14 CFR 171.311 - Signal format requirements.

View all text of Subpart J [§ 171.301 - § 171.327]

§ 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

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/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.

(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).

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

AA 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 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:

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.

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.

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:

I21

I22

DME transponder inoperative or not available.

Only IA mode or DME/N available.

FA mode, Standard 1, available.

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.