Ball spin rate measurement

A method of determining a spin rate of a projectile in flight, the method comprising:

• transmitting a microwave signal toward a projectile in flight;
• receiving a reflected signal from the projectile in flight, the reflected signal being a reflected portion of the microwave signal incident on the projectile in flight;
• mixing the received signal with a comparison signal based on the transmitted microwave signal to yield a difference signal;
• demodulating the difference signal to generate a demodulated signal comprising at least one periodic component indicating phase variations between the received signal and the comparison signal;
• detecting a first periodic component of the demodulated signal, the first periodic component comprising a repeating bipolar pulse having a first portion during which an apparent speed of the projectile is greater than a preceding speed of the projectile, the bipolar pulse having a second portion during which the apparent speed of the projectile is less than the preceding speed of the projectile;
• determining a period of the first periodic component;
• and
• determining the spin rate of the projectile in flight based on the period of the first periodic component.

A system for determining a spin rate of a projectile in flight, the system comprising:

• at least one hardware processor;
• and
• a memory including instructions that, when executed by the at least one hardware processor, cause the system to perform operations comprising:
• receiving a phase-demodulated difference signal of a projectile in flight from a Doppler radar system;
• detecting a first periodic component of the phase-demodulated signal, the first periodic component comprising a plurality of bipolar pulses, each of the plurality of bipolar pulses having a first portion during which an apparent speed of the projectile is greater than a nominal speed of the projectile between consecutive ones of the bipolar pulses, and each of the plurality of bipolar pulses having a second portion during which the apparent speed of the projectile is less that the nominal speed of the projectile between consecutive ones of the bipolar pulses;
• determining a period of the first periodic component;
• and
• determining the spin rate of the projectile in flight based on the period of the first periodic component.