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106 Satellite Newsgathering On its transmission side, the satellite also produces an IPFD toward the Earth, which is a measure of its `illumination' of the Earth's surface, and this varies according to the transmit footprint of the satellite. Again, the satellite operator publishes information relating to the IPFD, which in turn determines both the location and the size of receive antenna required to produce the correct level of input signal to the receiver. 2.15 Losses 2.15.1 Free space loss As the signal travels through space, it suffers attenuation. The ratio of the attenuation of the signal transmit- ted to the amount of signal received at an antenna is termed `free space loss' (FSL) or `free space attenua- tion' (FSA). The path between the Earth station and the satellite is termed the `slant range', and as the latitudinal position of the Earth station increases, the elevation angle to the satellite decreases, thus increas- ing the slant range. Thus, as the slant range lengthens, the loss increases. Free space loss is frequency dependent, increasing at higher frequencies, and affecting both the uplink and the downlink path losses. At elevation angles less than 5°, a significant part of the path will pass through the Earth's atmos- phere and will therefore be subject to terrestrial interference and more noise picked up from the `hot' surface of the Earth compared to the `cold' background of space. This increases the attenuation of the signal. In addition, at such a low angle of elevation, the link may be subjected to `scintillation'; this is a rapid fluctuation in amplitude and phase caused by the signal traveling a longer path through the atmos- phere and ionosphere.