Aviation Regulations Logo
§ 171.311
Signal format requirements.
The signals radiated by the MLS must conform to the signal format in which angle guidance functions and data functions are transmitted sequentially on the same C-band frequency. Each function is identified by a unique digital code which initializes the airborne receiver for proper processing. The signal format must meet the following minimum requirements:
(a) Frequency assignment. The ground components (except DME/Marker Beacon) must operate on a single frequency assignment or channel, using time division multiplexing. These components must be capable of operating on any one of the 200 channels spaced 300 KHz apart with center frequencies from 5031.0 MHz to 5090.7 MHz and with channel numbering as shown in Table 1a. The operating radio frequencies of all ground components must not vary by more than ±10 KHz from the assigned frequency. Any one transmitter frequency must not vary more than ±50 Hz in any one second period. The MLS angle/data and DME equipment must operate on one of the paired channels as shown in Table 1b.
Table 1a—Frequency Channel Plan
Channel No.
Frequency (MHz)
500
5031.0
501
5031.3
502
5031.6
503
5031.9
504
5032.2
505
5032.5
506
5032.8
507
5033.1
508
5033.4
509
5033.7
510
5034.0
511
5034.3
*    *    *    *    *
598
5060.4
599
5060.7
600
5061.0
601
5061.3
*    *    *    *    *
698
5090.4
699
5090.7
Table 1b—Channels
Channel pairing
DME parameters
DME No.
VHF freq. MHz
MLS angle freq. MHz
MLS Ch. No.
Interrogation
Reply
Freq. MHz
Pulse codes
Freq. MHz
Pulse codes µs
DME/N µs
DME/P Mode
IA µs
FA µs
* 1X
1025
12
962
12
** 1Y
1025
36
1088
30
* 2X
1026
12
963
12
** 2Y
1026
36
1089
30
* 3X
1027
12
964
12
** 3Y
1027
36
1090
30
* 4X
1028
12
965
12
** 4Y
1028
36
1091
30
* 5X
1029
12
966
12
** 5Y
1029
36
1092
30
* 6X
1030
12
967
12
** 6Y
1030
36
1093
30
* 7X
1031
12
968
12
** 7Y
1031
36
1094
30
* 8X
1032
12
969
12
** 8Y
1032
36
1095
30
* 9X
1033
12
970
12
** 9Y
1033
36
1096
30
* 10X
1034
12
971
12
** 10Y
1034
36
1097
30
* 11X
1035
12
972
12
** 11Y
1035
36
1098
30
* 12X
1036
12
973
12
** 12Y
1036
36
1099
30
* 13X
1037
12
974
12
** 13Y
1037
36
1100
30
* 14X
1038
12
975
12
** 14Y
1038
36
1101
30
* 15X
1039
12
976
12
** 15Y
1039
36
1102
30
* 16X
1040
12
977
12
** 16Y
1040
36
1103
30
▽17X
108.00
1041
12
978
12
17Y
108.05
5043.0
540
1041
36
36
42
1104
30
17Z
5043.3
541
1041
21
27
1104
15
18X
108.10
5031.0
500
1042
12
12
18
979
12
18W
5031.3
501
1042
24
30
979
24
18Y
108.15
5043.6
542
1042
36
36
42
1105
30
18Z
5043.9
543
1042
21
27
1105
15
19X
108.20
1043
12
980
12
19Y
108.25
5044.2
544
1043
36
36
42
1106
30
19Z
5044.5
545
1043
21
27
1106
15
20X
108.30
5031.6
502
1044
12
12
18
981
12
20W
5031.9
503
1044
24
30
981
24
20Y
108.35
5044.8
546
1044
36
36
42
1107
30
20Z
5045.1
547
1044
21
27
1107
15
21X
108.40
1045
12
982
12
21Y
108.45
5045.4
548
1045
36
36
42
1108
30
21Z
5045.7
549
1045
21
27
1108
15
22X
108.50
5032.2
504
1046
12
12
18
983
12
22W
5032.5
505
1046
24
30
983
24
22Y
108.55
5046.0
550
1046
36
36
42
1109
30
22Z
5046.3
551
1046
21
27
1109
15
23X
108.60
1047
12
984
12
23Y
108.65
5046.6
552
1047
36
36
42
1110
30
23Z
5046.9
553
1047
21
27
1110
15
24X
108.70
5032.8
506
1048
12
12
18
985
12
24W
5033.1
507
1048
24
30
985
24
24Y
108.75
5047.2
554
1048
36
36
42
1111
30
24Z
5047.5
555
1048
21
27
1111
15
25X
108.80
1049
12
986
12
25Y
108.85
5047.8
556
1049
36
36
42
1112
30
25Z
5048.1
557
1049
21
27
1112
15
26X
108.90
5033.4
508
1050
12
12
18
987
12
26W
5033.7
509
1050
24
30
987
24
26Y
108.95
5048.4
558
1050
36
36
42
1113
30
26Z
5048.7
559
1050
21
27
1113
15
27X
109.00
1051
12
988
12
27Y
109.05
5049.0
560
1051
36
36
42
1114
30
27Z
5049.3
561
1051
21
27
1114
15
28X
109.10
5034.0
510
1052
12
12
18
989
12
28W
5034.3
511
1052
24
30
989
24
28Y
109.15
5049.6
562
1052
36
36
42
1115
30
28Z
5049.9
563
1052
21
27
1115
15
29X
109.20
1053
12
990
12
29Y
109.25
5050.2
564
1053
36
36
42
1116
30
29Z
5050.5
565
1043
21
27
1116
15
30X
109.30
5034.6
512
1054
12
12
18
991
12
30W
5034.9
513
1054
24
30
991
24
30Y
109.35
5050.8
566
1054
36
36
42
1117
30
30Z
5051.1
567
1054
21
27
1117
15
31X
109.40
1055
12
992
12
31Y
109.45
5051.4
568
1055
36
36
42
1118
30
31Z
5051.7
569
1055
21
27
1118
15
32X
109.50
5035.2
514
1056
12
12
18
993
12
32W
5035.5
515
1056
24
30
993
24
32Y
109.55
5052.0
570
1056
36
36
42
1119
30
32Z
5052.3
571
1056
21
27
1119
15
33X
109.60
1057
12
994
12
33Y
109.65
5052.6
572
1057
36
36
42
1120
30
33Z
5052.9
573
1057
21
27
1120
15
34X
109.70
