The athermalized panchromatic imaging system (APIS) was the low-resolution refractive camera proposed by the Laboratorio de Instrumentación Espacial as a CubeSat payload. APIS flew on-board OPTOS CubeSat designed and developed by INTA using the methodology of European Cooperation for Space Standardization and space qualification tests. APIS had two main objectives: to analyze the performance degradation of commercial off-the-shelf (COTS) components due to space radiation and to verify in-flight functionality of the passive athermalization system. We summarize the design, manufacturing, and assembly integration and verification phases of the instrument, as well as the analysis of the radiation tests. Additional studies are included, such as thermal behavior, tolerances and sensitivity analysis, signal-to-noise ratio, and ghost images, as well as their implications during the design process. Three main goals were achieved during the mission lifetime: (1) the viability of a small refractive Earth observation camera on-board a CubeSat, (2) the validation for low Earth orbits of a passive athermalization system, and (3) the use of COTS elements, such as commercial glasses and detectors based on complementary metal–oxide–semiconductor technology, on a 2-year Earth observation mission.
The disposal of couples of images of the same landscape acquired with two spatial resolutions gives the opportunity
to assess the in-flight Modulation Transfer Function (MTF) of the lower resolution sensor in the common spectral
bands. For each couple, the higher resolution image stands for the landscape so that the ratio of the spectra obtained
by FFT of the two images, gives the lower resolution sensor MTF. This paper begins with a brief recall of the method
including the aliasing correction. The next step presents the data to be processed, provided by the Instituto Nacional
de Tecnica Aeroespacial (INTA). The model of the AHS MTF is described. The presentation of the corresponding
AHS results naturally follows. Last part of the paper consists in a comparison with other measurements:
measurements obtained with the edge method and laboratory measurements.
The Remote Sensing Laboratory at INTA owns and operates an 80-band airborne hyperspectral line-scanner radiometer, alias AHS. This instrument is based on previous airborne hyperspectral scanners as MIVIS and MAS. This instrument has been installed in the INTA´s aircraft (CASA C-212), and integrated with a GPS/INS.
The acquired imagery is processed and archived by INTA. For this purpose, a processing chain has been implemented at the INTA premises in Madrid. In this chain, raw data (level 0 product) is transformed to at-sensor radiance (level 1b) and later to geolocated at-sensor radiance (level 1c). Other processing levels, as atmospherically corrected reflectance, brightness temperature or surface emissivity could also be produced. The radiometric calibration is based on laboratory measurements using an integrating sphere for the reflective bands, and on in-flight blackbodies measurements for the thermal bands. The geolocation procedure is based on processing of GPS/INS data synchronized with the imagery collection. Finally, direct parametric georeferencing is achieved by means of the commercial software PARGE.
The resulting system is available to the international remote sensing community through specific agreements (contractual or based on joint collaborations).
As an example of the use of this system, an on-going project to evaluate water stress in olives in southern Spain is presented. In this project, high resolution thermal radiometry is used to evaluate tree temperature, while reflective bands are used for identifying individual trees and for simultaneous monitoring of plant health.