company's 4th External Cygnus Deployment mission after commands were sent to the Cygnus spacecraft from Orbital ATK's Mission Control at their Dulles headquarters. After departing from the Space Station, Cygnus was elevated to an altitude of 450 km before deploying the satellites. Note: The Orbital ATK CRS-8 mission was launched on November 12, 2017. - On this External Cygnus Deployment mission, NanoRacks deployed the following 14 satellites: 14) 15)
- ISARA of NASA/JPL
- PropCube-Fauna of NPS (Naval Postgraduate School)
- Lemur-2 (8 CubeSats) of Spire Global
- AeroCube 7 OCSD-B and -C (2 CubeSats) of The Aerospace Corporation
- Asgardia-1 of Asgardia Space
- CHEFSat of NRL (Naval Research Laboratory).
This historic and innovative satellite deployment service is a part of the first-ever program in which an ISS Commercial Resupply Vehicle is able to deploy satellites at an altitude higher than the ISS after completing its primary cargo delivery mission. Flying at 450-500 km provides an open door for new technology development as well as an extended life for CubeSats deployed in low-Earth orbit.
Experiment complement: (ISARA, CUMULOS)
ISARA (Integrated Solar Array and Reflectarray Antenna)
ISARA (the payload name is the same as the mission name) is a new type technology providing 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 ISARA 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 ISARA, 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 the ISARA make this a very useful CubeSat HGA technology for many applications. 16)
Figure 7: 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 7, 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 8: ISARA CAD model and photographs of components (image credit: NASA/JPL)
Figure 8 shows a CAD model of the flight configuration along with photographs of the panels, feed and hinges. The deployment mechanism 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 9 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 9: Measured ISARA EM (Engineering Model) antenna pattern at 26 GHz (image credit: NASA/JPL)
In summary, the ISARA antenna 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 ISARA antenna 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.
CUMULOS (CubeSat Multispectral Observation System)
CUMULOS is a is a secondary payload on the 3U CubeSat hosting the primary NASA/JPL ISARA mission. The CUMULOS payload was integrated on a noninterference basis and care was taken to ensure low risk to the primary payload mission. CUMULOS will research optimal methods for the operation of passively cooled COTS sensors and cameras and determine their suitability to perform weather and environmental monitoring missions (Ref. 2).
CUMULOS may be thought of as a miniature weather satellite with a special focus on lowlight studies. A primary engineering research goal is to study the use and performance of commercially available uncooled infrared (IR) cameras in space.
Three separate cameras comprise the CUMULOS payload: 1) a visible (VIS) CMOS camera, 2) a SWIR, InGaAs camera, and 3) a longwave infrared (LWIR), vanadium oxide microbolometer. The VIS camera uses an Aptina MT9M001C12STM CMOS chip mated to a ruggedized Schneider Xenoplan lens. The SWIR camera is a FLIR Tau SWIR 25 equipped with a vented lens from Stingray Optics. The LWIR camera is a FLIR Tau 640 using the vendor's lens choice. The visible and LWIR cameras are passively cooled. The SWIR focal plane is temperature stabilized using an internal TEC (Thermo-Electric Cooler).
The cameras underwent TVAC (Thermal Vacuum) and vibration testing independently, and with the integrated ISARA payload. Basic calibration tests such as darks, flat fields, linearity and MTF (Modulation Transfer Function) measurements were performed. 17)
Together, the sensors will be used as a small-aperture, staring payload suitable for testing the performance of passively cooled sensors to perform spaceborne weather and environmental monitoring missions. The sensor specifications are shown in Table 1. The host spacecraft was originally scheduled to be placed in 575 km circular, 97.7° inclination, sun-synchronous orbit with a 10:30 am equatorial crossing time by a Falcon 9 launch vehicle. - Late breaking decisions may enable an earlier launch on the OA-8 ISS resupply flight during Fall, 2017. In this option, after a 6-week dock, the upper stage would then place the host spacecraft into a 500 km circular orbit with 52° inclination. The orbit-dependent terms in Table 1 and Figure 5 will change a bit as a result.
