In radar surveillances systems we have PSR ( primary surveillance radar) and SSR ( secondary surveillance radar) And they provide informations to be use it by air traffic controller, one of these information is velocity :
PSR system : the velocity is Doppler speed.
SSR : send out interrogation at 1030 MHz and reply by the target at 1090 MHz, so it is not Doppler frequency.
My questions how does the SSR Radar calculate the speed ?


1 Answer 1


Even primary radar can't determine velocity from just one return. The Doppler shift only gives you the velocity component that's toward or away from the antenna site. Any motion side-to-side can't be determined by Doppler shift. Hence, most civilian radars ignore Doppler shift completely.

Rather, both primary and secondary radar determine speed the same way: by analyzing the difference in position of multiple returns from the same target. If a particular target's position during this sweep is 2000 feet away from its position during the last sweep, it must be traveling 2000 feet per 8 seconds (most common radars rotate one turn every 8 seconds; if your radar is different, adjust accordingly), which works out to be (let's see... carry the two...) about 150 knots.

  • 2
    $\begingroup$ And, of course, by "carry the two", I really mean "type '2000 feet per 8 seconds to knots' into Google". $\endgroup$ Commented Mar 4, 2023 at 2:06
  • $\begingroup$ Interesting. Is there any logic for determining velocity of an aircraft who is performing a DME arc around the station? $\endgroup$
    – wbeard52
    Commented Mar 4, 2023 at 15:02
  • $\begingroup$ @wbeard52 For an aircraft that's turning, the calculated velocity will be less than its true velocity. But the difference is rarely big enough to be worth considering, since radar only gives you ground speed anyway, and most planes only worry about airspeed. $\endgroup$ Commented Mar 4, 2023 at 17:23
  • $\begingroup$ And if the aircraft is Mode S equipped with Enhanced Surveillance (most today are), the Mode S ground station can just request the velocity (speed/heading) via Mode S datalink. It's the same data as broadcast via ADS-B. $\endgroup$
    – Gerry
    Commented Mar 4, 2023 at 19:59

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