# HPT: A High Spatial Resolution Multispectral Sensor for Microsatellite Remote Sensing
## 1 Introduction  
Artificial satellites can be categorized according to their masses, e.g., picosatellites (<1 kg), nanosatellites (1-10 kg), microsatellites (10-100 kg), minisatellites (100-1000 kg), and large satellites (>1000 kg). With the rapid development of electronic and communication technology, the adage \"smaller, faster, and cheaper\" summarizes the trend of satellite development activities, which has resulted in a significant increase in the number of nano/microsatellites launched over recent years, especially for Earth observation and remote sensing purposes [1]. Nano/microsatellites have great potential as remote sensing platforms because of the cost-effective implementation of satellite constellations and formations, which increase their overall temporal resolution and ground coverage [2]. However, nano/microsatellites have many limitations related to their small size, which lead to restrictions on their payload instruments in terms of design, power consumption, and data rate. For optical remote sensing, the spatial and spectral resolutions of an optical payload instrument are limited primarily by its aperture size, which is constrained by the dimensions of the satellite. For any optical system, irrespective of whether it is a spaceborne or ground-based instrument, its spatial resolution is limited theoretically by the diffraction of light, and its spectral resolution is limited practically by the signal-to-noise ratio (SNR). Both the diffraction and the SNR can be upgraded with larger apertures; thus, nano/microsatellites have fundamental disadvantages compared with larger satellites in terms of optical remote sensing [3]. Figure 1 shows a diagram of the distribution of high (<10 m) and moderate (10-100 m) spatial resolution multispectral Earth observation satellites/sensors launched in the previous few decades. The mass of a satellite and the decade of its launch are indicated by the area and the color of the circle, respectively. As can be seen, satellite sensors with three or four spectral bands show a trend toward higher ground resolutions and smaller satellite sizes. Another evident trend is the increase in the number of spectral bands for a series of large satellites, such as the Landsat and WorldView flagship series. Aside from the multispectral Earth observation satellites, only two satellites, Earth Observing-1 (EO-1) and Project for On-Board Autonomy-1 (PROBA-1), are experimentally equipped with high spatial resolution hyperspectral sensors, although additional operational hyperspectral missions are planned for launch before 2020 [4]. Hyperspectral sensors, which are also known as imaging spectrometers, are capable of obtaining images in hundreds of spectral bands, and they can provide abundant information in relation to a wide variety of Earth observation and remote sensing applications. Although the challenges in adapting high spatial resolution hyperspectral sensors to small satellites have been studied theoretically in terms of the resolution, quality, and volume of hyperspectral data, the practical feasibility appears low for satellites <100 kg, even when applying state-of-the-art technology [5; 6]. The hyperspectral sensor considered in the above studies was a push-broom/line-scanning spectrometer, which had been conventionally mounted on large satellites or on manned aircraft. This type of hyperspectral sensor has a two-dimensional (2D) detector array, where one axis records spatial information and the other axis records spectral information. The spatial axis is arranged perpendicular to the satellite track; thus, by scanning the Earth's surface along the path of movement of the satellite, a three-dimensional (3D) hyperspectral data cube is obtained. Consequently, both the resolution and the quality of the acquired data are constrained considerably by the ground track velocity of the satellite and the aperture size of the sensor. These constraints make it difficult to realize effective deployment of high spatial resolution hyperspectral sensors on nano/microsatellites.