WGS-84 is Not a Single Datum
The WGS-84 reference frame has been refined multiple times through the GPS control segment. Each realization aligns more closely with the International Terrestrial Reference Frame (ITRF) at a specific epoch:
| Realization | Epoch | Consistent with |
|---|---|---|
| G730 | 1994 | ITRF92 |
| G873 | 1997 | ITRF94 |
| G1150 | 2002 | ITRF2000 |
| G1762 | 2013 | ITRF2008 |
| G2139 | 2021 | ITRF2014 |
WGS-84 vs NAD83 in Aviation
NAD83 and WGS-84 are often treated as equivalent by GIS users, but they are not the same reference frame. NAD83 is fixed to the North American tectonic plate; WGS-84 is Earth-centered and moves with the GPS control segment. The difference in CONUS is approximately 1.0—.0 m depending on location and epoch.
For non-aviation applications, this difference is often acceptable. For ICAO Annex 15 compliance, aeronautical data must be in WGS-84 specifically —using NAD83-referenced coordinates without explicit WGS-84 transformation can produce systematic errors exceeding the ICAO threshold accuracy requirements for Cat III approaches.
Epoch-Dependent Divergence
North America moves approximately 2.5 cm/year relative to the ITRF geocentric frame due to tectonic motion. A coordinate surveyed in 2005 (epoch 2005.0) and published using the same realization will appear shifted relative to a coordinate surveyed in 2025 (epoch 2025.0) if the plate motion is not accounted for. For high-precision aviation applications (GBAS, precision RNP AR), this is a real consideration.