2005 October 17 East Coast Geodetic VLBI Meeting

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Instrumental effects in geodetic VLBI

Brian Corey

(with copious cribbing from Alan Rogers’s 1991 Chapman Conference paper on “Instrumentation Improvements to Achieve Millimeter Accuracy”)

2005 October 17 East Coast Geodetic VLBI Meeting

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Topics

Antenna reflections and focus effects Feed cross-polarization Spurious signals Baseband filters Cable cal Correlation/fringe-fitting errors Correlation/fringe-fitting repeatability Challenges to fringe-fitting algorithms

2005 October 17 East Coast Geodetic VLBI Meeting

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Antenna effects

Unstable antenna structure (Ft. Davis effect) Multiple reflections within antenna structure

Typical effect on MBD: 1-10 ps bias <1 ps variable with pointing and focus changes

Multiple reflections within feed For 10% voltage reflection, effect on MBD

measured over 720 MHz: <22 ps bias <1 ps variable with pointing and focus changes

2005 October 17 East Coast Geodetic VLBI Meeting

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Antenna effects (cont’d)

Dispersion in a defocused paraboloid For 1-cm change in feed

position, group-phase delay dispersion = ~4 ps, independent of BW.

2005 October 17 East Coast Geodetic VLBI Meeting

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Feed cross-polarization

Cross-polarization on unpolarized source MBD effect depends on product of LCP responses

and parallactic angle difference between antennas -15 to -20 dB typical xpol for non-VLBA antennas For -15 dB xpol, MBD error ~10 ps at X-band

Cross-polarization on linearly polarized source For 10% polarized source and -15 dB xpol feed, X-

band MBD effect ≤ 10 ps, depending on feed and source polarization position angles.

2005 October 17 East Coast Geodetic VLBI Meeting

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Feed cross-polarization (cont’d)

Cures: Apply xpol corrections to MBDs

Determine correction terms by Measuring xpol in special dual-pol experiments, or Estimating from geodetic MBD data directly, using

dependence on parallactic angle difference Will not work for polarized sources unless

polarization source maps are available Fix the feeds (like that’s going to

happen…) or build new antenna/feed systems (VLBI2010!).

2005 October 17 East Coast Geodetic VLBI Meeting

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Spurious phase calibration signals

Spurious signals interfere coherently with phase cal

-20 dBc spur can shift MBD measured over 720 MHz by 22 ps (worst case)

Causes: Additive signals Intermodulation Images Crosstalk And on and on…

2005 October 17 East Coast Geodetic VLBI Meeting

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Spurious phase calibration signals (cont’d)

Typical spur levels in current systems are -40 dBc, but there are exceptions (worse!).

Methods for reducing effects of spurious signals on MBD: Clean up receivers and backends Offset receiver LO from integer MHz Stabilize receiver LO phase ( more of effect

of spur gets soaked up in clock, provided spur amplitude and phase are stable)

Correct for spurs at analysis stage (e.g., phasedoctor)

2005 October 17 East Coast Geodetic VLBI Meeting

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A new kind of spurious signal

2005 October 17 East Coast Geodetic VLBI Meeting

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A new kind of spurious signal (cont’d)

2005 October 17 East Coast Geodetic VLBI Meeting

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Example of S-band spur spectrum

Onsala S-band pcal amplitude spectrum

2005 October 17 East Coast Geodetic VLBI Meeting

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Example of S-band spur spectrum (cont’d)

Onsala S-band spur power spectrum, relative to pcal

2005 October 17 East Coast Geodetic VLBI Meeting

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Baseband filters

Delays through analog filters of same type may differ by ~5 ns at 8 MHz BW.

Delay may change with temperature by ~10%. With single pcal tone at band-edge, delay

change appears as phase change of ~1°, or 4 ps in MBD.

Cures: Use phase cal tone near center of band. Use multiple phase cal tones to measure changes in

filter phase/delay characteristics (but beware spurious signals).

Go digital!

2005 October 17 East Coast Geodetic VLBI Meeting

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Mismatched baseband spectra in the extreme

Gilcreek-Kokee channel X5 in RD0505: DRF & xpower spectrum

Impedance mismatch in Gilcreek samplerformatter cable causing severe distortion in baseband spectrum

2005 October 17 East Coast Geodetic VLBI Meeting

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Extreme baseband spectra (cont’d)

RD0505 X-band closure MBDs for 4 stations: Gc (A), Ap (G), Ny (N), and Wz (V)

SNR = 100-300 for all scans

2005 October 17 East Coast Geodetic VLBI Meeting

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Cable calibration

Short-term stability of typical cable cal system = 1-3 ps rms

Cable cal adds 1-3 ps noise to corrected delay.

2005 October 17 East Coast Geodetic VLBI Meeting

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Correlation/fringe-fitting errors

From Alan Rogers (1991):

2005 October 17 East Coast Geodetic VLBI Meeting

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Correlation/fringe-fitting errors (cont’d)

(Non)dependence of total X-band MBD on residual SBD –MBD differences from repeated processings using

different clock models

Residual SBD (ns)

RMS MBD diff= 4 ps

2005 October 17 East Coast Geodetic VLBI Meeting

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Correlation/fringe-fitting repeatability

Distributions of R1113 X-band observable differences from repeated processings

2005 October 17 East Coast Geodetic VLBI Meeting

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Correlation/fringe-fitting repeatability (cont’d)

Dependence of R1113 X-band MBD differences from repeated processings on SNR

σ[ ΔMBD/ σ(MBD) ] = 0.12-0.16

0.14 is expected value if two sets of input data are disjoint by 2%

But input data for two processings are typically disjoint by ~1% mildly inconsistent.

2005 October 17 East Coast Geodetic VLBI Meeting

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Fringe-fitting challenges

How do you handle a case like this??

2005 October 17 East Coast Geodetic VLBI Meeting

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Fringe-fitting challenges (cont’d)