How is radar separation assured between primary and secondary targets? The 2019 Stack Overflow Developer Survey Results Are InWhy can’t air traffic control radars determine the altitudes of primary targets?How do ATC radars know where to position a primary target?Could an aircraft 'hide' in the aircraft shadow of another aircraft without being detected or noticed?How can ADS-B replace primary radar when FlightRadar24, using ADS-B, is so inaccurate?Would a homemade lawn chair balloon be visible on ATC and collision avoidance radar?How do stealth aircraft get ATC?What is the purpose of wide area multilateration (WAM)?Does a secondary surveillance radar need the support of a primary surveillance radar?How does radar receive Mode C information?How does ATC locate a stealth aircraft if its transponder fails?Why can’t air traffic control radars determine the altitudes of primary targets?

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How is radar separation assured between primary and secondary targets?



The 2019 Stack Overflow Developer Survey Results Are InWhy can’t air traffic control radars determine the altitudes of primary targets?How do ATC radars know where to position a primary target?Could an aircraft 'hide' in the aircraft shadow of another aircraft without being detected or noticed?How can ADS-B replace primary radar when FlightRadar24, using ADS-B, is so inaccurate?Would a homemade lawn chair balloon be visible on ATC and collision avoidance radar?How do stealth aircraft get ATC?What is the purpose of wide area multilateration (WAM)?Does a secondary surveillance radar need the support of a primary surveillance radar?How does radar receive Mode C information?How does ATC locate a stealth aircraft if its transponder fails?Why can’t air traffic control radars determine the altitudes of primary targets?










1












$begingroup$


Air traffic control radars come in two flavours, with most ATC centers using both and displaying the output of both on a single screen:




  • Primary surveillance radar (PSR) is a true radar system, which detects aircraft (and also non-aircraft objects, such as birds, balloons, weather, parts detached from aircraft, the ground...) by bouncing radio waves off them. PSR is valuable because it can detect not just aircraft with functioning and responding transponders, but also aircraft without functioning transponders and the aforementioned non-aircraft-but-potentially-of-interest-to-or-regarding-aircraft objects. However, in order to maximise the amount of airspace covered per 360º sweep, the radar beam from most civilian PSR transmitters is fan-shaped; at any given moment, the beam covers (for all intents and purposes) just a single azimuth, but is spread out over a wide range of elevation angles. The radar has no way to determine the elevation angle of a returning reflected signal, which makes most PSR systems useless for discerning altitude, and introduces some imprecision and a variable amount of inaccuracy into the ground position of a target (as the object producing the return could be located anywhere along a vertical arc segment at the target’s slant distance in the plane of the radar beam).1


  • Secondary surveillance radar (SSR) isn’t, strictly-speaking, radar at all. Instead of bouncing radio waves off the local flying-object population, it, in essence, calls out “Is anyone there?” in the blind; transponder-equipped aircraft hear this call, and the aircraft’s transponder then broadcasts the aircraft’s position, altitude, airspeed, direction, flight number, aircraft type, licence, registration, proof of insurance, etc., allowing the SSR system to plot the exact position of the aircraft generating the return, along with the aforementioned other various interesting pieces of information. This makes SSR extremely useful, especially in congested airspace, but it can’t detect anything without a transponder, or with a broken transponder, or with their transponder turned off. SSR is planned to eventually be replaced by ADS-B, which works in essentially exactly the same way, except that the aircraft’s transponder broadcasts all of the aforementioned information in the blind, without being asked, thus eliminating the need to have a large, expensive SSR transmitter for each ATC facility.

Here’s an illustration of the difference in the determination and display of target positional information between primary and secondary radars:



Image Description Here



Separating secondary targets from each other is (assuming that the air traffic controllers and pilots concerned are paying attention and doing what they’re supposed to), obviously, trivial; you just need to avoid having two or more aircraft occupy the same position at the same altitude at the same time. Separating primary targets from each other is also fairly trivial, despite the fuzzy positional information and the lack of altitude information; two or more aircraft occupying different vertical arc segments (and, thus, separate from each other) will show up as two or more separate targets, so the trick is to keep two or more aircraft from occupying the same arc segment. If you’re dealing with just primary targets, separation between aircraft can be assured by keeping the dots on the screen away from each other.