5035.8
516
1058
12
12
18
995
12
34W
5036.1
517
1058
24
30
995
24
34Y
109.75
5053.2
574
1058
36
36
42
1121
30
34Z
5053.5
575
1058
21
27
1121
15
35X
109.80
1059
12
996
12
35Y
109.85
5053.8
576
1059
36
36
42
1122
30
35Z
5054.1
577
1059
21
27
1122
15
36X
109.90
5036.4
518
1060
12
12
18
997
12
36W
5036.7
519
1060
24
30
997
24
36Y
109.95
5054.4
578
1060
36
36
42
1123
30
36Z
5054.7
579
1060
21
27
1123
15
37X
110.00
1061
12
998
12
37Y
110.05
5055.0
580
1061
36
36
42
1124
30
37Z
5055.3
581
1061
21
27
1124
15
38X
110.10
5037.0
520
1062
12
12
18
999
12
38W
5037.3
521
1062
24
30
999
24
38Y
110.15
5055.6
582
1062
36
36
42
1125
30
38Z
5055.9
583
1062
21
27
1125
15
39X
110.20
1063
12
1000
12
39Y
110.25
5056.2
584
1063
36
36
42
1126
30
39Z
5056.5
585
1063
21
27
1126
15
40X
110.30
5037.6
522
1064
12
12
18
1001
12
40W
5037.9
523
1064
24
30
1001
24
40Y
110.35
5056.8
586
1064
36
36
42
1127
30
40Z
5057.1
587
1064
21
27
1127
15
41X
110.40
1065
12
1002
12
41Y
110.45
5057.4
588
1065
36
36
42
1128
30
41Z
5057.7
589
1065
21
27
1128
15
42X
110.50
5038.2
524
1066
12
12
18
1003
12
42W
5038.5
525
1066
24
30
1003
24
42Y
110.55
5058.0
590
1066
36
36
42
1129
30
42Z
5058.3
591
1066
21
27
1129
15
43X
110.60
1067
12
1004
12
43Y
110.65
5058.6
592
1067
36
36
42
1130
30
43Z
5058.9
593
1067
21
27
1130
15
44X
110.70
5038.8
526
1068
12
12
18
1005
12
44W
5039.1
527
1068
24
30
1005
24
44Y
110.75
5059.2
594
1068
36
36
42
1131
30
44Z
5059.5
595
1068
21
27
1131
15
45X
110.80
1069
12
1006
12
45Y
110.85
5059.8
596
1069
36
36
42
1132
30
45Z
5060.1
597
1069
21
27
1132
15
46X
110.90
5039.4
528
1070
12
12
18
1007
12
46W
5039.7
529
1070
24
30
1007
24
46Y
110.95
5060.4
598
1070
36
36
42
1133
30
46Z
5060.7
599
1070
21
27
1133
15
47X
111.00
1071
12
1008
12
47Y
111.05
5061.0
600
1071
36
36
42
1134
30
47Z
5061.3
601
1071
21
27
1134
15
48X
111.10
5040.0
530
1072
12
12
18
1009
12
48W
5040.3
531
1072
24
30
1009
24
48Y
111.15
5061.6
602
1072
36
36
42
1135
30
48Z
5061.9
603
1072
21
27
1135
15
49X
111.20
1073
12
1010
12
49Y
111.25
5062.2
604
1073
36
36
42
1136
30
49Z
5062.5
605
1073
21
27
1136
15
50X
111.30
5040.6
532
1074
12
12
18
1011
12
50W
5040.9
533
1074
24
30
1011
24
50Y
111.35
5062.8
606
1074
36
36
42
1137
30
50Z
5063.1
607
1074
21
27
1137
15
51X
111.40
1075
12
1012
12
51Y
111.45
5063.4
608
1075
36
36
42
1138
30
51Z
5063.7
609
1075
21
27
1138
15
52X
111.50
5041.2
534
1076
12
12
18
1013
12
52W
5041.5
535
1076
24
30
1013
24
52Y
111.55
5064.0
610
1076
36
36
42
1139
30
52Z
5064.3
611
1076
21
27
1139
15
53X
111.60
1077
12
1014
12
53Y
111.65
5064.6
612
1077
36
36
42
1140
30
53Z
5064.9
613
1077
21
27
1140
15
54X
111.70
5041.8
536
1078
12
12
18
1015
12
54W
5042.1
537
1078
24
30
1015
24
54Y
111.75
5065.2
614
1078
36
36
42
1141
30
54Z
5065.5
615
1078
21
27
1141
15
55X
111.80
1079
12
1016
12
55Y
111.85
5065.8
616
1079
36
36
42
1142
30
55Z
5066.1
617
1079
21
27
1142
15
56X
111.90
5042.4
538
1080
12
12
18
1017
12
56W
5042.7
539
1080
24
30
1017
24
56Y
111.95
5066.4
618
1080
36
36
42
1143
30
56Z
5066.7
619
1080
21
27
1143
15
57X
112.00
1081
12
1018
12
57Y
112.05
1081
36
1144
30
58X
112.10
1082
12
1019
12
58Y
112.15
1082
36
1145
30
59X
112.20
1083
12
1020
12
59Y
122.25
1083
36
1146
30
** 60X
1084
12
1021
12
** 60Y
1084
36
1147
30
** 61X
1085
12
1022
12
** 61Y
1085
36
1148
30
** 62X
1086
12
1023
12
** 62Y
1086
36
1149
30
** 63X
1037
12
1024
12
** 63Y
1087
36
1150
30
** 64X
1088
12
1151
12
** 64Y
1088
36
1025
30
** 65X
1089
12
1152
12
** 65Y
1089
36
1026
30
** 66X
1090
12
1153
12
** 66Y
1090
36
1027
30
** 67X
1091
12
1154
12
** 67Y
1091
36
1028
30
** 68X
1092
12
1155
12
** 68Y
1092
36
1029
30
** 69X
1093
12
1156
12
** 69Y
1093
36
1030
30
70X
112.30
1094
12
1157
12
** 70Y
112.35
1094
36
1031
30
71X
112.40
1095
12
1158
12
** 71Y
112.45
1095
36
1032
30
72X
112.50
1096
12
1159
12
** 72Y
112.55
1096
36
1033
30
73X
112.60
1097
12
1160
12
** 73Y
112.65
1097
36
1034
30
74X
112.70
1098
12
1161
12
** 74Y
112.75
1098
36
1035
30
75X
112.80
1099
12
1162
12
** 75Y
112.85
1099
36
1036
30
76X
112.90
1100
12
1163
12
** 76Y
112.95
1100
36
1037
30
77X
113.00
1101
12
1164
12
** 77Y
113.05
1101
36
1038
30
78X
113.10
1102
12
1165
12
** 78Y
113.15
1102
36
1039
30
79X
113.20
1103
12
1166
12
** 79Y
113.25
1103
36
1040
30
80X
113.30
1104
12
1167
12
80Y
113.35
5067.0
620
1104
36
36
42
1041
30
80Z
5067.3
621
1104
21
27
1041
15
81X
113.40
1105
12
1168
12
81Y
113.45
5067.6
622
1105
36
36
42
1042
30
81Z
5067.9
623
1005
21
27
1042
15
82X
113.50
1106
12
1169
12
82Y
113.55
5068.2
624
1106
36
36
