What Material Will Block GPS Signal
5 things to know about blocking a GPS signal
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Dense metal, thick concrete and heavy tree cover all weaken or kill a GPS signal.
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Aluminum foil works because it forms a rough Faraday cage around the device.
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Parking garages, tunnels and downtown blocks do the same thing by accident, every day.
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Found a device you did not put there? Photograph it before you wrap or move it.
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GPS jammers are a separate thing entirely, and they are illegal for everyone in the US.
Short answer: almost any dense conductive material will block a GPS signal. Aluminum foil, sheet metal and thick concrete are the reliable ones. Wood, glass and plastic barely matter.
That is the physics, and the physics is simple. What people usually want to know sits underneath the question, so let us deal with it first.
Most readers land here for one of two reasons. Either a tracker they own keeps dropping out. Or they suspect somebody has put a device on their vehicle. Both problems come down to the same handful of materials. The answers, though, run in opposite directions.
If you think someone is tracking you, start here
Blocking a signal is a technical fix for what may not be a technical problem. An ex-partner or anyone else following your movements is stalking. People whose job is to help you plan around that safely are one call away.
National Domestic Violence Hotline: 1-800-799-7233, or text START to 88788, thehotline.org. Stalking Prevention, Awareness, and Resource Center: stalkingawareness.org.
RF bug detector
Foil and concrete stop a signal you have already found. A detector is how you find it. Radio frequency detection reads the transmission a cellular tracker sends out. A phone app cannot do that.
- Reads live RF, so it sees active cellular trackers
- Sweeps wheel wells, bumpers, the cabin and the OBD2 port
- Works on a parked car in a few minutes
Why anything blocks GPS at all
A GPS receiver does not transmit to find itself. Instead, it listens. Satellites roughly 12,500 miles up broadcast a timing signal. The receiver compares several of those signals at once and works out where it sits.
Those signals arrive faint. By the time they reach the ground, they sit below the background radio noise. Hence the need for a reasonably clear view of the sky. Put something dense and conductive in the way and there is nothing left to hear.
Worth separating two different failures here. Any receiver can lose its fix, meaning it cannot hear the satellites. The same unit can also lose its report. That means it has a fix but cannot reach the cell network. Metal and concrete tend to do both at once. For more on the underlying mechanism, see how GPS trackers work.
5 materials that block GPS signals
Aluminum foil
Foil is the one everybody has heard about, and it does work. Wrap a device completely and the signal stops.
The mechanism is a Faraday cage. A conductive shell in an electromagnetic field develops currents in its own surface. Those currents produce a field that cancels the incoming one. Whatever sits inside hears close to nothing.
- Coverage is everything. A gap, a seam or a torn corner leaves a way in, and partial wrapping gives partial blocking.
- Two layers beat one. Household foil is thin and tears easily once it is folded around a hard object.
- The block is total, not partial. A wrapped tracker does not report a wrong location. Instead, it goes quiet.
That last point matters more than it looks. Silence is itself a signal. Whoever watches the account sees a device that stopped reporting at a particular place and time. That tells them roughly as much as a live position would.
Forests, hills and heavy tree cover
Wet foliage absorbs the frequencies GPS runs on. Dense canopy and steep valley walls both cut down how much sky a receiver sees. Accuracy drops first. The fix disappears later.
Anyone who hikes has watched this happen. The blue dot drifts, then lags, then gives up under thick timber or in a narrow canyon.
Practical version for owners: a unit under a truck bed liner reports less reliably than the spec sheet suggests. Same for a heavily wooded storage yard. Carry a paper map for the backcountry, and expect gaps in the log for equipment parked under cover.
Sheet metal and metal boxes
A car is a metal box with glass in the roofline. That is why placement decides whether a tracker performs. A sealed steel toolbox is effectively a Faraday cage you already own.
Trunks are the classic mistake. Deep inside a trunk, under a metal parcel shelf, a unit fights the vehicle for every fix.
Concrete, especially underground
Concrete blocks GPS well. Reinforced concrete blocks it better, because the steel rebar does most of the work.
An underground parking structure is the strongest everyday example. Walls, ceiling and the ground above combine, and almost nothing gets through. Anyone who has watched navigation die on a garage ramp has seen it.
Above ground, concrete also reflects. A signal that bounces off a wall before reaching the receiver arrives late. Your receiver reads that delay as extra distance. Engineers call this multipath. A position can look plausible and still be wrong by a block.
Tall buildings and urban canyons
A street lined with towers leaves a receiver a narrow strip of visible sky. Steel, glass and concrete absorb some signals and bounce others. So a receiver ends up working from a mix of direct and reflected paths.
The result is rarely a clean loss. Rather, the position wanders a hundred feet and settles on the wrong side of the street. Rideshare drivers deal with this every shift.
If it is your tracker, this is a placement problem
Half the support calls we take about a "faulty" tracker are placement calls. The device is fine. The device is buried.
- Give the unit as much sky as the mounting allows, and keep dense metal off the top face.
- Avoid deep trunk placements, spare wheel wells and sealed steel boxes.
- Expect gaps in underground parking. They close a minute or two after the vehicle comes back up the ramp.
- Judge a unit over a week of logs, not over one afternoon in a garage.
One honest limit. No placement beats physics. A magnetic tracker under a steel chassis has the vehicle above it and the road below. That unit still reports. Mounted on a plastic panel or a wheel arch, it does better than under the floor pan.
