Create adsb.sigmf-ext.md

adsb/adsb.sigmf-ext.md

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+# ADS-B Extension v1.0.0
+
+The Automatic Dependent Surveillance-Broadcast (`adsb`) namespace extension
+defines dynamic properties of ADS-B signals extending `annotations`.
+
+## 1 Global
+
+`adsb` does not extend [Global](https://github.com/sigmf/SigMF/blob/main/sigmf-spec.md#global-object).
+
+## 2 Captures
+
+`signal` does not extend [Captures](https://github.com/sigmf/SigMF/blob/main/sigmf-spec.md#captures-array).
+
+## 3 Annotations
+
+The following names are specified in the `adsb` namespace and should be used in
+the `annotations` object:
+
+|name|required|type|unit|description|
+|----|--------|----|----|-----------|
+|`downlink_format`|true|int|N/A|Indicates if an ADS-B signal is a Mode S Short (11) or a Mode S Extended (17) signal.|
+|`message_type`|true|int|N/A|Indicates the type of data in a Mode S Extended signal.  The message type code range is from 0 to 31. The type of messages are aircraft identification (1-4), surface position (5-8), airborne position with barometric (9-18), airborne velocities (19), airborne position with GNSS (20-22), testing (23), reserved (24-27, 30), Emergency/Airborne Collision Avoidance System (ACAS) status (28), trajectory change (29), and aircraft operational status (31).  A signal with a Mode S Short downlink format does not contains a message and is represented by 0.
+|`ICA_address`|true|double|N/A|The International Civil Aviation Organization (ICAO) address of the ADS-B signal.|
+|`binary`|true|string|N/A|The binary signal, either 56 bits (Mode S Short) or 112 bits (Mode S Extended).|
+
+## 4 Examples
+
+No `adsb` examples.