42
1043
30
82Z
5068.5
625
1106
21
27
1043
15
83X
113.60
1107
12
1170
12
83Y
113.65
5068.8
626
1107
36
36
42
1044
30
83Z
5069.1
627
1107
21
27
1044
15
84X
113.70
1108
12
1171
12
84Y
113.75
5069.4
628
1108
36
36
42
1045
30
84Z
6069.7
629
1108
21
27
1045
15
85X
113.80
1109
12
1172
12
85Y
113.85
5070.0
630
1109
36
36
42
1046
30
85Z
5070.3
631
1109
21
27
1046
15
86X
113.90
1110
12
1173
12
86Y
113.95
5070.6
632
1110
36
36
42
1047
30
86Z
5070.9
633
1110
21
27
1047
15
87X
114.00
1111
12
1174
12
87Y
114.05
5071.2
634
1111
36
36
42
1048
30
87Z
5071.5
635
1111
21
27
1048
15
88X
114.10
1112
12
1175
12
88Y
114.15
5071.8
636
1112
36
36
42
1049
30
88Z
5072.1
637
1112
21
27
1049
15
89X
114.20
1113
12
1176
12
89Y
114.25
5072.4
638
1113
36
36
42
1050
30
89Z
5072.7
639
1113
21
27
1050
15
90X
114.30
1114
12
1177
12
90Y
114.35
5073.0
640
1114
36
36
42
1051
30
90Z
5073.3
641
1114
21
27
1051
15
91X
114.40
1115
12
1178
12
91Y
114.45
5073.6
642
1115
36
36
42
1052
30
91Z
5073.9
643
1115
21
27
1052
15
92X
114.50
1116
12
1179
12
92Y
114.55
5074.2
644
1116
36
36
42
1053
30
92Z
5074.5
645
1116
21
27
1053
15
93X
114.60
1117
12
1180
12
93Y
114.65
5074.8
646
1117
36
36
42
1054
30
93Z
5075.1
647
1117
21
27
1054
15
94X
114.70
1118
12
1181
12
94Y
114.75
5075.4
648
1118
36
36
42
1055
30
94Z
5075.7
649
1118
21
27
1055
15
95X
114.80
1119
12
1182
12
95Y
114.85
5076.0
650
1119
36
36
42
1056
30
95Z
5076.3
651
1119
21
27
1056
15
96X
114.90
1120
12
1183
12
96Y
114.95
5076.6
652
1120
36
36
42
1057
30
96Z
5076.9
653
1120
21
27
1057
15
97X
115.00
1121
12
1184
12
97Y
115.05
5077.2
654
1121
36
36
42
1058
30
97Z
5077.5
655
1121
21
27
1058
15
98X
115.10
1122
12
1185
12
98Y
115.15
5077.8
656
1122
36
36
42
1059
30
98Z
5078.1
657
1122
21
27
1059
15
99X
115.20
1123
12
1186
12
99Y
115.25
5078.4
658
1123
36
36
42
1060
30
99Z
5078.7
659
1123
21
27
1060
15
100X
115.30
1124
12
1187
12
100Y
115.35
5079.0
660
1124
36
36
42
1061
30
100Z
5079.3
661
1124
21
27
1061
15
101X
115.40
1125
12
1188
12
101Y
115.45
5079.6
662
1125
36
36
42
1062
30
101Z
5079.9
663
1125
21
27
1062
15
102X
115.50
1126
12
1189
12
102Y
115.55
5080.2
664
1126
36
36
42
1063
30
102Z
5080.5
665
1126
21
27
1063
15
103X
115.60
1127
12
1190
12
103Y
115.65
5080.B
666
1127
36
36
42
1064
30
103Z
5081.1
667
1127
21
27
1064
19
104X
115.70
1128
12
1191
12
104Y
115.75
5081.4
668
1128
36
36
42
1065
30
104Z
5081.7
669
1128
21
27
1065
19
105X
115.80
1129
12
1192
12
105Y
115.85
5082.0
670
1129
36
36
42
1066
30
105Z
5082.3
671
1129
21
27
1066
15
106X
115.90
1130
12
1193
12
106Y
115.95
5082.6
672
1130
36
36
42
1067
30
106Z
5082.9
673
1130
21
27
1067
15
107X
116.00
1131
12
1194
12
107Y
116.05
5083.2
674
1131
36
36
42
1068
30
107Z
5083.5
675
1131
21
27
1068
15
108X
116.10
508
1132
12
1195
12
108Y
116.15
5083.8
676
1132
36
36
42
1069
30
108Z
5084.1
677
1132
21
27
1069
15
109X
116.20
1133
12
1196
12
109Y
116.25
5084.4
678
1133
36
36
42
1070
30
109Z
5084.7
679
1133
21
27
1070
15
110X
116.30
1134
12
1197
12
110Y
116.35
5085.0
680
1134
36
36
42
1071
30
110Z
5085.3
681
1134
21
27
1071
15
111X
116.40
1135
12
1198
12
111Y
116.45
5086.6
682
1135
36
36
42
1072
30
111Z
5085.9
683
1135
21
27
1072
15
112X
116.50
1136
12
1199
12
112Y
116.55
5086.2
684
1136
36
36
42
1073
30
112Z
5086.5
685
1136
21
27
1073
15
113X
116.60
1137
12
1200
12
113Y
116.65
5086.8
686
1137
36
36
42
1074
30
113Z
5087.1
687
1137
21
27
1074
15
114X
116.70
1138
12
1201
12
114Y
116.75
5087.4
688
1138
36
36
42
1075
30
114Z
5087.7
689
1138
21
27
1075
15
115X
116.80
1139
12
1202
12
115Y
116.85
5088.0
690
1139
36
36
42
1076
30
115Z
5088.3
691
1139
21
27
1076
15
116X
116.90
1140
12
1203
12
116Y
116.95
5088.6
692
1140
36
36
42
1077
30
116Z
5088.9
693
1140
21
27
1077
15
117X
117.00
1141
12
1204
12
117Y
117.05
5089.2
694
1141
36
36
42
1078
30
117Z
5089.5
695
1141
21
27
1078
15
118X
117.10
1142
12
12.5
12
118Y
117.15
5089.8
696
1142
36
36
42
1079
30
118Z
5090.1
697
1142
21
27
1079
12
119X
117.20
1143
12
1206
12
119Y
117.25
5090.4
698
1143
36
36
42
1080
30
119Z
5090.7
699
1143
21
27
1080
15
120X
117.30
1144
12
1207
12
120Y
117.35
1144
36
1081
30
121X
117.40
1145
12
1208
12
121Y
117.45
1145
36
1082
30
122X
117.50
1146
12
1209
12
122Y
117.55
1146
36
1083
30
123X
117.60
1147
12
1210
12
123Y
117.65
1147
36
1084
30
124X
117.70
1148
12
1211
12
** 124Y
117.75
1148
36
1085
30
125X
117.80
1149
12
1212
12
** 125Y
117.85
1149
36
1086
30
126X
117.90
1150
12
1213
12
** 126Y
117.95
1150
36
1087
30
Notes:
* These channels are reserved exclusively for national allotments.
