@misc{indiciaeabf93dac7f69,
  title = {Differential optical proximity detector},
  author = {Johnston, A. R. and Shimada, K. and Tippins, H. H., Jr.},
  year = {1977},
  url = {https://ntrs.nasa.gov/citations/19770000274},
  note = {Source identifier: 19770000274}
}