chase&track question ( newbie question )

[mspo100]

Celestia has several commands to navigate through the space : go to, center,follow,sync,lock,track and chase ( I??m talking about space navigation on keystrokes, not on a script file ).
The "track" and "chase" commands are difficult for me because I??m not able to understand how they work when you wanna go for a space ride.
Somebody knows to give me an example about how to manage these two commands ?
Example : "Follow" Galileo spacecraft and "track" the next object it??ll be encountering................... How do I make this ?

[scalbers]

On a somewhat related note I have had some uneven results with the 'track" feature. This feature is absent with 1.3.1 running under SuSe Linux. WIth 1.3.2 it works on my notebook machine running windows. I recently downloaded 1.3.2 on another windows PC and the track selector was there, yet it didnt appear to track Dione as viewed by Cassini during the most recent flyby. Does anyone know if the 'track' feature is more solid these days, and for example with version 1.4?

[fsgregs]

The "track" command is a great feature. It is designed to follow an object as it moves through space and watch it pass by, without chasing it. Think of it as a camera that pans with the object as it passes you.

The confusion lies with the use of Track with other commands. It is not supposed to be used with other commands. For example, let's say you are orbiting the Sun and have selected comet Halley from the solar system browser menu. Click "goto", and Celestia will lock onto Halley, go to it, and "Follow" it. That means where it goes, you go. You never see it move because you are pacing it.

Now try this. Press the "L" key to speed time up to 10X faster. You won't notice much difference in the view of Halley. Now ... press the [Esc] key. This will cancel the "Follow" command and Halley will begin to drift away. Your camera view is not swinging to follow it. Now ... press the [T] key. Voila! You view immediately swings to track Halley as it drifts off. For a great effect, reverse time by pressing the [J] key. Halley will now come toward you and whiz past you.

Using the Track command this way, you can watch asteroids and comets move. It is great for spacecraft. By speeding up or slowing down time, you can watch them pass by at any speed you wish. Just remember to cancel any other navigation command with the Esc key before tracking anything.

The Chase command is similiar to the Follow command. You will pace the object you are locked on. The difference is that you will see the stars move in the background. To illustrate, select and follow any planet or moon. Back away a bit with the [End] key. Speed up time to 100x faster. Now, press the [Shift+"] keys to activate the Chase command. You will still follow the object but now, you will see the stars moving behind it.

Hope this has helped. Check out the complete Celestia Users Guide, available on the main website and on the Motherlode documentation page. It explains all these commands.

Regards

Frank G

[cpotting]

I have found that the following anology helps me in keeping the various modes straight in my head.

Instead of an observer floating in space, I think of a news helicopter flying around with a cameraman onboard, scooping the "big story". There are six basic commands, four which you give to the pilot, and two to the cameraman.

When you tell our cameraman to centre the UFO, he points his camera at the UFO and stands rigid. As the UFO flies towards the Pentagon, it moves across the screen and eventually out of view.

So you tell the cameraman to "track that UFO". Now he locks his sights on it, and no matter which way your helicopter twists and turns, our fearless, limber and somewhat underpaid cameraman shifts, twists and hangs out the door to make sure that the spinning flying saucer stays centred in the camera view for our audience to see.

When you tell the pilot to follow the UFO, he flies along making sure that he is always the same distance and compass direction from the bird. No matter where the UFO goes; we stay 1000m SSE of it so it is never too far away and the sun is never in the camera.

When you tell the pilot to chase the UFO, he goes into acrobatic-daredevil mode - and gives our cameraman a break: anywhere the UFO darts to, we maintain our distance and relative angle; the cameraman can sit down because the UFO will stay in the same place relative to the helicopter. Essentially, we are "formation flying" with the UFO.

When you tell the pilot to lock onto the UFO and White House, he gives the cameraman the chance for an Emmy award. The helicopter pilot keeps moving so that the angle between us, the UFO and the White House remains the same, so both the President (on the lawn in his PJ's) and the UFO remain in the picture as the UFO flies on towards the Pentagon.

And, finally, when you tell the pilot to synchronise with the UFO, we go into "airsick" mode: we whizz around and around the UFO as it spins so that to us the same spot on the UFO is always facing us, and the cameraman can zoom in on the "made in Taiwan" sticker he noticed earlier.

Substitute the Celestia view for the picture our cameraman is sending, and your favourite planet or satellite for the UFO and perhaps this might help in visualising the effects of the various commands.

[BlindedByTheLight]

The Chase command is similiar to the Follow command. You will pace the object you are locked on. The difference is that you will see the stars move in the background.

Thanks Frank. Gotta question for ya, though. Why do we see the stars move in the background? In what why is CHASING a different kind of "tracking" that we see the stars moving in the background?

Thanks,
S

[fsgregs]

Steve:

In track mode, we do not stay the same distance away from the object. It simply whizzes by and gets farther and closer to us, per our position in front or behind it.

In chase mode, we pace it and always stay the same distance away from it. Chase mode is like Follow ... we pace the object.

Frank

[BlindedByTheLight]

Steve:

In track mode, we do not stay the same distance away from the object. It simply whizzes by and gets farther and closer to us, per our position in front or behind it.