** These channels may be used for national allotment on a secondary basis. The primary reason for reserving these channels is to provide protection for the secondary Surveillance Radar (SSR) system.
▽ 108.0 MHz is not scheduled for assignment to ILS service. The associated DME operating channel No. 17X may be assigned to the emergency service.
(b) Polarization.
(1) The radio frequency emissions from all ground equipment must be nominally vertically polarized. Any horizontally polarized radio frequency emission component from the ground equipment must not have incorrectly coded angle information such that the limits specified in paragraphs (b) (2) and (3) of this section are exceeded.
(2) Rotation of the receiving antenna thirty degrees from the vertically polarized position must not cause the path following error to exceed the allowed error at that location.
(c) Modulation requirements. Each function transmitter must be capable of DPSK and continuous wave (CW) modulations of the RF carrier which have the following characteristics.
(1) DPSK. The DPSK signal must have the following characteristics:
bit rate
15.625 KHz
bit length
64 microseconds
logic “0”
no phase transition
logic “1”
phase transition
phase transition
less than 10 microseconds
phase tolerance
±10 degrees
The phase shall advance (or retard) monotonically throughout the transition region. Amplitude modulation during the phase transition period shall not be used.
Graphic of The phase shall advance (or retard) monotonically throughout the transition region. Amplitude modulation during the phase transition period shall not be used.
(2) CW. The CW pulse transmissions and the CW angle transmissions as may be required in the signal format of any function must have characteristics such that the requirements of paragraph (d) of this section are met.
(d) Radio frequency signal spectrum. The transmitted signal must be such that during the transmission time, the mean power density above a height of 600 meters (2000 feet) does not exceed −100.5 dBW/m 2 for angle guidance and −95.5 dBW/m 2 for data, as measured in a 150 KHz bandwidth centered at a frequency of 840 KHz or more from the assigned frequency.
(e) Synchronization. Synchronization between the azimuth and elevation components is required and, in split-site configurations, would normally be accomplished by landline interconnections. Synchronization monitoring must be provided to preclude function overlap.
(f) Transmission rates. Angle guidance and data signals must be transmitted at the following average repetition rates:
Function
Average data rate (Hertz)
Approach Azimuth
13 ±0.5
High Rate Approach Azimuth
1 39 ±1.5
Approach Elevation
39 ±1.5
Back Azimuth
6.5 ±0.25
Basic Data
(2)
Auxiliary Data
(3)
1 The higher rate is recommended for azimuth scanning antennas with beamwidths greater than two degrees. It should be noted that the time available in the signal format for additional functions is limited when the higher rate is used.
2 Refer to Table 8a.
3 Refer to Table 8c.
(g) Transmission sequences. Sequences of angle transmissions which will generate the required repetition rates are shown in Figures 2 and 3.
Graphic of (g) Transmission sequences. Sequences of angle transmissions which will generate the required repetition rates are shown in Figures 2 and 3.
Graphic of EC15SE91.006
(h) TDM cycle. The time periods between angle transmission sequences must be varied so that exact repetitions do not occur within periods of less than 0.5 second in order to protect against synchronous interference. One such combination of sequences is shown in Figure 4 which forms a full multiplex cycle. Data may be transmitted during suitable open times within or between the sequences.
Graphic of (h) TDM cycle. The time periods between angle transmission sequences must be varied so that exact repetitions do not occur within periods of less than 0.5 second in order to protect against synchronous interference. One such combination of sequences is shown in Figure 4 which forms a full multiplex cycle. Data may be transmitted during suitable open times within or between the sequences.
(i) Function Formats (General). Each angle function must contain the following elements: a preamble; sector signals; and a TO and FRO angle scan organized as shown in Figure 5a. Each data function must contain a preamble and a data transmission period organized as shown in Figure 5b.
Graphic of (i) Function Formats (General). Each angle function must contain the following elements: a preamble; sector signals; and a TO and FRO angle scan organized as shown in Figure 5a. Each data function must contain a preamble and a data transmission period organized as shown in Figure 5b.
(1) Preamble format. The transmitted angle and date functions must use the preamble format shown in Figure 6. This format consists of a carrier acquisition period of unmodulated CW transmission followed by a receiver synchronization code and a function identification code. The preamble timing must be in accordance with Table 2.
Graphic of (1) Preamble format. The transmitted angle and date functions must use the preamble format shown in Figure 6. This format consists of a carrier acquisition period of unmodulated CW transmission followed by a receiver synchronization code and a function identification code. The preamble timing must be in accordance with Table 2.
