Search

Planet currently operates a constellation of over a hundred satellites that collect a current image of the Earth each day. These satellites were launched over several years and cover several evolutions in design. This presentation will describe the process of updating the calibration of all the Planet Dove satellites using the archive of data in preparation for a new product release to customers in the summer of 2021. The on-orbit calibration methodology will be described combining simultaneous crossovers and lunar observations as well as results using the updated calibration. Research of per-scene harmonisation with a Sentinel-2 reference will also be shown demonstrating improvements in interoperability across different satellite generations.

Planet Labs currently operates a fleet of 21 high resolution Earth observation satellites known as SkySats. These sub-meter resolution satellites have high intra-day revisit rate capabilities, can image scenes in a range of viewing angles, create 3D scene composites and take videos even in regions traditionally difficult to observe due to low satellite capacity. The radiometric calibration of other satellite models within Planet’s fleet (Dove Classic, Dove R and SuperDove) is currently achieved using simultaneous crossovers with Sentinel-2. Radiometric calibration of SkySat imagery using Sentinel-2 poses unique challenges as it has relatively wide spectral bands and frequently takes images off-nadir. In order to achieve optimal radiometric calibration, SkySats are tasked to image Radiometric Calibration Network (RadCalNet) and Pseudo Invariant Calibration (PIC) sites daily to generate numerous near-simultaneous crossovers of these sites with Sentinel-2. Using a reference satellite to cross-calibrate is a common approach in radiometric calibration, however bidirectional reflectance distribution (BRDF) effects may affect accuracy especially if the viewing angles of the satellites are substantially different. Here we will discuss some of the current challenges and effects of applying this approach to SkySat imagery.

Planet currently operates a constellation of over a hundred satellites that collect a current image of the Earth each day. These satellites were launched over several years and cover several evolutions in design. The latest design are the Superdove satellites. One of the key advancements is a new payload with eight spectral bands, six of which are interoperable with Sentinel-2 and two of which are unique. This presentation will describe the on-orbit calibration methodology developed for this new design, results of the initial calibration and highlight some of the future improvements going forward.

Maintaining the radiometric accuracy of the 140-satellite Planet Dove constellation has required the development of a more automated approach to on-orbit radiometric calibration given the number of satellites and the operational behavior. In nominal operations, Dove Satellites are non-tasking and are nadir pointing. A methodology has been developed that utilizes a hybrid approach to combine lunar calibration and cross calibration to enable on-orbit absolute radiometric calibration of each individual Dove satellite. The cross-calibration approach utilizes instantaneous crossovers in spectrally characterized pseudo-invariant calibration sites with RapidEye, Landsat8, and other Dove satellites. Lunar Calibration utilizes an implementation of the ROLO model [1] and daily moonshots taken by each satellite during both the waxing and waning moon phases. This approach is automated, with new crossovers for each satellite processed and stored daily, and moonshots on a monthly cadence. This has allowed for regular monitoring of the radiometric calibration for each satellite and regular updates to ensure calibration accuracies. Initial validation of this approach using 40 Dove satellites shows an uncertainty of 5% at 1-sigma is achieved across all satellites using instantaneous crossovers with Landsat8 as a validation dataset. This paper describes the absolute radiometric calibration approach, how this method will scale to the larger 140 satellite constellation, and a discussion on the initial results. [1] Hugh H. Kieffer and Thomas C. Stone, "The Spectral Irradiance of the Moon", Astronomy. J. 129, 2887-2901 (2005)