[Paper Review] Starling Formation-Flying Optical Experiment: Initial Operations and Flight Results

Justin Kruger, Soon S. Hwang+1 more|arXiv (Cornell University)|Jun 10, 2024

Spacecraft Dynamics and Control4 citations

TL;DR

This paper presents the first in-orbit demonstration of autonomous, multi-spacecraft angles-only navigation using the Starling Formation-Flying Optical Experiment (StarFOX). By leveraging onboard cameras and an inter-satellite link, the Absolute and Relative Trajectory Measurement System (ARTMS) enables maneuver-free, distributed navigation with relative positioning uncertainties as low as 0.6% of target range, achieving autonomous initialization and simultaneous absolute and relative orbit determination for a swarm of four CubeSats.

ABSTRACT

This paper presents initial flight results for distributed optical angles-only navigation of a swarm of small spacecraft, conducted during the Starling Formation-Flying Optical Experiment (StarFOX). StarFOX is a core payload of the NASA Starling mission, which consists of four CubeSats launched in 2023. Prior angles-only flight demonstrations have only featured one observer and target and have relied upon a-priori target orbit knowledge for initialization, translational maneuvers to resolve target range, and external absolute orbit updates to maintain convergence. StarFOX overcomes these limitations by applying the angles-only Absolute and Relative Trajectory Measurement System (ARTMS), which integrates three novel algorithms. Image Processing detects and tracks multiple targets in images from each satellite's on-board camera. Batch Orbit Determination computes initial swarm orbit estimates from bearing angle batches. Sequential Orbit Determination leverages an unscented Kalman filter to refine swarm state estimates over time. Multi-observer measurements shared over an intersatellite link are seamlessly fused to enable absolute and relative orbit determination. StarFOX flight data presents the first demonstrations of autonomous angles-only navigation for a satellite swarm, including multi-target and multi-observer relative navigation; autonomous initialization of navigation for unknown targets; and simultaneous absolute and relative orbit determination. Relative positioning uncertainties of 1.3% of target range (1$σ$) are achieved for a single observer under challenging measurement conditions, reduced to 0.6% (1$σ$) with multiple observers. Results demonstrate promising performance with regards to ongoing StarFOX campaigns and the application of angles-only navigation to future distributed missions.

Motivation & Objective

• To enable autonomous, distributed navigation for small satellite swarms without reliance on external orbit knowledge or translational maneuvers.
• To overcome the weak observability of target range in angles-only navigation through multi-observer, multi-target measurement fusion.
• To demonstrate autonomous initialization of relative navigation for unknown, non-cooperative targets using only bearing angle measurements.
• To achieve simultaneous absolute and relative orbit determination using only optical bearing data and intersatellite communication.
• To validate the performance of a scalable, low-cost navigation system for future deep space and Earth-orbiting distributed space missions.

Proposed method

• The ARTMS framework integrates three modules: Image Processing (IMP), Batch Orbit Determination (BOD), and Sequential Orbit Determination (SOD).
• IMP uses time-tagged images and kinematic modeling to detect and track multiple targets, computing bearing angles via multi-hypothesis methods.
• BOD estimates initial swarm orbits using iterative batch least squares and sampling of weakly observable target ranges from bearing angle batches.
• SOD employs an adaptive unscented Kalman filter to refine state estimates by fusing multi-observer measurements over an intersatellite crosslink.
• Inter-satellite link data sharing enables seamless fusion of measurements across observers, improving robustness and convergence.
• The system operates autonomously on-board, requiring no ground intervention for orbit initialization or navigation updates.

Experimental results

Research questions

• RQ1 Can autonomous, maneuver-free angles-only navigation be achieved for a satellite swarm using only onboard optical sensors and intersatellite communication?
• RQ2 Can the system initialize relative navigation for an unknown target without prior orbit knowledge or external updates?
• RQ3 To what extent can multi-observer measurements improve relative positioning accuracy in angles-only navigation?
• RQ4 How accurately can absolute orbit states be estimated using only inter-satellite bearing angles and no GNSS or ground contact?
• RQ5 What are the performance limitations of angles-only navigation under real in-flight conditions such as poor visibility and timing errors?

Key findings

• StarFOX achieved the first in-orbit demonstration of autonomous, maneuver-free angles-only navigation for a satellite swarm using only onboard optical sensors and intersatellite links.
• Relative positioning uncertainties were reduced to 1.3% of target range (1σ) with a single observer and further improved to 0.6% (1σ) with multiple observers.
• The system successfully performed autonomous initialization of relative navigation for unknown targets, demonstrating in-flight state estimation without prior orbit knowledge.
• Absolute orbit errors were refined from several kilometers to less than 1 km using only bearing angle measurements and no external updates.
• Despite challenges from poor target visibility and time synchronization, the majority of pre-flight objectives were met during the initial experiment period.
• The results validate ARTMS as a scalable, low-cost solution for autonomous navigation in future distributed space missions.