(i) Digital codes. The coding used in the preamble for receiver synchronization is a Barker code logic 11101. The time of the last phase transition midpoint in the code shall be the receiver reference time (see Table 2). The function identification codes must be as shown in Table 3. The last two bits (I11 and I12) of the code are parity bits obeying the equations:
I6 + I7 + I8 + I9 + I10 + I11 = Even
I6 + I8 + I10 + I12 = Even
(ii) Data modulation. The digital code portions of the preamble must be DPSK modulated in accordance with § 171.311(c)(1) and must be transmitted throughout the function coverage volume.
(2) Angle function formats. The timing of the angle transmissions must be in accordance with Tables 4a, 4b, and 5. The actual timing of the TO and FRO scans must be as required to meet the accuracy requirements of §§ 171.313 and 171.317.
(i) Preamble. Must be in accordance with requirements of § 171.311(i)(1).
Table 2—Preamble Timing 1
Event
Event time slot begins at—
15.625 kHz clock pulse (number)
Time (milliseconds)
Carrier acquisition:
(CW transmission)
0
0
Receiver reference time code:
I1 = 1
13
0.832
I2 = 1
14
0.896
I3 = 1
15
0.960
I4 = 0
16
1.024
I5 = 1
17
2 1.088
Function identification:
I6
18
1.152
I7
19
1.216
I8
20
1.280
I9
21
1.344
I10 (see table 1)
22
1.408
I11
23
1.472
I12
24
1.536
END PREAMBLE
25
1.600
1 Applies to all functions transmitted.
2 Reference time for receiver synchronization for all function timing.
Table 3—Function Identification Codes
Function
Code
I6
I7
I8
I9
I10
I11
I12
Approach azimuth
0
0
1
1
0
0
1
High rate approach azimuth
0
0
1
0
1
0
0
Approach elevation
1
1
0
0
0
0
1
Back azimuth
1
0
0
1
0
0
1
Basic data 1
0
1
0
1
0
0
0
Basic data 2
0
1
1
1
1
0
0
Basic data 3
1
0
1
0
0
0
0
Basic data 4
1
0
0
0
1
0
0
Basic data 5
1
1
0
1
1
0
0
Dasic data 6
0
0
0
1
1
0
1
Auxiliary data A
1
1
1
0
0
1
0
Auxiliary data B
1
0
1
0
1
1
1
Auxiliary data C
1
1
1
1
0
0
0
(ii) Sector signals. In all azimuth formats, sector signals must be transmitted to provide Morse Code identification, airborne antenna selection, and system test signals. These signals are not required in the elevation formats. In addition, if the signal from an installed ground component results in a valid indication in an area where no valid guidance should exist, OCI signals must be radiated as provided for in the signal format (see Tables 4a, 4b, and 5). The sector signals are defined as follows:
(A) Morse Code. DPSK transmissions that will permit Morse Code facility identification in the aircraft by a four letter code starting with the letter “M” must be included in all azimuth functions. They must be transmitted and repeated at approximately equal intervals, not less than six times per minute, during which time the ground subsystem is available for operational use. When the transmissions of the ground subsystem are not available, the identification signal must be suppressed. The audible tone in the aircraft is started by setting the Morse Code bit to logic “1” and stopped by a logic “0” (see Tables 4a and 4b). The identification code characteristics must conform to the following: the dot must be between 0.13 and 0.16 second in duration, and the dash between 0.39 and 0.48 second. The duration between dots and/or dashes must be one dot plus or minus 10%. The duration between characters (letters) must not be less than three dots. When back azimuth is provided, the code shall be transmitted by the approach azimuth and back azimuth within plus or minus 0.08 seconds.
(B) Airborne antenna selection. A signal for airborne antenna selection shall be transmitted as a “zero” DPSK signal lasting for a six-bit period (see Tables 4a and 4b).
Table 4a—Approach Azimuth Function timing
Event
Event time slot begins at—
15.625 kHz clock pulse (number)
Time (milliseconds)
Preamble
0
0
Morse code
25
1.600
Antenna select
26
1.664
Rear OCI
32
2.048
Left OCI
34
2.176
Right OCI
36
2.304
To test
38
2.432
To scan 1
40
2.560
Pause
8.760
Midscan point
9.060
FRO scan 1
9.360
FRO test
15.560
End Function (Airborne)
15.688
End guard time; end function (ground)
15.900
AA1 The actual commencement and completion of the TO and the FRO scan transmissions are dependent on the amount of proportional guidance provided. The time slots provided shall accommodate a maximum scan of plus or minus 62.0 degrees. Scan timing shall be compatible with accuracy requirements.
Table 4b—High Rate Approach Azimuth and Back Azimuth Function Timing
Event
Event time slot begins at—
15.625 kHz clock pulse (number)
Time (milliseconds)
Preamble
0
0
Morse Code
25
1.600
Antenna select
26
1.664
Rear OCI
32
2.048
Left OCI
34
2.176
Right OCI
36
2.304
To test
38
2.432
To scan 1
40
2.560
Pause
6.760
Midscan point
7.060
FRO scan 1
7.360
FRO test pulse
11.560
End function (airborne)
11.688
End guard time; end function (ground)
11.900
1 The actual commencement and completion of the TO and the FRO scan transmissions are dependent on the amount of proportional guidance provided. The time slots provided will accommodate a maximum scan of plus or minus 42.0 degrees. Scan timing shall be compatible with accuracy requirements.
(C) OCI. Where OCI pulses are used, they must be: (1) greater than any guidance signal in the OCI sector; (2) at least 5 dB less than the level of the scanning beam within the proportional guidance sector; and (3) for azimuth functions with clearance signals, at least 5 dB less than the level of the left (right) clearance pulses within the left (right) clearance sector.
Table 5—Approach Elevation Function Timing
Event
Event time slot begins at:
15.625 kHz clock pluse (number)
Time (milliseconds)
Preamble
0
0
Processor pause
25
1.600
OCI
27
1.728
To scan 1
29
1.856
Pause
3.406
Midscan point
3.606
FRO scan 1
3.806
End function (airborne)
5.356
End guard time; end function (ground)
5.600
1 The actual commencement and completion of the TO and FRO scan transmissions are dependent upon the amount of proportional guidance provided. The time slots provided will accommodate a maximum scan of −1.5 degrees to + 29.5 degrees. Scan timing shall be compatible with accuracy requirements.
