Here am I sitting in my tin can... how do I know where's prograde?

[Laie]

It's very convenient to have a navball that can tell you where prograde, radial, normal are.

I just wonder, is it as easy in in real life? Is there a mechanism and apparatus to track and update it in real time? Or do you have to determine your position every so often, calculate your trajectory, and derive prograde from that?

[Xd the great]

16 minutes ago, Laie said:

It's very convenient to have a navball that can tell you where prograde, radial, normal are.

I just wonder, is it as easy in in real life? Is there a mechanism and apparatus to track and update it in real time? Or do you have to determine your position every so often, calculate your trajectory, and derive prograde from that?

There are computer systems that derive your current velocity from a variety of factors, such as the movement of stars. Then there are systems that use these info to determine prograde and retrograde and etc.

[kerbiloid]

Gyroscopes. Though, you have to keep calculating correction values. THough this happens automatically when you have a radionavigation.

GPS and other radionavigation systems.

Without GPS, you have an optical sight with reticule.
You can look at the Earth, and realize in which direction its details have shifted a minute later.

To find normal direction you have an infrared horizon sensor.

Spoiler

Detail 14 here, right bottom corner

It contains a rotating head and infrared sensor.
As the Earth is the major source of infrared radiation, the head makes a full turn and notices the edges of a big IR-hot circle.
This gives the direction to the Earth center, i.e. normal vector.

Astro- and sun sensors can give you latitude/longitude and Sun direction. Can be used only on demand (not permanently) as require the whole ship re-orientation.

A laser altimeter to measure altitude.
A corner reflector to have it measured by an Earth-based laser altimeter.

Historically they were also trying particle counters and other devices giving prograde direction by measuring the particles flow and other physical effects.

Edited July 25, 2018 by kerbiloid

[Bill Phil]

Once you find position throughout a period of time, you can differentiate and find the velocity.

[Reactordrone]

You could use a Vzor periscope. When you've got it pointed straight down you get even lights around the periphery of the scope then you can track a feature on the ground to determine your heading relative to where your ship is pointed.

[magnemoe]

In deep space you use the sun and stars. First find the sun, this give you one axis, now rotate around it until you find some specific bright stars, this give you orientation. 
if you can not see earth you know its position at this time, radio it and you get an reply this give you distance and relative speed, samples over time give your vector with increasing accuracy. 
 

[YNM]

Wait and See, as always !

 

A few stuff to look at :

- Ground

- Stars (light abberation)

- Comms transmission, from doppler shifts.

[kerbiloid]

4 hours ago, magnemoe said:

In deep space you use the sun and stars. First find the sun, this give you one axis, now rotate around it until you find some specific bright stars, this give you orientation. 

First find the Sun by a star sensor..
In simples case it's like a box with light sensor inside and two perpendicular slots on top. Preferrably with an opaque spot (mask) in the very middle of eash slot.
Or it may be a cylinder with 4 or 8 radial slots on its end face and the mask in center.
In any case better have it gimballed in two perpendicular planes.
Rotate your ship pointing with the box roughly to the presumed Sun direction.
Tilting the box in two planes, get two angles when the sunlight is maximum, then drops minimum (i.e. when the Sun gets covered with the mask).
Now you have the Sun direction.

Now use your star sensor to define position of navigation stars.
It can be a rotating 2d scanner which makes a temporary 2d map of the sky making a full turn. Then the board computer compares this map with a star map in memory.

Or you can use an old school way: a mini-telescope gimballed in two perpendicular planes.
You can use it also as a Sun sensor, covering its end face with a blend , having those 8 radial slots covered with sun-proof glass, and a mask in the center.
In this case you have 2-in-1.

After getting Sun direction, rotate your craft with star sensor perpendicular to the Sun direction.
Remove the sun-proof blend, getting a naked telescope glass.
Make a full turn. Let the sensor notice the brightest point in the sky.
This is Canopus. It's almost perpendicular to ecliptics.

Rotate your craft with the star sensor roughly to Canopus.
Tilting the star sensor with the gimbal in two planes, point exactly at the brightest place inside its field of vision.
Now you have Canopus direction.

On desire repeat this with other stars.

Edited July 25, 2018 by kerbiloid

[wumpus]

The earliest flights were completely automatic (Vostok, Mercury flights were designed automatic and obviously did so with the earliest primate crew) so that puts a huge damper on available means to discover prograde (GPS, stars, etc).  Gyroscopes were obviously available, and I'd hope that a means of tracking the Sun was possible (I'd expect liftoff to coincide with enough time so that the Sun was visible until orbital insertion) for gyroscopic correction.  Also if the Sun was visible, certainly the Earth was as well (Astronauts could obviously see the Earth at night, but I'd wonder about 1950s sensors).

Astronauts in the Apollo missions did check the stars while traveling to the Moon and updating the computers.  There's some question as to whether or not this was necessary or an improvement (which may have been pure propaganda, telling the Soviets that US missiles were uncannily accurate). https://www.youtube.com/watch?v=X2J-5QJC1qc [Vintage space discusses the need to wear eyepatches thanks to this procedure].

