US Air Force Space Command, effectively world lead for Space Situational Awareness responsibilities, has been recommended by the US National Research Council to tackle a series of current and pending problems that face the international community in relation to space debris and collision avoidance. These problems can be summed up as the need for
- high fidelity physics-based modeling of space objects / space environment interactions
- advances in space surveillance
- improved abilities for predicting and avoiding collisions
- development of algorithms for autonomous satellite GNC and rendezvous, and for distributed control across formations
Australia's contribution to the SSA effort is primarily based on ground-based sensors for space surveillance - joint hosting of USAF C-band radar and the DARPA Space Surveillance Telescope at Exmouth in WA, with S&T support from DSTO; laser tracking by EOS on Mt Stromlo ; classical orbit determination algorithm development by RMIT in support of the EOS work; and the complementary tracking afforded by UNSW Canberra's new Falcon telescope, part of the growing global Falcon Telescope Network.
A significant worldwide gap exists with respect to the high-fidelity modeling, which in turn can feed into advanced orbit prediction and collision avoidance. Autonomous satellite formation GNC is a further gap. UNSW Canberra is well placed to contribute to filling these gaps, and thus provide significant innovative Australian SSA contribution, complementary to the ground-based surveillance.
Following world best practice for aerospace research , UNSW Canberra is developing an SSA research program that closely couples physics-based supercomputer simulations of the interaction between spacecraft/debris and the near-Earth environment (spacecraft aerodynamics, or astrodynamics), benchmark-quality ground-based experiments (by means of coupling satellite thrusters with thermal vacuum chambers to create a rarefied gas satellite "wind tunnel"), and orbital flight experiments.
The simulations will guide the development of the ground-based and orbital experiments, and will reconstruct those experiments to develop full insight; the ground-based experiments and associated diagnostics will enable real data on the details of physics to be obtained, providing partial validation of the simulations; the full performance of the simulations in being able to predict space object motion will be validated by the flight experiments.
The SSA research program consists of:
Simulations (lead - Dr Melrose Brown):
- development of high-fidelity physics-based simulation capability – UNSW Canberra is collaborating with University of Strathclyde, enhancing their Direct Simulation Monte Carlo (DSMC) supercomputer code dsmcStrathFOAM, by adding charged particle capability to that code to enable ionosphere aerodynamics to be modeled. This represents a new contribution. An aerodynamics database for classes of space objects and altitudes will be developed.
- physics-based atmosphere modeling capability is being acquired, through collaboration with Los Alamos National Laboratory leveraging LANL's IMPACT program.
- UNSW Canberra's surrogate modeling approach, developed by the Multidisciplinary Design Optimisation group, will be used to build surrogate models of both the aerodynamics and atmosphere databases. This is also a new contribution.
Ground-based experiments (lead - Dr Philippe Lorrain):
- Development of a satellite "wind tunnel" is being explored at UNSW Canberra, utilising the rarefied gas plume from satellite thrusters mounted in thermal vacuum chambers.
- Diagnostic development will include both aerodynamic force measurements (probably laser based) and non-intrusive flowfield measurements (extending UNSW Canberra's high speed flow diagnostics expertise to electron beam density measurement capability).
Orbital flight experiments (lead – Prof Russell Boyce):
- Aerospace Corporation demonstrated with a formation of three cubesats in 2012 that significant trajectory modification can be achieved through differential aerodynamic drag between each cubesat (through deployment or retraction of solar panels), to the extent that formation reversal can be achieved over several weeks.
- UNSW Canberra and UNSW Kensington have jointly proposed a higher-fidelity version of that experiment during the AFRL-led, DSTO/UNSW-involved three-cubesat mission known as Biarri, to be launched in 2015. UNSW Canberra has proposed that the cubesats of the Biarri formation be held at specific different attitudes for a 2-day period, and their relative positions accurately determined via UNSW Kensington's on-board differential GPS receiver technology. The relative trajectories will be reconstructed with UNSW Canberra's simulation capability. Ground assessment of the photometry will monitor satellite dynamics using FTN.
- UNSW Canberra will develop and fly a cubesat formation that will provide benchmark-quality validation of our simulation capability. A formation of four fundamentally different aerodynamic shapes will be flown, deployed from cubesats – for example, inflation of a sphere, unfurling of a square sail. Their relative motion, measured accurately by on-board differential GPS and complemented by ground-based surveillance including the Falcon Telescope Network, will be used to validate our simulation capability. This mission is an innovative contribution to SSA.
- The Falcon telescope optical surveillance will produce light curves (optical signatures) for prescribed motions of the satellites, enabling a rigorous approach to the science of ground based characterisation of space object motion.
- In preparation for this formation flight, earlier missions will be flown, enabling experience and expertise with space hardware and operation to be acquired and tuned prior to the main mission, and enabling comparison of cubesat hardware from two different vendors; the first and second flight, performed in partnership with DSTO, also demonstrating the ability to deploy / unfurl shapes from cubesats in orbit; and the third flight providing the flight demonstration of UNSW Canberra's space-based diode laser instrumentation.