b..
Concerning a precision radar approach (PAR), it is the point where the glide path intercepts the landing surface.

(See ICAO term TOUCHDOWN.)

TOUCHDOWN [ICAO]- The point where the nominal glide path intercepts the runway.
Note: Touchdown as defined above is only a datum and is not necessarily the actual point at which the aircraft will touch the runway.
TOUCHDOWN RVR-
(See VISIBILITY.)
TOUCHDOWN ZONE- The first 3,000 feet of the runway beginning at the threshold. The area is used for determination of Touchdown Zone Elevation in the development of straight‐in landing minimums for instrument approaches.
(See ICAO term TOUCHDOWN ZONE.)
TOUCHDOWN ZONE [ICAO]- The portion of a runway, beyond the threshold, where it is intended landing aircraft first contact the runway.
TOUCHDOWN ZONE ELEVATION- The highest elevation in the first 3,000 feet of the landing surface. TDZE is indicated on the instrument approach procedure chart when straight‐in landing minimums are authorized.
(See TOUCHDOWN ZONE.)
TOUCHDOWN ZONE LIGHTING-
(See AIRPORT LIGHTING.)
TOWER- A terminal facility that uses air/ground communications, visual signaling, and other devices to provide ATC services to aircraft operating in the vicinity of an airport or on the movement area. Authorizes aircraft to land or takeoff at the airport controlled by the tower or to transit the Class D airspace area regardless of flight plan or weather conditions (IFR or VFR). A tower may also provide approach control services (radar or nonradar).
(See AIRPORT TRAFFIC CONTROL SERVICE.)
(See APPROACH CONTROL FACILITY.)
(See APPROACH CONTROL SERVICE.)
(See MOVEMENT AREA.)
(See TOWER EN ROUTE CONTROL
SERVICE.)
(See ICAO term AERODROME CONTROL
TOWER.)
(Refer to AIM.)
TOWER EN ROUTE CONTROL SERVICE- The control of IFR en route traffic within delegated airspace between two or more adjacent approach control facilities. This service is designed to expedite traffic and reduce control and pilot communication requirements.
TOWER TO TOWER-
(See TOWER EN ROUTE CONTROL
SERVICE.)

TPX‐42- A numeric beacon decoder equipment/ system. It is designed to be added to terminal radar systems for beacon decoding. It provides rapid target identification, reinforcement of the primary radar target, and altitude information from Mode C.
(See AUTOMATED RADAR TERMINAL
SYSTEMS.)
(See TRANSPONDER.)

TRACEABLE PRESSURE STANDARD-The facility station pressure instrument, with certifica-tion/calibration traceable to the National Institute of Standards and Technology. Traceable pressure standards may be mercurial barometers, commis-sioned ASOS or dual transducer AWOS, or portable pressure standards or DASI.