Question Universal Pointing MFD Inertial Attitudes

[sunshine135]

I had a question around how the Space Shuttle handles inertial attitudes around Universal Pointing MFD. I almost have this figured out, but need some help.

In Wikipedia, I read the following regarding the inertial frame:

If the goal is to keep the shuttle during its orbits in a constant attitude with respect to the sky, e.g. in order to perform certain astronomical observations, the preferred reference is the inertial frame, and the RPY angle vector (0|0|0) describes an attitude then, where the shuttle's wings are kept permanently parallel to the earth's equator, its nose points permanently to the [ame="http://en.wikipedia.org/wiki/Vernal_equinox"]vernal equinox[/ame], and its belly towards the Northern [ame="http://en.wikipedia.org/wiki/Polar_star"]polar star[/ame]

I noticed when I engage 0,0,0 and ITEM 18 on the Universal Pointing MFD that indeed, the wings are level to the equator, the nose does point to the vernal equinox, but I am perplexed that the top and not the "belly" of the shuttle points towards Polaris.

Is the Shuttle this way because the Star Tracker is located on the upper nose of the vehicle, or is the INS on the Universal Pointing MFD out by 180 degrees on the roll vector?

Thanks,

 

[SiameseCat]

That sounds like a bug in Universal Pointing MFD. It certainly doesn't have anything to do with the star trackers.

 

[sunshine135]

SiameseCat- I tried this with two separate scenarios: The first pic is with the Endeavor on Orbit Scenario. The second pic is from the flight deck in the STS-2 On Orbit Scenario. I have the same result.

Also, isn't the Vernal Equinox between Omega and Iota in the constellation of Pisces? The Universal Pointing MFD is placing the nose of the Shuttle around Orion and Taurus.

---------- Post added 10-11-12 at 09:44 AM ---------- Previous post was 10-10-12 at 02:37 PM ----------

UPDATE: I see that the Universal Pointing MFD uses the same code as the Attitude MFD. When I engage Attitude MFD, it also points the shuttle in this manner. The issue appears to be in the code for the MFD. I'm wondering if this is worth a fix though. It appears that attitude MFD has been out since 2003, and it also appears that I may be the only person who has brought this any attention. Another possibility may be that I just don't know what I am talking about, and at RPY values of 0/0/0 the Shuttle does point the nose between Taurus and Gemini, the top of the Shuttle points towards Polaris, and the wings are level with the ecliptic and not the Earth's equator. This goes against what Wikipedia says, but that has been known to be wrong as well. If anyone is familiar with inertial attitudes and the Shuttle though, your understanding in this area would be appreciated.

Thanks,

 

[Cras]

I think....THINK...I am not sure, but that Attitude MFD and the Shuttle use two different reference systems. Shuttle was M50...and Attitude MFD I think might be J2000. That maybe what is going on.

I think. I really have no clue. I use Attitude MFD all the time to keep a star fixed attitude but I do it in a way that keeps my nose and wings in line with the velocity vector. I have played around with Universal Pointing only very little, and I was not aware that it claimed to be as accurate as the real Ops 201

 

[sunshine135]

Cras,

On Universal Pointing, the engine that drives it is the same as Attitude MFD. Is the M50 the same as the 1950 inertial attitudes? I seem to recall that there are three separate types: 1950, 2000, and TOD. TOD would be the most accurate, based on the way things are today. 2000 would be based on Vernal Equinox position in 2000. I am assuming 1950 is based on the 1950 equinox position, but I seriously doubt that it was between Gemini and Taurus.

Either way, I am trying to determine if it is accurate to the actual Shuttle. Why this is even important to me is simple- if you attempt to perform a maneuver that needs an inertial attitude based on a real Shuttle checklist, your attitude may be off considerably. I believe the Wikipedia information was a bit too generic, so I'm curious if I could get verification from someone who may be familiar with the Inertial Navigation on the Shuttle. If no one knows, it isn't the end of the world. It just brings up the question of usability of Inertial Shuttle References.

One other note Cras- you are using Attitude MFD to hold an LVLH attitude. This is different than a star-fixed or inertial attitude which would reference a point on the celestial sphere, not the Earth.

 

[SiameseCat]

I think....THINK...I am not sure, but that Attitude MFD and the Shuttle use two different reference systems. Shuttle was M50...and Attitude MFD I think might be J2000. That maybe what is going on.

I think. I really have no clue. I use Attitude MFD all the time to keep a star fixed attitude but I do it in a way that keeps my nose and wings in line with the velocity vector. I have played around with Universal Pointing only very little, and I was not aware that it claimed to be as accurate as the real Ops 201

Attitude MFD (and Universal Pointing) uses Orbiter's inertial reference frame (which is left-handed). The real shuttle uses the right-handed M50 reference frame.

There are actually quite a few issues with Universal Pointing MFD; apart from using the wrong inertial frame, there are a few cases where it gets the LVLH attitude wrong. IIRC, it's also inefficient in maneuvering to a specified LVLH attitude.

