Navigation without communication?

[Artlav]

How would a ship in space determine it's position?

Let's take Apollo 13 for example. What if the explosion or whatever bad luck required damaged the communication equipment instead of O2 tanks, and cut the ship off from any communications with Earth. The crew knows their trajectory was altered slightly, and they know their initial trajectory, which leads into solar orbit by way of the Moon.
How can they return home?
Is that even possible?



Then, moving to pre-designed vessel.
In deep space there are plenty of ways to navigate, based on the same principles orbital determination works from Earth, you just look up bodies with known trajectories a few times at known intervals, and do the math.

What about navigating near a planet, what kind of instruments would it need?
There are ways of determining altitude, like radar or laser altimeter, but what about velocity?
Some kind of feature tracking to determine how fast and which way are we moving in X and Z, change of altitude to get Y, seems possible.
But what if the planet is a featureless gas giant or Venus-like ball?
Noting which stars it occlude and which way?
Noting the motion of vertical vector on the stellar background?

There i'm out of ideas.
What about you?

 

[SiberianTiger]

What about navigating near a planet, what kind of instruments would it need?
There are ways of determining altitude, like radar or laser altimeter, but what about velocity?
Some kind of feature tracking to determine how fast and which way are we moving in X and Z, change of altitude to get Y, seems possible.
But what if the planet is a featureless gas giant or Venus-like ball?

Some rough velocity information can be obtained from ion flow or magnetic gradient sensors, I think. Much like the way a pitot tube works.

 

[statickid]

ha! this subject has actually been what i've been working on in orbiter for the past several weeks! I've been trying do entire flights using only the surface mfd/hud, the map, and math programs i wrote on my TI calc. mainly making calculations with altitude and orbital speed, sometimes i use the ascent profile, but not usually. Also just started working on finding prograde/retrograde direction based on what i calculate to be my position in orbit. so far i can launch to a circular orbit, transform the orbit to any desired shape, and i'm testing a reentry profile calculator later on today. I figure these would only really work around the earth or moon currently. actually, they only work around leo and a little + right now. I also working on a burn time calculator

---------- Post added at 09:59 AM ---------- Previous post was at 09:53 AM ----------

I've also been using visual observation of star locations, the orbital hud, and geometry to calculate do some maneuvers or figure out my location. for example i was able to return to earth from the moon several times by observing when it was at my zenith, then making a prograde "puff" so that my AP would be marked on the opposite side of the moon, then i watch the ascent profile and watch my altitude over the moon. when i seem to be reaching Ap, i make a long burn to create a trajectory towards home

 

[Andy44]

This has been discussed before. IIRC Apollo had a pretty advanced capability for independent navigation using angle and horizon information and handheld instruments.

Without any comm gear or ability to signal at all, all you have is information from passive sensors, such as star sensors, earth horizon sensors, sun sensors, and of course, your own eyes. Using the Surface MFD is cheating, because there is no way to sense velocity or altitude in a vacuum without using an active sensor such as a radar or laser.

So you've got an instrument that can measure angles, including the angular sizes of the earth and moon. You've got index markings on the windows to tell you the angles relative to stars or other objects. And you've got a good clock, such as the Omega Speedmaster wristwatch the Apollo crews wore, which can tell you how much time elapses between observations, and can be used to time engine firings manually (see Kevin Bacon's character use one starting around 3:55 in the below vid). You've also got a pretty good prediction of what your orbit was before your comms went down.

This may be enough information to determine your orbit. The more observations, and the higher the quality, the better the solution. Hopefully your onboard computer can be used to help process the observations to save time, and also to propagate your orbit into the future so you can plan maneuvers to get home.

[ame="http://www.youtube.com/watch?v=33u3MAzl_-s"]YouTube- Apollo 13 - Day 6[/ame]

 

[statickid]

Without any comm gear or ability to signal at all, all you have is information from passive sensors, such as star sensors, earth horizon sensors, sun sensors, and of course, your own eyes. Using the Surface MFD is cheating, because there is no way to sense velocity or altitude in a vacuum without using an active sensor such as a radar or laser.

I don't consider surface hud cheating for my purposes because i'm just trying to work out ideas for independent navigation where i assume that i have working equipment like the radar or laser sensors you mention. with my calculator i can determine a rough estimate of what my velocity *ought* to be because i can get a pretty good small body ellipse model from taking several altitude measurements over time. actually the main thing that seems highly suspect about what i was doing to me is determining what is "exactly straight down" without the surface hud. what could be used to do this? a gyro and clock? only star positions?

 

[Andy44]

determining what is "exactly straight down" without the surface hud. what could be used to do this? a gyro and clock? only star positions?

If you define "straight down" as the center of the nearest body, you find it by taking angles from horizon to horizon and splitting it in two. This is easy to do with commercial horizon sensors.

---------- Post added at 09:44 PM ---------- Previous post was at 09:40 PM ----------

Also, the angular width of the earth (or whatever body) gives you the altitude above that body, using a little trig math.

By taking these measurements over and over at regular intervals, you can determine the rate at which your altitude is changing, which can be used to determine the shape fo your orbit. By watching to see which stars appear and disappear around the limb of the earth at specific times, you can determine your orbit plane and possibly your velocity.

All these measurements depend on how accurate your sensors are at determining the limb of the earth through atmospheric haze. An airless body like the Moon would be easier to make observations around.

 

[statickid]

oh yeah! duh! *slaps own forehead*

 

[Andy44]

Here's the thread where this was discussed before: http://www.orbiter-forum.com/showth...hlight=Apollo+navigation+tblaxland#post148083

Here's a post by tblaxland which describes how Apollo was capable of doing what I'm describing, including a link to a pdf:

Yes, the Apollo Guidance Computer, in junction with the on-board sextant and IMU, could do orbit determination by doing a series of measurements of the positions of background stars relative to the Earth and the Moon (the Colossus software contained a rudimentary model of the Moons orbit). The AGC only had capability to perform targeting calculations for certain manoeuvres, but most importantly it could do return-to-Earth targeting. Read more in this MIT report (warning 8 MB pdf): http://ocw.mit.edu/NR/rdonlyres/Scie...9_mit_role.pdf

 

[Hielor]

Since you can determine "straight down" using horizon sensing, you can establish a sort of coordinate system, which you could use to determine your angular velocity around the planet via finding the angular velocity of stars in the starfield...

 

[kwan3217]

The Apollo guidance system was originally designed with just such a out-of-contact scenario in mind - not from some onboard failure like Apollo 13, but from active jamming by the Soviets. Remember that this was the space race, part of the Cold War. The system was originally designed so that everything from post-TLI to reentry could be done purely by optical sightings.

Once it became evident that the Soviets were not going to jam these missions, the requirements were relaxed and navigation was mostly done from the ground, but the capability was always there and was extensively tested on Apollo 8. Jim Lovell was in charge of that and became famous for his accuracy.