Optimal Attitude Guidance for the EXACT and IMPRESS Cubesats using Graph Methods with Pruning

This work demonstrates a high-level mission planning method for maximizing data output from a pair of scientific CubeSat missions. The proposed approach identifies the optimal sequence of attitude maneuvers to perform in order to maximize total downlinked data over the mission, while considering constraints on available power. Many scientific satellite missions consist of at least three target attitudes: pointing solar panels towards the Sun for power, pointing an antenna towards a ground station to transmit data, or pointing a payload towards a point of scientific interest. While careful mechanical design of the mission may enable all three (or more) target attitudes to be achieved simultaneously in certain cases, in general a decision must be made about which target to point to at what time in order to optimally achieve mission objectives and satisfy mission constraints. In this work, we develop a mission planning method that maximizes the volume of data downlinked to the ground over the mission time horizon while respecting constraints on battery level. The optimization problem is posed as an integer program over the space of attitude trajectories and subject to battery constraints. The solution of this problem is an attitude sequence that can be used as a reference for a low-level attitude controller to track. Previous work on this problem suffered from slow solution time for complex mission scenarios which constrained the realism of simulations performed for validation, so in this work we build on our prior approach by leveraging more advanced pruning and search methods to improve optimizer efficiency. We demonstrate the proposed approach on two CubeSats: IMPRESS and EXACT, both currently in design and sharing many mechanical specifications. Both CubeSats are controlled by low-bandwidth actuators and have three main attitude targets: the Sun for power, the Crab Nebula or the Sun the scientific mission, and ground stations for communication. Using simulated orbit data, we show the effectiveness of this method in squeezing mission performance out of both CubeSats while maintaining on-board power. Additionally, the proposed method can run faster than real-time for time horizons of several orbits, enabling a high level of autonomy in orbit

The Undergraduate CubeSat Experience at the University of Minnesota

Building a satellite is a large undertaking with a lot of moving parts. Undergraduate students have complicated schedules with even more moving parts. Running a team of 60+ undergraduates toward the goal of launching a satellite is therefore quite the managerial challenge. Detailed on this poster are some specific challenges, along with strategies for mitigating them, that the UMN Small Satellite Research Lab faces in their work toward launching two small satellites

Evaluation of Low-Cost, Centimeter-Level Accuracy OEM GNSS Receivers

This report discusses the results of a study to quantify the performance of low-cost, centimeter-level accurate Global Navigation Satellite Systems (GNSS) receivers that have appeared on the market in the last few years. Centimeter-level accuracy is achieved using a complex algorithm known as real-time kinematic (RTK) processing. It involves processing correction data from a ground network of GNSS receivers in addition to the signals transmitted by the GNSS satellites. This makes RTK-capable receivers costly (in excess of $10,000) and bulky, making them unsuitable for cost- and size-sensitive transportation applications (e.g., driver assist systems in vehicles). If inexpensive GNSS receivers capable of generating a position solution with centimeter accuracy were widely available, they would push the GNSS revolution in ground transportation even further as an enabler of safety enhancements such as ubiquitous lane-departure warning systems and enhanced stability-control systems. Recently manufacturers have been advertising the availability of low-cost (<$1,000) RTK-capable receivers. The work described in this report provides an independent performance assessment of these receivers relative to high-end (and costly) receivers in realistic settings encountered in transportation applications

Minnesat: GPS Attitude Determination Experiments Onboard a Nanosatellite

This paper presents an overview of the attitude determination experiments onboard the University of Minnesota nanosatellite, Minnesat. Minnesat is designed as a test bed for conducting ultra-short baseline GPS attitude determination experiments in Earth orbit. The primary scientific mission of the Minnesat project is to design, develop, and validate an ultra-short baseline GPS attitude determination (AD) system. Minnesat is equipped with a set of sensors to support two independent AD systems that are referred to as the Primary AD System and the GPS AD System. The Primary AD System blends measurements of inertial sensors with measurements of a three-axis magnetometer to estimate Minnesat’s attitude. The GPS AD System blends measurements of inertial sensors with differential carrier phase GPS measurements to estimate Minnesat’s attitude. The Primary AD System is used as a truth source to validate the GPS AD System

A Design and Feasibility Study for the Detector Assembly of the Experiment for X-ray Characterization and Timing (EXACT ) CubeSat Project

The Experiment for X-ray Characterization and Timing (EXACT) is intended to be a proof of concept for the use of inexpensive small satellite technology to fill the need for long term solar Hard X-Ray monitoring with co-observation as a high energy counterpart to existing solar X-ray monitoring satellites (e.g. GOES), as well as provide precise timing measurements of solar HXR's. EXACT is a three-axis-stabilized, 3U CubeSat that makes use of an existing University of Minnesota Aerospace Department designed Gamma Ray Burst sensor that is intended for studies in deep space navigation using Gamma-ray burst timing for relative ranging of spacecraft. The high timing resolution of the GRB sensor, as well as the heritage of the pulse-read-out circuitry, makes it a good candidate for use on EXACT if it can be shown to suit the energy range and resolution requirements in the HXR regime. The GRB detector design incorporates square scintillators of Thallium-doped Cesium Iodide (CsI(Tl)), each connected to one Avalanche Photodiode (APD) for read-out to a pulse processing board. This paper will detail the design of the current GRB detector, report the results of the laboratory evaluation of that design, and make recommendations for modifications to the current design in order to fulfill the instrumentation requirements of EXACT

Inertially aided vector matching for opportunistic navigation in space

In this work, an estimator is developed for the joint estimation of orientation and position from astrophysical signals of opportunity, particularly pulsars. The filter is based on a combination of vector-matching techniques for estimating attitude and time-difference of arrival navigation for estimating position. The filter functions by computing the probability of association for each arriving photon with each signal source of interest, and using the association probabilities to perform the measurement update. The probability of association of a photon with a signal source is derived, as well as the probability of association with background. The estimation techniques proposed are tested using Monte Carlo analysis techniques. The accuracy of the resulting estimates is compared to other pulsar navigation techniques. The results of the simulation studies indicate that the technique proposed here generally outperforms other time difference of arrival estimation techniques