Orbiting VLBI Link Stability

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 Experiments: VT134, VT136, VT800

The link stablility for the 97 May experiments are summarized.

97 June Experiments: VT800, VT804, VT151, VT154, VT805, VT806

The link stablility for the 97 June experiments are summarized.

97 July 05 -- VT809 -- 1921-293

Tracking in Green Bank runs from 97 July 5 4h03m to 7h22m. This tracking pass appeared to be successful, due to extensive work on the Maser Lo Reciever during the holiday. Earlier July tracking passes had partial failures due to instability in timing reference.

97 July 13 -- V025c -- 1921-293

Tracking in Green Bank runs from 97 July 13 18h36m to 23h17m. Data recording on VLBA tape not scheduled to start until 20h49m. Satellite signal was present at beginning of pass, but slowly faded during pass. Signal slowly returned and good downlink data recorded from 22h43m to end of pass. No problems found in any tests of tracking station. Station pointing towards satellite seemed OK. Both uplink power detected at HALCA and downlink power measured at Station showed same characteristics: gradual decline in detected power, no satellite signal in Green Bank, then by gradual increase in detected power at both satellite and Green Bank.

97 July 16 -- VS06m -- J1939-15

Tracking in Green Bank runs from 97 July 16 17h11m to 19h40m. Data recording on S2. Tracking station problems with Two-Way-Timing module data communication prevented good data recording at beginning of pass. Failure of internal test fixture (Satellite simulator) confused testing after hardware repair. Decided to attempt tracking satellite, despite failure of internal tests. Data recorded during last few minutes appeared OK.

97 July 18 -- VT812 -- 1921-293

Tracking in Green Bank runs from 97 July 18 02h53m to 5h16m. Computer/Operations problem caused a loss of downlink data from 4h43m to end of tracking pass.

97 July 22 -- V083c -- 1413+135

Tracking in Green Bank runs from 97 July 22 01h10m to 3h53m. Satellite downlink signal weak/intermittent during last 4 minutes of tracking pass.

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 22