You bring a GPS receiver indoors and it goes blind. The device that locked onto eight satellites in the parking lot can't find one at your desk. If you develop GPS-enabled products, run a timing server, test trackers, or just need a working GPS reference at a bench, this is a daily frustration with a well-understood engineering fix.
This guide explains why GPS fails indoors, why the popular quick fixes mostly don't work, and how to build the antenna chain that does: an outdoor antenna, amplification in the right place, filtering, and power delivery over the coax. By the end you will know exactly which parts your situation needs.
Why GPS fails indoors (it's worse than you think)
GPS signals are astonishingly weak by the time they reach Earth. After travelling 20,000 km from the satellite, the signal arrives at about -130 dBm, which is roughly one ten-millionth of a billionth of a watt. It is already below the thermal noise floor; your receiver only recovers it through clever signal processing.
Now put a building in the way. Typical attenuation numbers:
- Glass window: 1 to 4 dB (low-E coated glass: 20 dB or more)
- Wood-frame wall: 5 to 15 dB
- Brick or concrete: 10 to 30 dB
- Metal roof or foil-backed insulation: effectively a GPS-proof box
A signal that starts with almost no margin loses 10 to 30 dB on the way to your desk. That is why "near the window" sometimes sort-of works and the middle of the building never does. No receiver setting fixes this. The signal simply is not there.
The fix: receive outdoors, use the signal indoors
The engineering answer is to stop trying to receive GPS where GPS does not exist. Put the antenna where the signal is, on the roof or outside a window with clear sky view, and bring the signal to your bench over coax. Done correctly, the receiver on your desk performs as if it were sitting outside.
"Done correctly" is the part this guide is about, because a long cable run introduces two new problems: cable loss and interference. Solve both and the setup works beautifully. Here is the complete chain, then each piece.
Antenna (roof, clear sky view) → filtered LNA at the antenna → coax run → filtered bias tee at the receiver → your GPS device
Quick answer: parts for your setup
| Your situation | What you need |
|---|---|
| The complete GNSS indoor-lab chain, antenna end | Pre-filtered GNSS Low Noise Amplifier with Bias Tee, 25 dB Gain |
| The complete GNSS indoor-lab chain, receiver end | GNSS Filtered Bias Tee, 1100–1700 MHz |
| GPS L1 only, short cable run, active antenna | 1575 MHz GPS L1 Band Pass Filter with DC Bypass |
| L1-only build with a longer run | Filtered GPS L1 1575 MHz LNA with DC Pass Through powered by the GPS L1 Filter with USB Bias Tee |
| Bench LNA where filtering is handled separately | GNSS Low Noise Amplifier, GPS L1–L5, 27 dB Gain |
| Passive splitting of the signal to one receiver at a time, interference cleanup | GNSS 1100–1700 MHz Bandpass Filter |
Step 1: Amplify at the antenna, never at the bench
Every metre of coax between the antenna and the first amplifier eats your signal. Budget RG-58 loses several dB per 10 metres at GPS frequencies; even good cable adds up over a 15 to 30 metre run from roof to lab. Here is the rule that decides whether your system works:
The first amplifier in the chain sets the noise performance of everything after it. Put the gain at the antenna and the cable loss afterwards barely matters. Put the cable first and the signal you lost can never be recovered, no matter how much you amplify later.
This is why the right part for the antenna end is a masthead amplifier, and why it should be a filtered one:
Filtered GPS L1 1575 MHz LNA with DC Pass Through 21 dB of gain at 1575 MHz with a noise figure under 1 dB (typically 0.8 dB), built behind an L1 filter that rejects 1615 MHz by 40 dB. The spec that sets it apart is linearity: +29 dBm output IP3, which is what lets it sit in a strong-signal environment, near cellular sites, without generating distortion products of its own. It passes DC through, so an active antenna downstream of it stays powered too.
Why filtered? Because your roof is the worst RF environment in the building. Rooftops host cellular antennas, and even when they don't, the antenna up there has line-of-sight to every base station in the area. An unfiltered amplifier boosts all of it, overloads, and desensitizes exactly when you need it most. The filter in front of the amplifier keeps the out-of-band energy from ever reaching it.
Step 2: Power the masthead amplifier from the bench
The amplifier on the roof needs DC power, and the elegant way to deliver it is up the same coax that carries the signal down. That is the job of a bias tee at the receiver end:
1575 MHz GPS L1 Band Pass Filter with USB Bias Tee, +3.3 V Sits at your receiver, injects +3.3 V up the coax from any USB port to power the line amplifier and antenna, and adds a final stage of L1 filtering before the receiver. [Add to Cart]
Pairing the masthead unit with the filtered bias tee gives you the full chain: filtered, amplified at the antenna, powered over a single cable, and filtered again at the receiver. This is the same architecture used in professional GNSS timing installations, built from two boxes.
