Satellites kept wandering through this book like a guest who never got introduced. GPS whispered to your phone from below the noise floor. Satellite TV doubled its channels with two polarizations. A Starlink "dish" turned out to be a plate of tiny antennas, and the mirror in the sky put a ceiling on how high a broadcast could bounce. Time to introduce the guest properly, because the whole subject comes down to one sentence: a satellite is a radio tower that refuses to stand on the ground. Everything else follows from how high you build it.
The parked giant#
Gravity offers one truly magical altitude. Orbit close to a planet and you must race around it; orbit far and you may amble. At about 22,000 miles up, over the equator, the ambling pace works out to exactly one lap per day, which is precisely how fast the Earth itself turns. A satellite parked there keeps perfect step with the ground below, so from your backyard it simply hangs, year after year, at one fixed spot in the sky. Engineers call the altitude geostationary, and it is beachfront property: the slots are numbered, argued over, and assigned country by country.
Parking is what makes the giant useful. A dish can be bolted to a wall, aimed once with a wrench, and never touched again; that is why a whole street's satellite dishes all stare, sunflower-fashion, at the same blank-looking patch of sky. And from that height one transmitter sees a third of the planet, which makes it the ultimate broadcast tower: one voice, a continent of listeners. The price is the distance. Over 22,000 miles, the inverse-square toll is ferocious (hence the dishes), and even light needs time: about a quarter of a second for the trip up to the satellite and back down, and since a reply must climb all the way up and back again, nearly half a second slips past between one person finishing and the other's answer landing. That little stumble in old transatlantic TV interviews, where both people start talking at once? That was never the people. That was the speed of light, being merely very fast.
The racing swarm#
Come down close, a few hundred miles up, and everything reverses. Signals arrive strong and sprightly, the round trip costs a few thousandths of a second, and a modest antenna suffices at both ends. But close means fast: down here gravity insists on a lap every ninety minutes, so any one satellite streaks from horizon to horizon in minutes and is gone. There is no parking in the basement.
The modern answer is to stop caring: launch a swarm. With a few thousand satellites crisscrossing overhead, there is always one above you, and your dish plays the game from A quilt of cells in mirror image: the cells do the moving while you sit still, and your connection is handed from satellite to satellite every few minutes without dropping a beat. Tracking something that fast with a motor would be misery, which is why the Starlink plate is the phased array from the field guide: hundreds of little patches steering the beam electronically, silently, with nothing to wear out.
The in-between#
And the middle floors? That is where GPS lives, about 12,500 miles up, lapping the planet twice a day. The height is a compromise chosen with care: high enough that each satellite sees most of a hemisphere and a couple dozen of them cover the world many times over (your phone needs several in view at once to fix a position), low enough that their whispered time signals survive the trip at all. What they do with that perch, and how a whisper of pure time becomes a blue dot on a map, is the business of Echoes that measure.
Go see one#
Here is the loveliest part: the low swarm is visible to the naked eye. Go out on a clear evening in the first hour or two after sunset, when your sky is dark but the sky a few hundred miles up still catches sunlight, and watch. Sooner or later a "star" will detach itself and glide, steady and silent, across the whole sky in a few minutes. That is a satellite, sunlit against the dusk, a radio tower sailing overhead at seventeen thousand miles an hour. And when you're ready to hear one instead, the epilogue's fifteen-dollar dongle will catch a weather satellite as it passes and hand you a photograph of the Earth, taken minutes ago, received by you.