# An Overview of CubeSat Missions and Applications
## 3 CubeSats as Earth Observation and Earth Science Platforms
### The Role of CubeSats in Earth Observation Missions
Therefore, ionospheric measurements can provide even more information on the dynamics of the Earth's atmosphere as a whole. Therefore, over the years, a variety of CubeSat missions have been dedicated to the study of the ionosphere with some examples being the Radio Aurora Explorer (RAX) [81], the Dynamic Ionosphere CubeSat Experiment (DICE) [82], the Triplet Ionospheric Observatory-CubeSat for Ion, Neutral, Electron and MAgnetic Fields (TRIO-CINEMA) [83], and the Scintillation Observations and Response of the Ionosphere to Electrodynamics (SORTIE) [84], among a variety of others. To perform their missions, these ionospheric research CubeSats can be equipped with a multitude of different instruments such as Langmuir probes, Retarding Potential Analyzers (RPAs), Ion Drift Meters (IDMs), magnetometers, and radio occultation receivers [85]. Perhaps the most widespread method used by CubeSats in the field of atmospheric monitoring is that of Global Navigation Satellite System Radio Occultation (GNSS-RO). The basic operational principle of this technique is that as the signal emitted by the GNSS satellites travels through the Earth's atmosphere and ionosphere, it is refracted as presented in the simplified illustration of Figure 7. The magnitude of the GNSS signal refraction  
Figure 6: A simplified illustration of the basic operating principle of Global Navigation Satellite System Reflectometry (GNSS-R) for terrain mapping and altimetry. depends on parameters such as the atmospheric temperature, pressure, water vapor content, and electron density. This way, a CubeSat (or any other satellite) in LEO that is equipped with specifically designed GNSS receiving elements can use the signals it is receiving from satellites belonging to GNSS constellations, such as GPS or Galileo, in order to collect data about the state of the atmosphere and ionosphere from various altitudes. This data can be subsequently used for weather-related applications, enabling near-real time weather monitoring and improving prediction capabilities, as well as for ionosphere condition monitoring [86; 87]. The most prominent example of GNSS-RO weather monitoring CubeSats are the Low Earth Multi-Use Receiver (LEMUR) platforms, which are developed and operated by Spire Global, Inc. (Vienna, VA, USA). These satellites form the cornerstone of a LEO constellation of more than 100 CubeSats, operating in orbits with altitudes between 400 km and 650 km. The vast majority of LEMUR CubeSat platforms have a 3U form factor and their main payload comprises an in-house developed GNSS receiver which is connected to large, side mounted GNSS antennas which are used to receive the occulted GNSS signals. Furthermore, apart from monitoring atmospheric conditions via GNSS-RO, the LEMUR platforms are also equipped with AIS receivers for the monitoring of maritime traffic and Automatic Dependent Surveillance-Broadcast (ADS-B) for aircraft tracking [88; 89; 90]. Regarding the GNSS-RO ionosphere monitoring data provided by the LEMUR platforms, it must be noted that a comparative study [91] between the Spire constellation and ground-based ionosphere monitoring systems such as the Global Ionospheric Radio Observatory (GIRO) and Arecibo Incoherent Scatter Radar (ISR) found a good correlation in the data generated by the different systems. Furthermore, it is noted in the same study that Spire constellation's global coverage and its direct access to the top side of the ionosphere represent significant advantages compared to ground-based systems. Another example of a GNSS-RO weather monitoring CubeSat constellation is the Community Initiative for Continuous Earth Remote Observation (CICERO), which comprises 6U CubeSats and is operated by GeoOptics, Inc. (Pasadena, CA, USA), with these satellites also being utilized in the mission of remote sensing via GNSS-R.