The NASA VLBI 2010 Proof-of-concept Demonstration and Future Plans

The next generation geodetic VLBI instrument is being developed with a goal of 1 mm position uncertainty in twenty-four hours. We have implemented a proof-of-concept system for a possible VLBI2010 signal chain, from feed through recorder, on the Westford (Massachusetts, USA) 18-m and MV-3 (Maryland, USA) 5-m antennas. Data have been obtained in four 512 MHz bands spanning the range 3.5 to 11 GHz to investigate the sensitivity and phase delay capability of the system. Using a new phase cal design, the phases have been aligned across four bands spanning 2 GHz with an RMS deviation of approximately eight degrees. Several components of the system will be improved for the prototype version of VLBI2010, including the feed, digital backend, and recorder, and these will be installed on a 12-m antenna that has been purchased and is ready for installation at the Goddard Space Flight Center outside of Washington, D.C., USA, site of the MV-3 antenna. 1. Proof-of-concept System Recognizing that spatial and temporal fluctuations in the atmosphere delay are a major component of the error in position determination, the VLBI2010 Committee carried out a large number of simulations to arrive at design goals for the next generation geodetic VLBI antenna system [1]. A primary goal is the implementation of fast slewing antennas that attain high delay precision per observation. Taking into account existing and anticipated data recording capabilities, these translate to an antenna diameter of 12 m or larger and delay precision of approximately 4 psec. With funding from NASA’s Earth Surface and Interior Focus Area, a proof-of-concept (PofC) project was initiated to demonstrate that the VLBI2010 concept is attainable. Obtaining good sensitivity from relatively small antennas in a short time requires a large data acquisition rate. By spreading the data acquisition over four bands spanning 2-14 GHz instead of the two bands used by the current geodetic VLBI systems at S-band and X-band, enough phase precision is expected to be obtained to estimate both the phase delay and the dispersive phase deviation due to the ionosphere for each scan. Of course the large total bandwidth increases the probability that radio frequency interference (RFI) will lie within the observed frequency range of the feed and receiver. Thus the band selection has been made tunable by using four separate UpDown Converters (UDC), one for each band. In addition to offering some RFI avoidance capability, the flexibility is important to be able to accommodate placing bands at S and X for compatibility with legacy antennas that are not able to upgrade, while having the option to use a higher minimum frequency to avoid RFI when only stations with the VLBI2010 broadband capability are participating. The large instantaneously available bandwidth of the PofC front-end is made possible with the ETS-Lindren 3164-05 dual linearly-polarized broadband antenna. While this antenna has neither IVS 2010 General Meeting Proceedings 23 Arthur Niell and the Broadband Development Team: NASA VLBI2010 Proof-of-concept a frequency-independent beam size nor phase center, it was commercially available at the initiation of the project and was affordable. A new feed is under development with the desired properties. Thus the solution for obtaining enough sensitivity and delay precision from fast, and therefore likely to be small, antennas is a dual polarization, wideband feed followed by four independent local oscillators and digital backends, recording at 2 Gbps on four Mark 5B+ recorders (Figure 1a). In order to increase the sensitivity, the feed and low noise amplifiers are cooled to 20 K. Phase and noise calibration signals are injected immediately following the feed. A new phase cal generator was designed and implemented to replace the obsolete S/X phase cal [2]. The amplified RF signals for each polarization are brought down on fiber optic cable to the control room where the signals are split, down-converted [3], digitized, and recorded. All of the control room electronics are contained in a single rack (Figure 1b). Figure 1. a) Configuration for the VLBI2010 system: LNA – low noise amplifiers; UDC – UpDown Converter (flexible local oscillator); iBOB/DBE – digital back end; Mk5B+ – high speed (2 gigabits per second) digital recorder; X and Y – two orthogonal components of linear polarization. b) Rack at Westford containing all of the VLBI2010 proof-of-concept backend equipment. The PofC systems were installed on the Westford (Massachusetts, USA) 18-meter antenna and the MV-3 5-meter antenna at Goddard Geophysical and Astrophysical Observatory (Maryland, USA). Figure 2 shows the Dewar mounted on MV-3. The optimum focus setting for Westford was determined by varying the distance of the Dewar from the vertex of the antenna for successive VLBI scans. The optimum focus is frequency dependent, and there appears to be a dependence on polarization at the higher frequencies. We expect these effects to be significantly reduced with the next generation of feed. 24 IVS 2010 General Meeting Proceedings Arthur Niell and the Broadband Development Team: NASA VLBI2010 Proof-of-concept Figure 2. MV-3 5-m antenna at GGAO with the VLBI2010 proof-of-concept Dewar mounted at the Cassegrain focus. −2 0 2 4 0 1 2 3 4 focus am p (m an ua l p ca l) V 3.4GHz H 3.4GHz V 5.5GHz H 5.5GHz V 8.5GHz H 8.5GHz V 9.5GHz H 9.5GHz Figure 3. Amplitude as a function of focal distance for the Westford antenna. Frequency increases from top to bottom at focus = 0.