 

[sevenperforce]

I think I heard this in a Scott Manley vid, but prior to WWII the British Interplanetary Society proposed a whole plan to go to the moon using clustered black powder rockets. They figured that spin-gravity would be needed in transit, so they rigged up a special geared observation mechanism to allow the crew to make accurate star observations while in a rotating cylinder.

[Bill Phil]

8 hours ago, magnemoe said:

In deep space you use the sun and stars. First find the sun, this give you one axis, now rotate around it until you find some specific bright stars, this give you orientation. 
if you can not see earth you know its position at this time, radio it and you get an reply this give you distance and relative speed, samples over time give your vector with increasing accuracy. 
 

You can also use asteroids.

https://en.m.wikipedia.org/wiki/Deep_Space_1#Autonav

[magnemoe]

53 minutes ago, Bill Phil said:

You can also use asteroids.

https://en.m.wikipedia.org/wiki/Deep_Space_1#Autonav

Smart, it should also work for planets who is larger and easier to see in an small telescope? 
On the other hand if you are in the astroid belt they are closer and planets can be in bad alignments, close together or be to close to the sun. 
 

[K^2]

I see gyroscopes mentioned a few times in this thread. I'm curious to how people imagine using these for finding prograde direction.

Gyroscope is a purely local instrument. Therefore, you should immediately realize that it cannot measure something that is frame-dependent, like orientation or direction of travel. It can measure changes in both of these, of course. And keeping track of changes in orientation is sufficient for you to know how the craft is oriented with respect to some fixed point, and that's good enough for orientation, so long as you can correct for drift once in a while.

You can also use gyros to keep track of changes in velocity using gyrointegration. The idea is that you place an off-axis gyro that can precess freely, and count the number of turns it makes. With constant angular velocity of the gyro itself, this has a fixed proportion to change of velocity along direction perpendicular to precession axis. However, this is where we run into a bit of bother. This measures changes relative to a free-falling frame. If you place a gyro off-axis on Earth, you know that it precesses like a top, because standing on the ground is equivalent to accelerating upwards at a rate of 1g. A ship in orbit is in free-fall. A gyrointegrator shows a flat zero throughout the flight, while the velocity with respect to ground changes.

People have done navigation by gyros alone. Early ICBMs certainly did that. But the idea there isn't to find where prograde is. It's simply to have the rocket follow a precomputed flight path, using gyros for orientation and gyrointegration, and adjusting thrust and orientation to stay on the program. The program flight is pre-computed according to a numerical model, so we could compute the prograde direction from that if everything's going according to plan, but if the rocket ever deviates from the plan, you will no longer be able to use information available to find prograde.

Navigation by stars is equally problematic. It tells you orientation of the craft, but you aren't going to measure the direction of travel. You have to have external inputs for that. If you can track stars and Earth, you can make that work. Parallax of the Moon might be measurable as well.

More precise navigation can be done with radio. Back in the day, if new re-entry procedure had to be established, the craft was "sighted" from Earth via radio or even visually to establish its heading and speed, and the correct timing and orientation for re-entry burn could be simply dictated to the pilot once the math was cranked out. Now things are a bit easier with all sorts of automation. GPS, of course, is fantastic for that in LEO. It has more than enough precision for most orbital operations. Once you leave Earth, things get more complicated.

The coolest nav project that's out there, undergoing testing, IIRC, is navigation by pulsars. These are basically nature's GPS. They are far enough out that they might as well be static in the sky, and their pulse timing is absurdly precise. By simply counting pulses from several known pulsars, you can measure your position anywhere in Solar System or well outside of it to within a few hundred meters. You probably want additional correction for landing, but otherwise, this is good enough for interplanetary navigation.

[mikegarrison]

4 hours ago, K^2 said:

I see gyroscopes mentioned a few times in this thread. I'm curious to how people imagine using these for finding prograde direction.

Gyroscope is a purely local instrument. Therefore, you should immediately realize that it cannot measure something that is frame-dependent, like orientation or direction of travel. It can measure changes in both of these, of course. And keeping track of changes in orientation is sufficient for you to know how the craft is oriented with respect to some fixed point, and that's good enough for orientation, so long as you can correct for drift once in a while.

It's been a long time since I learned the basics of inertial nav systems, and I have forgotten most of it. But I know they were/are able to calculate not just position but also orientation using dead reckoning from an initial position and orientation. If you have position over time, you have a velocity vector. If you have orientation then you can point to that velocity vector. (Of course they didn't just use gyroscopes, they also incorporate accelerometers.)

As you say, INS does depend on knowing your position and orientation when you start, and it can drift over time due to the steady accumulation of error.

[kerbiloid]

5 hours ago, K^2 said:

I see gyroscopes mentioned a few times in this thread. I'm curious to how people imagine using these for finding prograde direction.

