Precision Clock for Moon Bouncing

Abstract

Earth-Moon-Earth communications, also known as moon bouncing, consist on transmitting a signal to the Moon and receiving its echo. Although not used for communications anymore, it still poses an interesting challenge to overcome due to the large distance that separates both bodies. Similarly to that practice is the one of doing radar on the moon's surface. To do so, an high phase coherence between the transmitted and the received signal is needed in order to correlate both to make an image. This is accomplished by using a very stable clock reference. The goal of this thesis is to create a stable and precise time reference, usable for moon bouncing, while being of lower cost compared to alternatives in the market. A GPS Disciplined Oscillator (GPSDO) will be developed to achieve that goal. It consists on a local Oscillator (in this case an OCXO) enslaved to the Pulse Per Second (PPS) signal of a GPS thus taking advantage of both the short term stability of the first and the long term stability of the latter. Its working principle is very similar to that of a Phase Locked Loop (PLL) differing mainly on the loop filter where the GPSDO uses a microcontroller to run an algorithm that adjusts the local oscillator based on readings from its inputs. Firstly, a block diagram based on a common PLL will be established and later improved to meet the required resolution and applications. The components will be selected taking into account the strict time restrictions and the system will be assembled. A PID algorithm will then be developed to control the loop. Finally, the system will be tested and compared with the expected result and other time references. The author's objective in this thesis is to achieve a short-term frequency stability of 0.0218 PPB, which corresponds to the variation of a quarter of the carrier period (of frequency 2,45GHz) during the time it takes for the signal to go both ways between the Earth and the moon.