News The Space Shuttle for Flightgear 3.6

zerofay32

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So flightgear is some alternate reality where the shuttle was equipped with a SSVP and docks to the Russian segment... :flowers:

Kidding aside, very nice work btw.
:thumbup:
 

Thorsten

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So flightgear is some alternate reality where the shuttle was equipped with a SSVP and docks to the Russian segment.

As should be very apparent from the rather botched job that is the ISS 3d model - I'm not a 3d modeler :lol: (I wasn't actually sure that the location was supposed to be in the model, I more or less went with a rough visual comparison).

The usual pattern is that this eventually annoys a decent 3d modeler to the degree that he chimes in and fixes things though. And at that point I can actually see what is what in the 3d model and define proper docking points...
 

zerofay32

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Also, while docking with on the -rbar is a valid procedure, it was only used on the first couple assembly missions IIRC. Once the Destiny Lab was installed (STS-98) all subsequent shuttle dockings where a +vbar approach and docking. The shuttle would approach on the -rbar until ~600ft from the station (post Columbia, would then perform the RPM) then would perform a 1/4 fly-around to the +vbar, and then approach and dock on the +vbar. The station would be in a LVLH type attitude and the shuttle docks to the nose of the station. (in your example, the "nose" of the ISS is pointed away from earth. The vertical mast with solar panels is on the zenith side and while was a planned component in the final ISS, it was canceled. The Power Science Platform I think it was called.)
 
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Thorsten

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Tinkering with the rendezvous state vector filters on SPEC 33 - anyone has an idea what kind of accuracy to expect from the radar in ranging/angle or from the star tracker?
 

Urwumpe

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Tinkering with the rendezvous state vector filters on SPEC 33 - anyone has an idea what kind of accuracy to expect from the radar in ranging/angle or from the star tracker?

What to really expect or what is specified as requirements there?

I had posted a link to a NASA report somewhere in SSU, about the problems they had with the star trackers for rendezvous, which might be interesting there.
 

Thorsten

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Anything really.

I've found a radar webpage (Shuttle unrelated) that gave me a ranging accuracy estimate of 20-50 m for various types of radar, which sounds pretty miserable for doing a rendezvous with docking in space, I know the figure of merit for an inertial nav is about a mile of drift per hour, but surely knowing one is in orbit and experiencing periodical motion must push the error down quite a bit,... so at this point I'm pretty much happy with any number for an order of magnitude for the various sensors.

Only for COAS I know how to do decent error estimates :lol:

(Of course in the end GPS will be the magic device again like for area navigation..)
 

Urwumpe

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Anything really.

I've found a radar webpage (Shuttle unrelated) that gave me a ranging accuracy estimate of 20-50 m for various types of radar, which sounds pretty miserable for doing a rendezvous with docking in space, I know the figure of merit for an inertial nav is about a mile of drift per hour, but surely knowing one is in orbit and experiencing periodical motion must push the error down quite a bit,... so at this point I'm pretty much happy with any number for an order of magnitude for the various sensors.

Only for COAS I know how to do decent error estimates :lol:


Here is the one I remembered:



https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110003998.pdf
 

Thorsten

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Someone over in the FG forum managed to dig out a good number for the star tracker accuracy - it's in the Rendezvous workbook where it gives the 1 sigma error band to about an arcmin (which is vrey precise...)

Geez, hunting down these numbers surely is tedious...
 

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First tests with the brand-new mountable COAS frame - ISS visible through the recticie.



Still need to polish the alignment, hook it up properly to the electric system ad such, but it sure adds to the proximity ops experience.
 

DaveS

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thammond

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Someone over in the FG forum managed to dig out a good number for the star tracker accuracy - it's in the Rendezvous workbook where it gives the 1 sigma error band to about an arcmin (which is vrey precise...)

Geez, hunting down these numbers surely is tedious...

Thorsten, I found some specs for the Ku radar that may be of some help.

Range: 100 ft Min - 25.5 nm Max with 3 sigma accuracy of 80 ft or 1 % and Data Resolution of .3 ft

Range Rate: 0-148 ft/s (Decreasing range), 0-75 ft/s (increasing range) with 3 sigma accuracy of 1 ft/s and Data Resolution of 0.05 ft/s

Angle: 60 degree Max diameter with 3 sigma Accuracy of 8 mrad (0.46 degree) and Data Resolution of 0.09 degree
 

Thorsten

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Looks nice, just one thing though: The COAS glass has a purple tint to it, it's not clear like the HUD glass.

Interesting... and easy to do, thanks.

Thorsten, I found some specs for the Ku radar that may be of some help.

That's terrific, thanks.

So the 20 m the radar page mentioned are in fact pretty close to the 80 ft of range.And we now know why the star tracker with an arcmin of resolution is the preferred angular measuring device.

Nice.

---------- Post added 03-01-19 at 06:19 AM ---------- Previous post was 02-28-19 at 08:32 PM ----------

Actually, I wonder whether the purple tint isn't the mountable filter used for daylight sightings that's mentioned in the Star Tracker/COAS workbook.
 

