  The Extremely Low-Resource Optical Identifier (ELROI) beacon is a concept for
a milliwatt optical "license plate" that can provide unique ID numbers for
everything that goes into space. Using photon counting to enable extreme
background rejection in real time, the ID number can be uniquely identified
from the ground in a few minutes, even if the ground station detects only a few
photons per second. The ELROI concept has been validated in long-range ground
tests, and orbital prototypes are scheduled for launch in 2018 and beyond. We
discuss the design and signal characteristics of these prototypes, including a
PC-104 form factor unit which was integrated into a CubeSat and is currently
scheduled to launch in May 2018, and basic requirements on ground stations for
observing them. We encourage others to consider observing our test flights.
