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Author Topic: How will people navigate in deep space?  (Read 482 times)

Offline chris

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How will people navigate in deep space?
« on: 14/10/2016 09:34:29 »
On Earth we have GPS to help us find our way around. How might spacefarers of the future find their way around?


 

Offline alancalverd

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Re: How will people navigate in deep space?
« Reply #1 on: 14/10/2016 10:48:39 »
The advantage of flying over driving is that you can in principle travel in a straight line to your destination. On earth this actually means a great circle which is a bit more complicated over long distances but entirely feasible with inertial navigation or even a slide rule and stopwatch if you know the upper winds. Semiorbital (ballistic missile) flight is even easier because you are less affected by wind. We use GPS because it is easier, helps avoid dense traffic zones, accommodates course changes, and is sufficiently accurate to put you on the glide path right down to the deck, but people were flying around the world and droppnig bombs on more or less the right place for a hundred years before GPS. 

Flying in deep space is even easier! All you need do is calculate where your target is going to be by the time you get there, allow for or avoid any significant gravitational fields en route (you can see them - they have bloody great stars in the middle!) and just point yourself in the calculated direction. No wind, no competing traffic, no altitude restrictions, high ground, military manouvers, overworked controllers, parachutists, aerobatic displays, lost students (there but for the grace of god...), Amazon drones, royal flights, emergency helicopters (same thing round here!) or Heathrow control zone....and as far as we know, no GPS glide paths have yet been promulgated for Alpha Centauri so you just choose a flat field and drop anchor.  Since your target will have been determined by visual or radio astronomy, and there's little fog in space, positioning for approach is a doddle: just keep the planet in the middle of the windscreen until you reach orbital height.

Or use autopilot.
« Last Edit: 14/10/2016 10:58:52 by alancalverd »
 

Offline evan_au

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Re: How will people navigate in deep space?
« Reply #2 on: 14/10/2016 11:34:33 »
If you are in radio/laser communication with a base at a known location, it is pretty easy to work out the distance to that point by the round-trip time of signals, and your velocity by the Doppler shift of the signals.

If you are approaching a planet fairly closely, it is  possible to work out your distance and velocity using radar.

Within the Solar system, it is possible to triangulate from the (known) positions of the planets.

When traveling to Proxima Centauri, it is possible to triangulate from the (known) positions of nearby stars.

There is always "dead reckoning" - record how fast you accelerated, in which direction, and for how long; from this you can calculate your position and velocity. But it's wise to cross-check with some of the other methods, because any errors that creep into dead reckoning tend to build up over time.
 

Offline jeffreyH

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Re: How will people navigate in deep space?
« Reply #3 on: 14/10/2016 16:15:33 »
If you are in radio/laser communication with a base at a known location, it is pretty easy to work out the distance to that point by the round-trip time of signals, and your velocity by the Doppler shift of the signals.

If you are approaching a planet fairly closely, it is  possible to work out your distance and velocity using radar.

Within the Solar system, it is possible to triangulate from the (known) positions of the planets.

When traveling to Proxima Centauri, it is possible to triangulate from the (known) positions of nearby stars.

There is always "dead reckoning" - record how fast you accelerated, in which direction, and for how long; from this you can calculate your position and velocity. But it's wise to cross-check with some of the other methods, because any errors that creep into dead reckoning tend to build up over time.

Time dilation might invalidate your dead reckoning.
 

Offline Colin2B

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Re: How will people navigate in deep space?
« Reply #4 on: 14/10/2016 23:14:16 »
Time dilation might invalidate your dead reckoning.
Ah, but you are clever and know the maths so you can incorporate the adjustment into your calculations.
 

Offline alancalverd

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Re: How will people navigate in deep space?
« Reply #5 on: 15/10/2016 00:30:15 »
I still can't see the problem. In deep space, you can always see your destination and there is nothing to prevent you flying directly towards it. Things only get complicated if you are trying to use gravitational slingshot acceleration, but that is all precalculated - the first 4,700,000,000 miles (Earth to Pluto) constitutes a "Standard Instrument Departure" that you purchase from your chart supplier and use for getting from the airport/spaceport to your cruising route under radar control. At that point the controller says "leaving my zone - change to en route navigation" and you fly direct to your next waypoint which, in the case of interstellar navigation, is in fact your destination and clearly visible throughout the flight.   
 

