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Thursday, October 11, 2012

MONOPULSE TRACKING ... Part 1




Auto track is very important especially for Non Stationary satellite tracking and one of the most accurate methods is MONOPULSE.


The word MONOPULSE is made up of MONO + PULSE  i.e. ‘ single pulse ‘  and is based on  the fact that there are multiple elements which receive same signal simultaneously. These signals are processed in monopulse processor which carries out mathematical operations on them to derive Tracking Errors which represent the angular difference between Physical position of antenna axis and the Target from which the signal is emanating.








In the simplest form there are two receiving elements ( a RED and a BLUE  ) which are symmetrically placed about the mechanical axis of rotation as shown in adjoining figure.






The two elements A and B have their characteristic beam patterns placed slightly away from the actual axis but are symmetrically placed about the axis (  thick blue vertical arrow ) as shown in this figure.





A few sidelobes are seen in the extended range pattern below.  

We divert for a few moments from the topic of Monopulse to clarify the pattern.
The regular antenna pattern shows sidelobes only in the upper ( positive ) portion of graph as seen in the actual antenna  pattern result in figure below.
. This is because when the pattern is measured in field it is the voltage output of RF detector which simply gives output proportional to the amplitude of the signal irrespective of the phase. So it is always positive.

In reality the detector input has a negative polarity in the 1st sidelobe and again positive polarity in 2nd sidelobe and so on for subsequent sidelobes. So the actual input to detector for the element 'A'  ( or actual output from the feed ) is as shown in figure on right.    

 Detector output rectifies negative voltage and so the output is always positive as inthe figure on top.

One should keep this in mind because the monopulse concept requires a clarity on phase of the pattern and so we will use only the patterns similar to this figure and not the patterns which are represented only in positive region.

The feed outputs A and B are given to a ' Monopulse Comparator ' which ouputs  addition and subtraction of the two inputs viz ( A + B )  and  ( A – B ) which are called SUM and ERROR ouputs shown graphically below.

We will study the actual composition of Monopulse comparator in a subsequent post. For the present we understand that the input to it a vector quantity as shown in above figure rather than only the positive humps seen during pattern plotting.

When the outputs from two elements A and B are vectorially added and subtracted the resultant two waveforms are shown in next  two figures. Both outputs are simultaneously plotted in third figure.   








We concentrate  the ( A – B ) output in the vicinity ( between two thick RED lines ) of ON TARGET direction ( marked with a thick blue vertical arrow ) .  

Notice that the output is ZERO near target and is proportionately positive as we go away from axis in one direction and is proportionately negative as we go away in other direction. 
Moreover the output is very linear in w.r.t. the angular outness and therefore can be used to measure the angular offset from target.

 If this voltage is given to the control system then the control system will always bring the antenna towards the target automatically. 

This method is used for what is called as “ AUTO TRACK “ mode of antenna


One more very important observation to be made is what happens to the ( A - B ) output if angular difference is large?

Notice that the output changes direction beyond the zone confined with RED lines in previous figure. This will result in false indication of angle outness beyond this zone and will in fact drive the antenna away from target and bring it to rest near zero output which is far away from the real target. This called as a ‘falsely locked antenna position’  and control system should have measures to avoid this situation.


   
  
The above figures are theoretical figures created using an EXCEL sheet. On the right is an actual output of one of a  real systems during optimization in a open antenna test range situated between two mountains in India.





We have seen here the basic principles of MONOPULSE TRACKING and we will see how the things are actually achieved and what the different precautions we have to take in our next post.


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