Why Long RF Cables Reduce GNSS Accuracy | GPS Signal Loss Explained

Introduction

Global Navigation Satellite Systems (GNSS) such as GPS, GLONASS, Galileo, and BeiDou rely on extremely weak signals sent by satellites orbiting more than 20,000 kilometers above Earth. When these signals reach a GNSS antenna, their power levels are frequently as low as -130 dBm or less.

In such settings, every component in the RF signal route is important. RF cable length is an often-overlooked factor. Long RF coaxial connections between the GNSS antenna and receiver can drastically impair signal quality, resulting in lower location accuracy.

This blog describes why lengthy RF cables reduce GNSS accuracy, the RF mechanisms that cause the loss, and best practices for avoiding these difficulties.

GNSS Signal Sensitivity and RF Path Loss.

GNSS signals primarily function in the L-band, including:

• L1 (1575.42 MHz)

• L2 (1227.60 MHz)

• L5 (1176.45 MHz)

At the following frequencies:

• Signal power is quite low.

• Noise margin is low.

• Cable attenuation becomes crucial.

Even little losses in the RF route can degrade the signal-to-noise ratio (SNR), which has a direct impact on GPS performance.

How Long RF Cables Reduce GNSS Performance

1. Insertion Loss increases with cable length.

RF coaxial cables use frequency-dependent attenuation. Signal loss increases with cable length.

Typical loss at the GNSS L1 frequency (1575 MHz):

Cable Type	Loss per Meter
RG174	~1.5 dB/m
RG58	~0.6 dB/m
LMR100	~0.5 dB/m
LMR195	~0.2 dB/m

A 5-meter RG174 cable can provide 7-8 dB of loss, which is particularly detrimental to GNSS signals.

2. Reduced carrier-to-noise ratio (C/N₀).

GNSS receivers use C/N₀ (Carrier-to-Noise Density Ratio) to track satellites.

Long radio frequency (RF) cables

• Reduce the signal strength.

• Don't reduce thermal noise correspondingly.

• Result in lower C/N₀.

Lowering C/N₀ leads to:

• Slower satellite acquisition.

• Poor tracking stability.

• Increased positional error

3. Increased the Time to First Fix (TTFF).

Using weaker signals:

• Receivers take longer to acquire satellites.

• Cold-start TTFF rises.

• Warm and hot starts are less reliable.

This is particularly troublesome for:

• Vehicle tracking

• Emergency Services

• Asset monitoring systems

4. Noise Figure Degradation

The noise figure (NF) of the GNSS front-end is substantially influenced by losses prior to the first low-noise amplifier (LNA).

Long RF wires installed before the LNA:

• Add loss directly to the system noise figure.

• Cannot be restored through later amplification.

• This is a fundamental RF constraint outlined in Friis' formula.

5. Increased susceptibility to interference.

Longer cables can:

• Act as unintentional antennas.

• Collect EMI from neighboring electronics.

• Enhanced vulnerability to LTE, Wi-Fi, and DC-DC converter noise.

This interference significantly reduces GNSS accuracy and stability.

Effect on GNSS Positioning Accuracy

As the RF cable losses grow, the receiver experiences:

• Fewer satellites in visible.

• Poor satellite geometry (high DOP).

• Higher pseudorange errors.

• Reduced positioning precision

Active GNSS Antennas Are Not a Complete Fix

Active GNSS antennas have an inbuilt LNA to compensate for cable losses. Although useful, they have limitations:

• LNAs have limited gain (usually 20-28 decibels).

• Excessive cable loss still reduces SNR.

• Power loss over lengthy wires lowers LNA performance.

Long wires can still outweigh the advantages of an active antenna.

Typical Symptoms of Excess RF Cable Length

• GNSS fix works outside but fails indoors.

• Positional jumps or drifts

• Slow starting time.

• Satellite count was reduced.

• Poor performance in urban canyons.

Best Practices to Reduce GNSS Cable Loss

1. Keep the RF cable as short as possible.

Place the GNSS receiver adjacent to the antenna.

Avoid any unwanted cable loops.

2. Use low-loss coaxial cables.

Rather than RG174, consider:

• LMR100

• LMR195

• LMR240 (for long runs)

3. Choose Active GNSS Antennas Wisely

• Ensure an adequate gain.

• Verify electricity delivery via cable.

• Avoid overamplification.

4. Place the LNA near the antenna.

• Amplify the signal before the cable fails.

• Improve the overall noise figure.

5. Ensure proper impedance matching.

• Use 50 ohm wires and connectors.

• Avoid adaptors and superfluous connectors.

Cable Length Guidelines for GNSS Systems

Cable Type	Recommended Max Length
RG174	≤ 1–2 m
RG58	≤ 3–5 m
LMR100	≤ 5–8 m
LMR195	≤ 10–15 m

Applications Where Cable Length is Critical

• Vehicle Tracking Systems

• Precision Agriculture

• Surveying Equipment

• Unmanned aerial vehicles and drones

• Maritime navigation

• Timing and synchronization systems.

Conclusion

Long RF links impair GNSS accuracy by increasing attenuation, noise figure degradation, and interference susceptibility. Because GNSS signals are inherently weak, even a few decibels of additional loss can result in slower fixes, fewer monitored satellites, and incorrect location.

To ensure consistent and accurate GNSS performance:

• Keep the RF cables short.

• Choose low-loss coaxial cable.

• Use active antennas properly.

• Optimize the overall RF system architecture.

In GNSS systems, each decibel counts.

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