Signal format requirements.

  1. Law
  2. CFR 14
  3. Aeronautics and Space
  4. Federal Aviation Administration, Department of Transportation
  5. Navigational Facilities
  6. Non-Federal Navigation Facilities
  7. Microwave Landing System (MLS)
  8. Signal format requirements.

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§ 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.

Expand Table

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
Expand Table

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:

Expand Table
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.

(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/m2 for angle guidance and −95.5 dBW/m2 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:

Expand Table
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.

(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.

(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).

Expand Table

Table 2 - Preamble Timing1

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.

Expand Table

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).

Expand Table

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 scan1 40 2.560
Pause 8.760
Midscan point 9.060
FRO scan1 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.

Expand Table

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 scan1 40 2.560
Pause 6.760
Midscan point 7.060
FRO scan1 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.

Expand Table

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 scan1 29 1.856
Pause 3.406
Midscan point 3.606
FRO scan1 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:

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.

(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.

Expand Table

Table 6 - Angle Scan Timing Constants

Function Max value oft(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
Expand Table

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.

Expand Table

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
Expand Table

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.

Expand Table

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° −2°
20 −4°
21 −8°
22 −16°
23 −32°
24 Approach azimuth proportional coverage limit (positive limit) (0° to + 62°) 2° 2°
25 4°
26 8°
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°) 1° 1°
14 2°
15 4°
16 8°
17 16°
18 32°
19 64°
20 128°
21 256°
22 Back azimuth magnetic orientation (0° to 359°) 1° 1°
23 2°
24 4°
25 8°
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° −2°
14 −4°
15 −8°
16 −16°
17 −32°
18 Back azimuth proportional coverage positive limit (0° to + 42°) 2° 2°
19 4°
20 8°
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:

Expand Table
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.


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