Introduction
"If a satellite signal cannot reach the receiver, it cannot be used."
Signal blockage is the most straightforward form of GNSS degradation: a physical object stands between the satellite and the receiver antenna, preventing the signal from arriving. Unlike multipath, where a corrupted signal still reaches the receiver, a blocked signal is simply absent. The consequences - reduced satellite count, degraded geometry, and ultimately a weaker or failed position solution - are directly related to how many satellites are blocked and where they are in the sky. Understanding blockage, elevation masking, and their effect on DOP is fundamental to designing reliable GNSS systems.
Physical Obstruction
GNSS signals operate in the L-band (approximately 1.1–1.6 GHz) and are severely attenuated by solid materials. Unlike lower-frequency radio signals, L-band GNSS signals do not diffract significantly around large obstacles. Structures that cause blockage include:
- Buildings: The most common and impactful source of blockage in urban environments. Tall buildings can block entire quadrants of the sky, particularly at mid-elevation angles where many usable satellites reside.
- Bridges and overpasses: Even momentary passage under a bridge causes a complete loss of all satellite signals simultaneously. RTK receivers must re-initialise after passing under a structure, which can take tens of seconds to minutes depending on receiver and environment.
- Dense vegetation (tree canopy): Unlike solid structures, foliage does not completely block signals but attenuates them significantly. Signal strength drops by 10–20 dB or more through a dense tree canopy, making tracking marginal and cycle slips frequent.
- Terrain (mountains, cliffs, embankments): Natural terrain masks low-elevation satellites, which is a particular concern in valleys, canyons, and at the base of steep slopes.
- The vehicle or platform itself: For mobile systems, the host vehicle's body can block satellites on one side as the vehicle turns or pitches, creating dynamic blockage patterns that change with orientation.
Satellite Masking Angles
Receivers allow the user to define an elevation mask angle - a minimum elevation below which satellite signals are excluded from the navigation solution. Satellites near the horizon travel through a much longer path of atmosphere, making their signals more susceptible to tropospheric and ionospheric delay errors. They are also more likely to be affected by ground-bounce multipath. For these reasons, a mask angle of 10–15 degrees is commonly used in open-sky environments.
However, raising the mask angle in an attempt to exclude poor-quality signals has a critical side effect: it further reduces the number of available satellites. In an already-constrained urban environment, increasing the mask angle from 10 to 25 degrees can eliminate several satellites and critically degrade geometry. The engineer must balance signal quality against satellite count - raising the mask to improve individual measurement quality risks collapsing the overall navigation solution by leaving too few satellites in the solution.
Impact on Satellite Count
A 3D GNSS position fix requires a minimum of four satellites - three to resolve the three spatial coordinates and one to solve for the receiver clock offset. With exactly four satellites, there is no redundancy, and a single bad measurement directly corrupts the solution with no means of detection. With five or more satellites, the receiver can apply integrity monitoring and detect gross errors through residual analysis.
In a deep urban canyon, single-constellation receivers regularly fall below four usable satellites for significant periods. Research in major city environments has recorded epochs where fewer than four GPS satellites were visible for well over half of observation time on east-west-oriented streets. Multi-constellation receivers extend resilience but cannot guarantee adequate availability in the worst environments. The practical rule is that the more satellites available above the solution threshold, the more robust and reliable the navigation solution becomes.
Impact on Geometry and DOP
Satellite count alone does not determine solution quality. The geometric distribution of the visible satellites across the sky is equally important, captured by the Dilution of Precision (DOP) metric. DOP is a dimensionless multiplier - it scales the measurement noise into position error. A PDOP of 2 means position errors are approximately twice the measurement noise; a PDOP of 8 means errors are eight times larger.
Signal blockage degrades DOP in a specific and predictable way: because buildings block low and mid-elevation satellites on the sides of the street, the receiver is left tracking only high-elevation satellites clustered near the zenith. Even with several satellites visible, they are all located in a small region of the sky. This clustering produces poor geometry: the resulting DOP values spike sharply, and the vertical component of the solution becomes particularly uncertain because high-elevation satellites contribute little discrimination in the height dimension.
| DOP Value | Rating | Practical Interpretation |
|---|---|---|
| < 1 | Ideal | Rarely achieved; excellent all-sky distribution |
| 1–2 | Excellent | Open-sky multi-constellation conditions |
| 2–5 | Good | Acceptable for most survey and navigation applications |
| 5–10 | Moderate | Accuracy degraded; use with caution |
| > 10 | Poor | Solution unreliable; expect large position errors |
System Design Implications
Engineers must account for blockage and masking when defining system requirements. Key considerations include:
- Conduct sky-view surveys at representative deployment sites before finalising accuracy requirements
- Use multi-constellation receivers to maximise the satellite pool available in obstructed environments
- Implement DOP thresholds in system software to suppress or flag position outputs when geometry is poor
- Design antenna placement to minimise platform self-masking on mobile systems
- Where blockage is unavoidable, integrate inertial sensors or other positioning modalities to bridge GNSS outages
The relationship between blockage, satellite count, geometry, and accuracy is tightly coupled. A well-designed system monitors all these factors continuously and degrades gracefully rather than producing confident but wrong positions when conditions deteriorate.