Time Difference Detection of Atomic Clock-Based Reference Signal from a CubeSat in Orbit

Satellite signal delays and errors depend on the amount of ionosphere Total Electron Content (TEC) in the signal path. The measurement of TEC has been mostly done using Global Navigation Satellite System (GNSS) signals. Signals from a satellite orbiting in lower altitude with lower frequencies than GNSS may provide more precise measurement of Total Electron Content (TEC). Space Precision Atomic Clock Timing Utility Mission (SPATIUM) is a program to do three-dimensional ionosphere mapping by a satellite constellation in Low Earth Orbit (LEO) measuring TEC with Ultra High Frequency (UHF) radio signals. The first technology demonstration satellite called SPATIUM-I, a 2U CubeSat, was deployed from the International Space Station in October 2018. The satellite emits a 467-MHz reference signal generated by a chip scale atomic clock. The signal is Spread Spectrum (SS)-modulated with 250 chips in 4 ms. This paper explains the methodologies and their results of detecting the signal time delay on the ground using a Software-Defined Radio (SDR). To improve the precision, a Global Positioning System (GPS) clock was used to put a time stamp on the satellite signal every 1 s. The time difference of two consecutive signals is the key method for deriving the TEC difference between two signal paths.