They call them ghosts because they appear where they should not. In electronic warfare the phrase ghost signal is shorthand for any phantom or misleading electromagnetic signature that looks real to sensors yet originates from a different path, a different source, or a hardware quirk. Understanding the taxonomy of ghost signals is practical tradecraft. It tells you whether you are chasing meteorology, hardware, bad filtering, or an adversary trying to fool your system.

Types of ghost signals and how they show up

1) Atmospheric ducts and long range propagation Under the right atmospheric conditions VHF and UHF energy can be guided in a horizontal layer and travel far beyond line of sight. That trapped propagation lets distant transmitters or radars show up as clear, persistent echoes in locations where you would not expect them. On the water this effect is operationally significant because shipboard radars and coastal sensors can detect one another across hundreds of kilometers when ducts form. If you see a strong, geographically inconsistent contact with consistent kinematics and no corroborating local radar returns consider ducting as a top hypothesis.

2) Multipath and radar ghost tracks Close in, multipath reflections from terrain, structures, or the sea surface create delayed, attenuated copies of the true return. Those copies can look like independent targets with plausible velocity and track history and thereby generate false tracks in automatic trackers. In practical terms a single aircraft can spawn a twin or a false contact mirrored across a reflecting surface. Simulation and operational guides emphasize that ghost tracks inherit the kinematics of the parent echo which is why trackers will sometimes cement a ghost into a persistent track.

3) Spurious emissions and intermodulation products When transmitters are nonlinear, co-located, or poorly filtered they produce harmonics, mixing products, and parasitic outputs that appear on frequencies well outside the intended band. Those spurs can be mistaken for separate transmitters, or they can desensitize receivers and cause confusing overlays on spectrum displays. Regulatory frameworks treat these as spurious emissions and provide limits and test criteria because they create real interference and real operational risk.

4) Receiver artifacts, image frequencies, and SDR “ghosts” Receiver architectures are not immune to creating ghosts. Superheterodyne designs suffer image frequency problems if the front end lacks sufficient selectivity. Wideband direct sampling and low cost SDR front ends trade complexity for flexibility and can show aliasing, image responses, and front end overload that appear as strong, coherent signals at unexpected frequencies. The SDR and hobbyist communities call many of these artifacts ghost signals because they mimic real stations and can be persistent until the RF chain or software is corrected.

5) Intentional deception and test/measurement reillumination Not all ghosts are accidental. Re-illumination systems, target simulators, or deliberate deceptive ECM can create echoes and false tracks on purpose. Likewise, test setups that do not isolate transmit and receive paths properly can generate internal reflections that masquerade as targets. The existence of hardware and test-system causes for ghost returns is recognized in the literature on radar test systems and target simulators.

How to triage a ghost signal in the field

1) Correlate across sensors The simplest discriminator is independent corroboration. If a contact appears on one radar but not on another with different geometry or frequency, treat it skeptically. Multistatic and passive sensors are especially valuable because ducts and multipath are geometry dependent. When in doubt, check the same bearing at another frequency or platform before committing weapons or countermeasures.

2) Inspect kinematics and track-level consistency Ghost tracks often share the kinematic signature of a real target. A mirrored path, identical velocity bins, or instantaneous appearance and disappearance as environmental conditions change point to multipath or ducting. Track smoothing and anomaly detection tuned to kinematic plausibility help algorithms avoid locking onto ghosts.

3) Check RF hygiene and front-end behavior For signals that look like distant stations but appear on multiple nonadjacent frequencies, suspect spurious emissions or receiver artifacts. Apply front-end attenuation, add or change bandpass/preselector filters, and watch for disappearance or movement of the ghost. Software-defined receivers often expose ghosts when you sweep sample rate, gain, or center frequency. If the apparent source moves with your tuning or with changes to the RF front end it is almost certainly a receiver artifact.

4) Monitor environmental and meteorological data Ducting is forecastable to some degree. Incorporate radiosonde, surface layer, and marine boundary layer indices into your EW decision aid so that when the atmosphere is favorable for trapping you raise the suspicion level for long-range ghosting. Operational research programs have emphasized the value of forecasting duct conditions to avoid both false positives and unexpected exposure.

5) Isolate intermodulation and spurious sources on-site If you control the RF environment, sequentially switch transmitters, change power levels, or physically separate co-located radios to see if suspected spurs vanish. Intermodulation shows strong dependence on local RF power and co-location. Good RF planning and proper filtering are practical ways to remove persistent spurious emitters.

Operational mitigations and design hardening

  • Use preselectors and narrowband front end filtering ahead of mixers to reduce image frequency susceptibility and front end overload. That is basic receiver design but it remains the most effective mitigation against receiver-generated ghosts.

  • Employ sensor fusion. Cross-checking returns across frequencies and sensor types is a force multiplier against environmental ghosts. Multistatic geometries and passive RF collections reduce single-point failures caused by ducts or multipath.

  • Add algorithmic defenses. Track confirmation thresholds, track-to-detection gating based on expected radar geometry, and multipath-aware association logic help prevent trackers from promoting ghosts to fusable tracks. Simulators and model-based filters are useful tools to exercise those defenses in realistic scenarios.

  • Ensure transmitter and site filtering to reduce intermodulation. When multiple high power transmitters operate nearby, diligent filter design, physical separation, and careful power management reduce the creation of spurious products that will show up as ghost emissions. Compliance with spurious emission limits is not only a regulatory issue but an operational one.

Practical checklist for hobbyists and engineers who chase ghosts

1) Reproduce the phenomenon while changing one variable at a time: gain, sample rate, antenna, filter, and center frequency. If the signal moves with a hardware or software parameter it is likely a local artifact. 2) Use a different receiver or antenna and see if the signature persists. Diversity rules out local hardware faults. 3) Look up local weather and ducting indices if you see surprising long-range signals on VHF/UHF. Coastal and high pressure conditions commonly produce tropospheric ducting. 4) If you operate transmitters, verify your emissions mask and spurious spectrum with a spectrum analyzer. Reducing harmonics and improving filtering removes a surprising number of ghosts.

Final thoughts

Ghost signals are an unavoidable part of working in the real electromagnetic world. They are caused by physics, by imperfect hardware, and sometimes by deliberate human action. The right approach mixes environmental awareness, robust RF engineering, and disciplined sensor fusion. On this Halloween night think of ghosts as signals that tell you where to tighten your filters, harden your tracking logic, or improve your forecasting inputs. Ignore them at your peril, but do not panic. Find the root cause, apply the appropriate countermeasure, and incorporate the lesson into system design and operational procedures.