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Author Topic:   Using a sextant for deep space navigation
Robert Pearlman
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posted 06-20-2018 12:21 PM     Click Here to See the Profile for Robert Pearlman   Click Here to Email Robert Pearlman     Edit/Delete Message   Reply w/Quote
NASA release
Deep Space Navigation: Tool Tested as Emergency Navigation Device

A tool that has helped guide sailors across oceans for centuries is now being tested aboard the International Space Station as a potential emergency navigation tool for guiding future spacecraft across the cosmos. The Sextant Navigation investigation tests use of a hand-held sextant aboard the space station.

Sextants have a small telescope-like optical sight to take precise angle measurements between pairs of stars from land or sea, enabling navigation without computer assistance. Sextants have been used by sailors for centuries, and NASA's Gemini missions conducted the first sextant sightings from a spacecraft. Designers built a sextant into Apollo vehicles as a navigation backup in the event the crew lost communications from their spacecraft, and Jim Lovell demonstrated on Apollo 8 that sextant navigation could return a space vehicle home. Astronauts conducted additional sextant experiments on Skylab.

"The basic concepts are very similar to how it would be used on Earth," says principal investigator Greg Holt. "But particular challenges on a spacecraft are the logistics; you need to be able to take a stable sighting through a window. We're asking the crew to evaluate some ideas we have on how to accomplish that and to give us feedback and perhaps new ideas for how to get a stable, clean sight. That's something we just can't test on the ground."

The investigation tests specific techniques, focusing on stability, for possibly using a sextant for emergency navigation on space vehicles such as Orion. With the right techniques, crews can use the tool to navigate their way home based on angles between the moon or planets and stars, even if communications and computers become compromised.

"No need to reinvent the wheel when it comes to celestial navigation," Holt says. "We want a robust, mechanical back-up with as few parts and as little need for power as possible to get you back home safely. Now that we plan to go farther into space than ever before, crews need the capability to navigate autonomously in the event of lost communication with the ground."

Early explorers put a lot of effort into refining sextants to be compact and relatively easy to use. The tool's operational simplicity and spaceflight heritage make it a good candidate for further investigation as backup navigation.

YankeeClipper
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posted 07-29-2018 08:59 AM     Click Here to See the Profile for YankeeClipper   Click Here to Email YankeeClipper     Edit/Delete Message   Reply w/Quote
The sextant model shown under evaluation at 0:04-0:23 and 1:09-1:24 in the clip is the Celestaire Astra III Professional Marine Sextant modified with a 7×35 Prism Telescope (0509A).

ISS Research and NASA Marshall tweets on 03 July 2018 show photos of astronauts Alexander Gerst and Serena Auñón-Chancellor using a Celestaire Astra III Professional Sextant from the ISS Cupola.

ISS056-E-049856 taken on 28 June 2018 shows some details of the ISS experimental protocol Gerst was following.

Jim Behling
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posted 07-29-2018 09:51 AM     Click Here to See the Profile for Jim Behling   Click Here to Email Jim Behling     Edit/Delete Message   Reply w/Quote
I really don't understand this. Orion has star trackers and navigation base, so why isn't that used "to navigate autonomously in the event of lost communication with the ground." That is what Lovell did. He didn't use a hand held sextant, he used the sextant "star tracker" that was built into the CM navigation base. The shuttle had star trackers that did the same thing.

A computer is still needed, so if the spacecraft computers are down, how is the spacecraft going to be controlled?

YankeeClipper
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posted 07-29-2018 02:06 PM     Click Here to See the Profile for YankeeClipper   Click Here to Email YankeeClipper     Edit/Delete Message   Reply w/Quote
One of multiple, redundant, old school, mechanical backup/contingency aids providing a crew with every chance of navigating their return? Looks like they are trying to mitigate every conceivable scenario including obscured, damaged, compromised or malfunctioning star tracker optics, or low available power as in Apollo 13.

A handheld sextant is small, light, and requires minimal power — it's a "Hail Mary" play but better than not having it onboard.

Jim Behling
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posted 07-30-2018 12:13 PM     Click Here to See the Profile for Jim Behling   Click Here to Email Jim Behling     Edit/Delete Message   Reply w/Quote
Shuttle and Apollo used COAS as a backup to star trackers. This is only talking about loss of comm.

Still need a computer and working INU system to use the data.

YankeeClipper
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posted 07-30-2018 03:11 PM     Click Here to See the Profile for YankeeClipper   Click Here to Email YankeeClipper     Edit/Delete Message   Reply w/Quote
One problem at a time...

Although time-consuming, could manual positional calculations not be performed as a substitute for computerized ones?

While clearly sub-optimal, would it not be possible to manually adjust the attitude of the spacecraft so it was roughly aligned or oriented in the correct direction i.e. a planetary target kept centred in a certain window?

Jim Behling
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posted 07-30-2018 08:46 PM     Click Here to See the Profile for Jim Behling   Click Here to Email Jim Behling     Edit/Delete Message   Reply w/Quote
No. There are two things in play. Attitude knowledge and state vector.
  1. In cislunar space, attitude can be some what found. Going to Mars, not so easy. There isn't any near reference except the sun.

  2. A state vector, which is position and velocity in three dimensions at a specific time. Multiple star sightings over a time span would be required to derive the velocity components. That also means accurate time would need to be kept onboard.

