Two NCRC members, Matt KA1BQP and Bob WB4SON, participated in the April 2019 ARRL Frequency Measuring Test (FMT).  Both had results in the “Green Band”, along with 72 others out of the 112 global participants.  For this FMT, Green Band was better than 1 Hz accuracy. Matt and Bob turned in readings about 1/3 of that.

At these levels of precision, Doppler due to a constantly changing ionosphere becomes a major factor in off-the-air HF measurements, and can eat up most of the allowable error.