Earth+: on-board satellite imagery compression leveraging historical earth observations

TL;DR

Earth+ tackles downlink bottlenecks in large earth-observation constellations by enabling constellation-wide sharing of fresh, cloud-free reference images for change-based compression. It moves beyond satellite-local references to a constellation-wide repository of references uploaded via uplinks, enabling significantly fresher references and more accurate change detection. The method yields up to about 3.3× downlink bandwidth savings without increasing uplink bandwidth, compute, or storage, and scales with the number of satellites; however, it remains lossy and introduces ground-station coordination and deployment considerations. Overall, Earth+ demonstrates a practical pathway to dramatically increase data freshness and ground coverage under existing downlink constraints, while acknowledging limitations for lossless requirements.

Abstract

With the increasing deployment of earth observation satellite constellations, the downlink (satellite-to-ground) capacity often limits the freshness, quality, and coverage of the imagery data available to applications on the ground. To overcome the downlink limitation, we present Earth+, a new satellite imagery compression system that, instead of compressing each image individually, pinpoints and downloads only recent imagery changes with respect to the history reference images. To minimize the amount of changes, it is critical to make reference images as fresh as possible. Earth+ enables each satellite to choose fresh reference images from not only its own history images but also past images of other satellites from an entire satellite constellation. To share reference images across satellites, Earth+ utilizes the limited capacity of the existing uplink (ground-to-satellite) by judiciously selecting and compressing reference images while still allowing accurate change detection. In short, Earth+ is the first to make reference-based compression efficient, by enabling constellation-wide sharing of fresh reference images across satellites. Our evaluation shows that Earth+ can reduce the downlink usage by a factor of 3.3 compared to state-of-the-art on-board image compression techniques while not sacrificing image quality, or using more on-board computing or storage resources, or more uplink bandwidth than currently available.

Figures (19)

• Figure 1: Contrasting Earth+ with traditional satellite imagery compression. (a) The traditional approach compresses images by satellites using their local onboard information. (b) Earth+'s reference-based encoding uses reference images from any satellites in the constellation and uploads the reference images to the satellite to pinpoint the changed areas and only downloads their content.
• Figure 2: Illustration of a LEO satellite constellation: the satellites follow a sun-synchronous orbit, and the ground station downloads imagery data from a satellite when the satellite passes through it.
• Figure 3: An example illustrating why reference images need to be fresh. Comparing a captured image from Day 30 with a fresh reference from Day 27 reveals much fewer changes than when comparing it with an older reference from Day 1. Image © 2023 Planet Labs PBC.
• Figure 4: More changes need to be downloaded when the age of the reference image gets larger.
• Figure 5: Measuring the age of reference images under two strategies: updating the reference using historical images locally captured by the satellite ("Satellite-local") and updating the reference using images from any satellites in the constellation ("Constellation-wide"). It shows that the constellation-wide approach can reduce the average age of the reference image from 51 days to 4.2 days, a 12$\times$ reduction.