Report 202 · Defense Tech
The Army is fielding a backpack-sized system whose only job is to measure what its own command posts give off. That sounds like housekeeping until you read how direction finding works. A free Navy electronic warfare handbook lays it out: every emission has a direction the sender can't disguise, and two directions from two places cross at a location.
I was a Counter-IED and Electronic Warfare Officer. The habit that job builds is simple: before you key a radio, assume someone is listening, and assume that listening tells them where you are even if they can't understand a word. A lot of people outside the field think encryption covers them. It covers the content. It does nothing for the location, because the location isn't in the message. It's in the wave.
The news peg is small but telling. In late August, DefenseScoop reported that the Army would deliver nine Spectrum Situational Awareness Systems, or S2AS, to "prioritized Army units," with 46 expected by July 2027. The system, built by 3dB Labs, doesn't jam anything. The report says it "does not suppress the emission, but simply identifies it." It is pointed inward. Lt. Col. David Picard, product manager for Tactical Spectrum Warfare at Aberdeen Proving Ground, put it this way:
It's meant to look at ourselves
He also said that "Command posts typically give off a pretty significant signature." The article frames the reason directly: the electromagnetic spectrum has become a domain "where virtually every emission can be detected and targeted by a slew of fast, lethal weapons in a matter of minutes." The coverage explains why the Army wants this. It doesn't explain how an emission becomes a position. That's the part worth understanding.
The clearest public reference is the Naval Air Warfare Center Weapons Division's Electronic Warfare and Radar Systems Engineering Handbook (NAWCWD TP 8347), an unclassified engineering handbook released for public distribution. Its section 5-8 on signal sorting and direction finding is written about radar emitters, but the geometry applies to anything that radiates.
A transmitter can change a lot about its signal: frequency, timing, pulse pattern, power. The handbook's point is that one thing stays put. Angle of arrival, the direction the energy comes from, is set by where the transmitter physically is. In its words:
Angle of arrival is generally regarded as the best initial sorting parameter because it cannot be varied by the emitter from pulse to pulse.
It adds that angle of arrival "cannot be used for emitter identification, but is excellent for sorting signals." That's the key distinction. A listener may not know what you are or what you said. They can still tell that a signal keeps coming from the same direction. It's also why I keep separating "jamming-resistant" from "hidden" (Report 190). Hopping frequencies can make a link harder to jam, but a hop doesn't move the antenna.
One bearing is a line. The handbook lists three main ways to turn lines and timing into a location. Triangulation, where "the intersection of successive spatially displaced bearing measurements provides the emitter location." Azimuth and elevation from a single airborne sensor, where the measured line meets the ground. And time difference of arrival (TDOA), which "measures the difference in time of arrival of a single pulse at three spatially remote locations." The handbook notes that TDOA "can be used to obtain a fix for an emitter which radiates only a single pulse."
Geometry decides how good the fix is. For triangulation, the handbook says accuracy "is greatest for a long baseline and the broadside geometry," meaning the measuring points are far apart and the transmitter is off to the side of them, not straight ahead. Two bearings that cross at a sharp angle give a tight fix. Two that are nearly parallel smear the answer out along the line.
The handbook gives real ranges for the radar-warning world. Amplitude-comparison systems, the simple and cheap kind with four to six broad antennas, typically land at "3 to 10 degrees rms." Phase interferometers, which are more complex, are used when high accuracy "on the order of 0.1 to 1" degree is required. A 16-element circular array example reaches better than 1.7 degrees.
Degrees sound small. Here is what they mean on the ground. This is my arithmetic, simple trigonometry, for a transmitter 5 kilometres away:
A single cheap bearing narrows things to a wedge. A second bearing from somewhere else turns the wedge into a patch, and better equipment shrinks the patch. The handbook also gives the rule behind the precision: angular accuracy scales with the antenna's beamwidth divided by the square root of the signal-to-noise ratio. A stronger signal at the receiver produces a sharper bearing. So the loudest emitter in the area, a busy command post for instance, is also the easiest one to measure well.
Put that together and S2AS makes sense. If direction finding works on any emission, the defender's first problem is knowing what they're emitting. The DefenseScoop report says the system can identify "rogue" signals, "cell phones or radio chatter, for example," from units breaking their own emission rules, and that soldiers testing the Army's prototype command-and-control setup were trying a "hiding in the spectrum" tactic: matching their signatures "to the most prevalent local emissions." You can't hide in the noise if you don't know how loud you are. Picard's version: "You got to measure it, and that's what this system does."
The same logic runs both ways, for any radio on any side, including a drone controller. I'm not going to claim how any specific strike was located, because public accounts rarely say. The physics doesn't need a specific case. A controller that transmits is a point source with a direction, and the tools to measure that direction are well understood and published.
Fielded performance of any system. The accuracy figures above are the handbook's general ranges for radar-warning and ESM receivers, not specifications of S2AS or of any Russian, Ukrainian, or US direction-finding equipment.
Which units get S2AS, or how it works inside. DefenseScoop reports officials "declined to say which units the technology would be going to." Nothing here describes its internals.
Any specific strike. I have no primary source tying any particular attack to direction finding, and this report doesn't assert one.
Encryption protects what you say. It doesn't protect where you said it from. Direction of arrival is the one property a transmitter can't change on the fly, two bearings cross at a location, and stronger signals give sharper bearings. That's why the Army is buying a tool to measure its own command posts before an adversary does. When you read that a unit was found "by its emissions," that's not a mystery capability. It's geometry from a public handbook, done fast.
Scope note: this report explains publicly released direction-finding principles from a US Navy engineering handbook and public reporting. It describes no fielded system's capabilities and no technique beyond what is already public domain.
U.S. Army combat veteran (Counter-IED / Electronic Warfare), peer-reviewed researcher in microwave spectroscopy, and founder & CEO of Shroombiosis. Consults on laboratory operations, AI, and supplement formulation.