Satellite Swarm Operations
The focus of my doctoral dissertation is the optimization and maintenance of orbital trajectories for satellite swarms in Earth orbit. Satellite swarms are groups of spacecraft operating in close proximity (within a few dozen kilometers) to achieve a common task or goal, such as in-orbit manufacturing, spacecraft aggregation, and close-quarters robotic inspection of satellites.
To perform multi-variable optimization on the relative motion trajectories of all the spacecraft in the swarm, optimizing for minimal fuel using, the individual mission requirements of each swarm member (range, line-of-sight, etc.), and collision probability, I am using Genetic Algorithms and Sensor Fusion Kalman Filtering to converge to an optimal solution.
The scope of my research is limited to the trajectory generation, maintenance, and recon figuration for an arbitrarily numbered spacecraft swarm, up until the point of final rendezvous and docking within a few meters of a client spacecraft. This research strives to solve the as-yet untackled problem of multi-spacecraft coordination for in-space
manufacturing. It tests Genetic Algorithm and Filtering approaches in simulated trials for in-space manufacturing of an interplanetary spacecraft, with simulated sensor data and noise inserted into the system.
Dodona 3U CubeSat
Dodona is a 3U CubeSat, USC’s third CubeSat project; designed, built, integrated, and tested by a group of graduate and undergraduate students at the Space Engineering Research Center (SERC), under the supervision of Professor David Barnhart. As the Systems Engineering lead for the CubeSat, I oversaw the design, manufacture, and operations of the mission. I performed sizing of the power system to maintain positive payload power for a sun-synchronous dawn-dusk orbit, set requirements for subsystems, and coordinated with subsystem leads to troubleshoot critical hardware and software issues that arose during integration and testing.
Initial De-Tumble Operations
To perform the power simulations, taking into account the dynamic attitude of the spacecraft, I developed a comprehensive set of simulations for full spacecraft operations written in MATLAB, and interfacing with STK and LabView.
Upon reaching orbit and deploying the stowed solar arrays, the first satellite operation will be to de-tumble and dump excess angular momentum to stabilize its rotation and orient the petal solar arrays towards the Sun. Magnetic torque rods are used, which interact with the Earth’s magnetic field to impart a torque on the spacecraft, transferring the angular momentum of the spacecraft to the Earth.
Monte Carlo simulations were performed using LabView, MATLAB, and STK, to determine the energy used to de-tumble for a wide array of initial conditions. 48 attitude orientations, spread evenly over 4π Steradians, were considered, with a Gaussian spread of initial spin rates (up to 0.1 rad/s per axis) considered for a total of 14,400 cases. In all cases, the battery Depth of Discharge (DOD) was safely below 30% (manufacturer specifications).
The following video shows visualization of the LabView de-tumble simulation (done in STK), where the magnetic torque rods are used to de-tumble the satellite from the initial tipoff from the launch vehicle, until a safe threshold has been reached to active the reaction wheels. Note that the points where the satellite appears to jump are due to the fact that there are sensor drifts incorporated into the model, and the drift will only be corrected when the sun sensor gets a lock on the position of the Sun (the simulation uses perceived attitude not the truth values, for accurate modeling)
Far Field Radio Testing
In order to perform far-field radio testing of CubeSat flights components on a low budget, the Mobile Radio Test Unit (MRTU) was developed to transport sensitive flight hardware outside the cleanroom and to the field testing location, without exposing it to the environment. This enabled full link testing of the entire CubeSat communications system without needing to purchase a second transceiver solely for testing purposes. The MRTU consists of a rechargeable battery, a set of DC-DC power converters to provide multiple voltage rails for internal components, an ODroid XU-4 microprocessor to control the radio transceiver using UART communications, and a sealed “clean enclosure” with grommeted connectors that can be removed to facilitate transceiver installation in the cleanroom. It also contains a wide array of internal sensors for easy field diagnostics.
Given that the command station for UHF uplink and downlink is on USC’s main campus, by downtown LA, the furthest line-of-sight vantage point with road access was determined to be Griffith Observatory, overlooking most of the LA basin. At a straight-line distance of about 11.5km (7.15mi), this allowed for full verification of telemetry from the CubeSat’s health and status beacon to verify that all transmitted data can be received by the command and control station, using 6m (20ft) steerable yagii antenna. Since the transmission was in the amateur band (437.6MHz), an amateur radio technician license was used to legally transmit the signal. (callsign: KN6BSR)
The successful reception of all health and status beacons, using a custom receiver built using GNU Radio, validates that the communications system (1200bps AX.25 Stensat Radio Beacon) will close the link from an orbit altitude of 400km.
CONFERS
The Consortium for Execution of Rendezvous and Servicing Operations (CONFERS) is an industry-led initiative with initial seed funding provided by the Defense Advanced Research Projects Agency (DARPA) that aims to leverage best practices from government and industry to research, develop, and publish non-binding, consensus-derived technical and operations standards for OOS and RPO. These standards would provide the foundation for a new commercial repertoire of robust space-based capabilities and a future in-space economy.
From September 2017 – October 2019 the USC Space Engineering Research Center was contracted to perform fundamental research towards the past, present, and future state of the RPO and OOS industries, as well as provide technical details to form the foundation of any future standards or regulations for RPO and OOS. Throughout the course of this research, I analyzed all publicly available data on past RPO and docking missions in Earth orbit and Cis-Lunar space. Using this data, I created a set of unitless metrics to define safety criteria for spacecraft of any shape and size, so as not to restrict the future of spacecraft RPO and OOS to large monolithic spacecraft. The results from these two years of research were presented at the International Astronautical Congress and the International Association for the Advancement of Space Safety conferences.
As part of the first year initiative for the CONFERS project, The technical team at USC surveyed all previous published rendezvous and proximity operations past missions in both public and private sector settings (manned and unmanned), and created a database of flight identified methodologies, with the goal of finding commonalities between the missions to create safe guidelines for future RPO operations. Upon analysis of the database, no obvious common attributes in RPO were uncovered, which prompted the creation of a set of scale-able metrics that would focus on the risk aspect of the rendezvous safety, or more correctly avoidance of characteristics that would lead to collision between two objects. Evaluation of initial characteristics of a planned rendezvous yielded a safety posture that focused on two major areas, contact actions and remote influence/interference. One of these metrics considered the act of remote influence from a Servicing spacecraft onto a Client spacecraft, in the form of a thruster plume impingement. The following (simulated) video highlights the effect that the metric and guidelines seek to avoid.
GNU Radio | Software Defined Radio
Software Defined Radios (SDRs) are a modern and versatile way to access the radio spectrum and learn about how RF signals work and how to manipulate them. Unlike traditional radio equipment, which can cost thousands of dollars and can be quite bulky, software defined radios do away with the masses of hyper-specialized signal processing circuits, and instead take advantage of modern computing to process the signals in software. With an SDR, a laptop computer, and the right software, you can transmit, receive, and even decode RF data.
