A topic speaking about the work I am doing about TAEM and Autoland original guidances to not overload the main dev topic with that long term goal.
Documentations
*Space Shuttle TAEM guidance code sum up: https://ntrs.nasa.gov/citations/19920010688
*TAEM/Approach Handbooks there: https://forum.flightgear.org/viewtopic.php?f=87&t=38777
Overview
The last link mentionned above is pretty interesting to see the evolution of TAEM guidance and how it was handled.
The main document I used include the Optional TAEM Targeting (OTT) logic that has been used since STS-5 (before the HAC was a circle with less Energy options for test flights).
After STS-5, HAC could be flown with the different options we are used to see .
Overhead or Straight-In HAC; and Nominal Entry Point (7Nm in final) or Minimal Entry Point (4Nm in final)
Another option called - final radius shrinking - is included in that TAEM guidance version.
It allows the final HAC radius (2.3 Nm) to decrease up to 0.8 Nm if we are low during the HAC
The whole logic is organized through several functions that are called during all the TAEM phase at a rate between 160 and 980ms.
It ends at 10000 feet (Approach and Landing interface) where the Auto Land logic kicks in ( quite the same logic with tighter gains).
Let's go briefly through each functions.
The first function that is not mentionned is a frame coordinate converter from a Greenwhich frame into a runway centered frame.
1) TGXHAC
It is the initial TAEM computation where the HAC is defined following what we choose in the Spec 50 display (Runway / Overhead or Straight-In / NEP or MEP / etc)
Aim point is also taken into account (7500 feet or 6500 feet)
Another important factor that is calculated there is the Final Glideslope value for the A/L final path (starting at 10000 feet / 6Nm)
It can be either 18° for a heavy weight (more than 220000 lbs) or 20° for a lighter Shuttle.
That was changed later in the STS program to take into account the nominal mid-value for Speedbrake effectiveness ( 65° ).
That slope combination was then choosen.
In the dev branch, we can clearly see it now at Approach/Landing Interface in the HUD and PFD glideslope deviation.
Left HUD for Heavy / Right for Light
2) GTP (Ground Track Computations)
The distance to the runway is computed in that function.
*Left and Center Picture (Before and into the HAC): The range to go is the sum of the distance to be flown while aligning with the HAC entry point(ARCAC), the distance to be flown to that tangent WP1 (RTAN), the distance to be flown into the HAC up to the threshold (RPRED2)
*Right picture: Once close enough to the final runway course, distance forecasted becomes a direct distance to the runway threshold.
3) TGCOMP (General Computations)
All the computations for reference parameters are done there.
*The Altitude reference
A mix between linear and cubic segments.
Up to 40 Nmish in blue, a low slope linear profile ( 6° ish of slope)
Between A/L interface ( 6Nm) and 40 Nm, the green cubic segment where the slope increases up to the final Gamma targeted (18/20°)
At A/L interface, the red linear segment for final slope.
*The Specific Energy reference
That allows to shape the Nominal Energy path based on the True Airspeed and Altitude.
S-turn / Nominal / Low Energy boundaries.
Left picture is the Energy lines I took as a reference (closer to the latest Energy profiles flown in the later part of STS program )
Right picture shows that those lines are "just" some linear functions with some breakpoints here and there.
*Dynamic Pressure Reference Profile
It is basically the EAS targeted.
I adjusted it also to be closer to the latest QBAR profile flown (a tad higher, 305 psf targeted at A/L ie. 300 kts instead of 275 kts)
It will be coherent with the EAS visible in the Vert Traj display.
4) TGTRAN ( Transition between TAEM phases)
Here is handled the boundaries between the different part of the TAEM and Autoland ( Acquisition / HAC / Pre final / Outer Glide Slope / etc )
It works like a big Lego, and it will be easy for example to link it later to the RTLS logic with the phases 4 for 6.
The S-Turns are also commanded in that function for example.
A sanity check is done to turn away from the HAC. Once the Total Energy is closed to the Nominal One, IPHASE 0 is exited and we go back to the Acquisition logic (IPHASE 1)
5) TGNZC ( Nz Commanded)
The output for the Pitch AP is done there, under the form of a Radial Acceleration that will be converted later on to a Pitch Rate and sent into the AP loop.
All the functions calculated in the TGCOMP will be used there to limit that commanded Nz (NZC).
MIDVALUE function is like a Nasal clamp function.
The first NZC computed is for the altitude error, then it goes through an Energy error check, then QBAR, and finally a Max Nz filter to clamp the NZC between -0.5 g and +0.5g.
An example.
If we are low: positive NZC computed to Pitch Up and come back to the reference altitude.
If we are high on energy, a less positive NZC is then outputted to not go too high on energy.
If we are too close to QBAR boundaries, NZC is adjusted to stay away from a hazardous aerodynamical pressure condition.
Finaly, if we are approaching the max radial G's tolerated, the NZC is limited again to avoid breaking the wings.
A quite strong and intricated pitch command loop.
6) TGSBC (Speedbraked Commanded)
It handles the SB logic.
SB setting is fixed to 65° until Subsonic.
In Subsonic, position commanded is a function of reference Energy and Qbar errors until final where the logic is blended into a QBAR error only up to 3000 feet.
From there,a fix setting is commanded for Pre-Flare ( Highlighted in the second page there: https://forum.flightgear.org/viewtopic.php?f=87&t=38777&start=15).
7)TGPHIC (Bank commanded)
Here is done the bank commanded sent to the AP.
It depends on the Phase we are in.
From Left to Right.
*IPHASE 0 (S-turn): A constant bank is commanded away from the HAC
*IPHASE 1 (HAC acquisition): A bank proportionnal to the delta azimuth (DPSAC) with the WP1 is commanded (2.5 * delta azimuth)
*IPHASE 2 (In HAC): A bank proportionnal to the the HAC cross range and radial velocity is commanded.
*IPHASE 3 (Pre Final): A bank proportionnal to the final axis cross range / cross range variation /cross range integral is commanded.
For the Autoland phases , it is similar to the Pre Final with different Gains.