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Certain standard labels in the ARINC 429 specification have 20 significant bits. For example, Label 110 (GNSS Latitude) has a range of +/- 180 degrees, with a resolution of .000172 degrees. In order to accomplish this, how is the word formatted? Does the data run into the SDI field like so?

Bit # Another header
1-8 Label (110)
9 0.000171661377
10 0.0003433227539
11 0.0006866455078
12 0.001373291016
13 0.002746582031
14 0.005493164063
15 0.01098632813
16 0.02197265625
17 0.0439453125
18 0.087890625
19 0.17578125
20 0.3515625
21 0.703125
22 1.40625
23 2.8125
24 5.625
25 11.25
26 22.5
27 45
28 90
29 Sign (0/1)
30 SSM
31 SSM
32 Parity

Or does the sign bit move to bit 31, replacing the SSM instead?

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The extra bits of precision would use the SDI bits as you've suggested.

From ARINC 429 Part 1 dated May 17th 2004

2.1.4 Source/Destination Identifier

Bit Numbers 9 and 10 of numeric data words should be reserved for a data source/destination identification function. They are not available for this function in alpha/numeric (ISO Alphabet Number 5) data words (see Section 2.3.1.3 of this document) or when the resolution needed for numeric (BNR/BCD) data necessitates their use of valid data.

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