Slant Range: Why your satelite is further away.

35,786 km. That is the commonly described altitude of a geostationary satellite.

But its not that simple.

This figure is only correct if you are standing at the sub-satellite point (on the equator, directly beneath the satellite’s longitude).

Everywhere else on Earth, the signal travels further. Sometimes a lot further.

That real distance, from the satellite dish on earth to the geostationary satellite (in space) in a straight line, is called the ‘slant range’.

Altitude is measured straight up from the surface. Slant range is measured along the actual signal path.

The two only match in one place. Directly below the satellite.

The numbers

From London, the slant range to a geostationary satellite on the same longitude is roughly 38,500 km.

That is nearly 3,000 km more than is widely quoted in text books.

Now go further north.

At about 81 degrees latitude, the satellite reaches the horizon. The slant range stretches to around 41,700 km.

That adds almost 6,000 km to an actual real satellite link.

In practice you would not operate a dish with the satellite sitting exactly on the horizon. Buildings get in the way. So does terrain. So does the atmosphere. Every service works to a practical minimum elevation angle (the angle between the horizon and the satellite).

But it is a useful figure. It shows just how much the range can grow.

Does it actually matter?

For geostationary satellites, often less than you would expect.

Going from 35,786 km to 41,700 km costs around 1.3 dB of extra free-space path loss (the natural weakening of a radio signal as it spreads out over distance).

A well-designed link budget (the accounting sheet for every gain and loss between transmitter and receiver) will usually absorb that.

But not always.

On a tightly engineered link at Ku or Ka band, in poor weather, 1.3 dB is still worth having.

Round-trip propagation delay grows too.

About 239 milliseconds at the sub-satellite point. About 278 ms at the horizon limit.

You would notice that on a voice call. Particularly in the back and forth of a conversation.

It would not normally break the link.

And real-world latency will be higher again, once network routing and equipment processing are added on top.

For LEO, it is a different story

Take a satellite in low Earth orbit at 550 km altitude.

Directly overhead, its slant range is 550 km.

At a practical minimum elevation of about 5 degrees, that same satellite is around 2,200 km away.

Four times the distance. Roughly 12 dB of additional free-space path loss.

All else being equal, the signal arrives about sixteen times weaker.

At the theoretical 0-degree horizon, the range is closer to 2,700 km. Nearly five times the overhead distance, and about 13.8 dB more path loss.

That is one reason LEO constellations use so many satellites and such frequent handovers (passing your connection from one satellite to the next as they move across the sky).

It is not the only reason. Constellation size and handover rates also depend on coverage goals, capacity, orbit design, and the minimum elevation angle the service chooses to work with.

But the principle holds.

Keep the satellite high overhead. Keep the slant range short. Keep the link strong.

The point

Altitude is the headline figure.

Slant range is the number that goes into the link budget. The actual dish-to-satellite distance, at the elevation angle you are actually using.

If you are sizing a satellite link, that is the one worth checking.

I’m Craig Miles. Follow me on LinkedIn.