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ABSTRACT
It is well known that the amplitudes of the pulses reflected from the ionosphere vary in random manner often showing anomalous amplitudes of higher order echoes bigger than that of lower order echoes. Such anomalous reflections are observed during night time more frequently than during day time. Under conditions of anomalous reflections it is not possible to determine the correct values of reflection coefficient which is used (Appleton) for calculating the collisional frequency in the ionosphere even after taking running average of amplitudes of echoes for fairly long time during which the actual value of reflection coefficient may change appreciably. Attempt has, therefore, been made during the preparation of Thesis for Ph.D. to determine the collisional frequency of electrons with neutral particles under such conditions assuming that such anomalous reflections are caused by ripples in the ionosphere. It has been observed that the values of collision frequency of electrons in the undulatory state of the ionosphere are not very close to those during normal unrippled state of the ionosphere, for which the amplitudes of echoes decrease with the order of reflection.