page-61

863

Federal Aviation Administration, DOT

§ 171.311

Note 3:

Characters are encoded using the International Al-

phabet Number 5, (IA–5):

Note 4:

Coding for status bit:

0 = Function not radiated, or radiated in test mode (not reli-

able for navigation).

1 = Function radiated in normal mode (for Back Azimuth,

this also indicates that a Back Azimuth transmission follows).

Note 5:

Date items which are not applicable to a particular

ground equipment shall be transmitted as all zeros.

Note 6:

Coding for status bits:

0 DME transponder inoperative or not available.

0 Only IA mode or DME/N available.

0 FA mode, Standard 1, available.

1 FA mode, Standard 2, available.

Note 7:

The value coded shall be the actual beamwidth (as

defined in § 171.311 (j)(9) rounded to the nearest 0.5 degree.

Note 8:

When back Azimuth guidance is provided, Data

Words 4 and 6 shall be transmitted at intervals of 1.33 sec-
onds or less throughout the Approach Azimuth coverage and
4 seconds or less throughout the Back Azimuth coverage.

Note 9:

When Back Azimuth guidance is provided, Data

Word 5 shall be transmitted at an interval of 1.33 seconds or
less throughout the Back Azimuth coverage sector and 4 sec-
onds or less throughout the Approach Azimuth coverage sec-
tor.

Note 10:

Coding for status bit:

0 = Function not radiated, or radiated in test mode (not reli-

able for navigation).

1 = Function radiated in normal mode.

(4)

Minimum glidepath

the lowest

angle of descent along the zero degree
azimuth that is consistent with pub-
lished approach procedures and obsta-
cle clearance criteria.

(5)

Back azimuth status

shall represent

the operational status of the Back Azi-
muth equipment.

(6)

DME status

shall represent the

operational status of the DME equip-
ment.

(7)

Approach azimuth status

shall rep-

resent the operational status of the ap-
proach azimuth equipment.

(8)

Approach elevation status

shall rep-

resent the operational status of the ap-
proach elevation equipment.

(9)

Beamwidth

the width of the scan-

ning beam main lobe measured at the

3 dB points and defined in angular

units on the antenna boresight, in the
horizontal plane for the azimuth func-
tion and in the vertical plane for the
elevation function.

(10)

DME distance

shall represent the

minimum distance between the DME
antenna phase center and the vertical
plane perpendicular to the runway cen-
terline which contains the MLS datum
point.

(11)

Approach azimuth magnetic ori-

entation

shall represent the angle meas-

ured in the horizontal plane clockwise
from Magnetic North to the zero-de-
gree angle guidance radial originating
from the approach azimuth antenna
phase center. The vertex of the meas-

ured angle shall be at the approach azi-
muth antenna phase center.

OTE

: For example, this data item would

be encoded 090 for an approach azimuth an-
tenna serving runway 27 (assuming the mag-
netic heading is 270 degrees) when sited such
that the zero degree radial is parallel to cen-
terline.

(12)

Back azimuth magnetic orientation

shall represent the angle measured in
the horizontal plane clockwise from
Magnetic North to the zero-degree
angle guidance radial originating from
the Back Azimuth antenna. The vertex
of the measured angle shall be at the
Back Azimuth antenna phase center.

OTE

: For example, this data item would

be encoded 270 for a Back Azimuth Antenna
serving runway 27 (assuming the magnetic
heading is 270 degrees) when sited such that
the zero degree radial is parallel to center-
line.

(13)

Back azimuth proportional cov-

erage limit

shall represent the limit of

the sector in which proportional back
azimuth guidance is transmitted.

(14)

MLS ground equipment identifica-

tion

shall represent the last three char-

acters of the system identification
specified in § 171.311(i)(2). The char-
acters shall be encoded in accordance
with International Alphabet No. 5 (IA–
5) using bits b

through b

OTE

: Bit b

of this code may be recon-

structed in the airborne receiver by taking
the complement of bit b

(k)

Residual radiation.

The residual

radiation of a transmitter associated
with an MLS function during time in-
tervals when it should not be transmit-
ting shall not adversely affect the re-
ception of any other function. The re-
sidual radiation of an MLS function at
times when another function is radi-
ating shall be at least 70 dB below the
level provided when transmitting.

(l)

Symmetrical scanning.

The TO and

FRO scan transmissions shall be sym-
metrically disposed about the mid-scan
point listed in Tables 4a, 4b and 5. The
mid-scan point and the center of the
time interval between the TO and FRO
scan shall coincide with a tolerance of
plus or minus 10 microseconds.

(m)

Auxiliary data

—(1)

Addresses.

Three function identification codes are
reserved to indicate transmission of
Auxiliary Data A, Auxiliary Data B,

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