Ionospheric distortions are the largest source of errors in single-frequency solutions! And it's why the WAAS birds transmit a correction model, which all moder

Ionospheric distortions are the largest source of errors in single-frequency solutions! And it's why the WAAS birds transmit a correction model, which all modern (post-2004 or so) receivers can apply. Multi-frequency receivers can derive the corrections directly because the distortions affect the different frequencies in predictable ways, and they can work back to "ionosphere-free" pseudoranges, and base the rest of the solution on those. To your quoted comment, nicer receivers also tend to have a configurable "horizon mask" aka "elevation mask", so you can tune this rejection behavior. I could swear I've heard of some that let you configure the mask height _per azimuth_ but I can't find an explicit reference right now. Elevation masking is tricky because if you crank it up too high, you force yourself into poor-DOP geometries. But if you relax it too low, not only do you get heaping piles of ionospheric distortion, you also invite ground-clutter multipath. I think it's primarily used by stationary timing receivers, because they know their position is fixed, they're less susceptible to GDOP.