If, however, you’re dealing with a mix of primary and secondary targets, I see a problem arising: since positional information is determined differently for primary and secondary targets, an aircraft in a given position could show up in one of two different places on the screen, depending on whether it’s a primary or a secondary target:



Anatomy of an impending midair collision



It should be immediately obvious that the same effect could be produced if the primary and secondary targets were produced by different aircraft; two aircraft in dangerously close proximity to each other could appear to the controller to be separated by a safe distance, if one of the aircraft is a primary target and the other is a secondary target.



How to solve this problem?




1: Some PSR systems (primarily military systems, as it’s much easier to vector aircraft to intercept an intruder if you know its altitude and its exact location) do provide altitude information; some of these work by using a narrow “pencil beam”, which allows the determination of a target’s elevation angle at the cost of greatly-increased scan times, while others use a pair of radars at different heights and measure the difference in slant distances to the target between the two radars, allowing the target’s altitude to be determined using trigonometry.










share|improve this question









$endgroup$







  • 3




    $begingroup$
    Your description of SSR conflates the functionality of SSR and ADS-B. SSR returns the mode C ident and pressure altitude (only those two things, not lat/lon), and is usually integrated with the PSR.
    $endgroup$
    – Greg Hewgill
    2 hours ago















1












$begingroup$


Air traffic control radars come in two flavours, with most ATC centers using both and displaying the output of both on a single screen:




  • Primary surveillance radar (PSR) is a true radar system, which detects aircraft (and also non-aircraft objects, such as birds, balloons, weather, parts detached from aircraft, the ground...) by bouncing radio waves off them. PSR is valuable because it can detect not just aircraft with functioning and responding transponders, but also aircraft without functioning transponders and the aforementioned non-aircraft-but-potentially-of-interest-to-or-regarding-aircraft objects. However, in order to maximise the amount of airspace covered per 360º sweep, the radar beam from most civilian PSR transmitters is fan-shaped; at any given moment, the beam covers (for all intents and purposes) just a single azimuth, but is spread out over a wide range of elevation angles. The radar has no way to determine the elevation angle of a returning reflected signal, which makes most PSR systems useless for discerning altitude, and introduces some imprecision and a variable amount of inaccuracy into the ground position of a target (as the object producing the return could be located anywhere along a vertical arc segment at the target’s slant distance in the plane of the radar beam).1


  • Secondary surveillance radar (SSR) isn’t, strictly-speaking, radar at all. Instead of bouncing radio waves off the local flying-object population, it, in essence, calls out “Is anyone there?” in the blind; transponder-equipped aircraft hear this call, and the aircraft’s transponder then broadcasts the aircraft’s position, altitude, airspeed, direction, flight number, aircraft type, licence, registration, proof of insurance, etc., allowing the SSR system to plot the exact position of the aircraft generating the return, along with the aforementioned other various interesting pieces of information. This makes SSR extremely useful, especially in congested airspace, but it can’t detect anything without a transponder, or with a broken transponder, or with their transponder turned off. SSR is planned to eventually be replaced by ADS-B, which works in essentially exactly the same way, except that the aircraft’s transponder broadcasts all of the aforementioned information in the blind, without being asked, thus eliminating the need to have a large, expensive SSR transmitter for each ATC facility.

Here’s an illustration of the difference in the determination and display of target positional information between primary and secondary radars:



Image Description Here



Separating secondary targets from each other is (assuming that the air traffic controllers and pilots concerned are paying attention and doing what they’re supposed to), obviously, trivial; you just need to avoid having two or more aircraft occupy the same position at the same altitude at the same time. Separating primary targets from each other is also fairly trivial, despite the fuzzy positional information and the lack of altitude information; two or more aircraft occupying different vertical arc segments (and, thus, separate from each other) will show up as two or more separate targets, so the trick is to keep two or more aircraft from occupying the same arc segment. If you’re dealing with just primary targets, separation between aircraft can be assured by keeping the dots on the screen away from each other.



