When determining the effectiveness of a GPS system, it is necessary to analyze the contributions of error from various sources.  For the Earth-based system, the following error-sources affect the accuracy by the following factors:

• Satellite Clock Error (σc) – 3.5m
• Ephemeris Error (σe)  – 4.3m
• Ionospheric Delay Error (σi)  – 6.4m
• Tropospheric Delay Error (σt)  – 2.0m
• Receiver Noise Error (σn) – 2.4m
• Multipath Error (σm) – 3.0m
  

The total error of the system can be determined using the following formula:

These errors generate a total system error that does not meet the requirement. To accomplish a total system error of under 5.0m, a WAAS system was implemented.