# Baseline Sensitivity

Baseline sensitivity is the RMS thermal noise (ΔS) in the visibility amplitude in a single polarization on a single baseline. Adequate baseline sensitivity is required for VLBI fringe fitting. Baseline sensitivities between VLBA antennas, for typical observing parameters, are listed in column [6] of table 5.1.

Alternatively, the baseline sensitivity for two identical antennas, in the weak source limit, can be calculated using the formula (Walker 1995a; Wrobel & Walker 1999):

[display]\Delta S = {\rm SEFD} / [\eta_s \cdot ( 2 \cdot \Delta \nu \cdot \tau_{\rm ff} ) ^{1/2} ] \; {\rm Jy}[/display]

SEFD or "system equivalent flux density" is the system noise expressed in Janskys. [inline]\eta_s \le 1 \ \ [/inline] accounts for the VLBI system inefficiency (primarily quantization in the data recording). Walker (1995) and Kogan (1995b) provide the combination of scaling factors and inefficiencies appropriate for VLBA visibility data. The sensitivity values presented in table 5.1 were calculated using [inline]\eta_s = 0.8[/inline], while the EVN Sensitivity Calculator uses [inline]\eta_s = 0.7[/inline]. The bandwidth in Hz is [inline]\Delta\nu[/inline]. For a continuum target, use the channel bandwidth or the full recorded bandwidth, depending on the fringe-fitting mode; for a line target, use the channel bandwidth divided by the number of spectral points that span the channel. [inline]\tau_{\rm ff}[/inline] is the fringe-fit interval in seconds, which should be less than or about equal to the coherence time.

Moran & Dhawan (1995) discuss expected coherence times. The actual coherence time appropriate for a given observation can be estimated using observed fringe amplitude data on an appropriately strong and compact source.

For non-identical antennas 1 and 2, SEFD can be replaced by the geometric mean [inline]\sqrt{{\rm SEFD}_1 \times {\rm SEFD}_2}[/inline].

NOTE: When determining the sensitivity for a VLBA proposal, always use the EVN Sensitivity Calculator.