There is a persistent myth in GPS and GNSS system design: more gain ahead of the receiver is always better. Stack enough amplification in front and the problem goes away.
It does not work that way.
Why Gain Placement Matters
GPS signals arrive at the antenna at around -130 dBm, well below the thermal noise floor. The receiver relies on spread-spectrum processing gain to pull the signal out of the noise, not raw signal strength. This is why the LNA's position matters as much as its gain value.
Place the LNA close to the antenna and it sets the noise figure for the entire chain before cable loss degrades it. The Friis formula makes this clear: the first active element dominates. A GPIO Labs LNA with a 0.7 dB noise figure at the antenna port will hold that noise figure across a long cable run. Move the LNA to the far end of that same cable and the system noise figure rises by the full cable loss before the LNA even gets a chance to amplify.
The Problem With Too Much Gain
Once a low noise figure is established, additional gain stops improving sensitivity and starts creating a new problem: receiver overload.
Every GPS receiver has an ADC with a finite input range. Feed it too much signal and it clips. The correlator cannot recover from clipping, and the result looks exactly like a weak signal — poor acquisition, high time-to-first-fix, elevated position error. The AGC stage ahead of the ADC helps, but it has limits. A high-gain LNA sized for a long cable run, installed on a short one, can easily overwhelm it.
This is a common failure mode in GPS timing and survey installations, and it is almost never diagnosed correctly on the first pass.
Building the Gain Budget
Treat the receive chain as a link budget. Start at the LNA output and subtract every loss element to the receiver input:
- Cable loss at L1 (1575.42 MHz) runs roughly 0.1 to 0.5 dB per metre depending on cable type. A 20-metre RG-58 run costs around 6 dB. LMR-400 over the same distance costs closer to 1.5 dB.
- Connector loss adds 0.1 to 0.3 dB per connector. Multiple adapters can quietly consume 1 dB or more.
- Splitter loss is the largest single hit in multi-receiver installations. A passive two-way splitter costs 3.5 dB per port. A four-way costs roughly 7 dB. This must be accounted for explicitly, or receivers at the output ports will be underdriven.
Subtract the total from your LNA output gain. The result should land within the receiver's specified input range, with margin for temperature and aging.
Choosing the Right GPIO Labs LNA
GPIO Labs LNAs for GPS and GNSS applications are available across a gain range of roughly 20 to 36 dB, with noise figures from 0.6 to 0.9 dB. That range exists because installations vary.
A short bench run feeding one receiver needs 20 to 24 dB. A rooftop installation with 40 metres of cable feeding a four-way splitter needs 32 to 36 dB. The noise figure matters more than the gain for sensitivity. A GPIO Labs LNA at 0.7 dB noise figure with 24 dB of gain will outperform a generic part with a 3 dB noise figure and 36 dB of gain in any normal installation. The extra gain of the generic part adds nothing except overload risk.
The Short Version
Sum your cable loss, connector loss, and splitter loss. Select LNA gain to land the signal within the receiver's input range. Verify the AGC has headroom for your installation's signal level. If the level is marginal at any port, fix it with better cable or a smaller splitter tree before reaching for more gain.
The right gain is whatever puts the signal exactly where the receiver needs it. No more.
GPIO Labs designs low-noise amplifiers for GPS and GNSS front-end applications.