The duration of each pulse measured at the half amplitude point shall be at least 100 microseconds, and the rise and fall times shall be less then 10 microseconds. It shall be permissible to sequentially transmit two pulses in each out-of-coverage indication time slot. Where pulse pairs are used, the duration of each pulse shall be at least 50 microseconds, and the rise and fall times shall be less then 10 microseconds. The transmission of out-of-coverage indication pulses radiated from antennas with overlapping coverage patterns shall be separated by at least 10 microseconds.
Note:
If desired, two pulses may be sequentially transmitted in each OCI time slot. Where pulse pairs are used, the duration of each pulse must be 45 (±5) microseconds and the rise and fall times must be less than 10 microseconds.
(D) System test. Time slots are provided in Tables 4a and 4b to allow radiation of TO and FRO test pulses. However, radiation of these pulses is not required since the characteristics of these pulses have not yet been standardized.
(iii) Angle encoding. The encoding must be as follows:
(A) General. Azimuth and elevation angles are encoded by scanning a narrow beam between the limits of the proportional coverage sector first in one direction (the TO scan) and then in the opposite direction (the FRO scan). Angular information must be encoded by the amount of time separation between the beam centers of the TO and FRO scanning beam pulses. The TO and FRO transmissions must be symmetrically disposed about the midscan point listed in Tables 4a, 4b, 5, and 7. The midscan point and the center of the time interval between the TO and FRO scan transmissions must coincide with a tolerance of ±10 microseconds. Angular coding must be linear with angle and properly decoded using the formula:
\[ \theta = \frac{V}{2} \left( T_0 - t \right) \]
where:
θ = Receiver angle in degrees.
V = Scan velocity in degrees per microsecond.
T0 = Time separation in microseconds between TO and FRO beam centers corresponding to zero degrees.
t = Time separation in microseconds between TO and FRO beam centers.
The timing requirements are listed in Table 6 and illustrated in Figure 7.
Graphic of The timing requirements are listed in Table 6 and illustrated in Figure 7.
(B) Azimuth angle encoding. Each guidance angle transmitted must consist of a clockwise TO scan followed by a counterclockwise FRO scan as viewed from above the antenna. For approach azimuth functions, increasing angle values must be in the direction of the TO scan; for the back azimuth function, increasing angle values must be in the direction of the FRO scan. The antenna has a narrow beam in the plane of the scan direction and a broad beam in the orthogonal plane which fills the vertical coverage.
(C) Elevation angle encoding. The radiation from elevation equipment must produce a beam which scans from the horizon up to the highest elevation angle and then scans back down to the horizon. The antenna has a narrow beam in the plane of the scan direction and a broad beam in the orthogonal plane which fills the horizontal coverage. Elevation angles are defined from the horizontal plane containing the antenna phase center; positive angles are above the horizontal and zero angle is along the horizontal.
(iv) Clearance guidance. The timing of the clearance pulses must be in accordance with Figure 8. For azimuth elements with proportional coverage of less than ±40 degrees (±20 degrees for back azimuth), clearance guidance information must be provided by transmitting pulses in a TO and FRO format adjacent to the stop/start times of the scanning beam signal. The fly-right clearance pulses must represent positive angles and the fly-left clearance pulses must represent negative angles. The duration of each clearance pulse must be 50 microseconds with a tolerance of ±5 microseconds. The transmitter switching time between the clearance pulses and the scanning beam transmissions must not exceed 10 microseconds. The rise time at the edge of each clearance pulse must be less than 10 microseconds. Within the fly-right clearance guidance section, the fly-right clearance guidance signal shall exceed scanning beam antenna sidelobes and other guidance and OCI signals by at least 5 dB; within the fly-left clearance guidance sector, the fly left clearance guidance signal shall exceed scanning beam antenna sidelobes and all other guidance and OCI signals by at least 5 dB; within the proportional guidance sector, the clearance guidance signals shall be at least 5dB below the proportional guidance signal. Optionally, clearance guidance may be provided by scanning throughout the approach guidance sector. For angles outside the approach azimuth proportional coverage limits as set in Basic Data Word One (Basic Data Word 5 for back azimuth), proper decode and display of clearance guidance must occur to the limits of the guidance region. Where used, clearance pulses shall be transmitted adjacent to the scanning beam signals at the edges of proportional coverage as shown in Figure 8. The proportional coverage boundary shall be established at one beamwidth inside the scan start/stop angles, such that the transition between scanning beam and clearance signals occurs outside the proportional coverage sector. When clearance pulses are provided in conjunction with a narrow beamwidth (e.g., one degree) scanning antenna, the scanning beam antenna shall radiate for 15 microseconds while stationary at the scan start/stop angles.
(3) Data function format. Basic data words provide equipment characteristics and certain siting information. Basic data words must be transmitted from an antenna located at the approach azimuth or back azimuth site which provides coverage throughout the appropriate sector. Data function timing must be in accordance with Table 7a.
Table 6—Angle Scan Timing Constants
Function
Max value of t(usec)
To(usec)
V(deg/usec)
Tm (usec)
Pause time (usec)
Tt (usec)
Approach azimuth
13,000
6,800
0.02
7,972
600
13,128
High rate approach azimuth
9,000
4,800
0.02
5,972
600
9,128
Approach elevation
3,500
3,350
0.02
2,518
400
N/A
Back azimuth
9,000
4,800
−0.02
5,972
600
9,128
Table 7a—Basic Data Function Timing
Event
Event time slot begins at: 1
15.625 kHz clock pulse (number)
Time (milliseconds)
Preamble
0
0
Data transmission (bits I13-I30)
25
1.600
Parity transmission (bits I31-I32)
43
2.752
End function (airborne)
45
2.880
End guard time: end function (ground)
3.100
1 The previous event time slot ends at this time.
Table 7b—Auxiliary Data Function Timing—(Digital)
Event
Event time slot begins at:
15.625 kHz clock pulse (number)
Time (milliseconds)
Preamble
0
0
Address transmission (bits I13-I20)
25
1.600
Data transmission: (bits I21-I69)
33
2.112
Parity transmission (bits I70-I76)
82
5.248
End function (airborne)
89
5.696
End guard time; end function (ground)
5.900
Table 7c—Auxiliary Data Function Timing—(Alphanumeric)
Event
Event time slot begins at:
15.615 kHz clock pulse (number)
Time (milliseconds)
Preamble
0
0
Address transmission (bits I13-I20)
25
1.600
Data transmission: (bits I21-I76
33
2.112
End function (airborne)
89
5.696
End guard time; (end function ground)
5.900
(i) Preamble. Must be in accordance with requirements of § 171.311(i)(1).