Offline PmbPhy

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Re: How will people navigate in deep space?
« Reply #6 on: 15/10/2016 07:55:44 »
Quote from: alancalverd
Flying in deep space is even easier! All you need do is calculate where your target is going to be by the time you get there, allow for or avoid any significant gravitational fields en route (you can see them - they have bloody great stars in the middle!) and just point yourself in the calculated direction. No wind, no competing traffic, etc.
I strongly disagree. You're dismissing the very important fact that various things can affect the course of an objects trajectory. Such discrepancies are referred to as perturbations. For example; the presence of the planet Neptune was deduced from the perturbations caused by Uranus. Radiation pressure can have a considerable effect on large area/mass satellites. One such example was the satellite Echo.

See:
https://en.wikipedia.org/wiki/Project_Echo

If you'd like I can look more deeply into this and get back to you. Don't forget that one way to navigate in deep space is by the stars! :)
« Last Edit: 15/10/2016 09:09:58 by chris »
 

Offline evan_au

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Re: How will people navigate in deep space?
« Reply #7 on: 15/10/2016 12:52:57 »
As I understand it, to do navigation effectively in space, you need to measure (or be able to calculate) at least 10 things:
- Your orientation in space (3 dimensions, easily calculated from the angles to distant stars)
- Your position in space (3 dimensions)
- Your velocity in space (3 dimensions)
- Time/Duration (1 dimension, measured with a local clock)

Unfortunately, measuring round-trip time only gives you the radial component of distance from your base.
- the two transverse directions could be determined from very-long baseline interferometry (VLBI) - essentially a group of satellites forming a space-based telescope capable of measuring very fine angles.
- You can calculate your postion by dead reckoning if you accurately know your velocity and time, plus your initial position (mathematical integration)
- Or you could measure angles to nearby stars

And measuring Doppler shift only gives you the radial component of speed
- Again, VLBI can help measure the transverse components of velocity
- You can calculate your velocity by dead reckoning if you accurately know your orientation, acceleration and time, plus your initial velocity (mathematical integration)
- Alternatively, high-resolution spectroscopy can precisely measure your velocity relative to bright stars, in 3 dimensions

During the course of an interstellar flight, the positions of nearby stars will change, since they have their own independent velocities through the galactic disk. So you need to know the positions and velocities of these stars, too. That is a lot of unknowns, and you would want to take repeated measurements of a lot of stars to navigate accurately.

Fortunately, the Gaia satellite is collecting some of this information as we converse. You need the ability to refine these estimates as you are travelling, observing these stars over extended periods, from multiple angles.

But for a journey to another star, the main thing you need to know is your distance and velocity relative to your destination, so that simplifies the problem.

The space between the stars is mostly empty, but not entirely empty. A colision with a dust grain when traveling at interstellar speeds is likely to prove fatal.

Many a ship has foundered on uncharted shoals.
 

Offline alancalverd

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Re: How will people navigate in deep space?
« Reply #8 on: 15/10/2016 15:21:44 »

I strongly disagree. You're dismissing the very important fact that various things can affect the course of an objects trajectory. Such discrepancies are referred to as perturbations. For example; the presence of the planet Neptune was deduced from the perturbations caused by Uranus.

I mentioned local gravitational fields, the most significant of which are as I said marked by bloody great stars in the middle. Calculate or avoid, or see below. Planets aren't a problem bcause the question was about deep space, i.e not in a planetary system.

Quote
Radiation pressure can have a considerable effect on large area/mass satellites. One such example was the satellite Echo.

Again only a problem near a source of radiation, i.e a bloody great star. However if you insist on sailing in such dangerous waters, you can use "homing" (also known as a "banana approach"). Dead easy:

Flying in zero visibility and with no wind data, if you only have one nondirectional beacon available at your destination, you just keep the direction finder centered on it. You end up flying a curved approach - I think it's a Cornu spiral (that's the next level of instrument rating - I'll let you know when I qualify!) whose final vector takes you directly into wind at the beacon. Not the shortest route or the most efficient, but when all else fails and the brain has given up, it will get you there every time.  A star or a reflective planet is a nondirectional beacon.

Quote
If you'd like I can look more deeply into this and get back to you. Don't forget that one way to navigate in deep space is by the stars! :)
No! It's the one source you can't rely on because all our star maps either assume they are all fixed at infinity or once were where we think they were from parallax measurements. As we approach or recede from them so the apparent map represents their position at a different time from what you are now seeing!

Like it or not, the visual banana (keep the target on the crosswires) is the only reliable approach, and a STandard Approach via Reference points (yes, it's really called a STAR) won't work!
« Last Edit: 15/10/2016 15:28:43 by alancalverd »
 

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Re: How will people navigate in deep space?
« Reply #8 on: 15/10/2016 15:21:44 »

 

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