Table 1: CUMULOS sensor specifications
Mission concept: CUMULOS is designed to be used as a staring sensor. The basic collection sequence involves taking an image triplet, one frame per camera, as fast as possible to cover a region of interest. Figure 10 shows the pixel and swath of each camera projected onto the Los Angeles region. CUMULOS will obtain near-simultaneous frames with the three cameras. The individual cameras can frame rapidly, but downlink constraints will direct our focus towards taking small numbers of frames from each camera on selected targets of interest. With our current groundstation capabilities we can downlink one to two raw-file image triplets per ground-station contact. During operations, we will downlink thumbnail images and then select which full-frame JPEGs or raw files to retrieve from the 8 GB on-board storage. More full frames may be downlinked if JPEG compression and/or pixel aggregation are used. Use of the SDR radio, currently being tested on AC-7A, increases data downlink rates by a factor of 4. Note that the ISARA antenna is not connected to a radio, so we can't take advantage of its high gain capabilities with CUMULOS.
CUMULOS will leverage the improved pointing capabilities of the current generation Aerospace Corporation CubeSat bus, and add multispectral capabilities, to advance the point-and-stare nighttime imaging experiments we previously initiated with the visible cameras on AeroCube-4, -5, and -8. The three cameras were chosen for nighttime sensitivity and, in the case of the microbolometer, to test an uncooled microbolometer as a cloud and land surface temperature sensor.
Remote sensing applications to be investigated include: cloud detection and characterization, land and water surface temperature measurement, urban heat island measurement, hotspot detection (fires, gas flares, and volcanic activity), and nightlight detection with both the VIS and SWIR sensors. Attempts will be made to image dynamic weather events. While the nominal repeat cycles for all cameras are listed in Table 1, pointing off-nadir will allow more frequent revisit times. Particular attention will be paid to imaging weather at night to research capabilities similar to the VIIRS DNB (Day/Night Band).
The visible and SWIR cameras will be capable of imaging the Earth's surface and clouds when illuminated by moonlight and the airglow. The SWIR 0.9 to 1.7 µm band spans many excited OH emission lines in the atmospheric airglow (the Meinel bands). Much more illumination falls in the SWIR band than in the weak visible emissions detected by the VIIRS DNB under zero moonlight conditions in prior work. 18)
We want to study the utility of the wide-open SWIR band for weather missions and gain experience that can be applied to future weather sensors. The thermal waveband will be used to track cold clouds in the upper atmosphere as well as forest fires on the Earth's surface. Comparisons between bands will aid in performance assessment. Comparisons of CUMULOS data to VIIRS, MODIS (Moderate-resolution Imaging Spectroradiometer), and DMSP (Defense Meteorological Satellite Program) data, as well as data from the Japanese Space Agency CIRC (Compact Infrared Camera) and related on-orbit microbolometers, will be conducted to validate performance.
Calibration verification and on-orbit performance analysis are key parts of the mission goal of testing performance of commercial sensors. Radiometric calibration will be obtained from observations of stars, the Moon, and the Earth, and will be tied to that of other space sensors. Stellar observations with the VIS and SWIR sensors will be used to measure the operational point spread function.
Figure 10: Pixel footprints and fields of view for the three CUMULOS cameras projected onto The Aerospace Corporation and the Los Angeles region. The cameras are co-boresighted (image credit: The Aerospace Corporation)
Ka-band Ground Station
The ISARA Ka-band ground station, located on top of a nine story building at the Jet Propulsion Laboratory, is the key high data rate experimental link. The station was developed with simple requirements in mind, focused on verifying the Level 1 on-orbit mission requirement following the successful launch. The station utilizes an open loop recording device that allows the team to measure beam peak location, antenna gain and antenna patterns. A simplified block diagram of the station is shown in Figure 11. Since ISARA only transmits a simple carrier wave tone, there is no back-end radio that demodulates "data" (Ref. 11).
