  The Extremely Low-Resource Optical Identifier (ELROI) is a concept for an
autonomous, low-power optical "license plate" that can be attached to anything
that goes into space. ELROI uses short, omnidirectional flashes of laser light
to encode a unique ID number which can be read by a small ground telescope
using a photon-counting sensor and innovative extreme background-rejection
techniques. ELROI is smaller and lighter than a typical radio beacon, low-power
enough to run on its own small solar cell, and can safely operate for the
entire orbital lifetime of a satellite or debris object. The concept has been
validated in ground tests, and orbital prototypes are scheduled for launch in
2018 and beyond. In this paper we focus on the details of the encoding scheme
and data analysis that allow a milliwatt optical signal to be read from orbit.
We describe the techniques of extreme background-rejection needed to achieve
this, including spectral filtering and temporal filtering using a period- and
phase-recovery algorithm, and discuss the requirements for an error-correcting
code to encode the ID number. Worked examples with both simulated and
experimental (long-range ground test) data illustrate the methods used. We
present these techniques to describe a new optical communication concept, and
to encourage others to consider observing and analyzing our upcoming test
flights.
