Frequency Dependent Observing Strategies

by Justin Linford last modified Jul 16, 2020

Observations Above 12 GHz

The troposphere has a larger impact on frequencies above about 12 GHz. The higher the frequency, the more observers need to worry about weather. In the Technical Justification section of the VLBA proposal submission tool, proposers are asked to specify any restrictions on when an observation may occur, including under what weather conditions. If an observer needs to update these conditions, they should contact the VLBA Scheduling Officer () and explain any changes that need to be made. Occasionally, observers decide their original constraints were too demanding and will prevent a project from being observed in the desired time frame. In this case, the weather constraints can be relaxed and allow the observation to take place at the risk of the data being slightly degraded. Weather constraints also depend on the science targets and whether self-calibration can be used.

Because the troposphere affects high frequency waves more, small changes in the structure of the troposphere (turbulence) must be accounted for when observing at these frequencies. The primary way observers correct for this turbulence is to shorten the time between scans on calibrators and scans on science targets. See the Phase Referencing section of the Calibration Strategies chapter for more information on tropospheric coherence time and the associated switching time at high frequencies.

The effects of elevation are also larger at higher frequencies because observing at lower elevations means observing through more of the troposphere. Observers encouraged to use very short cycle times and sufficiently bright phase reference calibrators when performing observing at low elevations at frequencies above about 12 GHz.

While the VLBA is able to slew significantly faster than the VLA, there will still be gaps of a few to several seconds between the target and calibrator scans. For this reason, it is imperative that the observer chooses calibrators which are as close as possible to the science target when observing at higher frequencies.

Observations Between 1 and 3 GHz

Observations at frequencies between 1 and 3 GHz are often impacted by radio frequency interference (RFI). Because the VLBA antennas are widely separated, it is unlikely for multiple stations to experience the same RFI at the same time (unless a target happens to be close to a broadcasting satellite such as SirusXM, which broadcast between 2332 and 2345 MHz). However, more commercial systems are beginning to make use of frequencies in this range, which means each station is at greater risk of suffering from local RFI. Observers should be aware that portions of the observed bandwidth may be unusable at several stations. Plots of RFI for several VLBA observing bands are available on the VLBA RFI webpage. When possible, it is best to design the observing setup such that it avoids the worst of the known RFI (e.g., don’t observe near 2.3 GHz). Having a large number of spectral channels in each data channel can be useful for flagging RFI in the data while keeping as much of the bandwidth as possible.

Another concern at lower frequencies is the impact of the ionosphere. Fluctuations in the electron content of the ionosphere and turbulence within the ionosphere have larger impacts on observation below about 5 GHz. It is therefore recommended that observers use phase reference calibrators as close to the science target as possible, and no more than 4 degrees away. Whenever possible, it is best to use a phase reference target that will be in the primary beam while observing the science target.

Observations Below 1 GHz

At frequencies below 1 GHz, the fluctuations in the ionosphere have major impact on observations. At these frequencies, observers should use in-beam phase reference calibrators as close as possible to the science target(s). This will require at least 2 correlator passes (one at the location of the science target, and one at the location of the phase reference calibrator).