In largest airport a ground radar is used to handle safely aircrafts on taxiways and on runways, which should be the characteristics of this kind of radar ? What about its range and bearing resolution ?


It's been a while since my last lessons on radars, but..
The pulse length and the pulse repetition frequency play key roles in radar min and max ranges. The pulse the radar sends must be short enough to end (and the radar switch to detection mode) before the reflection returns.
To give a rough example, if the pulse length is 2 ms, the theoretical minimum range would be equal to pulse distance in 1 ms (1 there and 1 back). The pulse travels at light speed, so you can calculate the distance there.
Note that the pulse lengths differ a lot depending on radars, and presumably none of the are 1 ms long.
Free quote from the book Understanding Radar Systems:

Pulse length of 3 µs, which is a fairly typical pulse length for a long-range airborne surveillance radar, would give a theoretical range resolution of 450 m. However, other factors such as signal processing degrade the resoution, and in the given case the resolution would be around 750 m.

Note again, that radars are designed to serve a specific purpose, for example a long-range surveillance radar emphasizes range over resolution.
What it comes to fighters, most modern fighters have AESA radars instead of pulse radars, which work continuously and handle the signal differently.

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    $\begingroup$ Some things I just can't Google while driving. Expect an edit later.. $\endgroup$ – Sami May 26 '15 at 9:19
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    $\begingroup$ 1 ms would be absurdly long. Such pulse would be 300 km long $\endgroup$ – Jan Hudec May 27 '15 at 10:28
  • $\begingroup$ I know. That's why i said "none of the are 1ms". That was given as an example, could have just as easily chosen "x". $\endgroup$ – Sami May 27 '15 at 10:30

For surface movement radars (SMR) operating in the 9 - 9,5 GHz band, the pulse length is typically in the order of 40ns. This is equivalent to 12 meters. That would bring the theoretical minimum range to 6 if we ignore transmitter / receiver switching overhead time.

In practice there is no need for such a short range. Instead a pulse train is transmitted before the antenna is switched from the transmitter to the receiver. This allows the radar to increase the accuracy by averaging multiple reflections, at the price of an increase in minimum range. Typical minimal range will be in the order of 100 meters.

Other SMR systems use continuous wave radar and have a transmitter and receiver antenna in the radar head. These systems don't require to switch between transmitting and receiving and don't have a minimum range.

The range resolution is in the order of 3 meters, equivalent to the pulse rise time of about 10ns.

The azimuth resolution of a typical SMR is in the order of 0.25 degrees, which is equivalent to 10 meters at 2 km distance.

Apart from primary radar there are other classes of surveillance systems in use for airport surface detection.

  • Secondary surveillance radar (SSR) lacks the accuracy of the SMR and can only see transponders. But combined with the primary radar allows for identification of aircraft on the airport surface (i.e. putting identification labels on the aircraft's primary radar reflection).

  • Multilateration (MLAT) systems also rely on transponders but are more accurate than SSR for surface monitoring. These systems require a large number of receivers and are susceptible for radio reflections from buildings.

  • ADS-B relies on the aircraft (or airport vehicle) transmitting its position, speed and heading.

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  • $\begingroup$ Don't ground radar systems use secondary radar instead of primary radar for their use? I believe that this is why the AIM now says to have the transponder on while taxiing and many large airports require it. $\endgroup$ – Lnafziger May 27 '15 at 15:15
  • $\begingroup$ @Lnafziger SMR is primary radar, but often there is a SSR at/near the airport which is used for flight id / label correlation. The SSR alone is typically not able to provide the accuracy required for low visibility procedures on the surface. Multilateration systems (like ASDE-X in the USA) depend on the transponder as well but can be much more accurate than a SSR. $\endgroup$ – DeltaLima May 27 '15 at 15:23
  • $\begingroup$ @DeltaLima Would you expand on the meaning of the term "multilateration" in the context of your comment? Is that the term for the multiple pulses you mentioned in the answer, or something else? Thanks $\endgroup$ – Ralph J May 28 '15 at 4:32
  • $\begingroup$ @RalphJ Multilateration is a surveillance technology that relies on the time difference of arrival of a radio signal at different positions. It consists of a number of radio receivers positioned carefully over the area under surveillance. The receivers listen to transmissions from the aircraft's transponder. At a minimum there are 4 receivers, multilateration systems for airport surface surveillance typically have 20 or more. $\endgroup$ – DeltaLima May 28 '15 at 6:41
  • $\begingroup$ @DeltaLima Interesting -- had no idea ASDE X did that. So with all those receivers, it has a really precise location of the transponder antenna; is there also the radar as well to see other, non-transponder, vehicles on the airport surface? $\endgroup$ – Ralph J May 28 '15 at 13:09

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