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I am suspecting that it is easier to detect a satellite because in the space, there is no other thing. In other words, the background is cleaner.

Is it right?

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    $\begingroup$ Radar detection of a satellite needs a large antenna dish and a lot of power due to the radar equation. Doubling the distance needs 16 times transmitter power, 10 times the distance and 10000 times the power. So detecting an airplane in 40 km distance is much easier than a satellite in 400 km. $\endgroup$
    – Uwe
    Commented Nov 30, 2023 at 20:58

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In addition to Steve’s comments, there are some other factors

Target Reflectivity is affected by size (most satellites are smaller than most planes), material (planes are completely covered in metal), reflectivity (satellite PV panels are poor radar targets, as compared with aluminum airframes)

Geometry of the target. The best reflectors are “corner cubes”, like cat’s-eye bicycle reflectors, which rely on reflective surfaces at right angles to each other. Civilian aircraft have these surfaces in plenty (engine nacelles and pylons, vertical and horizontal stabilizers). Satellites, not so much.

The main lobe of a radar beam is narrow, shaped by the geometry of the antenna. The narrower the beam, the higher the signal strength and the longer the range of the radar. However, a narrow beam takes longer to scan a given quadrant. Long scan intervals mean the target may have moved between passes.

Scanning high elevation. Marine radar has an easy job since it only scans the horizon. But aviation radar needs to scan elevation as well, especially if the target is at high elevation. Scanning for satellites requires even higher elevation scans since they will likely be high elevation when they are close enough to be within range.

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Put all this together, and a satellite becomes a very difficult target. It is far away, fast, small, poorly reflective and at high elevation. A tight radar beam (to increase signal strength) means longer scanning intervals, reducing target detection.

Try catch a crow, flying high at night, with a narrow flashlight beam.

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  • $\begingroup$ Not entirely a fair comparison with the crow: for a typical satellite you know where to look and it has a very predictable path, unlike the crow. $\endgroup$
    – Ludo
    Commented Dec 1, 2023 at 8:06
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    $\begingroup$ @Ludo .. if you know where it is and its trajectory, you don't need radar. $\endgroup$
    – Woody
    Commented Dec 1, 2023 at 16:48
  • $\begingroup$ @Ludo you both make good points, but since the (very short) question asks about satellite "detection" rather than tracking, the crow at night with a flashlight analogy does seem to fit. Ideally the OP should have made that distinction clearer though, and explained what "detection" means here. $\endgroup$
    – uhoh
    Commented Dec 1, 2023 at 22:00
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    $\begingroup$ @uhoh the point I’m trying to make is that if you have capable radar, like the Smart-L, it’s not so hard. You scan the sky twice and you’ll have all the satellites: first scan will find it already, then you predict forward in time, and the second scan confirms. $\endgroup$
    – Ludo
    Commented Dec 2, 2023 at 17:05
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    $\begingroup$ @Ludo Oh I see. Yes I'm getting it. Radar gives you direction and range (localizes in 3D space) but also gives you range-rate via Doppler. Since we know Earth's gravity, that allows you to predict forward in time based on radar only, not some ephemeris of known objects. "for a typical satellite you know where to look" sounded to me like you meant using an ephemeris to point a radar beam (as one would point a flashlight) to say "yep it's where it's supposed to be". Now I understand it's a regular, thorough scan of a good chunk of sky, and "look" is what algorithms do to the massive dataset. $\endgroup$
    – uhoh
    Commented Dec 2, 2023 at 21:34
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No it's not easier. Radar technology is complicated and has many variables, but as a general rule it is harder to detect objects farther away than those that are closer. Satellites are almost always much farther away than airplanes. The only exceptions would be a relatively low flying satellite at say 300 km (185 miles) passing directly over a radar installation, which could be closer than an airplane on the horizon. Most satellites however orbit much higher than that and the distance to radar installations is much farther.

It is true that there is on average more "space" between detectable objects in Earth orbit (satellites and space debris etc.) than there is between objects in the atmosphere (aircraft, weather balloons, etc.). However aircraft radar has a huge advantage because of something known as secondary radar which can interrogate an airplane's transponder and help identify it. More recently ADS-B technology has made this type of identification even more effective. This type of identification doesn't exist for objects in space, which can make it very challenging to identify objects that are picked up on radar.

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Yes and no.

Modern long-range military radar has more than enough power to detect and track satellites in low-Earth orbit. For example, the Thales SMART-L has a range of 2000 km. It is not pointing straight up but over the horizon, but that is still enough range to cover LEO (I'm sure you can imagine why this would be useful...). Since satellites have very predictable paths, they are easy to find in the return scan.

However, a typical radar (e.g. for aviation or mobile military radars) have a range of at most a few hundred kilometer. These will not be able to detect a satellite reliably: even if their range was enough, a satellite would be at the very edge of their observation limits, and with the high relative ground speed of a satellite, they'd be very hard to impossible to track with such a radar.

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