Improving the NIST Ionospheric Measurement System [for international time transfer]

Two problems of the NIST Ionospheric Measurement System have been addressed. A new software lock on the received satellite frequency allows the system to lock more robustly in the presence of selective availability. We obtain about 30% more measurements. Also, biases in the measurements largely come from the front end antenna system. Preliminary results indicate that for the most part the problem is neither due to multi-path interference nor phase center offsets in the antennas, as was previously thought. There is indication that some of the effect is due to an interaction between the two quadrafiler helix antennas