A small circular disc on the dorsal spine of an F-15E Strike Eagle, just aft of the speed brake — that is the most significant piece of new hardware photographed on an American combat aircraft in months. The disc is a Controlled Reception Pattern Antenna, or CRPA, and it is part of BAE Systems’ Digital GPS Anti-jam Receiver, known as DIGAR. It was spotted in images of an F-15E operating during Operation Epic Fury. It is the first confirmed combat deployment of the system. In an operational environment where Iran has demonstrated credible electronic warfare capabilities — including GPS jamming and spoofing — a hardened navigation system is not an upgrade. It is a survival requirement.

How CRPA Defeats Jamming

A standard GPS antenna is omnidirectional — it receives signals from all directions equally. This makes it inherently vulnerable. A jammer on the ground or in the air can overwhelm the weak GPS signal (which arrives from satellites at roughly the power of a car headlight seen from 20,000 kilometres away) with noise. A spoofer can transmit a fake GPS signal that shifts the receiver’s calculated position by hundreds of metres — enough to send a precision-guided weapon into the wrong building. CRPA solves this through spatial filtering. The antenna contains multiple elements — BAE Systems’ implementation supports up to seven — arranged in a circular array. The receiver compares the signal arriving at each element, calculates the direction of any interference source, and electronically steers a null (a blind spot) toward the jammer. The legitimate satellite signals, arriving from above, pass through. The jamming signal, arriving from below or from the side, is suppressed. The mathematics behind this are well-established — adaptive beamforming has been used in radar and communications for decades — but miniaturising it into a conformal antenna that fits flush on the spine of a tactical fighter is a significant engineering achievement. The CRPA must operate across the full GPS frequency band, including the encrypted M-Code military signal, while withstanding the vibration, temperature extremes, and electromagnetic environment of a fast jet in combat.

Why Iran Forced the Issue

Iran has invested heavily in electronic warfare. Its air defence network includes systems capable of GPS jamming across wide areas — a relatively inexpensive way to degrade the accuracy of American precision-guided munitions. During Epic Fury, anecdotal reports have emerged of JDAM accuracy being affected in certain areas, suggesting that Iranian jamming has had at least some operational effect. DIGAR is the counter. By hardening the GPS receiver on the aircraft itself, the system ensures that the navigation solution feeding the weapons computer, the targeting pod, and the flight management system remains accurate even in a dense jamming environment. This matters not just for weapons delivery but for basic flight safety — a pilot who cannot trust their navigation in hostile airspace is a pilot in danger.

The Broader Upgrade Picture

DIGAR is part of a wider F-15E modernisation effort that includes the Radar Modernisation Programme (RMP), which replaces the legacy APG-70 radar with the APG-82 AESA, and the MUOS/SATURN communications upgrade. Together, these systems transform the F-15E from a 1980s airframe with 1990s avionics into a platform that can operate in a 2020s electronic warfare environment. The $13 million BAE Systems contract from 2022 covered installation on an initial batch of F-15Es at Cedar Rapids. The speed with which the upgrade moved from contract to combat deployment — less than four years — is unusually fast by Pentagon standards, and reflects the urgency of the GPS vulnerability problem. That small circular disc on the spine of the jet is not photogenic. It will never make a poster. But in the electromagnetic battle over Iran, it may be the most important piece of equipment on the aircraft. Sources: The Aviationist, BAE Systems, Air & Space Forces Magazine