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Technical Provisions for Mode S Services and Extended Squitter

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DRAFT - Working Paper ASP TSGWP11-01 <strong>for</strong> review by the TSG during the meeting in June 2011 in Paris<br />

Appendix CD CD-25<br />

CD.2.4.7.7 GLOBALLY UNAMBIGUOUS POSITION<br />

Globally unambiguous position decoding is typically used to initially establish the position of a target. Once the target’s<br />

position is determined it can be updated using local decoding. Local decoding may be used exclusively only when there<br />

is no possibility of message reception from targets farther than the ambiguous range of 180 NM. In applications where<br />

ADS-B messages are received more than 180 NM from the receiving station, it will be necessary to use globally<br />

unambiguous decoding.<br />

The CPR system includes a technique <strong>for</strong> globally unambiguous coding. It is based on a technique similar to the use of<br />

different pulse repetition intervals (PRI) in radars to eliminate second-time-around targets. In CPR, this takes the <strong>for</strong>m of<br />

coding the lat/lon using a different number of zones on alternate reports. Reports labeled T = 0 are coded using 15<br />

latitude zones <strong>and</strong> a number of longitude zones defined by the CPR coding logic <strong>for</strong> the position to be encoded (59 at<br />

the equator). The reports on the alternate second (T = 1) are encoded using 14 zones <strong>for</strong> latitude <strong>and</strong> N – 1 zones <strong>for</strong><br />

longitude, where N is the number used <strong>for</strong> T = 0 encoding. An example of this coding structure is illustrated in Figure<br />

CD-7.<br />

A user receiving reports of each type can directly decode the position within the unambiguous area cell <strong>for</strong> each report,<br />

since each type of report is uniquely identified. In addition, a comparison of the two types of reports will provide the<br />

identity of the area cell, since there is only one area cell that would provide consistent position decoding <strong>for</strong> the two<br />

reports. An example of the relative decoded positions <strong>for</strong> T = 0 <strong>and</strong> T = 1 is shown in Figure CD-8.<br />

Equator<br />

Greenwich<br />

meridian<br />

Draft<br />

T = 0 zone<br />

T = 1 zone<br />

Figure CD-7. Zone structure <strong>for</strong> globally unambiguous reporting.<br />

DRAFT - Working Paper ASP TSGWP11-01 <strong>for</strong> review by the TSG during the meeting in June 2011 in Paris

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