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If an aircraft has only Mode-A xpdr, then ATC with the help of Primary radar (for Position determination) and secondary radar (for receiving squawk), places a blip(only squawk code/callsign, position-from primary radar) on ATC screen (If a flight plan is there, then the controller can replace the provided squawk with his call-sign from the plan.)

If an aircraft has Mode-C xpdr, then ATC with the help of primary radar(position determination) and secondary radar (for receiving squawk and Altitude in FL), places a blip(squawk code/call sign, position-from Primary radar and altitude) on ATC screen.

if an aircraft has Mode-S xpdr, then ATC with the help of secondary radar ONLY can selectively interrogate the aircraft and aircraft sends a packet of data containing 24-bit ICAO address, position, FL, IAS, other parameters. ATC screen can display the required information on its screen (Callsign-now directly from the aircraft instead of taking from the flight plan, position-now received from aircraft, type of aircraft, FL, speed, trend-climbing or descending).

Why is ADS-B used when ATC can selectively interrogate like how Mode A/C xpdr was interrogated ?

Also, can you confirm that if an ATC screen is displaying more than callsign and altitude information, then ATC is using Mode-S xpdr only ?

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An ADS-B ground station is very very simple: it consists of a stationary antenna connected to a computer which can interpret the ADS-B packets.

In contrast, a traditional (primary) radar site has a rotating antenna system utilizing GHz-range radio waves of no little power. That requires a powerful amplifier, carefully designed and constructed waveguides, a motor to drive the antenna and consumables like motor oil and bearings—not to mention the hardware required to control the radar output and interpret the input. A secondary radar site is not as complicated but still requires rotating parts.

In short, ADS-B stations are cheaper to install and maintain. That means that the same amount of money can go into more ground stations and thus provide enhanced surveillance capability in areas where old-school radar would have been prohibitively expensive.

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  • $\begingroup$ "An ADS-B ground station is very very simple" – Scott Manley made a YouTube video about installing his own ADS-B receiver a while ago. A commenter on the video jokingly remarked that the hardest part was actually getting your hand on a Raspberry Pi. $\endgroup$
    – Jörg W Mittag
    Jun 3 at 10:04
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    $\begingroup$ @JörgWMittag you don't need a Pi; basically any computer that can run Linux (which is basically any computer) will work $\endgroup$
    – Someone
    Jun 3 at 16:05
  • $\begingroup$ @Someone you don't even need Linux. $\endgroup$
    – DeltaLima ♦
    Jun 6 at 12:09
  • $\begingroup$ A follow-up question would be... if ADS-B can do everything mode-S radar can do, was traditional secondary radar just an expensive design failure? $\endgroup$
    – fjch1997
    Jun 9 at 6:21
  • $\begingroup$ @fjch: Interesting question. I say no. ADS-B is dependent (hence the name) on the aircraft knowing its position in space. There have been ways for the pilot to know where they are for a long time, of course, but until GNSS receivers got small and cheap there wasn't a good way for a small aircraft like a Skyhawk to know where it was. $\endgroup$
    – randomhead
    Jun 9 at 12:22

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