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Author
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Topic: Gemini and Apollo navigation techniques
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Jim_Voce
Member Posts: 273
From:
Registered: Jul 2016
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posted 03-11-2018 05:02 AM
   
To my knowledge, all of the Earth orbital Gemini and Apollo missions did not have to rely on star-sighting of any kind to navigate in Earth orbit. All Earth orbital missions and Earth orbital docking missions only had to rely on radar readings (for rendezvous) and the Earth's horizon for lining up for reentry. But there was no actual star-sighting involved for these activities. It was only for heading out to the moon that the Apollo missions 8, 10, 11 and up to 17, had to rely on star-sighting to navigate. Is this correct? |
pollux
Member Posts: 54
From: London, England
Registered: Dec 2005
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posted 03-11-2018 05:38 AM
The excellent "Apollo Pilot" book, part of the Outward Odyssey series, details how Donn Eisele used the sextant and telescope to measure star angles, to be used for aligning the IMU during Apollo 7. |
Jim Behling
Member Posts: 1488
From: Cape Canaveral, FL
Registered: Mar 2010
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posted 03-11-2018 09:47 AM
Gemini took star readings. There was a sextant on some missions. All Apollo spacecraft missions took star readings since that is how the IMU was aligned. |
Jim_Voce
Member Posts: 273
From:
Registered: Jul 2016
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posted 03-12-2018 11:39 PM
Thank you Jim and Pollux for the good answers.To refine my question, I know that star readings were taken on Earth orbital mission Gemini and Apollo missions. The command module pilot had responsibility for this on the Apollo missions to align the Initial Measurement Unit in the command module. But on Apollo 7, I thought this was simply a test run for the lunar outbound missions. Did the aligning of the IMU during Apollo 7 serve any operational needs of Apollo 7 navigating in Earth orbit? In other words, was it a necessary task for Apollo 7's operations other than a test run of the IMU system for the lunar outbound flights? As for the Gemini missions, again I believe that the star sightings were just a test run for the Apollo lunar outbound missions and served no functional purpose for the navigational needs of the Gemini spacecraft in earth orbit. Is this correct? |
Jim Behling
Member Posts: 1488
From: Cape Canaveral, FL
Registered: Mar 2010
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posted 03-13-2018 06:49 AM
Star sightings on Gemini were also for rendezvous tracking. By the way, the Gemini IMU was aligned with horizon sensors. quote:
Originally posted by Jim_Voce:
...was it a necessary task for Apollo 7's operations other than a test run of the IMU system for the lunar outbound flights?
Yes, it was a necessary task. Star tracking isn't just for navigation, it is also for attitude knowledge. That is why the IMU has to be periodically realigned for drift. The shuttle had to do the same thing even though it never went to the moon. Without doing this task, the crew wouldn't know which way to point for deorbit burn. All Skylab ferry flights and ASTP had to use the star trackers. The IMU fed data to the 8 ball in the cockpit. Star trackers can provide precise attitude determination. They are used by many Earth orbiting satellites and most interplanetary spacecraft. LEO spacecraft that GEO location of items of interest such as weathersats, ones with laser altimeters, or mapping/recon sats need to know where they are pointing to provide this service. Star trackers fill this need. Interplanetary spacecraft don't have Earth references to provide attitude information and hence start trackers are used. As for star trackers for navigation, Apollo used the S-band signals and tracking stations determining range and range rate, which then was used for navigation. Mission control provided the bulk of the navigation data. Star sightings on Apollo were just to keep the IMU aligned. Apollo made sure that astronauts could navigate to get back to Earth if there was a loss of comm but they would not perform a mission based solely on onboard navigation. Only at real long distances are star trackers solely used for navigation. Mars missions use ground tracking for navigation and even Cassini. |
Jim_Voce
Member Posts: 273
From:
Registered: Jul 2016
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posted 03-17-2018 05:42 AM
Amazing knowledge Jim. Thank you. One comment. On the Gemini 12 mission when the rendezvous radar went out, I wondered if they did any star sighting for rendezvous purposes? But from what I have read, it has been said that Aldrin took out a sextant to measure the angle between the horizontal of the spacecraft and that of the Agena, ahead of and above the Gemini 12 spacecraft. Then Aldrin cross-checked his readings with his rendezvous chart and entered the necessary corrections into the onboard computer. But there is no mention of him doing any star sighting for the rendezvous purposes. |
Jim Behling
Member Posts: 1488
From: Cape Canaveral, FL
Registered: Mar 2010
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posted 03-17-2018 09:08 AM
Star sightings don't provide any benefit for rendezvous. Absolute position of the chaser spacecraft doesn't matter, only relative position to the target spacecraft. |
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