8.3 STATE ESTIMATION
State estimation is an important issue in encounters where both aircraft are equipped with a collision avoidance system. If the two aircraft have inconsistent views of the world due to sensor noise and they do not coordinate their maneuvers, it can lead to an induced collision. This section discusses several ways of performing state estimation.
Independent State Estimation
Di.erent aircraft receive separate sensor measurements, and noise in these measurements results in di.erent state estimates. Most of the time, the belief states between aircraft are relatively consistent. However, there is a risk that the two aircraft in the encounter will have di.erent and incompatible views of the world. For example, both aircraft may believe that they are at the higher altitude, and both issue climb advisories, potentially inducing collision.
TCAS uses independent state estimation, but when there is disagreement between aircraft that results in the aircraft choosing the same sense, it uses a tie-breaking scheme. Each aircraft is assigned a unique 24bit ICAO (Mode S) address, which is broadcast over the communication channel. If an aircraft encounters another aircraft with a lower ICAO address that selects the same sense, it will reverse its sense. Choosing the exact advisory that is compatible with the coordinated sense is done using individual sensor information. A similar strategy can be employed with logic generated by DP.
Fused State Estimation
If communication bandwidth were not an issue, aircraft could broadcast their measurements. Each aircraft would then use all of the measurements to update its individual state estimates, providing a uni.ed view of the airspace. Unfortunately, the current bandwidth allocated for coordination is insu.cient for transmitting accurate measurements. There is also the risk of measurements being corrupted in transit. Although fused state estimation is not feasible, it is worth mentioning because, in theory, it is the best possible strategy with perfect communication.

8.4 ACTION SELECTION
During
execution,
the
various
policies
computed
o.ine
(Section
8.2)
can
be
adapted
to
account
for
coordination messages between aircraft.
Independent Action Selection
The simplest strategy is to simply execute the policy directly using the current state estimate without any regard for the actual advisories being executed by the other aircraft. As mentioned earlier, this can lead to induced collisions if the aircraft have inconsistent views of the world or if the uncoordinated policy is used. To assess the value of coordination, the experiments in this section include independent action selection strategies.
Centralized Action Selection
One way to ensure the selection of compatible actions is to have the aircraft with the lower ICAO ad-dress dictate the actions for both aircraft. However, adopting this strategy would require changing the semantics of the current TCAS coordination messages, which only communicate the advisory sense complements, not actual advisories. Another weakness of this strategy is that it uses only the measurements obtained by the aircraft with the lower ICAO address to make decisions, but aircraft are able to measure their own altitude and vertical rate with greater accuracy.
A centralized strategy requires aircraft with higher addresses to trust those with lower ad-dresses. If the sensors on the aircraft with the lower address are faulty or if any other components of the collision avoidance system are not working properly, the action transmitted to the other aircraft can be disastrous. One way to help mitigate such situations is for the aircraft with the higher address to sanity-check the action it receives. If the cost of the prescribed action is sig-ni.cantly higher than the cost of other actions, then the aircraft should execute the lowest cost action. Selecting an action that is contrary to the one assumed by the other aircraft is very risky; it should only be done in extraordinary situations when the prescribed action is anticipated to lead to collision.
　
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