Matched template signal processing of a continuous wave laser for space debris ranging

Abstract

The ever-increasing congestion in Earth’s orbital space is a worrying environmental condition. The global community is reliant on various vital services provided by satellites orbiting the Earth. However, sharing these same orbits are pieces of space debris, including human-made objects, taking up valuable orbital space. As the congestion in Earth’s orbit increases, there is a higher probability of a collision occurring between the various orbiting objects. Each collision generates more space debris and contributes to a potential cascade chain reaction where eventually Earth’s orbit will be filled with space debris and satellites will no longer be safely launched into orbit. There are several proposed space debris removal methods in the literature, investigated with the aim of reducing the debris population. One promising method is using photon pressure from a ground-based continuous wave laser to gradually alter the orbit of a small piece of space debris. This thesis investigates the development of a ranging method using the same continuous wave laser source used to manoeuvre the space debris. This approach would allow the continuous wave laser method to simultaneously manoeuvre and measure the change in position of the piece of space debris. The experiments presented in this thesis amplitude modulated a continuous wave laser with a pseudo-random noise (PRN) code and used a matched filter analysis approach to measure the position and change in position of a target. The motion of the space debris causes a Doppler shift of the amplitude modulated code. The matched filter analysis generates a template PRN code where the properties of the template code can be altered to model the Doppler effect caused by a moving target space debris. The correlation between the template and amplitude modulated code indicates how similar the two codes are to each other. The parameter values of the altered template code with the highest correlation is an estimate of the time-varying delay of the reflected optical signal due to the space debris’ motion. The thesis investigated two detection schemes for potential space debris application. The first detection scheme is called the direct detection scheme where the optical intensity of the signal beam is directly measured. The experimental results showed that by taking advantage of the PRN code properties to integrate the signal for a more extended period, both signal detection and precision in estimating the time-varying delay was improved for low signal-to-noise ratio optical signals. The second detection method investigated in this thesis is called the coherent detection scheme. In this detection scheme, the optical signal, with the amplitude modulated PRN code, is interfered with a second brighter optical beam called a local oscillator. The interference aims to amplify the modulated signal and improve the signal-to-noise ratio of the optical signal for the matched filter analysis. The proposed laser ranging and analysis methods models using a 1.8 m telescope aperture with a 10 kW transmitting laser power. The aim of the analysis is to estimate the timevarying delay of a piece of space debris between 1 cm and 10 cm in size to within 1 mm/s. The estimated time-varying delay can be used for orbit prediction to determine potential collisions and the effect of the manoeuvring effort. Assuming the received signal power from a piece of space debris is equal to 3.5 fW, to achieve 1 mm/s precision the integration time needed for the direct detection scheme is 83 hours while for the coherent detection scheme is 250 s. The coherent detection scheme would be able to achieve the desired measurement precision within a flyover of the target debris. In addition, current space debris pulsed-laser ranging methods may not be able to operate during daylight hours due to the low signal-to-noise ratio of the reflected optical signal. For the coherent detection scheme, the optical signal can be amplified above the additive noise sources and potentially allow daylight operations. The bench-top experimental results in this thesis clearly showed the coherent detection scheme is superior to the direct detection scheme and potentially could overcome some of the challenges faced by current laser ranging methods for space debris ranging and maneuvering application.