- Posts: 2
- Thank you received: 1
if (tools.isZero(msg.getParity()) || msg.checkParity()) { // CRC is ok
[...] // Switch between different adsb message format
else if (msg.getDownlinkFormat() != 17) {
[...] Switch between other ModeS format whose DF20 and DF21 for CommB message
}
Please Log in or Create an account to join the conversation.
A. Passive Monitoring Challenges
[...] This system design is challenging for passive
observers such as OpenSky, which do not see the uplink
communication, namely the interrogations. [...]
Although Mode S messages
in SSR employ a cyclic redundancy check (CRC) to do so, the
checksum is XORed with the aircraft’s ICAO 24-bit address
or the interrogator ID (all-call replies) in Mode S replies to
save bits (see Figure 3). While this does not pose a problem
for the interrogator since it knows which address to expect,
a passive observer needs to learn the respective addresses
first in order to make use of the CRC.
3.1.1 Address Filter. To link a received message to an aircraft, we
need to extract the aircraft address from a data field that contains
the address XOR-ed with the parity of the message. Under the
assumption that the message did not suffer from bit errors, we can
extract the aircraft address from the parity by XOR-ing the parity
with the CRC of the received message body. Of course, with this
approach the parity can no longer be used to check the integrity
of the message
Please Log in or Create an account to join the conversation.