Abstract. Contrail cloudiness over Europe and the eastern part of the North Atlantic Ocean was analyzed for the two periods September 1979 - December 1981 and September 1989 - August 1992 by visual inspection of quicklook photographic prints of NOAA/AVHRR infrared images. The averaged contrail cover exhibits maximum values along the transatlantic flight corridor around 50 °N (of almost 2%) and over western Europe resulting in 0.5% contrail cloudiness on average. A strong yearly cycle appears with a maximum (<2%) in spring and summer over the Atlantic and a smaller maximum (<1%) in winter over southwestern Europe. Comparing the two time periods, which are separated by one decade, shows there is a significant decrease in contrail cloudiness over western Europe and a significant increase over the North Atlantic between March and July. Contrail cloud cover during daytime is about twice as high as during nighttime. Contrails are found preferentially in larger fields of 1000 km diameter which usually last for more than a day. Causes, possible errors and consequences are discussed.

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This paper studies the variation on the power budget, in an infrared optical link, for indoor channels with multipath propagation. The multkath propagation produces heavy distortion over the received signal and reduces the maximum baud rate for a given bit error rate.Focusing on IR wireless local area network (rR WLAN), we stu& some emitter and receiver configurations. In order to allow computer portability these link can be di&e or quasi-diffuse. These channels do not rely upon the line-ofthe-sight path. This works deals not only with the total received power, but with its time and spatial distribution. This paper considers the channel as characterized by its channel transfer function in a deterministic way. This is a good approximation if the variations in the environmental conditions are slow enough compared with the baud rate of the communication system. The source is an IR Emitter Diode @ED) with dgerent values of directiviq, pointing to a refrecting surface, white the receiver is a photodiode characterized through its Jeld of view (F.0.K) value. The results show that an optical Q-diffuse link improves the performances of a diffuse one. We use for the comparative analysis the RMS spread delay of the impulse response, the spatial distribution of received power and its standard deviation ?om the mean. We present extensive simulation results, an emitter configuration and the first measured results obtained using and APD as receiver. 924 7d.3.1

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