During rollout of an aircraft, in particular a transport airplane, on a runway of an airport, in the course of a landing phase, knowledge of the surface state of the runway is extremely important.  Indeed, the prediction of the aircraft's braking performance depends on this knowledge.  Indeed, in this case, it is possible to:
best estimate the distance required to stop the aircraft in a desire for safety; and
not overestimate the stopping distance required to immobilize the aircraft, and therefore not penalize, unduly, the usage operations in respect of the runway and the aircraft. 
Indeed, it is known that a lower than expected runway coefficient of friction or a greater than reported presence of contaminant form part of the main factors in respect of runway overshoot during landing. 
However, the braking performance of an aircraft on a contaminated runway is very difficult to predict because of the difficulty of ascertaining, in a reliable and accurate manner, the contribution of the runway state to the deceleration of the aircraft, in particular in terms of adhesion and spray drag and displacement drag in the case of thick contaminant. 
The contaminants may be any element settling on the runway, such as for example rubber deposited during previous landings, oil, rainwater forming a more or less uniform layer on the runway, snow, ice, etc.  Knowledge of the runway state may therefore be exploitable in order to improve landing systems. 
It is known that standard algorithms for runway state identification are based on a comparison between the current braking forces and reference braking forces (which represent the braking forces that the aircraft would have on reference states of the runway). 
However, such an estimation of the state of the runway may exhibit inconsistencies in certain situations. 
This standard solution is therefore not optimal.