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The Cubesat Radiometer Radio frequency interference Technology validation mission (CubeRRT) was developed to demonstrate real-time on-board detection and filtering of radio frequency interference (RFI) for wide bandwidth microwave radiometers. CubeRRT’s key technology is its radiometer digital backend (RDB) that is capable of measuring an instantaneous bandwidth of 1 GHz and of filtering the input signal into an estimated total power with and without RFI contributions. CubeRRT’s on-board RFI processing capability dramatically reduces the volume of data that must be downlinked to the ground and eliminates the need for ground-based RFI processing. RFI detection is performed by resolving the input bandwidth into 128 frequency sub-channels, with the kurtosis of each sub-channel and the variations in power across frequency used to detect non-thermal contributions. RFI filtering is performed by removing corrupted frequency sub-channels prior to the computation of the total channel power. The 1 GHz bandwidth input signals processed by the RDB are obtained from the payload’s antenna (ANT) and radiometer front end (RFE) subsystems that are capable of tuning across RF center frequencies from 6 to 40 GHz. The CubeRRT payload was installed into a 6U spacecraft bus provided by Blue Canyon Technologies that provides spacecraft power, communications, data management, and navigation functions. The design, development, integration and test, and on-orbit operations of CubeRRT are described in this paper. The spacecraft was delivered on March 22nd, 2018 for launch to the International Space Station (ISS) on May 21st, 2018. Since its deployment from the ISS on July 13th, 2018, the CubeRRT RDB has completed more than 5000 hours of operation successfully, validating its robustness as an RFI processor. Although CubeRRT’s RFE subsystem ceased operating on September 8th, 2018, causing the RDB input thereafter to consist only of internally generated noise, CubeRRT’s key RDB technology continues to operate without issue and has demonstrated its capabilities as a valuable subsystem for future radiometry missions.

Passive microwave temperature and water vapor sounding of the Earth’s atmosphere provides one of the most valuable quantitative contributions to weather prediction and is a key factor in initializing and validating climate models. Recent advances in the capabilities and robustness of small satellite components and systems provide an opportunity for NOAA, EUMETSAT and other operational agencies to explore the value of launching passive microwave sounder/imagers and complementary instruments on small spacecraft, including CubeSats, for relatively small investments. This provides the potential for deployment of microwave sounder constellations in low-Earth orbit (LEO) to substantially shorten revisit times. In this context, the first CubeSat-based multi-frequency microwave sounder to provide global data over a substantial period is the Temporal Experiment for Storms and Tropical Systems Demonstration (TEMPEST-D) mission. This mission was designed to demonstrate on-orbit capabilities of a five-frequency millimeter-wave radiometer to enable a future constellations of 6U CubeSats with low-mass, low-power millimeter-wave sensors to observe changes in convection and water vapor vertical profiles with revisit times on the order of minutes instead of hours. TEMPEST millimeter-wave radiometers provide observations at five frequencies from 87-181 GHz, with spatial resolution ranging from 12.5-25 km. To demonstrate technology necessary for deployment and operation of a CubeSat constellation of microwave sounders, the TEMPEST-D satellite was launched on May 21, 2018 from NASA Wallops to the ISS and successfully deployed into a 404-km orbit at 51.6° inclination on July 13, 2018. Now more than two years and nine months into its mission, the TEMPEST-D radiometer continues to provide science-quality data. The TEMPEST-D mission met all of its Level-1 requirements within the first 90 days of operations and achieved TRL 9 for both instrument and spacecraft systems. Validation of the TEMPEST-D brightness temperatures was performed over 50 days during a 13-month period through comparisons with GPM/GMI and MHS on NOAA-19, MetOp-A, MetOp-B and MetOp-C satellites. Results demonstrated calibration accuracy of TEMPEST-D within 1 K and stability within 0.6 K, as well as no evidence of any significant changes over time or with instrument temperature. TEMPEST-D brightness temperatures have been used to demonstrate data assimilation into NOAA numerical weather prediction models as well as atmospheric science parameter retrievals. In summary, on-orbit results show that TEMPEST-D is a very well-calibrated, highly stable radiometer, indistinguishable in performance from larger, more expensive operational sensors. Over its mission lifetime of nearly three years, TEMPEST-D has demonstrated the feasibility of deployment of a constellation of microwave sounders on CubeSats for relatively low cost and short timeline for implementation. A recently-completed CSU study, funded by NOAA, showed the potential for a CubeSat constellation of TEMPEST-based microwave sounders to perform temperature and moisture profiling with shorter refresh times. The InP HEMT low-noise amplifier technology developed for TEMPEST-D receivers for moisture profiling using 87-181 GHz frequencies can be enhanced by adding receivers with temperature profiling frequencies from 114-118 GHz range. The NOAA study demonstrated that a TEMPEST-based constellation of less than 12 CubeSats has the potential to greatly improve revisit times of current polar-orbiting operational microwave sensors.

