GNSS Radio Occultation
Definition status
This note defines GNSS Radio Occultation (GNSS-RO) as an empirical satellite-based technique for profiling the Earth’s ionosphere using radio signals from Global Navigation Satellite System (GNSS) transmitters observed by low-Earth-orbiting (LEO) receivers.
Boundary:
- This is a technique concept note, not a system-design note.
- It does not cover SBAS correction calculations, GBAS references, or avionics.
- It links to empirical results from GNSS-RO applications in the vault.
Working definition
GNSS-RO measures phase and amplitude changes of GNSS signals as they pass through the ionosphere during an occultation event — when a LEO satellite sets or rises behind the Earth relative to a GNSS transmitter. From these measurements, vertical profiles of ionospheric electron density and ionospheric total electron content (TEC) can be derived.
Key technique characteristics
| Characteristic | Description |
|---|---|
| Data source | LEO constellations with GNSS receivers (e.g., COSMIC-2, Spire) |
| Observable | Excess phase → bending angle → refractivity → electron density |
| Vertical resolution | Typically 200 m – 1 km |
| Horizontal footprint | Tangent-point region; hundreds of kilometres along ray path |
| Coverage | Global; especially valuable over ocean and data-sparse regions |
| Limitation | Spase temporal sampling; post-sunset gaps at low latitudes |
Relationship to the vault
Parent domain
- Ionospheric Model Validation — GNSS-RO provides the empirical ground truth against which models are validated
Sibling domains
- Total Electron Content (TEC) — TEC is the primary observable extracted from GNSS-RO profiles
- SBAS Integrity — GNSS-RO data informs ionospheric threat models that feed integrity design
- Source - GNSS Radio Occultation Technique — source scaffold for literature and datasets
Implementation connection
- IRI-2020 vs GNSS-RO Indonesia — concrete empirical application to Indonesia using Spire/COSMIC-2 podTec data
- Indonesian SBAS ION Paper Iterations — research paper series using GNSS-RO for SBAS threat screening
Aviation and SBAS implications
GNSS-RO has two distinct relevance levels for SBAS design:
-
Threat discovery — GNSS-RO can characterize ionospheric delay magnitude and spatial variability in regions lacking dense ground-station networks. This is valuable for pre-operational threat-model design.
-
Operational correction limitation — GNSS-RO does NOT provide the real-time, high-rate TEC measurements required for service-level SBAS ionospheric correction broadcasting. Ground GNSS receiver networks remain the operational standard for SBAS GIVE computation.
Critical boundary: The vault treats GNSS-RO as a threat-discovery and model-validation instrument, not as a direct SBAS correction source.
Data products referenced in the vault
- podTec (phase-derived TEC along occultation ray) — Spire/COSMIC-2 product used in the Indonesian study
- EUMETSAT/Radio Occultation Meteorology — operational weather RO; not ionospheric
- UCAR/CDAAC — primary archive for COSMIC series RO data
Current source anchors
- Source - GNSS Radio Occultation Technique — starter provenance scaffold
- Source - GNSS-RO Indonesia Empirical Study — study-specific source scaffold
Open provenance questions
- How does Spire podTec calibration compare to ground GNSS TEC for absolute accuracy?
- What is the relationship between RO-derived TEC along a tangent-point ray and the IPP-domain TEC that SBAS ground segments actually model?
- Should GNSS-RO validation of empirical models be separated from GNSS-RO threat-screening for SBAS?