I mentioned navigation in my last post. The contribution of “Coherent Radio Links” to navigation is not well known outside the professional community, but this has long been a focus of Micro-Space, ultralight system efforts. (We have achieved a level of coherent link operation - control uplink and telemetry downlink – in a one inch diameter rocket.)

Directional radio antennas allow tracking (and finding) rockets which fly beyond visual range. Interferometric techniques make greatly enhanced tracking resolution available – and this has been enabled by the coherent multichannel radio receivers Micro-Space has built and used. But among conventional radio tools only RADAR adds the range information needed for 3 dimensional tracking from a single instrument cluster.

High quality Doppler Shift data begins to fill in the missing data, without requiring a RADAR system (and RADAR works poorly with small – largely non conductive rocket bodies), because this effect results entirely from the changing, linear distance between the rocket and the receiver.

A modified receiver can capture and document the Doppler Shift from a stable telemetry transmitter. In fact it is surprisingly easy to do this with a well designed, crystal controlled transmitter! This is particularly surprising, since measurable radio frequency Doppler Shift requires motions at a “significant fraction” of The Speed of Light!

The Speed of Light is about one million times faster than the Speed of Sound. On the other hand, a well designed “crystal oscillator” will have a frequency stable to a small fraction of “one part per million” for a number of seconds.

Typical small rockets, accelerating to 500 feet per second in 2 seconds, will produce a very audible Doppler Shift in a UHF receiver with “Beat Frequency Oscillator” (used for CW – Code – and Single Sideband reception). At 432 MHz, the tone shift will be over 200 Hertz at 500 fps, increasing to 400 Hertz for near sonic velocity, and 600 to 800 Hz for achievable supersonic flight. Since “Sonic Boom” does not exist behind an accelerating rocket, this is a very nice way to verify predicted flight speeds.

But, as I will discuss soon, this is only the beginning of the possibilities!