This Conference Presentation, “Millimeter-wave sounder/imager on a CubeSat providing global atmospheric science observations for more than two years on orbit: Temporal Experiment for Storms and Tropical Systems-Demonstration (TEMPEST-D) Mission,” was recorded at SPIE Optics + Photonics 2021 held in San Diego, California, United States.

A 6–40 GHz antenna system for the CubeSat Radiometer Radio Frequency Interference Technology Validation mission is presented. The antenna system includes three bottom-tapered helical antenna elements that cover 6–12, 12–22, and 22–40 GHz, respectively. The design procedure for achieving continuous gain coverage versus frequency from 7 dBic at 6 GHz to 21 dBic at 40 GHz is discussed. The helical antenna elements are supported by a carefully designed meshed radome made of 1 mm-thick Ultem 1000 material for minimizing impacts to antenna performance. The entire antenna system has a volume of 2 cm (height) $\times7.3$ cm (width) $\times25$ cm (length), or 365 cm3, which is less than 0.5 U (500 cm3). The total mass of the antenna payload is 116 g. Comparisons between the simulation and measurement performance show good agreements. The final fabricated antenna/radome payload also passed typical CubeSat temperature, vacuum, and vibration requirements and was successfully deployed in space in August 2018.

The Temporal Experiment for Storms and Tropical Systems Demonstration (TEMPEST-D) mission is the first CubeSat-based multi-frequency microwave sounder to provide global data over a sustained period. The mission was designed to demonstrate on-orbit capabilities of a five-frequency millimeter-wave radiometer for a complete TEMPEST mission using a closely-spaced train of eight 6U CubeSats with identical low-mass, low-power millimeter-wave sensors to sample rapid changes in convection and surrounding water vapor every 3-4 minutes for up to 30 minutes. The TEMPEST-D satellite was launched on May 21, 2018 from NASA Wallops to the ISS and was successfully deployed on July 13, 2018, into a 400-km orbit at 51.6° inclination. The TEMPEST-D sensor has been operating nearly continuously since its first light data on September 5, 2018. On-orbit results indicate that TEMPEST-D is a very well-calibrated, highly stable radiometer, indistinguishable in performance from larger operational sensors.

To improve understanding of rapid, dynamic evolution of convective cloud and precipitation processes as well as the surrounding water vapor environment, we require fine time-resolution multi-frequency microwave sounding observations capable of penetrating inside the storm where the microphysical processes leading to precipitation occur. To address this critical observational need, the Temporal Experiment for Storms and Tropical Systems (TEMPEST) mission deploys a train of 6U CubeSats carrying identical low-mass, low-power millimeter-wave radiometers to sample rapid changes in convection and water vapor every 3-4 minutes for up to 30 minutes. These millimeter-wave radiometers observe at five frequencies from 87 to 181 GHz. By rapidly sampling the life cycle of convection, TEMPEST fills a critical observational gap and complements existing and future satellite missions. To demonstrate global, well-calibrated radiometric measurements to meet the needs of TEMPEST, the TEMPEST Technology Demonstration (TEMPEST-D) mission satellite was launched on May 21, 2018 on Orbital ATK’s CRS-9 mission to the ISS and deployed into a 400-km altitude and 51.6° inclination orbit by NanoRacks on July 13, 2018. TEMPEST-D has met all mission requirements on schedule and within budget. After achieving first light on September 5, 2018, the TEMPEST-D mission has achieved TRL 7 for both the instrument and spacecraft systems. TEMPEST-D brightness temperatures have been cross-calibrated with those of four NASA, NOAA and EUMETSAT reference sensors observing at similar frequencies. Results demonstrate that the TEMPEST-D on-orbit instrument is a very well-calibrated and stable radiometer with very low noise, rivaling that of much larger, more expensive operational instruments.

The CubeSat Radiometer Radio Frequency Interference Technology Validation (CubeRRT) mission is developing a 6U CubeSat system to demonstrate radio frequency interference (RFI) detection and filtering technologies for future microwave radiometer remote sensing missions. CubeRRT will perform observations of Earth brightness temperatures from 6–40 GHz using a 1 GHz bandwidth tuned channel and will demonstrate on-board real-time RFI processing. The system is currently under development, with an expected launch date in mid-2018 followed by a one year period of on-orbit operations. CubeRRT spacecraft and radiometer instrument testing as well as the mission concept of operations are described in this paper.

In this paper we discuss the necessary technology required to enable the future of spectrum resource constrained missions. We discuss the CubeSat Radiometer Radio Frequency Interference Technology (CubeRRT) validation mission and the development of its digital backend, necessary for performing on-board RFI detection and filtering for wideband high frequency radiometry. The CubeRRT mission will validate the on-board RFI filtering technology solving technological challenges such as bandwidth, data downlink volume, and RFI types. We present a few initial results of the backend spectrometer leading to full-system integration and test.