Mumbai University > Satellite Communication and Networks

Topic : Earth Station

Marks: 10M

Tracking:

• The following characteristics of the target are important.

  • Range of the target
  • Velocity
  • Azimuth angle
  • Elevation angle

• Depending on what sort of tracking is employed, we can have tracking in range, Doppler, Azimuth or Elevation or all combined.

• Just a narrow beam of antenna may not be sufficient for the accurate tracking of the target w.r.t. Above characteristics. Auxiliary methods of tracking for precise location of target have to be used.
• There are 3 techniques for angle tracking.
  • Lobe switching/ sequential lobing/ sequential switching.
  • Conical scanning/ switching
  • Monopulse tracking/simultaneous lobing

Lobe switching/ sequential lobing/ sequential switching:

• Here the direction of the antenna beam is rapidly switched between two positions. And hence the name lobe switching.

• The echo signals from the target will fluctuate at the switching rate, unless the target is exactly midway between the two directions. When once the latter condition is achieved, the strength of echo signals will be same for both the antenna position, but the angular tracking error is to be determined.

• This tracking error is applied to servomechanism unit which attempts to position the antenna beam on the target. When angular error is zero, the target is located along the reference direction. Thus sequential lobing is used for tracking a target accurately in only one plane.

Conical Switching:

• This is logical extension of lobe switching in which beam from parabolic antenna, mounted slightly off center, is rotated about the axis of the parabola as shown in figure.

• Since the revolution of solid is a cone as the tip of the pencil radiation pattern moves in circle, it gets the name conical scan.

• If a target is present within radiation, then the echo signal from the target will be amplitude modulated at a frequency equal to the rotation frequency of the beam. The conical scan modulation is extracted from the echo signal applied to a servo control system which positions the antenna on target both in azimuth & Elevation. Then, the conical scan modulation becomes zero and thus the target is tracked accurately.

Disadvantages:

1. The motion of the antenna is more complex in lobe switching & Conical Scanning where additional servo mechanism required.
2. More than one returned pulse is required to locate target accurately.
3. Amplitude comparison are not particularly accurate.
4. The conical scan is very susceptible to electronic counter measures because the scan rate is readily detectable in the RADAR signal & can be contaminated to compound the track servo mechanisms.

Monopulse Tracking:

• An ideal system would be one in which all the information obtained by conical scanning could be achieved with just one pulse. Such a system is called monopulse tracking. Angle tracking in modern RADARs is usually done using this technique.

• In an amplitude comparison monopulse system, four feeds are used with one parabolic reflector.These are four horn antennas displaced about the central focus of the reflector as shown in figure.

• The transmitter feeds the horns simultaneously so that sum signal is transmitted. The echo received by a receiver duplexer using a hybrid ring to provide signals.

• No difference will be recorded if the target is precisely in axial direction of the antenna. However once the target has been acquired, to lie within these four beams, any deviation of the target from the axial direction of the reflector will be shown by the presence of a vertical difference signal, a horizontal difference signal or both as shown in figure.