If, however, you’re dealing with a mix of primary and secondary targets, I see a problem arising: since positional information is determined differently for primary and secondary targets, an aircraft in a given position could show up in one of two different places on the screen, depending on whether it’s a primary or a secondary target:



Anatomy of an impending midair collision



It should be immediately obvious that the same effect could be produced if the primary and secondary targets were produced by different aircraft; two aircraft in dangerously close proximity to each other could appear to the controller to be separated by a safe distance, if one of the aircraft is a primary target and the other is a secondary target.



How to solve this problem?




1: Some PSR systems (primarily military systems, as it’s much easier to vector aircraft to intercept an intruder if you know its altitude and its exact location) do provide altitude information; some of these work by using a narrow “pencil beam”, which allows the determination of a target’s elevation angle at the cost of greatly-increased scan times, while others use a pair of radars at different heights and measure the difference in slant distances to the target between the two radars, allowing the target’s altitude to be determined using trigonometry.










share|improve this question









$endgroup$







  • 3




    $begingroup$
    Your description of SSR conflates the functionality of SSR and ADS-B. SSR returns the mode C ident and pressure altitude (only those two things, not lat/lon), and is usually integrated with the PSR.
    $endgroup$
    – Greg Hewgill
    2 hours ago













1












1








1


1



$begingroup$


Air traffic control radars come in two flavours, with most ATC centers using both and displaying the output of both on a single screen:




  • Primary surveillance radar (PSR) is a true radar system, which detects aircraft (and also non-aircraft objects, such as birds, balloons, weather, parts detached from aircraft, the ground...) by bouncing radio waves off them. PSR is valuable because it can detect not just aircraft with functioning and responding transponders, but also aircraft without functioning transponders and the aforementioned non-aircraft-but-potentially-of-interest-to-or-regarding-aircraft objects. However, in order to maximise the amount of airspace covered per 360º sweep, the radar beam from most civilian PSR transmitters is fan-shaped; at any given moment, the beam covers (for all intents and purposes) just a single azimuth, but is spread out over a wide range of elevation angles. The radar has no way to determine the elevation angle of a returning reflected signal, which makes most PSR systems useless for discerning altitude, and introduces some imprecision and a variable amount of inaccuracy into the ground position of a target (as the object producing the return could be located anywhere along a vertical arc segment at the target’s slant distance in the plane of the radar beam).1


  • Secondary surveillance radar (SSR) isn’t, strictly-speaking, radar at all. Instead of bouncing radio waves off the local flying-object population, it, in essence, calls out “Is anyone there?” in the blind; transponder-equipped aircraft hear this call, and the aircraft’s transponder then broadcasts the aircraft’s position, altitude, airspeed, direction, flight number, aircraft type, licence, registration, proof of insurance, etc., allowing the SSR system to plot the exact position of the aircraft generating the return, along with the aforementioned other various interesting pieces of information. This makes SSR extremely useful, especially in congested airspace, but it can’t detect anything without a transponder, or with a broken transponder, or with their transponder turned off. SSR is planned to eventually be replaced by ADS-B, which works in essentially exactly the same way, except that the aircraft’s transponder broadcasts all of the aforementioned information in the blind, without being asked, thus eliminating the need to have a large, expensive SSR transmitter for each ATC facility.

Here’s an illustration of the difference in the determination and display of target positional information between primary and secondary radars:



Image Description Here



Separating secondary targets from each other is (assuming that the air traffic controllers and pilots concerned are paying attention and doing what they’re supposed to), obviously, trivial; you just need to avoid having two or more aircraft occupy the same position at the same altitude at the same time. Separating primary targets from each other is also fairly trivial, despite the fuzzy positional information and the lack of altitude information; two or more aircraft occupying different vertical arc segments (and, thus, separate from each other) will show up as two or more separate targets, so the trick is to keep two or more aircraft from occupying the same arc segment. If you’re dealing with just primary targets, separation between aircraft can be assured by keeping the dots on the screen away from each other.



