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
“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.”
CAPSTONE is an
“enabler of both NASA’s Gateway operations involving multiple
spacecraft and eventually the ever-expanding commercial cislunar
“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
“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.
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.”
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
“use a chip-scale atomic clock on the cubesat to determine its
location using timing information from signals uplinked to it from