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In the example above, the natural key of the State ('BELGIUM') responsible for the DME and the natural
key of the collocated VOR (codeId, geoLat and geoLong) are included in the <Dme> element.
22 Edition Number: 4.5
AIXM PRIMER
Chapter 6. Data Integrity
6.1. Introduction
In order to aid system implementers to achieve the ICAO requirements for CRC (cyclic redundancy
check) protection, AICM has provision for CRC values, which are modelled by a 'VAL_CRC' attribute
in a number of AICM entities (navaids, aerodrome/heliport, etc.). The following section describes the
CRC algorithm. Appendix B provides sample Java code for implementing the CRC algorithm.
6.2. CRC Algorithm
The CRC checking is used in numerous systems to verify the integrity of the information during data
transmission. If the computed CRC bits are different from the original (transmitted) CRC bits, then there
has been an error occurred in the transmission. If there are identical, it can be assumed that no error occurred
(there is a chance 1 in 4 billion that two different bit streams have the same CRC32).
The idea is that the data bits are treated as a data polynomial and the CRC bits represent the remainder
of the division of the data polynomial by a fixed, known polynomial (called the CRC polynomial). The
CRC32 polynomial used here is the ICAO-approved CRC32Q which is defined as fallows:
1 + x + x^3 + x^5 + x^7 + x^8 + x^14 + x^16 + x^22 + x^24 + x^31 + x^32
In a binary representation, where the coefficients are represented by a logical 1 we have:
11010101100000101000001010000001
For the calculation of the value of the CRC in AIXM messages, the bit stream used is composed through
the concatenation of the XML fields concerned.
The algorithmic representation of the procedure of calculation of the CRC is represented below. The
input data of this procedure is the bits streams defines above. The output data of this procedure is the
remainder which contains the CRC (32 bits).
Edition Number: 4.5 23
AIXM PRIMER
Figure 6.1. CRC Algorithm Flowchart
24 Edition Number: 4.5
AIXM PRIMER
The function « ASCII_TO_BOOLEAN » converted the ASCII value of a character into a boolean array.
For example if you take the ‘A’ character, the Hexadecimal representation of this ASCII value is 0x41.
We have of binary representation:
01000001
the function « ASCII_TO_BOOLEAN » return for this character :
False, true, false, false, false, false, false, true.
To validate our algorithm, we developed a class java "CRC", which calculates value CRC of an introduced
data. Here a table, this gives the result of this class according to different stream from entry.
Table 6.1. Example data for CRC class
Latitude Longitude Height Geoid CRC
480637N - - - A5A7C704
- 0163411E - - A1AE5741
480637N 0163411E - - A1BA30EE
- - 782 - 6C297100
480637N 0163411E 782 - 6A259F4E
- - - 46.7 266D25C1
480637N 0163411E - 46.7 2F866D6D
480637N 0163411E 782 46.7 5E5DC940
6.3. Note with regard to the EAD Implementation of AIXM
In the current EAD implementation (Releases 3 & 4), the bit stream used for the calculation of the CRC
value in AIXM messages, composed by concatenating the following fields, in this order:
• Latitude (<geoLat>)
• Longitude (<geoLong>)
• Elevation (<valElev>)
• Geoid Undulation (<valGeoidUndulation>)
In cases where the elements representing the Elevation and the Geoid Undulation are “null” or the relevant
features do not have attributes to take these values, the concatenation of latitude and longitude is used.
Edition Number: 4.5 25
AIXM PRIMER
AIXM PRIMER
Appendix A. AIXM-AICM Mappings
A.1. Mapping of AIXM Features to AICM Entities,
plus natural key compositions
Note: For clarity, natural keys that are composed of a choice of relationships are listed without details
of those choices. For example, an <Ana> (radio navigation aid at an aerodrome/heliport) must be related
to an Aerodrome/Heliport and one of the following: TACAN, VOR, NDB, DME or MKR.
Edition Number: 4.5 27
AIXM PRIMER
Table A.1. AIXM-AICM Mappings
AIXM Feature Name AIXM Complex Type AICM Entity Natural Keys
Aha AerodromeHeliportAddressType ADDRESS Relationship to: AD_HP [Address
is for Aerodrome / Heliport]
Attribute: codeType
Attribute: noSeq
AerodromeHeliportColocation- AD_HP_COLLOCATION
Type
Ahc Relationship to: AD_HP [Aerodrome
/ Heliport - Co-location
is having as 1st Aerodrome /
Heliport]
Relationship to: AD_HP [Aerodrome
/ Heliport - Co-location
is having as 2nd Aerodrome /
　
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