If, however, you’re dealing with a mix of primary and secondary targets, I see a problem arising: since positional information is determined differently for primary and secondary targets, an aircraft in a given position could show up in one of two different places on the screen, depending on whether it’s a primary or a secondary target:



Anatomy of an impending midair collision



It should be immediately obvious that the same effect could be produced if the primary and secondary targets were produced by different aircraft; two aircraft in dangerously close proximity to each other could appear to the controller to be separated by a safe distance, if one of the aircraft is a primary target and the other is a secondary target.



How to solve this problem?




1: Some PSR systems (primarily military systems, as it’s much easier to vector aircraft to intercept an intruder if you know its altitude and its exact location) do provide altitude information; some of these work by using a narrow “pencil beam”, which allows the determination of a target’s elevation angle at the cost of greatly-increased scan times, while others use a pair of radars at different heights and measure the difference in slant distances to the target between the two radars, allowing the target’s altitude to be determined using trigonometry.










share|improve this question









$endgroup$




Air traffic control radars come in two flavours, with most ATC centers using both and displaying the output of both on a single screen:




  • Primary surveillance radar (PSR) is a true radar system, which detects aircraft (and also non-aircraft objects, such as birds, balloons, weather, parts detached from aircraft, the ground...) by bouncing radio waves off them. PSR is valuable because it can detect not just aircraft with functioning and responding transponders, but also aircraft without functioning transponders and the aforementioned non-aircraft-but-potentially-of-interest-to-or-regarding-aircraft objects. However, in order to maximise the amount of airspace covered per 360º sweep, the radar beam from most civilian PSR transmitters is fan-shaped; at any given moment, the beam covers (for all intents and purposes) just a single azimuth, but is spread out over a wide range of elevation angles. The radar has no way to determine the elevation angle of a returning reflected signal, which makes most PSR systems useless for discerning altitude, and introduces some imprecision and a variable amount of inaccuracy into the ground position of a target (as the object producing the return could be located anywhere along a vertical arc segment at the target’s slant distance in the plane of the radar beam).1


  • Secondary surveillance radar (SSR) isn’t, strictly-speaking, radar at all. Instead of bouncing radio waves off the local flying-object population, it, in essence, calls out “Is anyone there?” in the blind; transponder-equipped aircraft hear this call, and the aircraft’s transponder then broadcasts the aircraft’s position, altitude, airspeed, direction, flight number, aircraft type, licence, registration, proof of insurance, etc., allowing the SSR system to plot the exact position of the aircraft generating the return, along with the aforementioned other various interesting pieces of information. This makes SSR extremely useful, especially in congested airspace, but it can’t detect anything without a transponder, or with a broken transponder, or with their transponder turned off. SSR is planned to eventually be replaced by ADS-B, which works in essentially exactly the same way, except that the aircraft’s transponder broadcasts all of the aforementioned information in the blind, without being asked, thus eliminating the need to have a large, expensive SSR transmitter for each ATC facility.

Here’s an illustration of the difference in the determination and display of target positional information between primary and secondary radars:



Image Description Here



Separating secondary targets from each other is (assuming that the air traffic controllers and pilots concerned are paying attention and doing what they’re supposed to), obviously, trivial; you just need to avoid having two or more aircraft occupy the same position at the same altitude at the same time. Separating primary targets from each other is also fairly trivial, despite the fuzzy positional information and the lack of altitude information; two or more aircraft occupying different vertical arc segments (and, thus, separate from each other) will show up as two or more separate targets, so the trick is to keep two or more aircraft from occupying the same arc segment. If you’re dealing with just primary targets, separation between aircraft can be assured by keeping the dots on the screen away from each other.



