Transcript The GBT at K-Band - Green Bank Site — Science Website

The GBT at K-Band
Current Receiver

Two Dual-Feed Receivers in a single dewar








18.0 – 22.4 GHz
22.0 – 26.5 GHz
Feeds arranged in a square
Current cabling prevents using as a 4-feed 22-22.4 GHz Rcvr
Dual Polarizations
FWHM Beamwidth : 34” at 22 GHz
Feed separation ~ 3 arcmin (~5 beamwidths) in cross
elevation
Instantaneous Bandwidth : 4 GHz
Backends

DCR



Spectral Processor


Pointing and focus
Mapping strong sources (1/F noise)
Very limited -- narrow bandwidths, few spectral windows, coarse resolution
Spectrometer

800 MHz




200 MHz




1 or 2 spectral windows
Velocity Coverage per window : 2700at 22 GHz
Velocity Resolution : 0.08 to 0.3 km/s
50 MHz




1 or 2 dual-polarization spectral windows
Velocity Coverage per window : 11000 at 22 GHz
Velocity Resolution : 1.3 to 5 km/s at 22 GHz
1, 2, or 4 spectral windows
Velocity Coverage per window : 680 km/s
Velocity Resolution : 0.005 to 0.17 km/s
12.5 MHz



1, 2, or 4 spectral windows
Velocity Coverage per window : 170 km/s
Velocity Resolution : 0.001 to 0.04 km/s
Observing

Doppler Tracking
Hardware – 1st spectral window
 Software – all others


Switching modes


Total power
Frequency switching (few MHz)
 NOD – using telescope drives
 Subreflector Nodding (not yet deployed)

Observing types



On-Off
Track
On-The-Fly (crosses, rectangular maps, strips, …)
 Point maps
Antenna


Active Surface using an elevation-dependent lookup table
390 micron surface accuracy

Aperture Efficiency: 65% - 58% at 18 and 26.5 GHz
 Beam Efficiency : 89% – 79%
 Gain : 1.8 – 1.6 K/Jy
 Probably flat with elevation
Atmospheric Conditions

Opacity



Raises Tsys
Reduces TA
Effective System Temperature



Winds


Tsys’ = Tsys * exp(Tau*AirMass)
Proportional to sqrt(integration time)
Reduces effective aperture efficiency
Day/Nightime

Differential Heating/Cooling


Focus/Pointing/Surface Errors
Not an issue for K-band
Precipitable Water/Cloud Statistics
Atmospheric Conditions - Opacity
System Temperatures
25% Weather Conditions
Forecasting
Relative Effective Tsys

Effective System Temperature


Tsys’ = Tsys * exp(Tau*AirMass)  sqrt(integration time)
Relative Effective Tsys = REST = Tsys’/Tsys’Best=sqrt(t/t0)
Winds for a 5% loss of Efficiency
RFI
22.21 – 22.5 GHz : Shared protected band
 17.7 – 20.2 GHZ

 Fixed
Satellite Service
 Fixed Terrestrial Service
 Mobile Satellite Service

Car Avoidance Radar