# HPT: A High Spatial Resolution Multispectral Sensor for Microsatellite Remote Sensing
## 2 Sensor Characteristics  
### RISING-2 Microsatellite  
The RISING-2 microsatellite is a 50-kg platform developed jointly by the Tohoku and Hokkaido universities in Japan [8]. It constitutes a substantial improvement over the SPRITE-SAT, which was launched in 2009 and subsequently called the RISING satellite after its launch [9]. Tohoku University developed a satellite bus system for the RISING-2 platform, including a main onboard computer called the Satellite Central Unit (SCU). Hokkaido University developed the mission payload instrumentation for Earth observation, including a controller unit called the Science Handling Unit (SHU; manufactured by AD Co., Ltd., Sagamihara, Japan).  
The RISING-2 microsatellite was designed with the form of a 50-cm cube and a weight of about 43 kg, such that it could be adopted as a secondary small payload on an H-IIA launch vehicle. The satellite was launched successfully by the H-IIA from Tangeashima Space Center in Japan on 24 May 2014, along with the primary payload (the Advanced Land Observing Satellite-2 satellite) and three other secondary small payloads (nano/microsatellites, each \(<\)50 kg). The RISING-2 microsatellite was placed into a Sun-synchronous orbit at the altitude of 628 km with the local Sun time on the descending node of 12:00. Three years after its launch, the satellite remains active, although its operation has become less frequent.  
The three-axis active attitude control system of the RISING-2 microsatellite provides off-nadir pointing capability in order for the optical mission payload instruments to maintain focus on a specific location on the ground [10]. This capability not only enhances the revisit frequency for the selected site, it also increases the number of spectral bands available for the site by wavelength scanning with the LCTF of the HPT. The location of the site to be observed by the HPT is stored preliminarily in the memory of the SCU, and the HPT observation is executed at a scheduled time by sending the stored commands from the SCU to the SHU.  
The SHU controls both the power supply and the data acquisition by the mission payload instruments. Acquired images are stored temporarily in the 8 MB Static Random Access Memory (SRAM) of the SHU. As the data volume of an image taken by the HPT is about 650 kB, the HPT can capture up to 12 images sequentially. As described later, the HPT has the potential for a hyperspectralsensor that is capable of obtaining images in hundreds of spectral bands, but it is not realized because of the limitation of the number of images stored temporarily. The SRAM allows fast access, but it is a volatile memory unit, and it loses stored data when the power is off. After an observation sequence, the images are combined with ancillary data, and transferred to the 128-MB non-volatile flash memory of the SHU without image data compression. When the RISING-2 microsatellite is visible from the ground station located at Tohoku University, the data in the flash memory are downlinked via S-band telemetry at a data rate of 50 kbps [8], which means that it takes at least 1.7 min to downlink one image. Therefore, because of this limitation of the data rate, fewer than 10 images can be downlinked in one day.