  3. Pointing at a target doesn't work since they are not flying to where it is at but where it is going to be.

YankeeClipper
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posted 07-30-2018 11:08 PM     Click Here to See the Profile for YankeeClipper   Click Here to Email YankeeClipper     Edit/Delete Message   Reply w/Quote
Again this is obviously sub-optimal, and not conducive to extremely accurate readings, but is the time accuracy requirement beyond the performance capability of a latest generation quartz wrist chronometer e.g. Omega Speedmaster Skywalker X-33? Surely the timing drift error over a short period would not be so significant as to be insurmountable, and would likely be superceded by other errors associated with handheld measurement?

For information, reference, and comparative purposes here are three relevant NASA technical notes on Kollsman Space Sextant studies in the Gemini and Skylab eras:

  • TN D-4585 May 1968 - "A Hand-Held Sextant Qualified For Space Flight" by Bedford Lampkin & Donald Smith.

  • TN D-4952 Dec 1968 - "Sextant Sighting Measurements From On Board The Gemini XII Spacecraft" by Donald Smith & Bedford Lampkin.

  • TN D-8141 Feb 1976 - "Results of Skylab Experiment T00-2 Manual Navigation Sightings" by Robert Randle.

Robert Pearlman
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posted 07-09-2020 02:49 PM     Click Here to See the Profile for Robert Pearlman   Click Here to Email Robert Pearlman     Edit/Delete Message   Reply w/Quote
Astronauts add expertise, refine space station science in orbit

When NASA astronaut Serena M. Auñón-Chancellor tested the operation of a sextant aboard the International Space Station, she quickly realized that using the centuries-old navigation instrument while traveling more than 17,000 miles per hour in microgravity would require some adjustments.

Above: NASA astronaut Serena Auñón-Chancellor operating a sextant in the space station's cupola for the Sextant Navigation investigation.

Early explorers on Earth used the instrument to navigate at sea by the stars, but their ships weren't moving quite so fast.

Auñón-Chancellor, NASA astronaut Anne McClain, and ESA (European Space Agency) astronaut Alexander Gerst each used a sextant as a part of the Sextant Navigation investigation. The study demonstrated that a hand-held sextant could work well enough in space to serve as back-up navigation, particularly once astronauts figure out the right modifications.

On the ground, the astronauts trained for the experiment in "the dome," essentially a mock-up of the space station's largest window, the cupola. They were shown projections of stars with instructions to look for particular pairs to start their measurements.

"When we train on the ground, we are trying to picture what it is like to do something in space, but you really cannot unless you have been there before," Auñón-Chancellor says. "On orbit, the actual stars are moving relative to the station's field of view, and you don't have much time. After the first few sessions, we asked the investigators to give us several star pairs, so if we were unable to catch one, we could move on to a different pairing."

Above: ESA (European Space Agency) astronaut Alex Gerst inspecting, calibrating and operating Sextant Navigation.

Scientists who send research to the space station often benefit from such in-orbit feedback by highly trained astronauts who crew the microgravity lab.

"Some experiments need to follow a precisely prescribed set of procedures, but others very much include crew member feedback as a vital part of the experiment itself," says Sextant Navigation principal investigator Greg Holt, NASA Orion Navigation Lead at Johnson Space Center. "We knew that ours was one of those from the beginning and that the astronauts would be giving us a lot of feedback on how the experiment was run and on the instrument itself."

Another challenge in using a typically Earth-based tool in orbit is maintaining stability while taking measurements. "In microgravity, you are in constant motion unless you restrain yourself. Learning how to move in space is something like learning a sixth sense," Auñón-Chancellor said. "The investigators had ideas for how we could position ourselves to use the instrument, but we told them, we will figure it out and show you what worked." Auñón-Chancellor, for example, found she could brace herself against the side of the cupola using her knees.

The sextant sighting technique is extremely flexible and can change as vehicle type and crews change, she adds. This and other studies are helping to determine whether a sextant is accurate enough to serve as back-up navigation for the Orion spacecraft, a key component of NASA's Artemis program to explore the Moon in preparation for human missions to Mars. Once the instrument returns to Earth, Holt says investigators will use the astronauts' feedback along with formal data to develop future versions for potential use on these missions.

Other sextant input or hacks provided by the astronauts include a scratch in the arc that is used to measure angles so they could identify the zero point with their thumbs in low light, Holt said. And while the first star charts were paper printouts, red-text versions on a tablet proved a better option. Those easily could be delivered electronically and used with red light headlamps to preserve the user's night vision.

"We anticipated as much as we could beforehand, but there is nothing like actually having it up there and having crew members experiment with it to let us know what was working and what was not," Holt says.

Above: Detail of the sextant instrument used to test its capabilities as a navigation option aboard the International Space Station.

Potential future users of a sextant could benefit from what the astronauts learned during this investigation as well as from hundreds of years of history. Holt proposed the idea of navigating modern-day spacecraft with a sextant in 2012 after research and collaborative work with Emil Schiesser, who led Apollo navigation during the 1960s and 70s. Buzz Aldrin carried a handheld sextant on Gemini XII, and Apollo missions had one as an emergency crew return device.

"Alex and I are both big history buffs," Auñón-Chancellor says. "We loved pulling out a non-powered piece of equipment that early explorers used to navigate, one that we can still use on the most modern machine that exists. That melding of past exploration techniques with future exploration questions was amazing."

The space station's Microgravity Science Glovebox (MSG), which provides a sealed environment to contain liquids and particles in microgravity for science and technology experiments, is another tool on station primed for astronaut adaptation.

"Scientists have ideas about how to do the experiment and where to put everything in the glovebox, but up there everything floats and we have to figure it out," Auñón-Chancellor explains. "A lot of what we hack on-orbit for science is technique."

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