The problem with this, very simple Doppler Detection system, is that a good crystal oscillator will drift by one part per million in a few minutes under the best of conditions and, given the air temperature drop with altitude, the drift rate for the transmitter in a small rocket will be much faster than this. There is usually an initial delay of about 4 seconds before the adiabatically cooled, expanding air in the rocket begins to cool the crystal, and this is enough for the acceleration and “Fast Burn” velocity verification. Long before apogee is reached, substantial frequency changes mask the Doppler effect, although the various effects can be partially resolved by linear extrapolations in captured data.

It is feasible to make both temperature compensated Crystal oscillators, and temperature stabilized units. But keep in mind that anything you fly in a small rocket has a good chance of being severely damaged, and find a way to keep the circuit costs down. Really good crystal oscillators approach 1/100 part per million drift over many minutes (or even years, with very good temperature control), but stretch the budget as “Disposable” components.

The alternative is to give up on stabilized oscillators, and control this unit “remotely”. By transmitting a reference signal UP to the rocket, and making the transmitter “Phase Lock” to this reference, the frequency error goes to zero (first order approximation). The reference signal is also seen to have a Doppler Shift at the rocket receiver, and this becomes added to the Doppler Shift of the returned signal (just as it would if directly reflected as in a RADAR system), doubling the effect.

The Phase Lock process involves an electronic comparison between the weak received signal and the strong transmitter signal, with provision to adjust the transmitter so that these oscillations are synchronized. Since it is important to keep the transmitted signal out of the receiver, the synchronization is often implemented with a mathematical ratio between the frequencies. The transmitted signal could, for example, be at twice the frequency of the received reference. They would still be synchronized, but would use separate antennas and filters to keep the transmitted signal out of the receiver.

Things become more complicated – as will be discussed next – if one wants to combine Radio Control instructions with the reference signal and telemetry data with the signal transmitted from the rocket, but this is a very desirable achievement!