All the preceding methods guide on the target. Navigation guidance guides on a position: the weapon knows where it has to go (coordinates) and navigates there on its own, without having to observe anything of the target. This is the domain of cruise missiles, ballistic missiles and GPS-guided bombs — mainly against stationary targets.
Inertial navigation
Inertial navigation works like walking blindfolded and counting your steps and turns: you know where you are without seeing anything outside, but small errors build up.
In practiceCruise missiles and bombs with a JDAM kit use INS together with GPS. If GPS is jammed, they carry on using INS — less precisely, but they do not get lost. That is why GPS jamming makes such weapons less accurate but does not stop them (Other protective measures).
An Inertial Navigation System (INS) is fully self-contained navigation based on the laws of motion: gyroscopes measure every rotation, accelerometers every acceleration. From a known starting position the computer integrates acceleration → velocity → position, hundreds of times per second.
- Unjammable and invisible: an INS receives nothing and transmits nothing — there is literally no signal to disrupt or detect. That is why INS is the backbone of almost every guided weapon.
- But: drift. Every small measurement error is integrated twice and grows with time. Even good systems drift by hundreds of metres per hour. Fine for a short missile flight; useless without corrections for a two-hour cruise missile flight.
That is why the standard recipe is: INS as the basis + periodic position fixes from outside — from satellite navigation (Ranging navigation guidance), terrain matching (Terrain following guidance) or a datalink update. The combination is called "INS/GPS" and is found in almost every modern weapon: the INS provides continuous, unjammable guidance; the fixes keep the drift in check.
Ranging navigation guidance
With ranging navigation the weapon determines its position from range measurements to reference points with exactly known positions. The ubiquitous implementation is satellite navigation (GPS/GNSS): each satellite transmits its position and an extremely precise time signal; from the travel time the receiver calculates its range to each satellite (range = c × Δt — the same formula as Distance from a target to the radar), and from four or more of those ranges its own 3D position plus time follows.
- Accuracy: metres (better with military codes and augmentation) — anywhere in the world, in all weather, without any preparation of the target area.
- Vulnerability: the satellite signal is extremely weak on arrival and therefore easy to jam and — more sophisticated — to spoof (faking it, so that the receiver calculates a wrong position). GPS interference around conflict zones is now an everyday occurrence.
- Therefore never on its own: GPS serves as drift correction for the INS (Inertial navigation). If GPS drops out, the weapon flies on using INS with a slowly growing error instead of getting lost. Anti-jam antennas (nulling towards the jammer) increase resistance.
Historical forerunners of the same principle: radio beacons and hyperbolic systems (LORAN), and working the other way round: DME in aviation. The concept is always identical — ranges to known points cut out your position.
Terrain following guidance
Weapons and aircraft that want to stay low use the terrain itself as a reference. Two related techniques:
Terrain following / terrain avoidance
A terrain following radar (or radar altimeter, often FMCW — Frequency Modulated Continuous Wave radar) continuously measures the height above the ground and the profile ahead of the weapon, and steers so that a set minimum height is followed: up the hill, down into the valley. The aim: to stay below the cover of the radar horizon — a low-flying cruise missile is only seen by ground radars at very short range (earth curvature + ground clutter).
Terrain-referenced navigation (TERCOM / TRN)
TERCOM (Terrain Contour Matching) compares the measured height profile along the flight path with a stored digital terrain map: the unique "fingerprint" of the landscape gives away where you are, and the INS is corrected with it — entirely without satellites. In addition, DSMAC-like scene matching (comparing a camera image with reference photos) can make the terminal phase accurate to within metres.
- Strengths: independent of GPS (immune to GPS jamming), and flying low is itself a countermeasure against detection.
- Limitations: requires prepared terrain data for the route; works poorly over flat or changeable terrain (sea, desert, snow); and the radar altimeter is a (weak, but real) emission that sensitive ESM can detect.
A modern cruise missile combines everything in this section: INS as the backbone, GPS fixes where possible, TERCOM fixes where GPS is disrupted, terrain following to stay low and possibly scene matching or a seeker for the terminal phase. Layering is the defence against every individual countermeasure.