A word on why the bias tee is part of the recommendation rather than an option. Our amplifiers are designed for high linearity, with IP3 specifications above what is typical at these prices, because linearity is what keeps an amplifier clean next to strong cellular signals.
The cost of that linearity is bias current: these LNAs draw more than the antenna supply built into most GPS receivers is designed to provide. The receiver's bias output is sized for a small active antenna, not a high-linearity line amplifier. Powering the chain from a dedicated filtered bias tee removes the problem and adds a second filtering stage at the same time.
One precaution that applies to any bias tee: DC is present on the RF port facing the antenna side. Connect only devices designed to accept power over coax. Never connect a bias tee output directly to test equipment or a receiver input that lacks DC blocking.
Simpler cases: when you don't need the full chain
Short run, L1 only. If your antenna is just outside a window and your cable run is a few metres, you may not need a masthead LNA at all, especially with an active antenna that already contains one. What still helps in urban RF environments is filtering. If your receiver powers an active antenna through the coax, use the DC-passing filter so you don't cut that power:
1575 MHz GPS L1 Band Pass Filter with DC Bypass Passes GPS L1 (1575 ± 7.5 MHz) with 2.7 dB typical insertion loss and rejects out-of-band interference by more than 40 dB at 850 MHz and 1640 MHz. The DC bypass carries your receiver's antenna power through to the active antenna.
Medium run, L1 only. For an L1-only installation with a longer cable, this is the line amplifier to use:
Filtered GPS L1 1575 MHz LNA with DC Pass Through 21 dB of gain at 1575 MHz with a noise figure under 1 dB (typically 0.8 dB), built behind an L1 filter that rejects 1615 MHz by 40 dB. The spec that sets it apart is linearity: +29 dBm output IP3, which is what lets it sit in a strong-signal environment, near cellular sites, without generating distortion products of its own. It passes DC through, so an active antenna downstream of it stays powered too.
A note on powering it, and this applies to high-linearity amplifiers in general: linearity costs current. This LNA draws 130 mA at +3.3 V, which is more than the antenna supply on most GPS receivers can deliver (typically a few tens of milliamps, intended for a small active antenna). The receiver's bias voltage passes through, but you should not rely on it as the supply. Instead, power the chain from the receiver end with a filtered bias tee:
1575 MHz GPS L1 Band Pass Filter with USB Bias Tee, +3.3 V Sits at your receiver, injects +3.3 V up the coax from any USB port to power the line amplifier and antenna, and adds a final stage of L1 filtering before the receiver.
Multi-constellation receivers. Modern receivers track GPS, GLONASS, Galileo, and BeiDou together across roughly 1100 to 1700 MHz. An L1-only filter will cut off everything except GPS L1.
If your receiver is multi-constellation, and most modern modules are, use the full-band parts: the GNSS 1100–1700 MHz Bandpass Filter for filtering, or the full-band masthead GNSS LNA with Output Bias Tee and filtered GNSS bias tee above for the complete chain.
What about GPS repeaters?
A repeater receives GPS outside and re-transmits it inside the building, and it is the right tool for some facilities, such as hangars and vehicle test bays where the device under test must use its own antenna. Be aware that re-radiating GPS signals is regulated in most countries and typically requires a licence, because a repeater can disrupt GPS for your neighbours. For a lab bench or equipment rack where you control the cabling, the wired chain described above delivers a cleaner signal with no licensing questions. If you think your facility genuinely needs re-radiation, talk to us about the RF design first.
Troubleshooting: a 60-second diagnosis
- Works outside, dead inside: signal blockage. No setting fixes it; move the antenna outside.
- Works at the window, marginal lock, drops at busy hours: interference. Add filtering.
- Outdoor antenna installed but performance is poor: check where the gain is. If the amplifier is at the receiver end of a long cable, move it to the antenna end.
- Added a filter and things got worse: you have probably blocked DC to an active antenna. Use the DC bypass filter or add a bias tee.
- Masthead LNA dark: confirm the bias tee at the bench is powered and that nothing DC-blocking sits between it and the LNA.
- Multi-constellation receiver only tracking GPS: check whether an L1-only filter is in the chain.
Get the chain right the first time
GPIO Labs designs and manufactures every filter, LNA, and bias tee above, and our GNSS products are used by research institutions, aerospace organisations, and engineering teams who need a dependable GPS signal at an indoor bench. If you are unsure what your cable run, receiver, and antenna need, contact us with the details and we will spec the chain for you. RF is what we do.
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