DSS \xi Factor

Some observers may wish to have their project scheduled even if the weather is not ideal. For example, projects that use very short integration times are dominated by overheads, not the radiometer equation, and so they may wish to get a scoring boost in order to allow observing under a wider range of weather conditions. To implement this desire, we allow observers to modify the minimum effective system temperature, (T_{sys} e^{\tau})^{'}, where T_{sys} is the atmospheric system temperature, \tau is the opacity, and the prime denotes the minimum value. This value is set by the DSS using historical weather data and will depend on frequency. The atmospheric observing efficiency is given by \eta_{atm} = (\frac{(T_{sys} e^{\tau})^{'}}{T^{sys} e^{\tau}})^{2} and is one factor in ranking observing sessions. The atmospheric efficiency is a measure of the integration time required to acheive the same rms noise under the best weather conditions. For example, if \eta_{atm} = 0.5 then the weather conditions will require an integration time twice as long to achieve the same rms noise compared to the best weather conditions.

The minimum effective system temperature can be scaled by a factor \xi to either improve or degrade the atmospheric conditions for a particular session. The default is \xi = 1.0. The observer needs to use caution when modifying this parameter, especially because it will have a different effect at different frequencies. For example, doubling the minimum effective system temperature will have a much larger effect at 14 GHz than at 22 GHz. This is illustrated in the figure below where the cummulative distribution function of the effective system temperature, T_{sys} e^{\tau}, is plotted for several frequencies.

tsysprime cdf.jpg

Here is a table with recommendations for \xi.

14 GHz 22 GHz 35 GHz 45 GHz 50 GHz 90 GHz Comments
1.01 1.10 1.05 1.10 1.10 1.10 Moderate Effect
1.03 1.20 1.10 1.20 1.20 1.20 Significant Effect
1.05 1.30 1.15 1.30 1.30 1.30 Large Effect

-- DanaBalser - 2010-11-17

Topic attachments
I Attachment Action Size Date Who Comment
tsysprime_cdf.jpgjpg tsysprime_cdf.jpg manage 59 K 2010-11-17 - 16:59 DanaBalser cdf for the atmospheric effective system temperature
Topic revision: r2 - 2010-11-18, KarenONeil
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