Problem during Atlantis Landing? (during turn in HAC)

[ElPelado]

Have you noticed that there were some problems regarding the turn on the HAC? Atlantis was short on the 180 and 90. As far as I heard, it was because they approached the HAC in a slightly different path than they were supposed to, so the turn should have been a little longer but it wasn't, or something like that....
Does anyone have any more details?

 

[Arrowstar]

I did notice that, too. Not sure why, though.

 

[DaveS]

Not a problem. Was expected that they would be 1600 ft low on the HAC due to the winds.

 

[ElPelado]

If it was expected, why didn't they correct their path before so they could get at the right angle inside the hac?

 

[Urwumpe]

Wind conditions are not waiting for change until you land.

If wind changes after deorbit, you can't include the data into the reentry trajectory anymore.

 

[ElPelado]

Got it! Thanks!

 

[DaveS]

Wind conditions are not waiting for change until you land.

If wind changes after deorbit, you can't include the data into the reentry trajectory anymore.

Plus, those ground tracks are generated days prior to de-orbit. They're only slightly updated on entry day. Reason for the orbiter not following the pre-generated ground track is that it is on Closed Loop Guidance(CLG).

CLG means that the GPCs are actively monitoring the altitude and energy vs the range to go and makes the necessary corrections to stay on altitude and on energy.

 

[Donamy]

Didn't they have a problem with the GPS ?

 

[DaveS]

Didn't they have a problem with the GPS ?

Only Endeavour is 3-string GPS. Discovery and Atlantis is 1-string GPS and 3-string TACAN.

 

[Urwumpe]

Only Endeavour is 3-string GPS. Discovery and Atlantis is 1-string GPS and 3-string TACAN.

As translation:

Endeavour can use three GPS receivers at once for navigation permitting it to rely on GPS data for updating the inertial navigation during reentry.

Atlantis and Discovery have one GPS receiver only for testing GPS, but use three TACAN (The military version of the VOR) receivers for updating the inertial navigation instead.

A three-string GPS orbiter has no TACAN at all.