How will NASA's software for autonomous navigation in cis-lunar space be tested; what information is processed, what outputs evaluated?

Question

note: it's been a year and a half and Capstone is scheduled to be launched soon, so perhaps there is more information available now?

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NASA news item NASA Funds CubeSat Pathfinder Mission to Unique Lunar Orbit says:

The pathfinder mission represents a rapid lunar flight demonstration and could launch as early as December 2020. CAPSTONE will demonstrate how to enter into and operate in this orbit as well as test a new navigation capability. This information will help reduce logistical uncertainty for Gateway, as NASA and international partners work to ensure astronauts have safe access to the Moon’s surface. It will also provide a platform for science and technology demonstrations.

“This is an exciting opportunity for NASA to aggressively push forward towards the Moon in partnership with several American small businesses as a vanguard to Artemis and sustained human presence beyond low-Earth orbit,” said Jim Reuter, associate administrator for NASA’s Space Technology Mission Directorate. “This mission is highly ambitious in both cost and schedule – and taking that deliberate risk is part of the objective of this mission – alongside the rapid technological advancement in cislunar navigation and the opportunity to verify orbital trajectory assumptions and retire unknowns for future missions.”

The 12-unit CubeSat is about the size of a small microwave oven. Onboard is a communications system capable of determining how far CAPSTONE is from NASA’s Lunar Reconnaissance Orbiter and how fast the distance between the two spacecraft is changing. The inter-spacecraft information will be used to demonstrate software for autonomous navigation, allowing future missions to determine their location without having to rely exclusively on tracking from Earth.

"software for autonomous navigation, allowing future missions to determine their location without having to rely exclusively on tracking from Earth" is interesting to me. According to the answers to Can the James Webb Space Telescope basically manage its own orbit if necessary? the JWST won't know where it is or be able to self-station-keep without regular two-week astrometry and commands from Earth.

Of course by the time JWST is actually launched... :-)

Question: How will this demonstration of NASA's software for autonomous navigation in cis-lunar space work? What kinds of information will be available to use as inputs? And if answerable, what will it provide as outputs? How will it be established that the algorithms work successfully?

Answer 1

CAPS will cut the apron strings from earth-based navigation. It requires no sophisticated hardware, just well-proven S-Band RF ranging. Since it is only software, it adds no mass to the craft. It can be updated easily and may even be retrofitted to existing spacecraft while still in service. It has the potential to be networked for limitless network growth.

Think of it as all the best design features of GPS, the Internet and Starlink.

Background on spacecraft navigation:

During cruise phase, spacecraft navigation is accomplished using Earth Based Radio Frequency Ranging (signal flight time and Doppler). The Deep Space Network, Near Earth Network or similar antennae both transmit and receive the signal. Using orbital mechanics, the state vector is calculated for the spacecraft. This can be transmitted back to the spacecraft or, more often, only instructions for course corrections are transmitted, since the spacecraft doesn’t “care” what its exact position is anyway.

During the spacecraft’s target approach phase, fixes can be obtained from onboard optical images of the target against a known star background. The optical images provide a direct measure of spacecraft position relative to the target. Apollo used sextant angle measurements of lunar surface features. The New Millenium mission Deep Space 1 (DS-1) successfully demonstrated autonomous, onboard cruise navigation, using optical-only measurements

Earth Based Tracking depends on limited and expensive ground-based resources. The huge ground based antennae need to be duplicated around the world to give continuous coverage as the Earth rotates. Antennae need to be re-alligned frequently to acquire multiple spacecraft in sequence. As cis-lunar space becomes more populated with spacecraft, ground-based assets will become an oversubscribed bottleneck.

Thus, there is an incentive for an “autonomous” navigation system for cis-lunar space. In this case, “autonomous” means independence from Earth ground assets. The CAPSTONE mission will

“demonstrate inter-spacecraft ranging between the CAPSTONE spacecraft and the Lunar Reconnaissance Orbiter (LRO) currently in operation at the Moon.”

https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=5016&context=smallsat

and

“Demonstrate spacecraft-to-spacecraft navigation services that allow future spacecraft to determine their location relative to the Moon without relying exclusively on tracking from Earth.”

https://www.nasa.gov/smallspacecraft/capstone

CAPSTONE is an

“enabler of both NASA’s Gateway operations involving multiple spacecraft and eventually the ever-expanding commercial cislunar economy.”

https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=5016&context=smallsat

It will

“demonstrate … the core software for the Cislunar Autonomous Positioning System (CAPS)”

“At its foundation, CAPS … requires only relative radiometric ranging and Doppler measurements. In its most streamlined implementation, CAPS will be a software innovation that can be incorporated on any future spacecraft.”

CAPSTONE will

“establish a crosslink with the Lunar Reconnaissance Orbiter (LRO), testing the “CAPS” part of the mission’s name. They have also used a chip-scale atomic clock on the cubesat to determine its location using timing information from signals uplinked to it from ground stations.

https://spacenews.com/capstone-working-well-more-than-a-year-after-launch

How will this demonstration of NASA's software for autonomous navigation in cis-lunar space work?

The overall mission will include collaboration with the Lunar Reconnaissance Orbiter (LRO) operations team at NASA Goddard Space Flight Center to demonstrate inter-spacecraft ranging between the CAPSTONE spacecraft and LRO.

CAPSTONE will perform S-band radiometric ranging and Doppler with the LRO to fix its position, and validate this fix through ground resources. LRO has its own dedicated Earth ground station. Being a survey satellite, we can assume LRO has optimum real time positioning information.

What kinds of information will be available to use as inputs?

CAPS relies on radio and Doppler ranging information between spacecraft.

“… collection of inter-spacecraft ranging data, and execution of the CAPS navigation software system on-board the CAPSTONE spacecraft.”

https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=5016&context=smallsat

I could find no reference to position information being transmitted between spacecraft in the CAPSTONE system. However, it would be inefficient if the craft with the most reliable position information did NOT transmit its own position a part of the ranging process. Thus, a single ranging contact could provide a sphere of position for the receiving craft.

And … what will it provide as outputs? and How will it be established that the algorithms work successfully?

CAPSTONE will perform numerous crosslink communication passes with the Lunar Reconnaissance Orbiter (LRO) using its S-band telecommunication system. The tracking passes will occur when CAPSTONE is nearer to periapse, as LRO is in a polar, low lunar orbit. The availability of these passes will depend on a number of factors, including each spacecraft's power, their relative distances, lunar occultations, pointing constraints, and LRO ongoing science operational priorities

“These tracking passes will provide two-way, coherent range and Doppler measurements to the CAPS flight software onboard CAPSTONE. The flight software will demonstrate CAPS in flight, while also downlinking the CAPSTONE-LRO crosslink data to the ground for further refinement, validation, and further development…Eventually, during the enhanced mission phase, the goal is for the CAPS to provide full autonomous navigation onboard the CAPSTONE spacecraft thus demonstrating this capability for future cis-lunar mission applications”

https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=5016&context=smallsat

CAPSTONE will

“use a chip-scale atomic clock on the cubesat to determine its location using timing information from signals uplinked to it from ground stations.

https://spacenews.com/capstone-working-well-more-than-a-year-after-launch