---------- Post added at 02:46 PM ---------- Previous post was at 02:43 PM ----------

Cras,

On Universal Pointing, the engine that drives it is the same as Attitude MFD. Is the M50 the same as the 1950 inertial attitudes? I seem to recall that there are three separate types: 1950, 2000, and TOD. TOD would be the most accurate, based on the way things are today. 2000 would be based on Vernal Equinox position in 2000. I am assuming 1950 is based on the 1950 equinox position, but I seriously doubt that it was between Gemini and Taurus.

Either way, I am trying to determine if it is accurate to the actual Shuttle. Why this is even important to me is simple- if you attempt to perform a maneuver that needs an inertial attitude based on a real Shuttle checklist, your attitude may be off considerably. I believe the Wikipedia information was a bit too generic, so I'm curious if I could get verification from someone who may be familiar with the Inertial Navigation on the Shuttle. If no one knows, it isn't the end of the world. It just brings up the question of usability of Inertial Shuttle References.

One other note Cras- you are using Attitude MFD to hold an LVLH attitude. This is different than a star-fixed or inertial attitude which would reference a point on the celestial sphere, not the Earth.

J2000 and M50 are almost identical. The real problem is that Orbiter's frame is left-handed, and uses the ecliptic as a reference. The shuttle's frame is right-handed and equatorial.

 

[sunshine135]

Thank you very much SiameseCat. I think this answers my question. All inertial attitudes should be suspect with Universal Pointing MFD. This is not based on the author - rather it is based on an Orbiter limitation.

 

[Cras]

Sunshine, no what I do is I hold LVLH 0-0-0 until I cross the equator northbound then I set that as the reference attitude and switch to the inertial hold mode. If that makes sense, so the stars are fixed, but I am offsetting my yaw and roll so I am inline with the Earth...

I only do this sometimes, otherwise I am either offsetting myself based on an object, like the Sun for example. (Thinking of Payload bay door closing here), or just holding a full LVLH attitude.

And thanks Siamese Cat, I totally forgot about Orbiter being left-handed.....derp on my part.

With that being said, would it be possible to create a universal attitude MFD that takes into account a right handed equatorial inertial reference? Like a Right Handed version of Attitude MFD perhaps?

 

[sunshine135]

Cras,

I understand what you are saying now. You manually take into account the attitude, and then adjust your P,Y,R based on the offset at the equator. To make the wings level to the equator though, you would need an OM 90 to LVLH though right basically a prograde or retrograde orientation versus a wing level right? In either case- very smart indeed. So let me follow that up with a question: If I were to obtain an approximate wing-level attitude at the equator and I set up the date of the mission to be at the vernal equinox, then theoretically, if I pointed the nose of the Shuttle at the sun at the exact moment of the vernal equinox, would that not give me the appropriate offset for P, Y, R to determine what 0,0,0 would look like on Universal Pointing MFD for all missions? And going back to the original question, would it be belly up towards Polaris or belly down away from Polaris?

 

[SiameseCat]

Thank you very much SiameseCat. I think this answers my question. All inertial attitudes should be suspect with Universal Pointing MFD. This is not based on the author - rather it is based on an Orbiter limitation.

It's actually possible to hold the correct inertial attitude - the SSU attitude control code uses the M50 frame and I'm not aware of any inaccuracies. Universal Pointing MFD hasn't been updated for a long time, because I don't have the time to maintain it, and Attitude MFD provides most of the same functionality. From a technical standpoint, it should be fairly easy to update Universal Pointing MFD to function like the actual MM 201 displays.

---------- Post added at 06:00 PM ---------- Previous post was at 05:49 PM ----------

Cras,

I understand what you are saying now. You manually take into account the attitude, and then adjust your P,Y,R based on the offset at the equator. To make the wings level to the equator though, you would need an OM 90 to LVLH though right basically a prograde or retrograde orientation versus a wing level right? In either case- very smart indeed. So let me follow that up with a question: If I were to obtain an approximate wing-level attitude at the equator and I set up the date of the mission to be at the vernal equinox, then theoretically, if I pointed the nose of the Shuttle at the sun at the exact moment of the vernal equinox, would that not give me the appropriate offset for P, Y, R to determine what 0,0,0 would look like on Universal Pointing MFD for all missions? And going back to the original question, would it be belly up towards Polaris or belly down away from Polaris?

You can't really calculate a useful offset - adding P,Y,R angles doesn't work, and the axes are difference between the Orbitersim and M50 frames.

The correct 0,0,0 attitude is the nose pointing towards the vernal equinox and the belly pointing towards Polaris.

 

[sunshine135]

You can't really calculate a useful offset - adding P,Y,R angles doesn't work, and the axes are difference between the Orbitersim and M50 frames.

The correct 0,0,0 attitude is the nose pointing towards the vernal equinox and the belly pointing towards Polaris.

Ah, of course. I had to sleep on that for it to sink in completely. The Yaw Axis is at the ecliptic versus equatorial, everything would be offset.

Thanks again for the explanation.