79 lines
6.0 KiB
Plaintext
79 lines
6.0 KiB
Plaintext
AIVDM/AIVDO Sentence Layer
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AIVDM/AIVDO is a two-layer protocol. The outer layer is a variant of NMEA 0183, the ancient standard for data interchange in marine navigation systems; NMEA 0183 is described at [NMEA].
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Here is a typical AIVDM data packet:
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!AIVDM,1,1,,B,177KQJ5000G?tO`K>RA1wUbN0TKH,0*5C
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And here is what the fields mean:
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Field 1, !AIVDM, identifies this as an AIVDM packet.
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Field 2 (1 in this example) is the count of fragments in the currently accumulating message. The payload size of each sentence is limited by NMEA 0183’s 82-character maximum, so it is sometimes required to split a payload over several fragment sentences.
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Field 3 (1 in this example) is the fragment number of this sentence. It will be one-based. A sentence with a fragment count of 1 and a fragment number of 1 is complete in itself.
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Field 4 (empty in this example) is a sequential message ID for multi-sentence messages.
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Field 5 (B in this example) is a radio channel code. AIS uses the high side of the duplex from two VHF radio channels: AIS Channel A is 161.975Mhz (87B); AIS Channel B is 162.025Mhz (88B). In the wild, channel codes '1' and '2' may also be encountered; the standards do not prescribe an interpretation of these but it’s obvious enough.
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Field 6 (177KQJ5000G?tO`K>RA1wUbN0TKH in this example) is the data payload. We’ll describe how to decode this in later sections.
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Field 7 (0) is the number of fill bits requires to pad the data payload to a 6 bit boundary, ranging from 0 to 5. Equivalently, subtracting 5 from this tells how many least significant bits of the last 6-bit nibble in the data payload should be ignored. Note that this pad byte has a tricky interaction with the [ITU1371]requirement for byte alignment in over-the-air AIS messages; see the detailed discussion of message lengths and alignment in a later section.
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The *-separated suffix (\*5C) is the NMEA 0183 data-integrity checksum for the sentence, preceded by "*". It is computed on the entire sentence including the AIVDM tag but excluding the leading "!". This checksum is the XOR of all bytes in the payload.
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For comparison, here is an example of a multi fragment sentence with a nonempty message ID field:
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!AIVDM,2,1,3,B,55P5TL01VIaAL@7WKO@mBplU@<PDhh000000001S;AJ::4A80?4i@E53,0*3E
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!AIVDM,2,2,3,B,1@0000000000000,2*55
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Technically, NMEA0183 does not actually require that a !-led sentence be AIS. This format can be used for any encapsulated data. The syntax and semantics of fields 1-4 are fixed, and the fill-bit field and NMEA checksum are required, but the payload fields may contain any encapsulated data.
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It is, however, a safe bet that any such sentence containing an A or B channel code in field 5 is AIVDM/AIVDO.
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AIVDM/AIVDO 语句层
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AIVDM/AIVDO 是一种两层协议。外层是 NMEA 0183 的一种变体，NMEA 0183 是一种用于海上导航系统数据交换的古老标准；NMEA 0183 协议的详细描述可以参考 [NMEA]。
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以下是一个典型的 AIVDM 数据包：
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//
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!AIVDO,1,1,,,B>qc:0001R:t?tUFfnT03wj4h4Id,0*73
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!AIVDM,1,1,,A,E:hTsH1Pb7W@LHHqIHKqIHIJqpqDEpF8:eLFp1088;AP02,4*0E
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//
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!AIVDM,1,1,,B,177KQJ5000G?tO`K>RA1wUbN0TKH,0*5C
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下面是字段的含义：
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字段 1，!AIVDM，标识这是一个 AIVDM 数据包。
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字段 2（此示例中为 1）是当前正在累积的消息中的片段总数。由于每条 NMEA 0183 语句的最大有效负载大小为 82 字符，因此有时需要将有效负载拆分为多个片段语句。
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字段 3（此示例中为 1）是当前语句的片段编号，采用基于 1 的计数。如果片段总数为 1 且片段编号为 1，则该语句本身是完整的。
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字段 4（此示例为空）是多片段消息的序列消息 ID。
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字段 5（此示例中为 B）是无线电频道代码。AIS 使用两个 VHF 无线电频道的高端：AIS A频道是 161.975 MHz（87B），AIS B频道是 162.025 MHz（88B）。在实际应用中，也可能遇到频道代码 '1' 和 '2'；标准并没有规定这些代码的含义，但显然它们也有明显的区分。
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字段 6（此示例中的 177KQJ5000G?tOK>RA1wUbN0TKH`）是数据有效载荷。稍后我们将描述如何解码它。
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字段 7（此示例中的 0）是填充位的数量，用于将数据有效载荷填充到 6 位的边界，范围为 0 到 5。相应地，从这个数字减去 5 就是告诉我们在数据有效载荷的最后一个 6 位字节中需要忽略的最低有效位数。需要注意的是，这个填充字节与 [ITU1371] 对于无线电 AIS 消息的字节对齐要求有一个复杂的相互作用，稍后会详细讨论消息长度和对齐问题。
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以 * 分隔的后缀（*5C）是该语句的 NMEA 0183 数据完整性校验和，计算方式是对整个语句进行异或运算，包括 AIVDM 标签，但不包括前导的 ! 字符。
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为了便于对比，这里是一个多片段语句的示例，其中消息 ID 字段不为空：
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!AIVDM,2,1,3,B,55P5TL01VIaAL@7WKO@mBplU@<PDhh000000001S;AJ::4A80?4i@E53,0*3E
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!AIVDM,2,2,3,B,1@0000000000000,2*55
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从技术上讲，NMEA 0183 并不要求任何以 ! 开头的语句必须是 AIS 格式。该格式可以用于任何封装数据。字段 1 到字段 4 的语法和语义是固定的，填充位字段和 NMEA 校验和是必需的，但有效载荷字段可以包含任何封装的数据。
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然而，如果语句的字段 5 包含 A 或 B 频道代码，可以合理地认为它是 AIVDM/AIVDO 格式的。
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http://radarcape/aircraftlist.json?knownposonly=1&vpn=48.38&vpe=12.08&vps=48.19&vpw=10.97&altitude.from=1&altitude.to=25000
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http://radarcape/flightpath.json?icao=3C4D63&icao=3C5443&icao=300558&icao=3D41A3&icao=4B1A1D&icao=49130E&lookback=180
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http://radarcape/aircraftlist.json?select=avail
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http://radarcape/aircraftlist.json?knownposonly=1&vpn=49.19&vpe=16.09&vps=47.46&vpw=7.51&type=b74*,a38*
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http://radarcape/aircraftlist.json?icao=3f*,d-e*&altitude.from=1&altitude.to=25000
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http://radarcape/flightpath.json?icao=3D2515&lookback=600
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1A 34
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05 F8 C0 00 00 00
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00
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7D 00 14
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00 00 00 00 00 00 00 00 00 00 00
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Lat 32-bit1
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Latitude as 32-bit BAM 2, thus 232 ≡ 360°
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Lon 32-bit1
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Latitude as 32-bit BAM 2
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Alt 16-bit1
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Signed altitude meters
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