Aeronautical Information Manual (AIM), page 66

Index   65 -- Page 66 -- 67

aligned with the runway and the course may be wider,
resulting in less precision.

d. Usable off-course indications are limited to
35 degrees either side of the course centerline.
Instrument indications received beyond 35 degrees
should be disregarded.

e. The SDF antenna may be offset from the runway
centerline. Because of this, the angle of convergence
between the final approach course and the runway
bearing should be determined by reference to the
instrument approach procedure chart. This angle is
generally not more than 3 degrees. However, it should

be noted that inasmuch as the approach course
originates at the antenna site, an approach which is
continued beyond the runway threshold will lead the
aircraft to the SDF offset position rather than along
the runway centerline.

f. The SDF signal is fixed at either 6 degrees or
12 degrees as necessary to provide maximum
flyability and optimum course quality.

g. Identification consists of a three-letter identifi-
er transmitted in Morse Code on the SDF frequency.
The appropriate instrument approach chart will
indicate the identifier used at a particular airport.

1-1-12 Navigation Aids

Page 66 of the Aeronautical Information Manual (AIM.pdf)
AIM: Official Guide to Basic Flight Information and ATC Procedures

Index   65 -- Page 66 -- 67