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Inspired by this question, pre GPS all space craft used inertial navigation with periodic fixes from the ground. Inertial systems will drift with time, so the longer between last fix and landing the larger the dispersion in landing point or even a non survivable re-entry trajectory.

This means that while loss of fix capability is not an immediate problem, there would need to be some point at which you stop trying to fix the problem and commit to re-entry if you want to hit a landing target rather than 'Earth'*.

Are there published mission rules or other documentation for manned vehicles giving guidance for this, either as a hard 'land within X hours of last fix' or predicted landing dispersion increase over time?

*Knowing the expected DV for the planned re-entry burn, an astronaut in LEO with no external information but a working IMU should be able to manually align in direction of orbital motion and thrust retrograde with enough accuracy to get a reentry, probably even a survivable one but hitting recovery fleet or a runway might be optimistic.

GremlinWranger
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1 Answers1

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There have been a couple of questions about this already, but they are in regard to launch and landing:

For orbit, the main criteria for a state vector update for shuttle was as follows:

A4-101 ONBOARD NAVIGATION MAINTENANCE

ADEQUATE NAVIGATION AND MANEUVER DEFINITION WILL BE MAINTAINED ONBOARD TO PROVIDE RUNWAY LANDING CAPABILITY AFTER LOSS OF COMMUNICATION.

This rule reflects a management decision to provide for safe deorbit and landing capability in the event of a total loss of communications between the vehicle and the ground. The on-orbit navigation state must be maintained within the defined uncertainty. The maximum acceptable downtrack error is 20 nm, predicted to deorbit TIG. Twenty nm is thought to be the maximum safe energy error that can be steered out after 130,000 feet altitude (see rationale for Rule {A4-151}, IMU ALIGNMENT). The current onboard state vector is propagated to the next day prime deorbit TIG to determine if the resulting state is within limits. Periodic state vector updates are uplinked to the vehicle whenever the propagated downtrack error approaches the limit.

This is from the Flight Rules and I've left the weird formatting.

Addendum: GremlinWrangler pointed out that Flight Rule A11-52 LOSS OF BOTH VOICE AND COMMAND covers the "land within X hours of last fix" part of the question. That rule is complex, stuffed with acronyms, and has a long rationale, so rather than quote it in full, I'll show only the meat:

IF GPS IS NOT VERIFIED TO BE FUNCTIONING PROPERLY AND NO ALTERNATE VOICE OR FILE TRANSFER PATH EXISTS TO UPDATE THE ONBOARD STATE VECTOR, DEORBIT WITHIN THE NEXT 13 HOURS IF PERIGEE IS GREATER THAN OR EQUAL TO 120 NM; OR THE NEXT 9 HOURS IF PERIGEE IS GREATER THAN 105 NM, BUT LESS THAN 120 NM...

Please refer to the source for details.

Organic Marble
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    My metric brain immediately read "nm" as nanometers instead of nautical miles and thought "Wow that's pretty accurate!" – Darth Pseudonym Mar 20 '24 at 20:13
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    Is rule A11-52 also relevant here? It appears to mandate de-orbit in 13 or 9 hours depending on Perigee for loss of coms+GPS due inability to update IMU? Not related to question but it appears A8-110 gives a 25 hour time between angular updates to IMU via star alignment to avoid issues with re-entry angle of attack. – GremlinWranger Mar 21 '24 at 08:30
  • @GremlinWranger looks like yes! I skipped over that part of the question, and that rule completes the either/or part. I'll confirm & update when I get on a real pc. – Organic Marble Mar 21 '24 at 11:32