A global analysis of the temporal availability of PlanetScope high spatial resolution multi-spectral imagery

The PlanetScope CubeSat constellation is providing unprecedented global coverage, visible to near infrared, atmospherically corrected, 3 m imagery. The revisit interval between successive overpasses varies in space and time in a complex manner because of a variety of factors and particularly because of the different sensor orbits. The temporal availability of PlanetScope imagery is quantified in this study considering all of the publicly available images acquired globally for a 12 month period from December 1st 2019 to November 30th 2020. A total of 175.8 million images were acquired by the constellation that was composed of between 100 and 133 unique PlanetScope sensors each month and three sensor generations. The local morning overpass times of the three sensor generations were quantified and the most frequently occurring times were 10:16, 10:29, and 10:03 (to the nearest minute) with 90% of the images acquired with a range of morning overpass times of 2 h and 13 min, 1 h and 30 min, and 1 h and 50 min, for PlanetScope-0, PlanetScope-1, and PlanetScope-2, respectively. Maps, histograms and summary statistics of the total number of observations and revisit intervals are derived with respect to a global grid of 4.7 million land points spaced 5.6 km apart in the equal area sinusoidal projection. The annual and monthly number of PlanetScope observations and average revisit intervals did not vary in a geographically uniform manner. This is due to several factors including the different PlanetScope orbits, seasonal high latitude darkness at the time of sensor overpass, and because of the changing number of sensors on orbit as PlanetScope sensors were decommissioned and later generations became operational over the 12 month study period. In addition, the images in each frame of sensed data are not made available if they cannot be geolocated due to cloud and/or featureless or unstructured terrain precluding ground control matching. The PlanetScope constellation provided higher temporal resolution than provided by sensors such as Landsat-8 or Sentinel-2 although 9% of the global land grid locations, predominantly in the interior of Greenland and non coastal Antarctica, had no observations. Considering the 12 months of global observations, the median average revisit interval was only 30.3 h, and 9.6%, 71.8%, and 88.4% of the land points had average revisit intervals <24 h, <36 h, and < 48 h, respectively. Globally, the median minimum revisit interval was 25 s and the median maximum revisit interval was 9.15 days; 95.4% of the land grid points had a minimum revisit <180 s, and 89.1% had a maximum revisit <480 h (20 days). The PlanetScope images are labelled as "standard" or as "test" quality based on solar geometry, saturated pixel, and geolocation accuracy criteria. The median annual proportion of observations labelled as "standard" at each land grid point over the 12 months was 78.14%. A global cloud analysis was undertaken to quantify the probability of there being at least one and at least two cloud-free PlanetScope observations within 5, 7 and 10 day consecutive periods. Lower probabilities occurred in cloudy regions and where there were fewer observations. The global mean average probability of there being at least one cloud-free observation over the 12 study months was 0.84, 0.88 and 0.92 for the 5, 7 and 10 day periods respectively. The global mean average probability of there being at least two cloud-free observations was 0.65, 0.76 and 0.84 for the 5, 7 and 10 day periods respectively. The probabilities varied seasonally and the northern hemisphere winter (December-February) and spring (March-May) had lower and higher global mean average seasonal probabilities, respectively, than those derived over the 12 months. The high temporal global coverage provided by the PlanetScope constellation will benefit new applications in particular those concerned with assessment of rapidly changing phenomena and assessment of phenomena that cannot be resolved at moderate and coarse resolution.