# An Overview of CubeSat Missions and Applications
## 3 CubeSats as Earth Observation and Earth Science Platforms
### The Role of CubeSats in Earth Observation Missions
It is important to note that the CICERO satellites were initially envisioned as microsatellites with a mass of more than 100 kg each; however, technological leaps in GNSS-RO hardware miniaturization eventually allowed the project's satellites to acquire the more compact 6U CubeSat form [92; 93; 94]. Apart from the GNSS-RO technique, there are also other methods via which CubeSats are employed in the atmospheric and weather monitoring and research fields, such as radiometry. More specifically, CubeSats equipped with radiometers can be used to measure the electromagnetic radiation that is emitted, reflected, or scattered by the Earth's surface  
Figure 7: A simplified illustration of the Global Navigation Satellite System Radio Occultation (GNSS-RO) concept. and/or its atmosphere by operating across visible, IR, and microwave spectral bands. Most notably, microwave radiometers can be used to penetrate clouds in order to measure water vapor and precipitation (rain, snow, or hail). IR radiometers, on the other hand, provide information regarding temperature and humidity by detecting thermal radiation at varying altitudes. Finally, radiometers may also be used to monitor sea surface temperatures, soil moisture, and other variables that are known to influence weather patterns and extreme events like hurricanes and droughs. This way, satellites equipped with radiometers can provide global, real-time atmospheric and weather condition data [95; 96]. In that regard, an important mission was the Temporal Experiment for Storms and Tropical Systems Technology-Demonstration (TEMPEST-D) which involved the launch of a 6U CubeSat developed by NASA's JPL, Colorado State University, and Blue Canyon Technologies (Lafayette, CO, USA). TEMPEST-D was equipped with a millimeter-wave radiometer NASA that was capable of performing continuous measurements between 89 GHz and 165 GHz frequencies. Its mission was to demonstrate it's payload's capabilities in observing the time-evolution of clouds and studying the conditions that govern the transition of clouds from the non-precipitating to the precipitating state. The satellite was deployed from the ISS on 13 July 2018 and remained operational for a period of 3 years, until its eventual reentry, having successfully demonstrated the efficacy of its payload for the observation of cloud and precipitation processes [97; 98]. The success of TEMPEST-D paved the way for more missions in that field of atmospheric monitoring via microwave radiometry, such as NASA's Time-Resolved Observations of Precipitation structure and storm Intensity with a Constellation of Smallsats (TROPICS). The TROPICS constellation comprises four 3U CubeSats under the mission of observing tropical systems and collecting data regarding temperature, moisture, cloud-ice, and precipitation. The data collected by the TROPICS constellation will thus help scientists to monitor, study, and better understand the dynamic phenomena and processes of tropical storms. To perform their mission, the TROPICS CubeSats, which were designed and developed through the collaborative efforts of the Massachusetts Institute of Technology (MIT) Lincoln Laboratory and Blue Canyon Technologies, are equipped with a rotating, 12-channel passive microwave radiometer. This radiometer is capable of providing imagery between 90 GHz and 206 GHz, with the frequencies between 90 GHz and 118 GHz being used for temperature measurements and the frequencies between 183 GHz and 206 GHz being allocated to the performance of moisture measurements. For temperature measurements, the radiometer's average spatial resolution is approximately 40 km and for moisture measurements, 25 km. The data collected by the TROPICS constellation is made publicly available by NASA [12; 99]. Another notable NASA CubeSat mission that involved radiometry for atmospheric research was IceCube. IceCube was a 3U CubeSat equipped with a 883 GHz radiometer that was tasked with the mission of studying cloud-ice formation, a phenomenon that has great effect on the Earth's climate.