一、ADS-B versions

主要由0,1,2 三个版本

1.1 version changes
从0 - > 1 的改进是

Added Type Code 28, 19, and 31 messages
– TC=28 : Aircraft status - Emergency/priority status and ACAS RA Broadcast (紧急优先状态广播)
– TC=29 : Target state and status（目标状况和情况）
– TC=31 : Operational status（可选情况）

Introduced the “Navigation integrity category ( NIC )” and “Surveillance integrity
level ( SIL )” in addition to the “Navigation accuracy category ( NAC )” from the Version 0
– Type Code and an NIC Supplement bit ( NICs ) is used to define the NIC
– NIC Supplement bit included in TC=31 messages

从1 - > 2 的改进是

Re-defined the structure and content of TC=28 , TC=29 , and TC=31 messages.

Introduced two additional NIC Supplement Bit

NICa is defined in operational status messages ( TC=31 )

NICb is defined in airborne position messages ( TC=9-18 )

NICc is defined in operational status messages ( TC=31 )

Introduced an additional “Horizontal Containment Radius ( Rc )” within NIC=6 /TC=13

1.2 Identify the ADS-B Version
确定版本需要两步骤

1.Step 1: Check whether an aircraft is broadcasting ADS-B messages with TC=31 at all. If no message is ever reported, it is safe to assume that the version is Version 0
（检查所广播的信息中是否含有[TC=31],如果没有那么可以初步判定是0版本）

Step 2: If messages with TC=31 are received, check the version numbers located in
the 41-43 bit of the payload (or 73-75 bit of the message)
（如果接受信息中含有[TC=31],那么查看信息中第73-75比特的版本序列号）

二、Ehanced Mode-S

EHS（Mode-S Enhanced Surveillance）比ELS（Mode-S Elementary
Surveillance）在ATC（o Air Traffic Control）中能提供更多的信息。
在解密是遇到以下几个问题

这个信息到底来着哪个飞机？

这个信息到底是哪一种（BDS类型）？

解密的信息到底有多少可信度？

2.1 Enhanced Mode-S Basics

2.1.1 Downlink Format and message structure

此时首五个比特采用[DF=21]，[DF=20]。

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每部分分解以及解释.png
除了[DF]，前32比特在解密是没有包含什么有用的信息

2.1.2 Parity and ICAO address recovery

在ADS-B中ICAO地址是伴随广播出去的。而在EHS中我们在解密其他信息之前需要先解密ICAO地址，其隐藏在信息和检验和之中。

S模式下有两种奇偶校验和地址AP（Address Parity）和DP（Data Parity），不过大多数都是使用前者。

Address Parity

对于AP来说，还原ICAO需要做一下处理。

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2.1.3 BDS (Comm-B Data Selector)

BDS是一个用两位十六进制的数字，它能够定义我们查询的信息的类型。无论是ADS-B数据还是其他类型的s模型的信息都被分配有他们独特的BDS数字。

When SSR interrogates an aircraft, a BDS code is included in the request message (Uplink
Format - UF 4, 5, 20, or 21). This BDS code is then used by the aircraft transponder to
register the type of message to be sent. But when the downlink message is transmitted, its
BDS code is not included in the message (because the SSR knows what kind of message
it requested). Good news for them, but challenges for us.