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How does star navigation work?

Fuji_Abound wrote:

Would I be right however that if we could ever travel at anything like sensible speeds for space exploration an additional adjustment would be needed for time dilation?

Everything is relative, but a pulsar beats at the same frequency no matter how you perceive that frequency, so I guess no? not if everyone use the same pulsar.

The elephant is the circulation
ENVA ENOP ENMO, Norway

Pulsars are not all that accurate

Administrator
Shoreham EGKA, United Kingdom

Fuji_Abound wrote:

Would I be right however that if we could ever travel at anything like sensible speeds for space exploration an additional adjustment would be needed for time dilation?

That is needed already for GPS….

ESKC (Uppsala/Sundbro), Sweden

Peter wrote:

Pulsars are not all that accurate

And by that my knowledge of pulsar timing suddenly increased by a factor 1M Time to do the weekly hike on some local hill/mountain to keep the vascular system in shape

The elephant is the circulation
ENVA ENOP ENMO, Norway

To use Astro Navigation, you calculate the position of a body so as to determine exactly where it will be overhead on the earths surface at the desired time. If the body is exactly overhead the Elevation will be 90 degrees. As this is rather limiting, you calculate what the Elevation will be at a given position know as an Assumed Position. If the Elevation you measure is greater or less than that calculated, you will be displaced along the bodies Azimuth by 1 nm per minute of arc from the assumed position.
You do not measure the Azimuth, you calculate it to locate the body. You only ever measure Elevation or “Altitude” using the sextant. One observation gets you a single position line at 90 degrees to the Azimuth. 2 bodies will produce a sandwhich fix with lines at right angles whilst 3 bodies having Azimuths displaced by 60 degrees will produce a cocked hat. You can also take sightings of the ground using a sextant to determine range.
Because of aircraft and operator movement, a number of shots are taken and averaged over one or two minutes. Sometimes 3 or 4 seperate shots will be taken on the same body and the average taken (V Force technique)

Last Edited by Tumbleweed at 25 Aug 18:34

Was Amerigo Vespucci the first to discover where America was, using moon and star observations?
Other astronomers with his skills stayed at home at that time.

Maoraigh
EGPE, United Kingdom

In order to get anywhere with celestial navigation you need five things:

1. A device to accurately measure angles above the horizon
2. A clock giving you an absolute (current) time reading
3. Lookup tables or formulae to convert your angular and time readings into a Lat/Long position
4. A visible horizon
5. A visible astro body fitting your tables/formulae

At long sea voyages you usually have (4) and (5) often enough (unless stuck in fog, haze or under clouds for extended periods).
(3) you can prepare ahead of time before even taking off.
(2) used to be a tough problem but can be considered „solved“ for most practical purposes by investing 10 bucks in a quartz watch.

That leaves (1) as the only „real“ problem, which is typically taken on by relying on a sextant.
Unfortunately a full-fledged sextant can be quite a tricky beast to handle properly. Not to speak of the cost, weight and sensitivity to all kinds of environmental factors like temperature, humidity or shock.

One way to get around all these issues around classic sextants is something like the „Bris Sextant“ (https://en.m.wikipedia.org/wiki/Bris_sextant), which has no moving parts and is small enough to be worn on a necklace!

The basic idea is to give up on the goal of trying to accurately measure any angle, but rather only a certain set of convenient angles.
For example, rather than measuring the angle between the sun and the horizon right now and then recording 26.7 degrees and the current time one can also simply wait until the angle is exactly 30 degrees and record the time at that particular instance.

In many cases this trading the ability to establish a position at any time with complicated and costly machinery against doing the same only at particular times but with a great simplification in measuring and equipment can be quite beneficial.
Especially since „anytime“ isn’t „anytime“ anyway, since you always need a horizon and a clear sky, even with the best and most expensive sextant in the world…

And: By reducing your possible angular readings to, say, only 8 or 16 cases your lookup tables become a lot smaller, which can be quite an important benefit as well.

Unfortunately, waiting for the next convenient angular reading time is much less practical in a fast moving aircraft than a slow moving ocean vessel.
In the plane we simply can’t wait two hours for the next „positioning event”.
But for sea or even space navigation I find this basic principle quite interesting…

EDTF

Elon Musk’s Starlink system (space-based Internet communication) uses a star tracker for precision pointing. It has only little in common with celestial navigation using a sextant.

United States

…I beg to differ…

It does exactly what all stellar navigation systems do – establish a definitive vector in space. It just uses pattern matching instead of fiddling with a sextant to achieve this.

This can be used to determine the orientation in space of the instrument, and on Earth, together with time, the location.

There have been three types of answers in this thread

  • how to use a vector in space and time to get the position on Earth (a simple matter of applied geometry)
  • how to establish the vector in space (sextants, the horizon, star trackers etc.)
  • how to establish time (clock, orbital position of Earth, moon, planets or other satellites)
Biggin Hill

Peter wrote:

They did have a reasonably accurate clock on board, though only quartz crystal controlled, so probably within a few seconds.

For the length of an Apollo mission (only a few days), a quartz crystal controlled clock should remain within a second, especially with a TCXO.

Andreas IOM
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