Total Electron Content (TEC)
Definition status
This note defines Total Electron Content (TEC) as the line integral of electron density along the signal path between a GNSS satellite and a receiver. It is measured in TEC units (TECU), where 1 TECU = 10¹⁶ electrons/m².
Boundary:
- This is an observable concept note, not a model or system-design note.
- It does not cover algorithms for real-time SBAS ionospheric correction.
- It links TEC to integrity, validation, and empirical studies in the vault.
- It must not be used to publish operational GIVE, service-volume, availability, alerting, or procedure-eligibility claims without standards and service-provider evidence.
Working definition
TEC quantifies the total number of free electrons in a cylinder with a cross-sectional area of 1 m² along the propagation path. It directly determines the ionospheric delay experienced by GNSS signals through the dispersive nature of the ionospheric plasma.
TEC-to-delay conversion
At L1 frequency (1575.42 MHz):
| TEC (TECU) | Equivalent L1 delay (m) |
|---|---|
| 1 | 0.162 |
| 10 | 1.62 |
| 50 | 8.10 |
| 100 | 16.2 |
| 200 | 32.4 |
| 341 (p99: Indonesian RO) | 55.4 |
Conversion formula:
delay_L1 = (40.3 × TEC) / f_L1²
Relationship to the vault
Parent domain
- GNSS Radio Occultation — TEC is the primary observable extracted from RO occultations
- Ionospheric Model Validation — TEC is the variable used to compute model error metrics
Sibling domains
- SBAS Integrity — TEC uncertainty maps to ionospheric correction uncertainty, which impacts protection level and integrity risk
- Protection Levels — large TEC gradients can inflate protection levels
- Alert Limits — TEC anomalies may trigger integrity alerts
Implementation connection
- IRI-2020 vs GNSS-RO Indonesia — empirical TEC distributions from Indonesian GNSS-RO
- SBAS Ionospheric Threat — Empirical Evidence — threat quantification from observed TEC tails and gradients
- Source - GNSS-RO Indonesia Empirical Study — study-specific TEC statistics
TEC in SBAS context
Operational perspective
SBAS ground segments estimate ionospheric delay using dual-frequency ground receiver observations and broadcast corrections as gridded ionospheric vertical delays (GIVDs) or equivalent. The quality of these estimates determines:
- User position accuracy — corrected versus uncorrected delay
- Integrity risk — probability that the actual delay exceeds the broadcast bound (GIVE)
- System availability — whether protection levels remain below alert limits
Empirical perspective
GNSS-RO provides an independent TEC measurement that can:
- Validate or invalidate empirical ionospheric models (e.g., IRI-2020)
- Quantify the tail of TEC delay distributions for threat-model design
- Characterize spatial TEC gradients that threaten differential correction accuracy
Critical boundary: GNSS-RO TEC measures a column along one ray path at one time. SBAS GIVE requires spatial interpolation over many ground-receiver IPPs. These are complementary, not interchangeable.
Current source anchors
- SBAS Core Claim Routing — core claim-family routing
- SBAS Corrections and Integrity Separation — prevents TEC research claims from becoming operational correction claims
- Source - GNSS-RO Indonesia Empirical Study — empirical TEC distribution from Indonesian RO data
- Source - IRI-2020 Ionosphere Model — model-predicted TEC for comparison
- Source - ICAO Annex 10 Volume I GNSS SBAS — future standards-family routing for operational ionospheric requirements
- Source - RTCA DO-229 — future receiver/equipment extraction target
Open provenance questions
- What is the relationship between RO-derived TEC at a tangent point and the vertical IPP TEC used in SBAS grid interpolation?
- Should TEC statistics from sparse RO sampling be treated as conservative (overbounding) or biased (undersampling tails)?
- How do TEC conversion factors vary with geometry, receiver type, and signal frequency?
See also
- GNSS Radio Occultation
- Ionospheric Model Validation
- SBAS Integrity
- SBAS Corrections and Integrity Separation
- SBAS Core Claim Routing
- Protection Levels
- Alert Limits
- IRI-2020 vs GNSS-RO Indonesia
- SBAS Ionospheric Threat — Empirical Evidence
- Source - GNSS-RO Indonesia Empirical Study
- Source - IRI-2020 Ionosphere Model