Ionosphere error
-- dual freqency correction
-- IGS ionosphere model correction
-- broadcast (Klobuchar) model correction
-- no correction

Satellite clock error
-- IGS clock corrections
-- broadcast clock corrections
-- broadcast group delay correction

Orbit error
-- IGS/NGS precise (post-processing) orbits
-- IGS/NGS precise predicted orbits
-- broadcast orbits

Troposphere error
-- FSL/MAPS troposphere models
-- measured surface data
-- generic seasonal model (Herring)
-- no correction

Multipath error
-- L1
-- L2
-- antenna and site characteristics

Receiver errors
-- C/A and L1 code range
-- codeless L2 pseudorange (e.g. cross correlation)
-- interchannel bias

Geometric effects
-- DOP's -- PDOP, HDOP, VDOP, TDOP
-- positional effects (high vs. low latitudes)
-- accuracy
+++ horizontal position
+++ ellipsoidal height
+++ time

Robust processing effects (adjustment)
-- outlier detection
-- weighting strategies
-- alternative norm processing

Note: this list only covers elements significant (or potentially
significant) for meter-level results. The list would be
considerably longer for 1-10 cm level accuracies.