Parameters that define the orbit of a satellite✓Correct answer
Phase reversing elements in a Yagi antenna
High-emission heater filaments used in magnetron tubes
Encrypting codes used for spread spectrum modulation
Explanation
Keplerian elements are the small set of numbers that fix a satellite's orbit in space and time, so tracking software can predict where the spacecraft will be. They have nothing to do with antenna elements, tube filaments, or spread spectrum keys.