Time Delay of Arrival Based Orbit Determination of Geosynchronous Signals of Opportunity

Remote sensing is crucial for our understanding of the Earth’s climate, water cycle, land, and atmosphere. Signals of Opportunity (SoOp) has recently emerged as an innovative method of microwave remote sensing that reutilizes existing, non-cooperative, satellite communication signals as sources of illumination in bistatic radar. Knowledge of the transmitter position is required both for georeferencing the specular point and for accurately estimation the path delay for altimetric observables. This paper describes an experiment using a network of receivers distributed over a continental-scale area to perform time delay of arrival (TDOA) measurements to solve for the position of a geosynchronous satellite transmitting in S-band (2.3 GHz) using a communication signal with a 2 MHz bandwidth. Synchronization was provided by commercial off the shelf (COTS) GNSS timing receivers and the network required only conventional virtual private network (VPN) connections over the internet for communications. A local experiment (in which the path delay errors between receivers can be assumed to cancel) was used to determine the observation error of 30 m. Kinematic solutions for the geosynchronous source were then produced from 6 receivers distributed across the United States in April, 2021. Comparison with two line elements (TLE’s) were within the error bounds predicted by a dilution of precision (DOP) analysis. Fundamental feasibility of this approach was demonstrated with an expected improvement in accuracy through use of wideband signals and statistical orbit determination methods.