That's very simple, let me explain it to you.
You know your orbit elements, you know the normal vector. A gyroscope gives you your position vector. A pair of position vector measurements gives velocity vector.
Using both normal vector and velocity vector, you know your tangential vector.
Applying your tangential vector to your known orbit, you get an angle between your velocity vector and the local tangential vector of your orbit.

Would you also know how to use a gyroscope as a compass?

5 hours ago, K^2 said:

Early ICBMs certainly did that. But the idea there isn't to find where prograde is. It's simply to have the rocket follow a precomputed flight path, using gyros for orientation and gyrointegration, and adjusting thrust and orientation to stay on the program. The program flight is pre-computed according to a numerical model, so we could compute the prograde direction from that if everything's going according to plan, but if the rocket ever deviates from the plan, you will no longer be able to use information available to find prograde.

Nonsense.
ICBM use inertial system to point MIRVs at several targets. And they use exactly the inertial system for this. They don't do this "just following the plan" or another magic words, they do this after radio- and sometimes astro- navigation seance. SLBM even doesn't know her coordinates before the astrocorrection.

5 hours ago, K^2 said:

Navigation by stars is equally problematic. It tells you orientation of the craft, but you aren't going to measure the direction of travel. You have to have external inputs for that. If you can track stars and Earth, you can make that work. Parallax of the Moon might be measurable as well.

It is being used since.1950s with no problems. On both SLBM and spacecrafts. Probably they don't know that it's problematic.

5 hours ago, K^2 said:

More precise navigation can be done with radio. Back in the day, if new re-entry procedure had to be established, the craft was "sighted" from Earth via radio or even visually to establish its heading and speed, and the correct timing and orientation for re-entry burn could be simply dictated to the pilot once the math was cranked out.

Long ago, in 1950s they were using 2-3 omnidirectional stations for ballistic rockets. Their purpose is just to be beacons. The client has omnidirectional antennas and is happy with them.
In aviation about 10 radionavigation systems have been widely used just since 1960s. Does VOR-DME abbreviation tell something to you?
With navsats like GPS things go even easier.

5 hours ago, K^2 said:

The coolest nav project that's out there, undergoing testing, IIRC, is navigation by pulsars.

Oops, I just didn't realized the question was about interstellar ships.

Edited July 26, 2018 by kerbiloid

[Bill Phil]

They've done the pulsar thing on the ISS. They got within 7 km or so. Can be improved with time. 

That's a really interesting way of navigating.

[kerbiloid]

2 minutes ago, Bill Phil said:

They've done the pulsar thing on the ISS. They got within 7 km or so.

That's fine for interstellar or interplanetary ships. Of course it's fine for astronomy itself.
But ICBM have ~100 m accuracy without pulsars.

[Mad Rocket Scientist]

It's slightly off-topic, but the Apollo LM star tracker was really impressive:

 

[Bill Phil]

1 hour ago, kerbiloid said:

That's fine for interstellar or interplanetary ships. Of course it's fine for astronomy itself.
But ICBM have ~100 m accuracy without pulsars.

It's not really relevant to ICBMs. I don't think anyone implied that it was....

[kerbiloid]

5 minutes ago, Bill Phil said:

It's not really relevant to ICBMs. I don't think anyone implied that it was....

7 km is too much if compare with typical orbit corrections of ISS (3..4 km), and requires special equipment.

While star navigation requires a 1950s era device and is enough accurate even for ~100m ICBMs, that's what I meant.

Pulsars are a thing for deep space or for heavy navsats.

Edited July 26, 2018 by kerbiloid

[K^2]

2 hours ago, kerbiloid said:

You know your orbit elements

Let me stop you right there. If you know your orbital elements, the only instrument you need is a stop watch. If you find yourself needing a gyroscope, it already means you don't know the orbital elements. The rest of your reply is rendered irrelevant by this.

2 hours ago, kerbiloid said:

Does VOR-DME abbreviation tell something to you?

Of course. I've been trained to fly with VOR, ADF, and perform ILS approaches. The fact that I'm actually mentioning radio nav-aids in the very post you are quoting should also have been a clue that I'm aware of how these things work. In practice, however, inside-out navigation, where ship can track own position, came later. First systems tracked the craft from ground, and had instructions relayed to pilot if necessary.

If you have ever actually had to dial in radials while switching airways, you'd probably have some idea why. Having to deal with navigation system, while also trying to compute orbital elements, and control a ship would be suicide. Unless you have a computer to do most of these operations, you're much better off leaving it to the ground team.

P.S. I grew up near a Soviet missile base, and not far from Zvezdny to boot. Majority of physicists I've learned from growing up had been specifically trained to be responsible for making sure the warheads make it to their targets. If we're going to start arguing about Soviet ICBM navigation systems, you're going to lose.

[kerbiloid]

1 hour ago, K^2 said:

If we're going to start arguing about Soviet ICBM navigation systems

The thing which I will be not doing in any case.