Why not GPS only?
GPS is extraordinary, but it has three weaknesses that make GPS-only navigation unacceptable for a commercial airliner. First, GPS signals are extremely weak — they travel 20,000 kilometers from orbit and arrive barely above the noise floor. A deliberate jammer the size of a cell phone can block GPS reception across a large area, and sophisticated spoofing — broadcasting false GPS signals — can mislead a receiver into reporting a wrong position without the crew knowing anything is wrong. Second, GPS provides position only in discrete fixes, with no update between them — not the continuous, high-rate position and velocity the flight controls and autopilot need. Third, GPS answers only where am I: it says nothing about which way the airplane is pointing or how it is oriented, providing no attitude and no heading at all. The inertial system covers every one of these gaps. It cannot be jammed or spoofed, because it senses physics directly and needs no outside signal — no external source can fool an accelerometer or a ring laser gyro. It runs continuously at a high rate, filling every space between GPS fixes. And it supplies the attitude and heading GPS never can. That is why the airplane never relies on GPS alone: it blends GPS with inertial, taking the long-term accuracy of the one and the continuity, resilience, and completeness of the other.
GPS: how the signal path works
GPS satellites orbit the Earth at approximately 20,000 kilometers altitude, continuously broadcasting precise timing signals. The 787 has two GPS antennas mounted on top of the fuselage — one feeding the left INR and one feeding the right INR. Inside each INR is a GPS receiver that picks up signals from multiple satellites simultaneously — typically a dozen or more are in view at any time — and uses the timing differences between those signals to compute a precise three-dimensional position fix. All satellite selection is fully automatic.
The raw GPS position from each INR is passed to the IRU on the same side. The left INR feeds the left IRU, the right INR feeds the right IRU. Inside the IRU, the GPS position and the inertially-computed position are blended to produce the hybrid GPS-inertial position. This is the number that goes to the FMC — and the position value you will use almost every moment of your daily operation.
What the hybrid GPS-inertial position actually is
The hybrid position is not simply a choice between GPS and inertial — it is a mathematical blend engineered to take the best from each source. On its own, GPS is extremely accurate in absolute terms but can be slightly jittery second-to-second: satellite geometry changes, signal reflections cause momentary errors, atmospheric conditions affect signal travel time. On its own, inertial is beautifully smooth and stable second-to-second but drifts slowly over time as small measurement errors accumulate.
The two also work at completely different speeds, which is half the reason they suit each other. Pulling a position out of satellite signals takes real computation, so GPS fixes arrive slowly — conventionally about one a second. Inertial sensors have no such problem: they simply feel acceleration and rotation, and can report many times a second. So GPS delivers a slow trickle of absolutely correct positions, while the inertial delivers a fast, smooth stream that is going quietly wrong. Each covers precisely what the other lacks.
The hybrid filter uses the GPS fix to continuously correct the IRU's long-term drift, and uses the IRU's inertial data to smooth out the GPS's short-term jitter. The result is a position that is both accurate over long periods and stable from second to second — more reliable than either source alone.
You have likely seen this same blend in your own car or phone. Drive into a long tunnel and the GPS signal vanishes — yet the arrow on the map keeps moving, tracking you through the tunnel and around its bends. With no satellites in view, the device is dead-reckoning: estimating position from your speed and the turns it feels, exactly as an IRU does. The moment you emerge and the satellites reappear, GPS snaps the arrow back onto your true position, correcting whatever small error crept in underground. The 787 does this constantly, not just in tunnels: the IRU carries the position smoothly between GPS fixes, and GPS continuously trims away the IRU's drift. The IRU is the dead reckoning; GPS is the fix that corrects it; the hybrid filter stitches the two together, moment by moment.
What is the hybrid GPS-inertial position, and why is it better than either source alone?
It is a mathematical blend that takes the best of each. On its own, GPS is extremely accurate in absolute terms but slightly jittery second to second, as satellite geometry, signal reflections, and the atmosphere nudge the fix around. On its own, inertial is beautifully smooth and stable second to second but drifts slowly as tiny measurement errors accumulate. The hybrid filter uses the GPS fix to continuously correct the IRU's long-term drift, and uses the IRU's inertial data to smooth out the GPS's short-term jitter — a position both accurate over the long haul and rock-steady moment to moment. It is the same trick your phone plays in a tunnel: the map arrow keeps moving on dead reckoning while the satellites are gone, then snaps back onto truth when they reappear. The 787 does it constantly — the IRU carrying the position between fixes, GPS trimming away its drift — and the blend is the number the FMC uses almost every moment of every flight.