(ii) Data transmissions. Basic data must be transmitted using DPSK modulation. The content and repetition rate of each basic data word must be in accordance with Table 8a. For data containing digital information, binary number 1 must represent the lower range limit with increments in binary steps to the upper range limit shown in Table 8a. Data containing digital information shall be transmitted with the least significant bit first.
(j) Basic Data word requirements. Basic Data shall consist of the items specified in Table 8a. Basic Data word contents shall be defined as follows:
(1) Approach azimuth to threshold distance shall represent the minimum distance between the Approach Azimuth antenna phase center and the vertical plane perpendicular to the centerline which contains the landing threshold.
(2) Approach azimuth proportional coverage limit shall represent the limit of the sector in which proportional approach azimuth guidance is transmitted.
(3) Clearance signal type shall represent the type of clearance when used. Pulse clearance is that which is in accordance with § 171.311 (i) (2) (iv). Scanning Beam (SB) clearance indicates that the proportional guidance sector is limited by the proportional coverage limits set in basic data.
Graphic of (3) Clearance signal type shall represent the type of clearance when used. Pulse clearance is that which is in accordance with § 171.311 (i) (2) (iv). Scanning Beam (SB) clearance indicates that the proportional guidance sector is limited by the proportional coverage limits set in basic data.
Table 8a—Basic Data Words
Data bit #
Data item definition
LSB value
Data bit value
Basic Data Word No. 1
1
Preamble
N/A
1
2
1
3
1
4
0
5
1
6
0
7
1
8
0
9
1
10
0
11
0
12
0
13
Approach azimuth to threshold distance (Om−630m)
100m
100m
14
200m
15
400m
16
800m
17
1600m
18
3200m
19
Approach azimuth proportional coverage limit (negative limit) (0° to −62°)
−2°
20
−4°
21
−8°
22
−16°
23
−32°
24
Approach azimuth proportional coverage limit (positive limit) (0° to + 62°)
25
26
27
16°
28
32°
29
Clearance signal type
N/A
0 = pulse; 1 = SB
30
Spare
Transmit zero
31
Parity: (13 + 14 + 15. . . + 30 + 31 = odd)
N/A
N/A
32
Parity: (14 + 16 + 18. . . + 30 + 32 = odd)
N/A
N/A
Note 1: Transmit throughout the Approach Azimuth guidance sector at intervals of 1.0 seconds or less.
Note 2: The all zero state of the data field represents the lower limit of the absolute value of the coded parameter unless otherwise noted.
Basic Data Word No. 2
1
Preamble
N/A
1
2
1
3
1
4
0
5
1
6
0
7
1
8
1
9
1
10
1
11
0
12
0
13
Minimum glide path (2.0° to 14.7°)
0.1°
0.1°
14
0.2°
15
0.4°
16
0.8°
17
1.6°
18
3.2°
19
6.4°
20
Back azimuth status
see note 4
21
DME status
see note 6
22
23
Approach azimuth status
see note 4
24
Approach azimuth status
see note 4
25
Spare
Transmit zero
26
......do
  Do.
27
......do
  Do.
28
......do
  Do.
29
......do
  Do.
30
......do
  Do.
31
Parity: (13 + 14 + 15. . . + 30 + 31) = odd)
N/A
N/A
32
Parity: (14 + 16 + 18. . . + 30 + 32 = odd)
N/A
N/A
Note 1: Transmit throughout the Approach Azimuth guidance sector at intervals of 0.16 seconds or less.
Note 2: The all zero state of the data field represents the lower limit of the absolute range of the coded parameter unless otherwise noted.
Basic Data Word No. 3
1
Preamble
N/A
1
2
1
3
1
4
0
5
1
6
1
7
0
8
1
9
0
10
0
11
0
12
0
13
Approach azimuth beamwidth (0.5°−4.0°) See note 7
0.5°
0.5°
14
1.0°
15
2.0°
16
Approach elevation beamwidth (0.5° to 2.5°) See note 7
0.5°
0.5°
17
1.0°
18
Note: values greater than 2.5° are invalid
2.0°
19
DME distance (Om to 6387.5m
12.5m
12.5m
20
25.0m
21
50.0m
22
100.0m
23
200.0m
24
400.0m
25
800.0m
26
1600.0m
27
3200.0m
28
Spare
Transmit zero
29
......do
  Do.
30
......do
  Do.
31
Parity: (13 + 14 + 15. . . + 30 + 31 = odd)
32
Parity: (14 + 16 + 18. . . + 30 + 32 = odd)
N/A
N/A
Note 1: Transmit throughout the Approach Azimuth guidance sector at intervals of 1.0 seconds or less.
Note 2: The all zero state of the data field represents the lower limit of the absolute range of the coded parameter unless otherwise noted.
Basic Data Word No. 4
1
Preamble
N/A
1
2
1
3
1
4
0
5
1
6
1
7
0
8
0
9
0
10
1
11
0
12
0
13
Approach azimuth magnetic orientation (0° to 359°)
14
15
16
17
16°
18
32°
19
64°
20
128°
21
256°
22
Back azimuth magnetic orientation (0° to 359°)
23
24
25
26
16°
27
32°
28
64°
29
128°
30
256°
31
Parity: (13 + 14 + 15. . . + 30 + 31 = odd)
N/A
N/A
32
Parity: (14 + 16 + 18. . . + 30 + 32 = odd)
N/A
N/A
Note 1: Transmit at intervals of 1.0 second or less throughout the Approach Azimuth guidance sector, except when Back Azimuth guidance is provided. See Note 8.
Note 2: The all zero state of the data field represents the lower limit of the absolute range of the coded parameter unless otherwise noted.