Thorsten

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And here's the calibrated COAS (aft position) with daylight filter mounted...



It's surprisingly tedious to make the device actually useful - since (like a HUD), it is a combiner and hence a projection from infinity - so just painting the pattern onto the glass is not enough, because then the axis center and the apparent angular scale depends on where the viewpoint is.

Which of course makes it useless if the user dares to move the view - so in order to make it useful, the parallax correction has to be re-computed for every movement of the viewpoint. Which is not hard conceptually, but of course the offset and scale gains for each axis have to be set correctly, and to measure that out is a bit tedious and took me the better part of two hours (part of the work was to make sure the COAS pattern in FWD position is exactly aligned with the boresight of the HUD, and the pattern in AFT position upward is aligned with the off-axis values reported by the Z star tracker - so we want the target in the pattern center when the star tracker reports zero offset angles...)

In practice, the effect of the combiner is that if you're not at the optimal view point, the pattern will appear partially shifted outside the glass, as the pattern center will stay locked on the same point of the observed object (parallax correction for head movement for the distance to ISS shown here is negligibly small).

But now the angular scale can be used regardless of where the head is.

And it's all properly integrated into systems, it powers off when the circuit breaker is pulled, when the power switches are flicked or when the bus voltage drops below operating value.
 

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I think I finally got the rel nav filters under control (after some confusion what is what and updates what and what is displayed where...) So this is for fans of Shuttle avionics intricacies only. :lol:

***

We don't know precisely where the Shuttle is because the intertial navigation always drifts a little. So that mandates periodic state vector updates.

We don't know precisely where a rendezvous target is either for the same reason.

So the state vectors of Shuttle and target propagated by the navigation system have errors, and that leads to errors in relative quantities - range, proximity coordinates,...

On board sensors like star tracker or Ku antenna with radar ranging can measure relative quantities directly - all can do angular offsets, the radar can also do distance and rate of distance change. These pose constraints on the propagated state vectors, and if these constrains are used, they result in a filtered set of state vectors, from which a different set of relative coordinates can be derived.

But the system asks whether to incorporate a measurement by presenting the ratio - that's (filtered - propagated) value divided by some threshold - if the ratio is <1, the avionics believes it;s okay to proceed, if the ratio is > 1 the sensor might be malfunctioning and is better not incorporated - or the propagated value might be unusually bad of course.

Using the measurements and filters, the idea is to get closer to truth (which of course is easily obtained in the simulation...)

***

So here's an example of how the filters in REL NAV are operated to get a decent state vector (for comparison simulated truth is always shown in the lower left):




REL NAV is enabled and defaults to propagated state vectors. At this point the system thinks the range is 9189 ft, simulated truth would be about 9845 ft, so it misses by a good margin.





Radar ranging is now on and provides its own direct distance measurement of 9802 ft - this is well within the 80 ft uncertainty the radar has at this distance.



Now item 4 is used to show the filtered state vector and item 13 provides the angular data in addition to the range to the navigation filter. All residuals and ratios are okay, so items 17 and 20 are selected to automatically incorporate the constrains into the process. The system improves its estimate from 9189 to 9775 ft (which is doing quite a lot better...)

In the event, knowing truth we know the direct radar ranging measurement is in fact better - reality is that the propagated value is very low and the ranging measurement is low, whereas the filter thinks the propagated value might be too low and the measurement a bit too high - point being, it could be otherwise, we have no way of knowing in reality, so the filter does the best estimate given what we know.



Finally, transferring the filtered state vector to the propagated one zeros all residuals (by definition) and highlights the fact that zero residuals just mean that propagated and sensed state vector are identical, but they can both be wrong in the same direction.

Anyway, this is as good as it gets, so the range remains ~100 ft off. Luckily the station is in sight, and we can use COAS to do a size measurement once closer.

But never try to dock at night using radar ranging with the Shuttle...:lol:
 

Gingin

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Excellent, that will give a sense to different ST and RR pass during final rendez vous part :)
 

Thorsten

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I've tried a fly-around procedure today (putting the Shuttle in inertial attitude hold, using Earth's rotation to turn it around, using the COAS to keep the station centered using THC and RR to hold distance).

It was a fairly instructive exercise - it's not quite as easy as it sounds on paper, largely because of the control cross couplings - even in attitude hold mode, THC always causes some residual attitude changes while firing before additional firings null the motion, so the target in the COAS recticle moves for two rather distinct reasons and they have to be kept separate.

Also, initial orientation matters - turns out it's a very bad idea to initialize the motion chiefly with the forward-firing thrusters (in my defense, its's hard to notice what is going to fire when you're staring out of the overhead window and have the sense set to Z-axis...) - this depletes the FRCS fast, and once that has happened, translations are gone.

So I guess proximity-ops have to be very FRCS-propellant conservative, maneuvering around in a hurry is a recipe for running out of gas in a hurry.

It's really quite something to experience all these little details hands-on and not just read about them...
 
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