Abstract length < 1500 words. The dictum “small is the new big”, synonym of “when bigger isn't better”, increasingly applies to the traditional Earth observation (EO) industry whose business model is under pressure by an emerging EO...
moreAbstract length < 1500 words. The dictum “small is the new big”, synonym of “when bigger isn't better”, increasingly applies to the traditional Earth observation (EO) industry whose business model is under pressure by an emerging EO small-scale (< 500 Kg in weight) satellite constellation technology, aiming at sub-meter spatial resolution and sub-daily temporal resolution in a “seamless innovation chain” required by Space 4.0, whose ambitious objectives range from the business applications market to grand societal challenges, e.g., the United Nations Development Programme (UNDP) Sustainable Developments Goals (SDGs), such as migration, food security, climate change, terrorism, urbanisation and poverty. In spite of an increasing interest on EO small satellite constellations, according to some EO experts “the small satellite market is a failed business model in the short term at least. It is hype. It is here to hit established players (just like Uber – an excellent player with an innovative business model, to shake up the existing setup), but making huge losses in the process. It is quite obvious that we have already reached a point with the EO industry that the amount of EO data collected is way beyond than the need. So, the next time you get excited about 80 odd satellites launched by someone, just question, when and how will they make money?” To further investigate pros and cons of existing EO small satellite constellations, our analytic and pragmatic analysis starts from first principles proposed by the intergovernmental Group on Earth Observations (GEO), whose visionary goal of a Global Earth Observation System of Systems (GEOSS) implementation plan for years 2005-2015, unaccomplished to date, is systematic transformation of multi-sensor, multi-angular and multi-temporal EO big data cubes into timely, comprehensive and operational EO value-adding information products and services (VAPS). According to the joint GEO-Committee on Earth Observation Satellites (CEOS) Quality Accuracy Framework for EO (QA4EO) Calibration/Validation (Cal/Val) guidelines, necessary-but-not-sufficient pre-conditions for GEOSS “to allow the access to the Right Information, in the Right Format, at the Right Time, to the Right People, to Make the Right Decisions” are the four key principles of Availability/Accessibility and Suitability/Reliability applied to data and information products. Related to the four GEO-CEOS key principles are the foundational principles of Findability, Accessibility, Interoperability, and Reusability (FAIR) applied to data, products and processes to enhance the ability of machines to automatically find and use the data, in addition to supporting its reuse by individuals. Indeed, existing EO small satellite constellations, such as the PlanetScope constellation of 130+ Dove nano-satellites (standardized 3U CubeSat technology, 5 kg in weight, 3 m multi-spectral (MS) spatial resolution, time resolution > 10X Daily), and Planet’s SkySat constellation of 13 micro-satellites (125 kg in weight, 1 m MS spatial resolution, time resolution 2X Daily), guarantee augmented Suitability (in terms of very high spatial and temporal resolutions) and enhanced Availability/Accessibility (expected to be near real-time). On the other hand, the key principle of Reliability/Interoperability requires the remote sensing (RS) community to enforce mandatory GEO-CEOS QA4EO Cal/Val principles within and across small satellite constellations. Radiometric Cal is the transformation of dimensionless digital numbers (DNs) into a radiometric unit of measure to be community-agreed upon at increasing levels of information quality, specifically, top-of-atmosphere (TOA) radiance (TOARD) in range [0, infty), TOA reflectance (TOARF) in range [0, 1], surface reflectance (SURF) in range [0, 1], corrected for atmospheric topographic and/or adjacency effects, and surface albedo in range [0, 1], corrected for bidirectional reflectance distribution function (BRDF) effects. On theory, EO data Cal is a well-known “prerequisite for physical model-based analysis of airborne and satellite sensor measurements in the optical domain”. For example, EO data Cal is considered mandatory by the GEO-CEOS QA4EO Cal/Val guidelines. It guarantees interoperability (harmonization) of EO data acquired across time, space and sensors, in agreement with the FAIR criteria. On a daily basis, all sensory data we deal with are provided with a community-agreed physical unit of measure , e.g., hour, meter, kg, etc. Whereas physical variables can be investigated by physical model-based, statistical model-based or hybrid (combined deductive/ top-down/ physical model-based and inductive/ bottom-up/ statistical model-based) inference systems, uncalibrated sensory data provided with no physical meaning can be investigated by statistical data models exclusively. Although statistical models do not require physical variables as input, they can benefit from data Cal in terms of augmented robustness to changes in the input data set acquired across time, space and sensors. Irrespective of these unquestionable true-facts, the GEO-CEOS QA4EO Cal requirement remains largely neglected in the RS common practice. For example, in the large majority of papers published in the RS literature the word “radiometric calibration” is absent, which means these works cope with uncalibrated EO data provided with no physical unit of radiometric measure. One consequence is that, to date, statistical model-based and inductive learning-from-data EO image understanding (EO-IU) systems dominate the RS literature as well as commercial EO image processing software toolboxes, consisting of overly complicated collections of inductive learning-from-data algorithms to choose from based on heuristics. In EO small-scale (< 500 Kg in weight) satellite constellations, the lack of an on-board radiometric Cal sub-system, to be compensated by vicarious Cal strategies, is expected to have an impact on intra- and inter-constellation Reliability/Interoperability requirements. For example, a Planet Surface Reflectance (SR) Product is systematically generated by Planet from the PlanetScope constellation. Based on the Planet SR product documentation, the PlanetScope radiometric Cal metadata file misses a per-band offset (bias) Cal coefficient. As a consequence, TOARD, TOARF and SURF values estimated in sequence are affected by an unknown bias (additive error term), either positive, null or negative. Consisting of SURF values corrected for atmospheric effects, the PlanetScope SR Product is expected to ensure consistency across localized atmospheric conditions, minimizing uncertainty in spectral response across time and location. No Planet SR product is available for the Planet SkySat micro-satellite constellation. According to the Planet SkySat Imagery Product specification, SkySat DNs are radiometrically calibrated into absolute (at-sensor, TOA) radiance (TOARD) units, derived using vicarious Cal methods. This means that, first, PlanetScope and SkySat radiometrically calibrated products are not comparable in physical terms. Second, the PlanetScope SR Product is not comparable in physical terms and “difficult” to be compared (e.g., fused) in statistical terms with other EO data-derived products radiometrically calibrated into SURF values corrected for atmospheric, topographic and adjacency effects, such as the Sentinel-2 Correction Prototype Processor (Sen2Cor), developed and distributed free-of-cost by the European Space Agency (ESA) to be run on user side. In compliance with the GEO-CEOS QA4EO Cal/Val guidelines, the goal of the present work is to validate by independent means the absolute radiometric quality (in comparison with a “ground truth” or reference baseline) and robustness to changes in input data, acquired across time, space and sensors within and across constellations, of the PlanetScope SR Product, consisting of SURF values corrected for atmospheric effects for augmented Reliability/Interoperability. To validate the radiometric Cal of the PlanetScope SR Product, an off-the-shelf Satellite Image Automatic Mapper™ (SIAM™) lightweight computer program was adopted.