# An Overview of CubeSat Missions and Applications
## 3 CubeSats as Earth Observation and Earth Science Platforms
### The Role of CubeSats in Earth Observation Missions  
In general, the main sensory instruments that are utilized by all Earth observation satellites, regardless of size, can be classified into two main categories, optical sensors and microwave sensors. Optical sensors may include instruments operating in the visible (VIS) and/or infrared (IR) spectra or Light Detection And Ranging (LIDAR) equipment. Microwave sensors of Earth observation satellites, on the other hand, may include Synthetic Aperture Radars (SAR) and microwave radiometers [44]. Furthermore, Global Navigation Satellite System antennas and receivers can be utilized to collect information for the Earth's surface, as well as its atmosphere [45]. In recent years, the miniaturization of electronics and remote sensing equipment has enabled the development of miniaturized versions of most of the aforementioned sensory instruments, which are now capable of fitting inside the limited volume that is available to CubeSat platforms and has thus allowed CubeSats to undertake an important and increasing role in Earth observation missions, supplementing the traditional large Earth observation platforms. For Earth observation missions, the limited size and power generation capacity offered by CubeSat platforms presents a considerable hurdle that limits CubeSats to the utilization of mostly passive imaging instruments. More specifically, some of the most common sensory instruments found in Earth observation CubeSat missions include optical imagers and spectrometers operating in the visible, Near-Infrared (NIR), Short-Wave Infrared (SWIR), Medium-Wave Infrared (MWIR), Long-Wave Infrared (LWIR) or Far-Infrared (FIR) spectral bands. Imaging instruments operating in the visible and NIR spectra are commonly utilized in missions intended to capture photographs of the planet's surface, reconnaissance applications, or for the identification of chemical compounds. Imagers and spectrometersoperating in the longer-wavelength IR spectra, on the other hand (MWIR, LWIR, and FIR) are utilized in missions involving thermal energy emission monitoring, land, sea or atmospheric temperature mapping, night imaging, wildfire and volatile compound monitoring, as well as for mineral characterization. It must also be noted that multispectral and hyperspectral variants of imaging instruments such as cameras and spectrometers which simultaneously collect information from a large number of distinct spectral bands of the electromagnetic spectrum are also commonly found on Earth observation CubeSat missions. Such instruments are used in missions involving material, chemical pollutant, chlorophyll and microorganism detection, as well as volatile compound and moisture monitoring among others. Another type of instrument commonly found onboard Earth observation CubeSats are radiometers. Radiometers commonly operate in the IR and microwave regions of the electromagnetic spectrum and are used to perform atmosphere-related measurements such as temperature measurements, water vapor content and precipitation monitoring, as well as for the identification and monitoring of greenhouse gases. Furthermore, other types of passive microwave sensors such as scatterometers, altimeters, and Global Navigation Satellite System (GNSS) receivers are also commonly utilized by CubeSat platforms to collect reflected or occulted GNSS signals for missions that involve terrain mapping, altimetry, and atmospheric condition monitoring. Finally, some limited efforts to incorporate active microwave sensors, such as imaging microwave radars, on Earth observation CubeSat platforms are also underway [46; 47]. Table 3 presents some commonly recognized wavelength boundaries among the distinct regions covered by the visible, IR bands, and microwave bands of the electromagnetic spectrum. The high interest for the development of Earth observation CubeSat missions involving optical instruments over the years has facilitated the development of a multitude of COTS imaging solutions which are tailor-made for CubeSat missions.