If, however, you’re dealing with a mix of primary and secondary targets, I see a problem arising: since positional information is determined differently for primary and secondary targets, an aircraft in a given position could show up in one of two different places on the screen, depending on whether it’s a primary or a secondary target:



Anatomy of an impending midair collision



It should be immediately obvious that the same effect could be produced if the primary and secondary targets were produced by different aircraft; two aircraft in dangerously close proximity to each other could appear to the controller to be separated by a safe distance, if one of the aircraft is a primary target and the other is a secondary target.



How to solve this problem?




1: Some PSR systems (primarily military systems, as it’s much easier to vector aircraft to intercept an intruder if you know its altitude and its exact location) do provide altitude information; some of these work by using a narrow “pencil beam”, which allows the determination of a target’s elevation angle at the cost of greatly-increased scan times, while others use a pair of radars at different heights and measure the difference in slant distances to the target between the two radars, allowing the target’s altitude to be determined using trigonometry.







air-traffic-control radar separation






share|improve this question













share|improve this question











share|improve this question




share|improve this question










asked 3 hours ago









SeanSean

5,90632873




5,90632873







  • 3




    $begingroup$
    Your description of SSR conflates the functionality of SSR and ADS-B. SSR returns the mode C ident and pressure altitude (only those two things, not lat/lon), and is usually integrated with the PSR.
    $endgroup$
    – Greg Hewgill
    2 hours ago












  • 3




    $begingroup$
    Your description of SSR conflates the functionality of SSR and ADS-B. SSR returns the mode C ident and pressure altitude (only those two things, not lat/lon), and is usually integrated with the PSR.
    $endgroup$
    – Greg Hewgill
    2 hours ago







3




3




$begingroup$
Your description of SSR conflates the functionality of SSR and ADS-B. SSR returns the mode C ident and pressure altitude (only those two things, not lat/lon), and is usually integrated with the PSR.
$endgroup$
– Greg Hewgill
2 hours ago




$begingroup$
Your description of SSR conflates the functionality of SSR and ADS-B. SSR returns the mode C ident and pressure altitude (only those two things, not lat/lon), and is usually integrated with the PSR.
$endgroup$
– Greg Hewgill
2 hours ago










3 Answers
3






active

oldest

votes


















2












$begingroup$

Typically the surveillance information from multiple sources (PSR, SSR, ADS-B, multilateration, etc) are combined into a single "system track" using a Kalman filter. The ATM system has specific code to deal with crossing tracks where the labels may get confused, and controllers have procedures to fix this when it does happen.



The ATM system does its best to avoid displaying multiple targets for the same aircraft.






share|improve this answer









$endgroup$




















    1












    $begingroup$

    Mode C transponders only transmit a squawk code (same as mode A) and altitude, so the range error due to slant distance is the same for SSR as for PSR, and the two can be easily merged. Mode S adds some other details, but none that are relevant here.



    ADSB transmits lat/long (and many other things), so it has no slant range error. However, ADSB messages also contain the same squawk code and altitude as in mode A/C/S, which means ADSB targets can be trivially merged with their corresponding SSR targets.






    share|improve this answer









    $endgroup$




















      0












      $begingroup$

      I was told that separation with primary targets is usually not assured.



      The primary radar output usually contains a lot of ‘ground clutter’ like cars on the motorway and wind turbine rotors—it can filter out reflections of static objects, but these are moving and the system cannot be sure—so finding a relevant primary target in it is quite hard.



      So controllers only look for primary targets if they have another indication that an aircraft should be there—either because it is communicating over radio, but does not have transponder or it had failed, or because another pilot already reported an aircraft that is not communicating at all in the area (gliders, balloons and such often don't have radio at all, and you don't have to have it in the uncontrolled airspace).



