Mumbai University > EXTC > Sem 8 > Satellite Communication and Networks

Marks: 5M

Lobe Switching

In the case of lobe switching tracking methodology, the antenna beam is rapidly switched between two positions around the antenna axis in a single plane as shown in Figure. The amplitudes of the echo from the object to be tracked are compared for the two lobe positions.The difference between the two amplitudes is indicative of the location of the target withrespect to the antenna axis. When the object to be tracked is on the axis, the echo amplitudes for the two positions of the beam are equal and the difference between the two is zero.

Monopulse technique

In the monopulse technique the errors for driving the antenna system are derived by simultaneous lobing of the received beacon — hence the name static- split or monopulse. Several monopulse schemes such as amplitude comparison, phase comparison or amplitude and phase comparison are possible. The amplitude comparison technique is the simplest and is commonly used for satellite tracking.

Two horns are offset and mounted in a plane (figure a). The radiation patterns of individual horns are shown in figure (b) and the superposed pattern in figure (c). Two types of pattern can be distinguished — a sum pattern and a difference pattern. The difference pattern output with respect to the sum pattern is zero when the satellite is centered, otherwise the output is proportional to the tracking error.

Step Tracking

In the case of step track, antenna axes are moved in small incremental steps in an effort to maximize the received signal strength. Amplitude sensing is the basis of this tracking methodology. It is simple and low cost and RF phase stability is not important. It is best suited to small and medium Earth stations. The technique is susceptible to amplitude perturbations caused by scintillation, signal fading and so on. Tracking accuracy is primarily determined by the step size and signal to noise ratio. For a high signal-to-noise ratio, tracking error approaches the step size. Accuracy is sensitive to amplitude interference.

Intelligent Tracking

In the case of intelligent tracking, the satellite position is obtained by optimally combining antenna position estimate data obtained from a gradient tracking algorithm with the prediction data on satellite position obtained from a satellite position model. In the case of signal amplitude fluctuations, the antenna position may be updated by using prediction data from satellite position model. Intelligent tracking offers all advantages of step track. The technique is susceptible to amplitude fluctuations during initial acquisition. Full accuracy is achieved severalhours after acquisition. Intelligent track may be used in small, medium and large Earth stations, particularly those susceptible to scintillation and signal fades.