a. a terminal, such as delivery and order details. Mobile The troposcatter signal received at the distant fixed point may usage of SDM2 typically employs a bar code reader for work continuously vary in signal level due to the presence of the ran- progress and parts ordering/updating. domly varying parameters involved in the scatter process. Vari- Traffic channel data transfer is possible following either manual ations in signal level may reach 20dB with monthly, daily, and or automatic call set up, an error correcting protocol such as an X.25 hourly variations often being encountered (Orr, 1987). derivative is commonly used to prevent data corruption from peri- Providing sufficient effective radiated power output exists from odic maintenance messages. Otherwise, no defined protocols are the transmitter, the maximum attainable communication range is in required as an open channel is available as in the case of analogue the order of 800km. This is limited by the maximum height of the cellular, thus base computer interrogation may be performed with useful common scattering volume in the troposphere. Although no added security through data encryption techniques. critical frequency is involved with the scattering mechanism, the intensity of the scattered reflections decreases with increasing fre- quency, thus aerial gain and transmitter power must be correspond- 46.5 The use of troposcatter ingly increased with increase of frequency. 46.5.1 Introduction 46.5.4 Tropospheric ducting With the use of high transmission power combined with high gain Tropospheric ducting of v.h.f./u.h.f. signals leads to extended com- aerials, it is possible for v.h.f./u.h.f. signals to propagate beyond the munication range over a given geographical path. Ducting is the normal 'line of sight' range. This method makes use of the availa- result of the variation of the refractive index of the atmosphere bility of random irregularities in the refractive index structure of the found at the periphery of air masses of differing temperatures. A trop