Orbiting VLBI Link Stability for 97 May

For the VLBI observations using the Green Bank Earth Station, the experiment is summarized and the link stability is plotted below. The plots of link stability indicate the difference between the predicted and measured 14.2 GHz downlink from HALCA. This down link frequency difference is expected to be different from zero, due to errors in the satellite orbit prediction and troposhere and ionospheric effects. The frequency errors are typically on the order of a few Herz, indicating the frequency error at the satellite is a few parts in 15,000,000,000.

The frequency residuals are used to produce a time corrections file applied to the data at correllators and by the JPL orbit determination group to calculate the satellite orbital parameters.

For each tracking pass below, two plots are produced:
Residual Frequency
The difference between the predicted and measured down link frequency is ploted versus time. (The Time axis is truncated MJD, which is MJD - 50000.). The difference is usually a few Hertz, indicating the satellite orbit prediction is very good. The frequency is slope of a linear fit to the downlink cycles during a two second interval.
RMS Frequency
The uncertainty of the slope of the linear fit to the downlink cycles indicates the quality of the two way link to the satellite. Typically, the RMS level is better than 0.02 Hz.

97 May 20 -- VT134 -- 1823+568

HALCA First Fringes with NRAO 140ft. "First fringes" is the first alignment in time of astronomical signal from HALCA throught the Green Bank Earth Station and the NRAO 140ft telescope in Green Bank, WV. After this signal alignment, extreamly high angular resolution images of the radio sources are made. Observations from 97 May 20, 06h30m to 10h08m UTC.

97 May 22 -- VT136 -- 1519-273

HALCA First Fringes with NRAO VLBA. Observations from 97 May 22, 03h06m to 06h20m UTC. A detailed description of the observation can be obtained from Jim Ulvestad.

97 May 31 -- VT800 -- 1156+295

Fringes were found in this VLBA experiment. Unfortunately after the first 10 minutes of this experiment, the residual phase data were corrupted due to hardware problems. Green bank Tracking ran from 97 May 31 21h02m to 23h27m.

Other Web Sites

Return to the NRAO Green Bank Tracking Station page.

VSOP/HALCA has a home page. The VLBA Correlator NRAO Socorro is developing other hardware and software for the NASA OVLBI project. There is a Space VLBA Correlator home page as well.

Return to the Station Software Guide. The National Radio Astronomy Observatory (NRAO) is a facillity of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

Last update: 97 July 2