A safeguard inside the receiver
GPS satellites do not track each aircraft or decide which one may use the system. They broadcast timing signals and orbital information; a receiver uses signals from several satellites to calculate its own position and motion. The restriction is enforced locally: software in some receivers checks the calculated altitude and speed, then stops providing navigation if both cross its programmed limits.
That distinction matters. The satellite network keeps transmitting, and other receivers can continue working nearby. It is the particular unit’s firmware—the instructions built into its electronics—that refuses to produce a fix.
Why speed and altitude were paired
The limits grew out of export-control concerns: a compact navigation receiver could help guide a fast, high-altitude weapon. Requiring both unusual speed and altitude made the restriction target that kind of flight profile rather than ordinary aircraft, which might be high or fast without being both at once.
A commonly cited historical threshold is about 1,000 knots (roughly 1,150 mph) and 18 kilometers (about 60,000 feet), reached simultaneously. The exact behavior depends on the receiver and its design; the rule is not a physical boundary at which GPS signals disappear.
The tradeoff is in the chip
At extreme speed, a receiver must also keep up with rapidly changing signal frequencies and update its position quickly. Those are engineering challenges, but they are separate from the cutoff: a capable receiver can still be programmed to shut itself down when its limits are crossed.
So when a high-flying vehicle loses GPS under those conditions, the satellites have not lost sight of it. The navigation solution has been withheld by a rule embedded in the equipment.