      Note that the precision of an area radar is not that high, maybe a mile, so the slant error is often below the resolution anyway. And since the secondary radar collects transponder responses to the sweeps of the primary radar, it has no problem matching the secondary targets with the corresponding primary ones.






      share|improve this answer









      $endgroup$













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        3 Answers
        3






        active

        oldest

        votes








        3 Answers
        3






        active

        oldest

        votes









        active

        oldest

        votes






        active

        oldest

        votes









        2












        $begingroup$

        Typically the surveillance information from multiple sources (PSR, SSR, ADS-B, multilateration, etc) are combined into a single "system track" using a Kalman filter. The ATM system has specific code to deal with crossing tracks where the labels may get confused, and controllers have procedures to fix this when it does happen.



        The ATM system does its best to avoid displaying multiple targets for the same aircraft.






        share|improve this answer









        $endgroup$

















          2












          $begingroup$

          Typically the surveillance information from multiple sources (PSR, SSR, ADS-B, multilateration, etc) are combined into a single "system track" using a Kalman filter. The ATM system has specific code to deal with crossing tracks where the labels may get confused, and controllers have procedures to fix this when it does happen.



          The ATM system does its best to avoid displaying multiple targets for the same aircraft.






          share|improve this answer









          $endgroup$















            2












            2








            2





            $begingroup$

            Typically the surveillance information from multiple sources (PSR, SSR, ADS-B, multilateration, etc) are combined into a single "system track" using a Kalman filter. The ATM system has specific code to deal with crossing tracks where the labels may get confused, and controllers have procedures to fix this when it does happen.



            The ATM system does its best to avoid displaying multiple targets for the same aircraft.






            share|improve this answer









            $endgroup$



            Typically the surveillance information from multiple sources (PSR, SSR, ADS-B, multilateration, etc) are combined into a single "system track" using a Kalman filter. The ATM system has specific code to deal with crossing tracks where the labels may get confused, and controllers have procedures to fix this when it does happen.



            The ATM system does its best to avoid displaying multiple targets for the same aircraft.







            share|improve this answer












            share|improve this answer



            share|improve this answer










            answered 3 hours ago









            Greg HewgillGreg Hewgill

            6,48612534




            6,48612534





















                1












                $begingroup$

                Mode C transponders only transmit a squawk code (same as mode A) and altitude, so the range error due to slant distance is the same for SSR as for PSR, and the two can be easily merged. Mode S adds some other details, but none that are relevant here.



                ADSB transmits lat/long (and many other things), so it has no slant range error. However, ADSB messages also contain the same squawk code and altitude as in mode A/C/S, which means ADSB targets can be trivially merged with their corresponding SSR targets.






                share|improve this answer









                $endgroup$

















                  1












                  $begingroup$

                  Mode C transponders only transmit a squawk code (same as mode A) and altitude, so the range error due to slant distance is the same for SSR as for PSR, and the two can be easily merged. Mode S adds some other details, but none that are relevant here.



                  ADSB transmits lat/long (and many other things), so it has no slant range error. However, ADSB messages also contain the same squawk code and altitude as in mode A/C/S, which means ADSB targets can be trivially merged with their corresponding SSR targets.






                  share|improve this answer









                  $endgroup$















                    1












                    1








                    1





                    $begingroup$

                    Mode C transponders only transmit a squawk code (same as mode A) and altitude, so the range error due to slant distance is the same for SSR as for PSR, and the two can be easily merged. Mode S adds some other details, but none that are relevant here.



                    ADSB transmits lat/long (and many other things), so it has no slant range error. However, ADSB messages also contain the same squawk code and altitude as in mode A/C/S, which means ADSB targets can be trivially merged with their corresponding SSR targets.






                    share|improve this answer









                    $endgroup$



                    Mode C transponders only transmit a squawk code (same as mode A) and altitude, so the range error due to slant distance is the same for SSR as for PSR, and the two can be easily merged. Mode S adds some other details, but none that are relevant here.



