Correction creation
What three major error sources are modeled in WAAS?
Satellite clock error, orbit error, ionospheric error (plus modeled troposphere).
through what type of satellites are WAAS corrections broadcast
Corrections are applied to what part of the GNSS calculation?
The pseudorange measurements.
How long is “pass‑to‑pass” evaluated over?
This GPS system offers roughly 1–1.5 meter repeatable accuracy, making it more stable than standalone GPS but less stable than RTK.
What is WAAS repeatability?
What is the main atmospheric error that WAAS and SF1 model
The ionospheric delay
This frequency band is used to deliver WAAS/SF1 corrections?
what is L1
Does a rover apply correction before or after calculating its final position?
what is before? — corrections are applied to pseudoranges prior to solving.
Typical WAAS pass‑to‑pass precision:
What is under 3m
What is “repeatability” (YTY) accuracy?
Position variance within 1 year, 90% of the time.
Where are the master stations located in the WAAS system
what is one on each U.S. coast.
Why do WAAS & SF1 not require cellular service?
The corrections are delivered via satellite, not the internet.
Why can WAAS & SF1 not reach RTK-level accuracy?
Errors are modeled over a wide area, not individually corrected for each satellite-path like RTK does.
Why is WAAS PTP accuracy generally good?
Short‑term atmospheric conditions don’t change quickly, so modeled corrections remain consistent.
Why does WAAS have poor YTY repeatability?
Because the differential is wide‑area modeled, not locally measured.
Why are these systems called "wide area differential"
They use large, sparse network of stations to model errors across an entire region
How many WAAS satellites are currently used?
Three geostationary satellites (PRNs 131, 133, 135)
What is one limitation of wide-area correction in the linearized equations?
Residual local errors remain because corrections aren’t site-specific.
What real‑world field operation is MOST dependent on strong pass‑to‑pass performance instead of absolute accuracy?
Spraying or spreading — where swath overlap depends on short‑term consistency.
Why can small biases in the WAAS atmospheric model accumulate into meter‑level annual drift?
Long‑term model biases compound over time, shifting absolute position solutions even when PTP remains stable.
Why is wide‑area correction modeling necessary instead of using local corrections like RTK?
WAAS needs to cover extremely large regions, so model‑based corrections scale better than local base‑station measurements.
What is one advantage of satellite-delivered corrections over ground-based corrections?
More consistent coverage; not limited by cellular dead zones.
Why do linearized GNSS equations require at least four corrected pseudoranges?
Because the solution needs four unknowns (X, Y, Z, and clock bias), requiring at least four independent equations.
Under what conditions might WAAS pass‑to‑pass precision temporarily degrade despite good satellite coverage?
During ionospheric disturbances or sudden tropospheric changes (e.g., weather fronts).
Daily double This long‑term error source causes WAAS positions to shift by up to 2 meters over a year, even when day‑to‑day accuracy appears stable. It results from slowly changing biases in the wide‑area atmospheric and orbit models, combined with this specific limitation: WAAS does not measure local error at the user’s location.
What is the lack of localized atmospheric measurements (wide‑area model drift)?