Conventional RTLS typically relies on one of three general approaches:
- Direction Finding (DF): originated by John Stone Stone in 1902,
- Amplitude Ranging: originated by Lee de Forest in 1904, and
- Time of Flight (Time Difference of Arrival (TDOA), or Transponder Ranging are typical approaches): orignating in the 1920′s and 1930′s.
As DF came of age after the First World War, inventors sought techniques other than DF and Amplitude Ranging to implement RF-based location systems. Inventors began to develop Time-of-Flight based systems. But they also explored a wide variety of other less conventional approaches. This post will review four novel approaches to RF-based location all invented in the 1920′s and 1930′s:
- Synchronous Ranging: an early attempt at a ToF-based system,
- Differential Attenuation: a comparison between two different frequency signals with differing rates of attenuation to determine range,
- Multipath Interference Ranging: seeking an interference between a direct and reflected ray for maritime collision avoidance, and
- Transponder Ranging: a more sophisticated ToF-based system.