Lidars are active remote sensing instruments. They send very short laser pulses vertically into the atmosphere and collect the backscattered signal, which allows to derive the ozone profiles. Lidar measurements are restricted to cloud free nights, because the lidar radiation is rapidly extinguished by clouds, and daylight extremely deteriorates the signal to noise ratio. Typically, the integration time for an ozone measurement in the upper stratosphere is several hours, depending on atmospheric conditions and lidar system efficiency. Since the length of the night changes with latitude and season, the integration time becomes shorter from the equator towards the summer pole. Shorter integration times might reduce the accuracy of the lidars.

• lidar altitude range:         ~ 15km - 50km
• lidar vertical resolution:  ~ 1km                (15km - 40km altitude)
                                            ~ 4km                (> 40km altitude)
• lidar accuracy:                 3% - 5%            (15km -o 40km altitude)
                                           10% - 20%        (> 40km altitude)

Co-location: Only co-located satellite and lidar ozone profiles are used in the validation process. The criteria for temporal and geographical coincidence are as follows:

• Temporal coincidence: 
  Lidar profiles must be measured the night before or after the satellite overpass. This leads to a maximum time difference of 15 hours between both profiles.
• Geographical coincidence:
  satellie pixel cetres must be located within +- 2° in latitudinal and longitudinal distance from the ground station.