Technical Provisions for Mode S Services and Extended Squitter
Technical Provisions for Mode S Services and Extended Squitter
Technical Provisions for Mode S Services and Extended Squitter
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A-38 <strong>Technical</strong> <strong>Provisions</strong> <strong>for</strong> <strong>Mode</strong> S <strong>Services</strong> <strong>and</strong> <strong>Extended</strong> <strong>Squitter</strong><br />
between solutions is less, <strong>and</strong> varies according to the cosine of latitude. For example at 87 degrees latitude,<br />
the minimum distance between solutions is 280 NM. This is sufficiently large to provide assurance that the<br />
correct aircraft location will always be obtained. Currently no airports exist within 3 degrees of either pole, so<br />
the decoding as specified here will yield the correct location of the transmitting aircraft <strong>for</strong> all existing airports.<br />
h) A reasonableness test is applied to the resulting decoded position in accordance with §A.2.7.2.<br />
A.2.6.9.1 OVERVIEW<br />
DRAFT - Working Paper ASP TSGWP11-01 <strong>for</strong> review by the TSG during the meeting in June 2011 in Paris<br />
A.2.6.9 CPR DECODING OF RECEIVED POSITION MESSAGES<br />
The techniques described in the preceding paragraphs (locally <strong>and</strong> globally unambiguous decoding) shall be used<br />
together to decode the latitude/longitude contained in airborne, surface, intent <strong>and</strong> TIS-B position messages. The<br />
process shall begin with globally unambiguous decoding based upon the receipt of an even <strong>and</strong> an odd encoded<br />
position squitter. Once the globally unambiguous position is determined, emitter centered decoding shall be used to<br />
support subsequent decoding based upon a single position report, either even or odd encoding.<br />
A.2.6.9.2 EMITTER CENTERED LOCAL DECODING<br />
In this approach, the most recent position of the emitter shall be used as the basis <strong>for</strong> the local decoding.<br />
Note.— This produces an unambiguous decoding at each update since the transmitting aircraft cannot move more<br />
than 180 NM between position updates.<br />
A.2.7 REASONABLENESS TEST FOR CPR DECODING OF RECEIVED POSITION MESSAGES<br />
A.2.7.1 OVERVIEW<br />
Draft<br />
Note.— Although receptions of position messages will normally lead to a successful target position determination, it<br />
is necessary to safeguard against position messages that would be used to initiate or update a track with an erroneous<br />
position. A reasonableness test applied to the computed position resulting from receipt of a position message can be<br />
used to discard erroneous position updates. Since an erroneous globally unambiguous CPR decode could potentially<br />
exist <strong>for</strong> the life of a track, a reasonableness test <strong>and</strong> validation of the position protects against such occurrences.<br />
A.2.7.2 REASONABLENESS TEST APPLIED TO POSITIONS DETERMINED<br />
FROM GLOBALLY UNAMBIGUOUS DECODING<br />
A reasonableness test shall be applied to a position computed using the globally unambiguous CPR decoding per<br />
§A.2.6.7 <strong>for</strong> airborne participants or per §A.2.6.8 <strong>for</strong> surface participants. Upon receipt of the even or odd encoded<br />
position message that completes the globally unambiguous CPR decode, the receiver shall per<strong>for</strong>m a reasonableness<br />
test on the position decode by the following:<br />
If the receiver position is known, calculate the distance between the decoded position <strong>and</strong> the receiver position <strong>and</strong><br />
verify that the distance is less than the maximum operating range. If the validation fails, the receiver shall discard the<br />
decoded position, the even <strong>and</strong> odd position messages used to per<strong>for</strong>m the globally unambiguous CPR decode, <strong>and</strong><br />
reinitiate the globally unambiguous CPR decode process.<br />
DRAFT - Working Paper ASP TSGWP11-01 <strong>for</strong> review by the TSG during the meeting in June 2011 in Paris