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Navigation Aids

2. General aviation operators requesting ap-

proval for special procedures should contact the local

Flight Standards District Office to obtain a letter of

authorization. Air carrier operators requesting

approval for use of special procedures should contact

their Certificate Holding District Office for authoriz-

ation through their Operations Specification.

c. Transponder Landing System (TLS)

1. The TLS is designed to provide approach

guidance utilizing existing airborne ILS localizer,

glide slope, and transponder equipment.

2. Ground equipment consists of a transponder

interrogator, sensor arrays to detect lateral and

vertical position, and ILS frequency transmitters. The

TLS detects the aircraft’s position by interrogating its

transponder. It then broadcasts ILS frequency signals

to guide the aircraft along the desired approach path.

3. TLS instrument approach procedures are

designated Special Instrument Approach Procedures.

Special aircrew training is required. TLS ground

equipment provides approach guidance for only one

aircraft at a time. Even though the TLS signal is

received using the ILS receiver, no fixed course or

glidepath is generated. The concept of operation is

very similar to an air traffic controller providing radar

vectors, and just as with radar vectors, the guidance

is valid only for the intended aircraft. The TLS

ground equipment tracks one aircraft, based on its

transponder code, and provides correction signals to

course and glidepath based on the position of the

tracked aircraft. Flying the TLS corrections com-

puted for another aircraft will not provide guidance

relative to the approach; therefore, aircrews must not

use the TLS signal for navigation unless they have

received approach clearance and completed the

required coordination with the TLS ground equip-

ment operator. Navigation fixes based on

conventional NAVAIDs or GPS are provided in the

special instrument approach procedure to allow

aircrews to verify the TLS guidance.

d. Special Category I Differential GPS (SCAT−


1. The SCAT−I DGPS is designed to provide

approach guidance by broadcasting differential

correction to GPS.

2. SCAT−I DGPS procedures require aircraft

equipment and pilot training.

3. Ground equipment consists of GPS receivers

and a VHF digital radio transmitter. The SCAT−I

DGPS detects the position of GPS satellites relative

to GPS receiver equipment and broadcasts differen-

tial corrections over the VHF digital radio.

4. Category I Ground Based Augmentation

System (GBAS) will displace SCAT−I DGPS as the

public use service.


AIM, Paragraph 5−4−7 f, Instrument Approach Procedures