If I may jump in ...
To get any braking at all, there must be some slippage between tyre and runway surface. It is usual to express this as a ratio (slip ratio) which is a percentage of the unbraked wheel speed.
The braking friction starts out being proportional to the slip ratio up to a maximum at about 20% slip and thereafter it deteriorates as the wheel goes into a true skid. The aim of the ABS is to keep the braked wheel as close as possible to this optimum slip ratio. To do this (at least in the systems I am familiar with) the actual wheel rotational speed is compared with an unbraked wheel speed derived from a measured nosewheel rotational speed.
The resulting braking friction coefficient is not constant; on dry concrete it varies from (about) 0.8 at low speed through 0.7 around 75 kts down to 0.65 at about 150 kts.
The braking force will be the appropriate coefficient times the wheel reaction at that speed. Obviously, immediately after touch down the braking will be at a minimum on both counts. The wheel reaction will be the aircraft weight less the aerodynamic lift. Even with all wheels on the ground this can be a considerable proportion of the actual weight - maybe as high as 50% with full flap.
Deploying lift dumpers drops this to something close to zero and at the same time just about doubles the aerodynamic drag - both very useful at a time when runway distance is being gobbled up.
When the aircraft gets to lower speeds the wheel reaction is close to the actual weight and the coefficient of braking friction is also near its maximum. In these conditions the torque needed to maintain the optimum slip may become greater than than available from the brakes. The braking force is then limited by maximum brake capability.
On the question of a landing distance monitor, I agree that it should be possible, but I'm not sure how reliable it might be given the possibility that the braking characteristics of any given runway can vary with time - how for example do you know how much friction you are going to get on that final bit of runway that is contaminated by rubber smears from repeated touchdowns and maybe also wet?
Last edited by Owain Glyndwr; 18th Oct 2012 at 09:23.