                    ADSB transmits lat/long (and many other things), so it has no slant range error. However, ADSB messages also contain the same squawk code and altitude as in mode A/C/S, which means ADSB targets can be trivially merged with their corresponding SSR targets.







                    share|improve this answer












                    share|improve this answer



                    share|improve this answer










                    answered 2 hours ago









                    StephenSStephenS

                    5,2431929




                    5,2431929





















                        0












                        $begingroup$

                        I was told that separation with primary targets is usually not assured.



                        The primary radar output usually contains a lot of ‘ground clutter’ like cars on the motorway and wind turbine rotors—it can filter out reflections of static objects, but these are moving and the system cannot be sure—so finding a relevant primary target in it is quite hard.



                        So controllers only look for primary targets if they have another indication that an aircraft should be there—either because it is communicating over radio, but does not have transponder or it had failed, or because another pilot already reported an aircraft that is not communicating at all in the area (gliders, balloons and such often don't have radio at all, and you don't have to have it in the uncontrolled airspace).



                        Note that the precision of an area radar is not that high, maybe a mile, so the slant error is often below the resolution anyway. And since the secondary radar collects transponder responses to the sweeps of the primary radar, it has no problem matching the secondary targets with the corresponding primary ones.






                        share|improve this answer









                        $endgroup$

















                          0












                          $begingroup$

                          I was told that separation with primary targets is usually not assured.



                          The primary radar output usually contains a lot of ‘ground clutter’ like cars on the motorway and wind turbine rotors—it can filter out reflections of static objects, but these are moving and the system cannot be sure—so finding a relevant primary target in it is quite hard.



                          So controllers only look for primary targets if they have another indication that an aircraft should be there—either because it is communicating over radio, but does not have transponder or it had failed, or because another pilot already reported an aircraft that is not communicating at all in the area (gliders, balloons and such often don't have radio at all, and you don't have to have it in the uncontrolled airspace).



                          Note that the precision of an area radar is not that high, maybe a mile, so the slant error is often below the resolution anyway. And since the secondary radar collects transponder responses to the sweeps of the primary radar, it has no problem matching the secondary targets with the corresponding primary ones.






                          share|improve this answer









                          $endgroup$















                            0












                            0








                            0





                            $begingroup$

                            I was told that separation with primary targets is usually not assured.



                            The primary radar output usually contains a lot of ‘ground clutter’ like cars on the motorway and wind turbine rotors—it can filter out reflections of static objects, but these are moving and the system cannot be sure—so finding a relevant primary target in it is quite hard.



                            So controllers only look for primary targets if they have another indication that an aircraft should be there—either because it is communicating over radio, but does not have transponder or it had failed, or because another pilot already reported an aircraft that is not communicating at all in the area (gliders, balloons and such often don't have radio at all, and you don't have to have it in the uncontrolled airspace).



                            Note that the precision of an area radar is not that high, maybe a mile, so the slant error is often below the resolution anyway. And since the secondary radar collects transponder responses to the sweeps of the primary radar, it has no problem matching the secondary targets with the corresponding primary ones.






                            share|improve this answer









                            $endgroup$



                            I was told that separation with primary targets is usually not assured.



                            The primary radar output usually contains a lot of ‘ground clutter’ like cars on the motorway and wind turbine rotors—it can filter out reflections of static objects, but these are moving and the system cannot be sure—so finding a relevant primary target in it is quite hard.



                            So controllers only look for primary targets if they have another indication that an aircraft should be there—either because it is communicating over radio, but does not have transponder or it had failed, or because another pilot already reported an aircraft that is not communicating at all in the area (gliders, balloons and such often don't have radio at all, and you don't have to have it in the uncontrolled airspace).



                            Note that the precision of an area radar is not that high, maybe a mile, so the slant error is often below the resolution anyway. And since the secondary radar collects transponder responses to the sweeps of the primary radar, it has no problem matching the secondary targets with the corresponding primary ones.







                            share|improve this answer












                            share|improve this answer



                            share|improve this answer










                            answered 57 mins ago









                            Jan HudecJan Hudec

                            40.4k4106196




                            40.4k4106196



























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