On/Off-Keying (OOK) data transmission in the atmospheric optical channel suffers from slow fading of the received
power. Fading is produced by index of refraction turbulence and transmitter and receiver terminal alignment errors
produced by tracking errors. Standard bit-error-ratio measurement techniques count the number of errors during a fixed
time. Variation of measurement duration can change the measurement result significantly. Further measurement times
are normally longer than fading events. Therefore no information about the distribution of bits in error in a data-stream
can be measured with standard bit-error-rate testers. However the error distribution is important for the design of error
correction methods like forward error correction. Therefore a special device has been developed to measure the error
distribution. A description of this device is given in this paper. Further first measurements results are presented.