  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.
