How are ADS-B surface position frames decoded

The position data in DF17 packets are transmitted as fragments in two different packets: "even" and "odd" packets. Decoding positions in DF17 can be done in two ways: global decoding and local decoding.

Global decoding involves using both an even and odd packet, which must not be received more than 20 seconds apart.

Local decoding requires the ADS-B receiver to utilize one packet (either even or odd) and the home position of the ADS-B receiver to decode the positions accurately.

For ground traffic, the resolution of position data within the DF17 packet is four times higher than for airborne traffic. Despite this higher resolution, the number of available bits in the DF17 packet remains the same. Consequently, ground positions can only be encoded relative to a master position. Fortunately, this limitation is not problematic since the range of an ADS-B receiver for ground traffic is inherently limited. The maximum range for unambiguous local decoding of ground traffic is approximately 45 nautical miles (NM). In contrast, for unambiguous global decoding, the range extends to around 1500 NM.

It's important to note that for ground traffic, a home position is always needed, as is the case with every ADS-B receiver.

You may check the RTCA DO-260 series and EUROCAE ED-102 series documents for how to decode.

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