"Lynndel K. Humphreys" wrote in
:

Is TACAN the controlling force for landing?

It's a navigation aid, not a controlling force.

Does it say "Here I am at
x altitude traveling a y velocity and decelerating at Z nm an hour In
order to get to point P I need to be at an angle of attack of such and
such degrees and need to veer to the left or right in order to remain
"on energy".

Boiling the process down to the essentials, it really goes like:

The navigation software says, "Here I think I am with position vector P
and velocity vector V".

The IMUs say, "Here you are decelerating at rate A".

The navigation software integrates acceleration over time to get
velocity, and velocity over time to get position, and says, "Here's my
new position vector P and velocity vector V".

The TACAN says, "Here you are at range R and azimuth Z relative to me,
the TACAN station."

The navigation software says, "Based on P and V and where I know that
TACAN station is, I was expecting different values of R and Z. So I'll
subtract what I expected from what I got. If that difference is too big,
I'm not going to trust the TACAN station. Otherwise, I'll update P and V
by some fraction of that difference."

The guidance software says, "Based on that updated P and V, here's the
angle of attack and bank angle we need to stay on energy and reach the
runway."

The control software says, "Based on our current attitude, here's how we
need to deflect the aerosurfaces and fire the RCS jets to get to that
angle of attack and bank angle."

That's simplified a bit - in addition to TACAN, the software can also use
DRAG H (altitude derived from acceleration), the air data probes, and MLS
to update the state vector. Or even just replace the entire state vector
with the GPS state vector. And there are multiples of each one: three
IMUs, three TACANs, four air data transducers, three MLSs, and the
software has to figure out which ones are "good".

( And it does this all by a little gremlin or gnome and
his pocket calculator.)

Gerbils and their AP-101S's, actually, but same difference...

--
JRF

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