A Critical Lesson We Learned from a Field GNSS Test

A Practical Lesson from a PPP Field Test

In high-precision GNSS systems, engineers often focus on satellite availability, signal quality, multi-constellation fusion, and algorithm performance such as PPP (Precise Point Positioning) or RTK. However, there is a fundamental factor that is frequently underestimated in real-world testing: antenna installation height and ground reflection (multipath effects).

This article shares a real testing experience that highlights how environmental conditions—especially ground reflections—can significantly impact positioning stability.

When an “Ideal” GNSS Environment Isn’t Ideal

During an internal evaluation of PPP performance, our FAE team selected a rooftop environment to ensure an “open-sky” scenario. The setup seemed ideal:

• Fully open rooftop environment
• No obvious signal blockage
• Excellent satellite visibility
• Suitable for PPP convergence testing

From a standard engineering perspective, this should have been close to an ideal test condition.

However, the results were unexpected:

• PPP convergence time was longer than expected
• Positioning solutions showed noticeable fluctuations
• Stability was worse than ground-based open-field tests

At first, we suspected issues with firmware, data links, or satellite conditions.

After deeper discussion with our R&D team, we identified the real cause: the environment itself—specifically, multipath effects from the rooftop surface.

The Root Cause: Multipath from the Rooftop Surface

The rooftop surface was primarily concrete, which acts as a strong reflector of GNSS signals.

As a result, the antenna received two different signal paths:

• Direct signals from satellites (the desired observations)
• Reflected signals bouncing off the rooftop before reaching the antenna

These reflected signals arrive slightly later than the direct signals and distort the measurements used for positioning.

When direct and reflected signals combine, they can introduce:

• Pseudorange errors
• Carrier-phase distortions
• Reduced positioning stability
• Longer PPP convergence times

Concrete, tiles, waterproof membranes, and other rooftop materials often have relatively high reflectivity, making rooftop environments surprisingly challenging for high-precision GNSS testing.

Open Sky Does Not Mean Clean Signals

This test highlighted a critical lesson in GNSS engineering:

Open Sky Does Not Guarantee Clean GNSS Conditions

Many engineers unconsciously assume:

• Open space = good GNSS environment
• Higher elevation = better positioning
• More visible satellites = higher accuracy

In reality:

• Open sky ≠ free from reflections
• Elevated locations ≠ low multipath environments
• Excellent visibility ≠ high-quality measurements

A rooftop can easily become a strong multipath environment, particularly when the antenna is placed directly on or very close to the reflective surface.

Why Antenna Height Matters More Than You Think

Antenna height is often viewed simply as a way to improve satellite visibility.

In reality, its role is much more important.

Antenna height directly affects:

1. Multipath Suppression

The closer an antenna is to a reflective surface, the stronger the reflected signals become.

Increasing antenna height reduces the influence of ground reflections and improves observation quality.

2. Observation Stability

Low-elevation satellite signals are particularly vulnerable to multipath effects.

Reducing reflected signal energy helps improve measurement consistency across the entire satellite constellation.

3. Carrier-Phase Quality

PPP and other high-precision positioning techniques rely heavily on carrier-phase observations.

Because carrier-phase measurements are extremely sensitive to signal distortions, multipath can significantly impact:

• PPP convergence speed
• Ambiguity resolution
• Position stability

Even relatively small changes in antenna height can noticeably improve performance in reflective environments.

How We Improved Our Testing Methodology

Following this experience, we updated our internal GNSS testing procedures.

1. Avoid Using Bare Reflective Surfaces

Concrete rooftops are no longer considered standalone reference environments for PPP testing.

2. Improve Antenna Isolation

Where necessary, we use:

• Antenna stands
• Mounting poles
• Ground planes
• RF-absorbing materials

to reduce the impact of reflections.

3. Control Antenna Height

For most testing scenarios, we maintain an antenna height of at least 1.5 meters above the surrounding surface.

4. Validate Across Multiple Environments

Performance is now evaluated across:

• Open grass fields
• Controlled rooftop installations
• Permanent reference stations
• Partially obstructed environments

This approach provides a more realistic assessment of real-world performance.

Why This Matters for PPP and High-Precision GNSS

For PPP and other carrier-phase-based positioning systems, environmental effects are particularly important.

PPP relies heavily on:

• Precise carrier-phase observations
• Long-term signal consistency
• Accurate error modeling

Multipath directly corrupts these observations.

Even relatively small measurement distortions can:

• Increase convergence time
• Reduce positioning stability
• Affect repeatability
• Lower overall accuracy

This leads to an important engineering reality:

Environmental errors often dominate algorithmic errors in high-precision GNSS systems.

Many performance issues that appear to be related to firmware, receivers, or correction services are actually caused by uncontrolled measurement environments.

Final Thoughts

This rooftop test revealed a simple but critical insight:

GNSS performance is not only determined by satellites in the sky, but also by the environment beneath the antenna.

Even in open-sky conditions, reflective surfaces can introduce significant multipath effects and degrade high-precision positioning performance.

For PPP and other carrier-phase-based systems, this means:

• Open sky is not enough
• Multipath control is essential
• Antenna placement matters as much as algorithm design
• Environmental control is critical for reliable testing

Ultimately, in high-precision GNSS, what limits performance is often not the system itself—but the environment we overlook.

📘 Recommended Reading

Multipath Effects Explained: Why GNSS Positioning Fluctuates in Urban Areas

Learn how reflected GNSS signals introduce positioning errors, affect carrier-phase measurements, and reduce positioning stability in challenging environments.