The Alpha-Beta tracker is shown in the figure below,
along with the open loop transfer function G(S) of the continuous equivalent
circuit. At sample intervals, T, a height error is measured and applied
with scaling to a rate integrator and a position integrator. In the
SEASAT-A altimeter the integrators will become digital accumulators.
The output of the rate integrator, which is height rate, is applied along
with the error signal (scaled by alpha) to the position integrator
whose output is the smoothed estimate of height. In the absence of an
error signal, the output of the rate integrator will cause the position
integrator to ramp up or down, corresponding to the last value of rate.
This is characteristic of the Type II servo - the ability to follow a
constant rate input without error, or lag.