Basic Data Word No. 5
1
Preamble
N/A
1
2
1
3
1
4
0
5
1
6
1
7
1
8
0
9
1
10
1
11
0
12
0
13
Back azimuth proportional coverage negative limit (0° to −42°)
−2°
14
−4°
15
−8°
16
−16°
17
−32°
18
Back azimuth proportional coverage positive limit (0° to + 42°)
19
20
21
16°
22
32°
23
Back azimuth beamwidth (0.5° to 4.0°) See note 7
0.5°
0.5°
24
1.0°
25
2.0°
26
Back azimuth status
See Note 10
27
......do
  Do.
28
......do
  Do.
29
......do
  Do.
30
......do
  Do.
31
Parity: (13 + 14 + 15. . . + 30 + 31 = odd)
N/A
N/A
32
Parity: (14 + 16 + 18. . . + 30 + 32 = odd)
N/A
N/A
Note 1: Transmit only when Back Azimuth guidance is provided. See note 9.
Note 2: The all zero state of the data filed represents the lower limit of the absolute range of the coded parameter unless otherwise noted.
Basic Data Word No. 6
1
Preamble
N/A
1
2
1
3
1
4
0
5
1
6
0
7
0
8
0
9
1
10
1
11
0
12
1
(13-30)
MLS ground equipment identification (Note 3)
13
Character 2
N/A
B1
14
B2
15
B3
16
B4
17
B5
18
B6
19
Character 3
N/A
B1
20
B2
21
B3
22
B4
23
B5
24
B6
25
Character 4
N/A
B1
26
B2
27
B3
28
B4
29
B5
30
B6
31
Parity: (13 + 14 + 15. . . + 30 + 31 = odd)
N/A
N/A
32
Parity: (14 + 16 + 18. . . + 30 + 32 = odd)
N/A
N/A
Note 1: Transmit at intervals of 1.0 second or less throughout the Approach Azimuth guidance sector, except when Back Azimuth guidance is provided. See note 8.
Note 3: Characters are encoded using the International Alphabet Number 5, (IA-5):
Note 4: Coding for status bit:
0 = Function not radiated, or radiated in test mode (not reliable 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:
I21
I22
0
0
DME transponder inoperative or not available.
1
0
Only IA mode or DME/N available.
0
0
FA mode, Standard 1, available.
1
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 seconds 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 seconds or less throughout the Approach Azimuth coverage sector.
Note 10: Coding for status bit:
0 = Function not radiated, or radiated in test mode (not reliable 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 published approach procedures and obstacle clearance criteria.
(5) Back azimuth status shall represent the operational status of the Back Azimuth equipment.
(6) DME status shall represent the operational status of the DME equipment.
(7) Approach azimuth status shall represent the operational status of the approach azimuth equipment.
(8) Approach elevation status shall represent the operational status of the approach elevation equipment.
(9) Beamwidth the width of the scanning 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 function 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 centerline which contains the MLS datum point.
(11) Approach 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 approach azimuth antenna phase center. The vertex of the measured angle shall be at the approach azimuth antenna phase center.
Note:
For example, this data item would be encoded 090 for an approach azimuth antenna serving runway 27 (assuming the magnetic heading is 270 degrees) when sited such that the zero degree radial is parallel to centerline.
(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.
Note:
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 centerline.
(13) Back azimuth proportional coverage limit shall represent the limit of the sector in which proportional back azimuth guidance is transmitted.
(14) MLS ground equipment identification shall represent the last three characters of the system identification specified in § 171.311(i)(2). The characters shall be encoded in accordance with International Alphabet No. 5 (IA-5) using bits b1 through b6.
Note:
Bit b7 of this code may be reconstructed in the airborne receiver by taking the complement of bit b6.
(k) Residual radiation. The residual radiation of a transmitter associated with an MLS function during time intervals when it should not be transmitting shall not adversely affect the reception of any other function. The residual radiation of an MLS function at times when another function is radiating shall be at least 70 dB below the level provided when transmitting.
(l) Symmetrical scanning. The TO and FRO scan transmissions shall be symmetrically 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, and Auxiliary Data C. Auxiliary Data A contents are specified below, Auxiliary Data B contents are reserved for future use, and Auxiliary Data C contents are reserved for national use. The address codes of the auxiliary data words shall be as shown in Table 8b.
(2) Organization and timing. The organization and timing of digital auxiliary data must be as specified in Table 7b. Data containing digital information must be transmitted with the least significant bit first. Alphanumeric data characters must be encoded in accordance with the 7-unit code character set as defined by the American National Standard Code for Information Interchange (ASCII). An even parity bit is added to each character. Alphanumeric data must be transmitted in the order in which they are to be read. The serial transmission of a character must be with the lower order bit transmitted first and the parity bit transmitted last. The timing for alphanumeric auxiliary data must be as shown in Table 7c.
(3) Auxiliary Data A content: The data items specified in Table 8c are defined as follows:
(i) Approach azimuth antenna offset shall represent the minimum distance between the Approach Azimuth antenna phase center and the vertical plane containing the runway centerline.
(ii) Approach azimuth to MLS datum point distance shall represent the minimum distance between the Approach Azimuth antenna phase center and the vertical plane perpendicular to the centerline which contains the MLS datum point.
(iii) Approach azimuth alignment with runway centerline shall represent the minimum angle between the approach azimuth antenna zero-degree guidance plane and the runway certerline.
(iv) Approach azimuth antenna coordinate system shall represent the coordinate system (planar or conical) of the angle data transmitted by the approach azimuth antenna.
(v) Approach elevation antenna offset shall represent the minimum distance between the elevation antenna phase center and the vertical plane containing the runway centerline.
(vi) MLS datum point to threshold distance shall represent the distance measured along the runway centerline from the MLS datum point to the runway threshold.
(vii) Approach elevation antenna height shall represent the height of the elevation antenna phase center relative to the height of the MLS datum point.
(viii) DME offset shall represent the minimum distance between the DME antenna phase center and the vertical plane containing the runway centerline.
(ix) DME to MLS datum point distance shall represent the minimum distance between the DME antenna phase center and the vertical plane perpendicular to the centerline which contains the MLS datum point.
(x) Back azimuth antenna offset shall represent the minimum distance between the back azimuth antenna phase center and the vertical plane containing the runway centerline.
(xi) Back azimuth to MLS datum point distance shall represent the minimum distance between the Back Azimuth antenna and the vertical plane perpendicular to the centerline which contains the MLS datum point.
(xii) Back azimuth antenna alignment with runway centerline shall represent the minimum angle between the back azimuth antenna zero-degree guidance plane and the runway centerline.