Does ISS drift?

[diegorodriguez]

Whenever on final approachng for docking to ISS, I follow the usual order in the Docking MFD.

1. Align (X cross in the center)
2. Rotate (arrow pointing upwards)
3. Offset (+ cross in the center)

The 1st maneuver makes the rest two possible. Failing to align means you cannot effectively rotate or eliminate port offset, the projection on axis x,y of your own position.

Fortunately, once you have tuned in your selected docking port, it's quite straightforward to get a good alignment just rotating around and killing rotation once you get the cross in the center. Your attitude gets fixed, so you should see stars standing still on your windshield.

What I've noted in respect to docking to ISS is that, even killing my own rotation after reaching a good alignment, I see a small but consistent drift along time that ends making my attitude bad and forcing me to re-do alignment interleaved with two-dimensional offsetting until the very capture. This makes the whole thing extra-annoying...

Moving the point of view onboard ISS I see there's a small rotation that makes me wonder whether it should be convenient to kill rotation on ISS before actually aligning my own vessel.

So... Why does ISS rotate? Is it so in real life?

 

[garyw]

Personally I always do a kill rot on the ISS as I do the TI burn. This leaves me able to line up for the RPM and docking without messing around with ISS attitude.

 

[diegorodriguez]

What is the TI burn?

 

[Urwumpe]

In real life, the ISS keeps a local velocity/local horizontal attitude, which means that the shuttle docking port is always pointing at front (and the Soyuz/ATV/progress docking ports to the aft).

Yes, this means the ISS will usually rotate at the same rate as it orbits.

 

[garyw]

What is the TI burn?

Terminal Initiation. The final OMS burn before arriving at the ISS.

http://www.shuttlepresskit.com/STS-98/rendezvous13.htm

 

[diegorodriguez]

Understood.

The shuttle front port is in right angle with the Soyuz/ATV/Progress aft ports and the whole complex is horizon levelled.

Why? Maybe to maintain antennas looking down?

 

[garyw]

Understood.

The shuttle front port is in right angle with the Soyuz/ATV/Progress aft ports and the whole complex is horizon levelled.

Why? Maybe to maintain antennas looking down?

Two reasons - First of all the atenna need to look UP as they point to TDRS which is in a geosync orbit.

Secondly, it's a good attitude for thermal distribution and for the solar arrays to obtain maximum benefit from sunlight.

The shuttle docking port (PMA-2) is not at a right angle to the aft port. It's at the front of the ISS complex.

 

[Fizyk]

The drift in Orbiter is caused mostly by gravity gradient torque if you have it enabled, I think. When it is disabled, everything stays in place.

 

[tblaxland]

Two reasons - First of all the atenna need to look UP as they point to TDRS which is in a geosync orbit.

Secondly, it's a good attitude for thermal distribution and for the solar arrays to obtain maximum benefit from sunlight.

The main reasons are to keep a torque equilibrium attitude (TEA) and to minimise drag. TEA balances out the gravity gradient torque and atmospheric torque so that the attitude can be held with minimum control inputs. The solar/thermal/antenna systems are all designed around that attitude.

TEA is also slightly off true LVLH, depending on the station configuration, was at about yaw=350.0°,pitch=350.6°,roll=0° (XVV) for Expedition 8. YVV (about yaw=271.1°,pitch=351.6°,roll=1.8° for Exp 6) is also a TEA but was only flown earlier in assembly at high solar beta angles before the rotating radiators were fully commissioned.

 

[Donamy]

Also, after the Columbia accident, the whole assembly rotates 180 degrees TEA,after docking, so the belly of the shuttle does not fly toward the front.

 

[-Pv-]

I use the prograde autopilot when docking with the Shuttle Fleet ISS which will turn the ISS to the correct cofiguration for docking. My experience is similar to what "diegorodriguez" complains about. As soon as I re-focus back on the Shuttle for manouvers, the position of the ISS begins to wander (even though I have Nonspherical and Gravity Gradiant turned off.) As soon as I refocus, The ISS AP is disabled and any slight drift the AP was correcting then I lost focus causes the ISS to drift and I have to chase it down. It doesn't seem to make any difference how long I retain focus on the ISS. As soon as I leave focus, any correction the ISS AP was performing when I change (in order to track prograde) will cause the ISS to continue to move as a result of the last undampped correction.

I'm in hopes the effort up into ISSU gyros will eventually solve this problem.
-Pv-

 

[zerofay32]

The "drifting" isn't caused by the ISS, it's the shuttle. If the ISS is rotating at orbital rate then the shuttle must also rotate along at the same rate. Once both vehicals are rotating and you are lined up for docking you will notice a translation away from Earth (along the R bar of the station). You have to account for this in your docking approach.

 

[-Pv-]

I've been using attitude MFD to null my position to the ISS. This helps, but doing the RPM is pretty tricky.
-Pv-

 

[tblaxland]

I've been using attitude MFD to null my position to the ISS. This helps, but doing the RPM is pretty tricky.
-Pv-

AttitudeMFD v3.2 has two special modes, RPM and TOR, that you may be interested in. These will each set the required pitch rate for the RPM and TORFA/TORVA manoeuvres respectively. From the documentation:

Also useful in this mode[Target Relative], if performing a rendezvous between a space shuttle and the ISS, are the RPM and TOR buttons. These buttons can be found on the second page of buttons by pressing the MNU button until they appear. Pressing these buttons will command the auto-pilot to maintain a positive pitch rate of 0.75deg/sec and orbital rate, respectively. Press the buttons again to deactivate.

You can also use AttitudeMFD to null out your relative velocity after arriving on the R-bar (TA button).

Another tip for using AttitudeMFD when rendezvousing: Switch focus to the ISS and activate the prograde auto-pilot. Switch back to the Shuttle. Once you are on or close to the V-bar and within, say 150m, set AttitudeMFD to Velocity mode, set a relative attitude offset of Pitch (P) = 90°, Yaw (Y) = 0°, Roll (R) = 0° and press HLD. You can then simply drive into the ISS using your linear thrusters without having to worry about your alignment. EDIT: Oh yeah, AttitudeMFD will automatically turn off the hold mode as soon as you dock.

 

[TMac3000]

Docking with ISS (particularly when GGT and NSG are enabled) is like making love to a woman during a 5.2 earthquake in a double-canopy jungle at midnight.

It's going to drift, and you have to keep accounting for that, and go very slow during the approach. It's a massive pain in the you-know-what, but that's the only way.

 

[diegorodriguez]

yes, the ISS found inside Orbiter is slowly rotating when GGT and NSG are enabled. That means that no attitude control is performed on the ISS model, unlike it's real life counterpart. So I guess it's not that unfair to uncheck GGT and NSG or to use AttitudeMFD to help docking. Just remember that after GGT and NSG have been disabled, a killrot must be performed to ensure a fixed to stars attitude is achieved.

Check http://www.spaceflight.esa.int/users/downloads/userguides/opsenv.pdf for real flight attitudes and coordinate systems.