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While reading the description of ADS-B given on the FR24 website (ADS-B How it works), I came across this sentence:

The farther away from the receiver an aircraft is flying, the higher it must fly to be covered by the receiver.

What is the relation between height and coverage in this case?

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up vote 13 down vote accepted

The ADS-B signal used by FR24 is transmitted on 1090 MHz. Signals at that frequency do not follow the curvature of the earth very well. They work best in line of sight. Aircraft far away from the receiver must be at high altitude to be above the horizon.

At 1000ft, the horizon is about 33 nautical mile away. Aircraft further away will be shielded below the horizon. For 40000ft, the horizon is about 220 nautical mile away.

L-Band signal horizon

Due to a slight refraction of the signal the practical range is about 15% further.

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+1 Great diagram. The only minor change I'd make is to make the 'Invisible' text horizontal (parallel to the dashed line). Currently looks like the annulus is the invisible region. – Hugh Mar 17 '14 at 0:38

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