In chase mode, we pace it and always stay the same distance away from it. Chase mode is like Follow ... we pace the object.

Frank

Thanks, Frank.

Tracking, I actually have down (perhaps I shouldn't have used the word "tracking", parenthetical or not to reference chase and following). But to say that both "chase" and "follow" always stay the same distance... doesn't actually distinguish the DIFFERENCE between them (aside from background star move-age), which is what I was asking.

In other words, I understand that the difference between CHASE and FOLLOW is, as you noted, the "stars move in the background" in one and not the other.

My question was: why? They both pace. They both stay the same distance away. Neither are revolving around the body (as in Sync Orbit). But the stars still move in the B/G with chase (kinda like Sync Orbit) - so obviously chase is rotating around the body in some way. But what governs the speed of this rotation?

I'm thinking FOLLOW always points the observer in the same "absolute" direction - irrespective of the revolutions of the body being followed. Whereas CHASE takes into account the revolutions of the body being chased and always keeps the observer's back at the center of that orbit?

That seems kinda complicated, though. Anyone want to weigh in?

[cpotting]

I'm thinking FOLLOW always points the observer in the same "absolute" direction - irrespective of the revolutions of the body being followed. Whereas CHASE takes into account the revolutions of the body being chased and always keeps the observer's back at the center of that orbit?

That seems kinda complicated, though. Anyone want to weigh in?

You have it. In FOLLOW, the frame of reference is centred on the target, but does not change orientation as the target moves. Therefore the vector between the observer and the target always remains the same. That is if you follow a planet, the "universal" direction between you and the planet does not change. This is why you do not see the stars in the background move.

In CHASE, the frame of reference is centred on the target and is oriented with the target's direction of movement. Sort of like "forward" always being the direction in which the planet is heading. As the planet orbits its sun, "forward" slowly changes. When you are in CHASE mode, the vector between the observer and the target remains the same as measured in that frame of reference (e.g. you remain "behind" the planet), but the frame of reference is slowly turning as the planet orbits. And so the stars appear to move in the background.

[BlindedByTheLight]

Thank you! Part of my problem was I was always perpincular to the target when I did chase, so I didn't grasp what I was doing intuitively. Behind behind the target as it moves makes it all the more clear (along with your very specific explanation).

The part is that the orbit lines don't quite mesh up all that well so the frame of reference is a bit skewed.

One more question, though... when I follow the earth, the view "rocks" side to side as the Earth travels around the the sun (with me following). Where is this rocking motion coming from?

Thanks,
S

[cpotting]

One more question, though... when I follow the earth, the view "rocks" side to side as the Earth travels around the the sun (with me following). Where is this rocking motion coming from?

Thanks,
S

I am assuming that you are seing this when you "chase" Earth, not "follow" it. I don't see how you would observe any rocking in FOLLOW mode.

Assuming that you are in CHASE mode, the behaviour you describe is because the alignment of the three axes. The Z axis extends along the direction of motion for the object (Earth). The Y axis is aligned with Earth's axis of rotation and the X axis is orthanormal to the other two (in this case, it points more or less towards the sun).

This means that in CHASE mode the X-Z plane, the plane that defines what laymen would intuitively refer to as "flat", is always aligned with the Earth's equator. The X and Z axes always lay on this plane, but not always in the same "universal" direction. As the Earth orbits Sol, the Earth's direction of travel (which defines the Z axis) rotates, and the X axis rotates to match. However, the Y axis (up) never changes - the Earth keeps pointing in the same direction. To see the effect, try the following

First, you need two people - one can do it, but keeping all the movements straight may be a little confusing.
Second you need either a globe or a flat disk like a plate. I think a plate will work better.

Now, your friend is going to hold the plate so it represent the X-Z plane when chasing the Earth. To do this he just needs to hold the plate out in front or himself and tilt it so it is not quite flat and then turn around slowly in a circle to represent the Earth orbiting the sun. The trick is that the flat of the plate should always point in the same direction - just as the Earth's axis of rotation always points towards Polaris. Tell him to think of the plate as a sattelite dish that he has to keep aligned while he turns around. Good, we know have a representation of Earth.

Now you are going "chase" the plate - walk right behind the plate, on the outside of the circle, wherever - it doesn't matter as long as you keep your position relative to the plate the same. You may find it easiest though to walk "behind" the plate as it orbits and keep looking at the plate edge-on. The second thing you must do is remember that the plate defines your Y axis. Keep tilting your head so that plate is horizontal to you (as if you were going to take a picture and you want to get an edge-on shot the plate and have it be horizontal in the frame).

Now, as your friend moves in the circle representing Earth's orbit and you follow along, you will find that you have to rock your head back and forth to keep that plate horizontal in your view. In fact, you should find that you have to squat and stretch up at times too. The more tilted the plate is, the more you have to squat and stretch and rock left and right.

Both of these effects are occuring in Celestia, but the squat and stretch is not as appearent because there is no ready reference for this motion. But as you rock your head back and forth, the room around you tilts this way, then that. That is the rocking motion you are seeing in Celestia.

I hope that helps.

Now put back your grandmother's bone china plate before your friend gets dizzy and drops it!