To keep costs in line with the project's budget, most of the devices employed in the station electronics are standard off-the-shelf components. The exceptions to this were the positioner, which was a refurbished unit that had recently been declared as excess by the NASA Deep Space Network (DSN), and the 70 cm diameter Ka-band parabolic dish antenna, which was a custom build to accommodate our operating frequency. Additional facilities work was also needed to accommodate the dish in its current location. The resulting station satisfies the requirements of the ISARA mission, but could easily be modified into a more traditional station as needed, leaving JPL with an asset for the future.
Figure 11: Simplified Ka-band ground station block diagram (image credit: NASA/JPL)
On-Orbit Experiment – Typical Pass: The ISARA CubeSat flies in Low Earth Orbit at 450 km with an approximately 90 minute orbital period and typically a 3-5 minute observation time per pass. In principle, two passes per day are available to measure telecom data rate, gain and antenna patterns. However, in practice it was found that one pass per day was actually usable.
Prior to a pass, the spacecraft and the Ka-band ground station are programmed to execute the required pointing maneuvers and data collection. On the spacecraft side, the ADCS (Attitude Determination and Control System) is programmed to execute the requisite spacecraft pitch, roll and yaw desired for the pass. For example, this could include staring at the ground station, conical beam peak search, pattern cuts to measure sidelobe peaks, etc. The high accuracy ADCS was used to achieve the required 0.2º antenna pointing accuracy. The ground station is programmed to power up and initialize the data acquisition system and point the ground station antenna toward the spacecraft. Experimental passes are operated autonomously and can be run at any time of day or night.
Following a pass, telemetry data is downloaded from the spacecraft through the UHF ground station. This data includes pointing data obtained from the ADCS subsystem, GPS location data and KBE telemetry that includes transmit power level. All of these data are time tagged so that they can be correlated with the ground station recorded data.
Ground station recorded data includes HGA and SGA signal level as well as a record of ground station pointing data. This data is correlated with the spacecraft pointing data in order to verify antenna pointing, plot antenna patterns as a function of angle relative to the spacecraft, etc. Note that during brief periods of a pass the solar array is not expected to generate power. At other times the solar array can be positioned for optimal solar power generation.
The NASA Level 1 requirements for the ISARA mission include both ground measurements and on-orbit results. The purpose of this is to directly verify that the Ka-band antenna on-orbit performance matches what was measured prior to launch. The pre-launch antenna performance will be detailed in a future publication, but is very similar to breadboard data that was reported previously.
The key Level 1 requirement is Ka-band antenna gain. Ground measurements showed the gain is 35.6 dBic, which is in very good agreement with antenna model simulations. The on-orbit passes described above confirmed this antenna gain through a series of conical scans that were used to search for the main beam.
Figure 12: On-orbit HGA and SGA (Standard Gain Antenna) signals recorded by the JPL Ka-band ground station (image credit: NASA/JPL)
Figure 12 shows signal levels recorded for a typical pass (March 25, 2018). The pulses in the figure correspond to a two second HGA/SGA switching period, so each pulse is 1 second in duration. The HGA signal has uncertainty values highlighted with red shading, while the SGA uncertainty is green. Note the quadratic shape of the high gain antenna main beam is clearly evident (a dashed line has been added to illustrate this). In contrast, the SGA signal level remains relatively constant, which is expected because the SGA beamwidth is much broader.
Averaging several such passes determined a measured on orbit gain of about 32.9 dB with an uncertainty of about 0.8 dB. This value compares favorably to the prelaunch measured gain of 33.6 dB. It should be noted that, due to practical issues that were being solved during the time these data were collected, it is not known exactly how close the measured peak is to the actual beam peak. At least one pass observed a peak gain of 33.4 dB with an uncertainty of 0.6 dB.
These measured gain values have been used in a detailed link budget of the kind JPL routinely uses to predict and assess the performance of spacecraft communications systems. This link budget assumed signal to noise ratio performance values consistent with the ISARA ground station equipment (e.g. a 70 cm ground station antenna). This link budget shows that such a system will achieve a 100 Mbit/s data rate while maintaining an average 5.7 dB link budget margin for a typical LEO mission.
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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).