What a hidden tracker actually looks like
People picture something smaller and more exotic than the reality. Most units are a black plastic slab about the size of a deck of cards. A magnetic case usually adds bulk rather than removing it.
A phone will not find one of these. Detector apps read the magnetometer, which measures magnetic fields. They flag the magnet in a mount and never see the tracker. We covered that in detail in is there an app to detect a GPS tracker.
You found a device. Do this before you wrap it
Most articles send you straight to foil next. Hold off for a moment. The order of operations matters more than the material.
- Leave it in place and photograph it.Shoot the device where it sits, wide enough to show the vehicle and the exact spot. Where a thing was hidden is often the most useful fact about it.
- Work out whose it is.Financed and leased vehicles sometimes carry lender units. Company vehicles usually carry fleet units. A rental may have one. Rule those out before assuming the worst.
- Think about your movements first.Wrapping or pulling a tracker tells whoever placed it that you found it. If that person is dangerous, the moment they learn you know is the moment worth planning for.
- Report it.Call local police with your photos. Bring the device only if they ask, and say plainly where and when it was found.
- Then contain it, if you still want to.A foil wrap in a sealed box keeps the device from reporting while you sort out the rest. Signal contained, evidence intact.
A device on your car can be evidence
Stalking cases turn on documentation. A photographed tracker with a date, a location and a police report number carries weight. A tracker in a drawer with foil around it carries much less.
National Domestic Violence Hotline: 1-800-799-7233, or text START to 88788. SPARC: stalkingawareness.org.
A tracker you wrap in foil is a tracker you have warned. Photograph it before you touch it.
Ryan Horban, founder, GPS Tracker Shop
Is it legal to block a GPS signal?
Two very different questions hide inside that one. Passive blocking is a question about property and consent. Jamming is a question about federal radio law, and the answer there is much simpler.
Start with the passive side. Nobody has a general right to interfere with equipment that is lawfully where it is. So the honest test is not whether blocking is legal in the abstract. Rather, ask who owns the device and whether it is there lawfully.
- A device placed on your vehicle without your consent. Handling or removing it is generally not treated as an offense. Many states make the original placement the crime. The rules still vary by state.
- A fleet tracker in a company vehicle. The vehicle is not yours. Disabling the unit is a disciplinary matter at minimum. The driver in the case below lost his job over exactly this.
- A lender's or rental company's unit. Written into the agreement you signed, as a rule. Interfering with it can breach that contract.
- A device placed under a court order. Leave it entirely and talk to a lawyer.
Our state-by-state guide to GPS tracking laws covers which statutes apply where. None of this is legal advice. Call a local attorney if real consequences are on the table.
Stop guessing whether something is there
A ten-minute sweep with an RF detector settles the question either way. That beats a roll of foil.
Shop bug detectorsGPS jammers: the line you should not cross
A jammer is not a blocker. Blocking is passive, and it affects one device inside one wrapper. A jammer actively transmits noise across the GPS band and takes down every receiver within range.
Passive blocking
- Foil, a metal box, a garage, a shielded pouch
- Affects only what sits inside the barrier
- Transmits nothing at all
- Legality depends on who owns the device
A GPS jammer
- An active transmitter, often sold as a lighter plug
- Knocks out every GPS receiver within range
- Can reach aircraft, emergency services and navigation systems
- Illegal to sell, buy, import or operate, with no exceptions
The federal position is blunt. The FCC says jamming devices violate federal law. No exemption exists for a business, a classroom, a home or a vehicle. Local police forces have no independent authority to use them either. Marketing, importing and selling them is prohibited alongside operating them. Criminal statutes on interference with satellite communications sit behind the civil penalties.
The trucker who jammed Newark airport
In August 2012, an FCC agent traced interference at Newark Liberty International Airport. The source was a red Ford pickup on the airport road. The FAA had complained the day before about disruption to a ground-based augmentation system. That equipment sharpens GPS accuracy for aircraft on approach.
The driver, Gary Bojczak, admitted the device was his. He had bought a cheap jammer and plugged it into the cigarette lighter of his company truck. The purpose was defeating the GPS tracking his employer had fitted. His signal was strong enough to interfere with the airport equipment.
The FCC proposed a forfeiture of $31,875 across three violations: illegal equipment, unlawful operation and interference with authorized communications. He surrendered the jammer, and he lost his job. A later consent decree cut the amount after he showed he could not pay the full sum.
Forfeiture the FCC proposed against one driver for blocking his employer's tracker.
Jammers had been interfering with that same Newark system since at least 2009. Truckers dodging employer tracking were the usual source. Worth sitting with that for a second. A hundred-dollar gadget bought to hide a lunch break degraded a landing aid. At one of the country's busiest airports.
The year illegal jammers first began disrupting Newark's GPS landing system.
So the practical rule is easy to remember. Wrap a device, box it, park underground, all fine as far as federal radio law is concerned. Transmit anything, and you are in a different category of trouble.
Sources
FCC, Jammer Enforcement: fcc.gov/general/jammer-enforcement
GPS.gov, Information About GPS Jamming: gps.gov/information-about-gps-jamming
FCC Order DA 16-184, Gary Bojczak consent decree: docs.fcc.gov
GPS.gov, Space Segment (satellite generations and design life): gps.gov/space-segment
Settle the question in ten minutes
If you suspect a device is on your vehicle, a proper RF sweep answers it. Foil is what you do afterwards, and only once the photographs exist.
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