Scheduling Considerations

Sessions

When the submitting a proposal for either VLA or VLBA observing time, you must enter the observations you plan in the form of “sessions” in the Proposal Submission Tool (PST).  Each session is a unit of observing time (target + resource + time) which will be considered by the Time Allocation Committee (TAC) when determining how much time to grant a proposal.  If your sessions are not properly set up in the PST, it can strongly impact the TAC’s decisions when assigning observing priority.  Users are encouraged to consult the guidelines for creating sessions for the VLA.  Most of these guidelines are also relevant for creating VLBA sessions.  One major difference between VLA and VLBA is that VLA sessions require you to enter a Local Sidereal Time (LST) range for each, while VLBA sessions require a Greenwich Sidereal Time (GST) range.  Also unlike the VLA, the PST does not calculate the GST range for a session; instead, proposers should calculate this using SCHED or the EVN PlanObs or some other tool. For more details on LST and GST, including a tool to convert between GST and LST, check out the US Naval Observatory's Sidereal Time page. 

Proposers are strongly encouraged to create “dummy” observing schedules with the SCHED software.  This will ensure that you properly account for considerations such as slew time, rise/set times, and calibrator scans.  Keep in mind that you may need scans on bandpass calibrators, fringe finders, polarization calibrators, and phase reference sources.  This is covered in more detail in the Exposure and Overhead chapter.

High Frequency Observations

High frequency observations are susceptible to atmospheric effects, such as air turbulence and water vapor. During the summer months, and especially during daytime, conditions appropriate for high frequency observing are limited. Other limitations may include needing night-time observing or observing during dry seasons for increased phase stability. With the stricter weather constraints on high frequency (> 12 GHz) observations, programs are in competition for less available time.

If your science requires high frequency observations, it may be best to propose for a semester when your target will be up at night.  However, this is not always possible, especially considering that not all VLBA stations will experience night and day at the same time.  Therefore, proposers are strongly encouraged to carefully consider which observing frequencies are absolutely necessary for their science during a particular semester.  If a proposal requires large amounts of daytime high frequency observations it may be extremely difficult to schedule successfully.  In such a a case, the TAC may allot a reduced number of hours or determine the project is unfeasible even if the proposal was highly ranked by the Science Review Panel (SRP).  

Low Frequency Observations

Radio frequency interference (RFI) from terrestrial sources is not as large a concern for VLBA observing as it is for VLA or single-dish observing.  While terrestrial RFI does impact low frequency observations more than high frequency observations, the VLBA antennas are located so far apart that the RFI does not (usually) correlate between baselines.  However, RFI is becoming more ubiquitous at low frequencies and similar RFI may be present at multiple antennas despite their geographic separation.  Strong RFI at a single antenna can still lead to reduced sensitivity (since the VLBA only has 10 antennas compared to the VLA’s 27).  Some satellite transmissions can lead to catastrophic loss of data across all (or nearly all) VLBA antennas, but such occurrences are relatively rare and tend to happen in the dedicated transmission bands.  However, the number of satellites is growing at high rate and users who are planning long-term projects are encouraged to take this into consideration when designing their observing plans.

Low frequency observations on the VLBA are affected by ionospheric variations more than on the VLA.  Because the VLBA antennas are so spread out, the ionosphere needs to be measured above each individual station.  While the Total Electron Content (TEC) measurements are made, using them is not 100% effective in nullifying phase issues the ionosphere creates in low frequency VLBI data.  If your science requires that the phases be as perfectly calibrated as possible at lower frequencies, you may need to use 13/4-cm dichroic system for some observations in addition to your science band(s) in order to accurately measure the effects of the ionosphere on your observations.  See VLBA Scientific Memo 23 for more details on this issue.

Low Declination Sources

Unlike the VLA, the VLBA does not have a single declination limit. Instead, each station has its own limits. See the VLBA Declination Limits section of the VLBA OSS for details on the limits for each station.

In general, NRAO recommends that a you observe a source when it will have an elevation above about 20 degrees at each participating station. This helps make calibration easier. However, depending on your science, you can try observing at lower elevations. An elevation of 10 degrees is the practical limit for most stations. Below 10 degrees, you will be more likely to encounter “spill-over” (signal reflected from or originating from the ground) at lower frequencies (<4 GHz), and you will need to deal with a large amount of atmosphere at higher frequencies (>10 GHz). Also, keep in mind that RFI tends to increase as the telescopes point to lower elevations because they will be more likely to look at man-made objects (cell phone towers, cars on nearby roads, etc.).

If you are observing at very low frequencies (< 1 GHz) or very high frequencies (>40 GHz), the practical elevation limit for each station is closer to about 20 degrees.

Your science goals, your observing frequency, and the nature of your target will determine how low you are willing to observe a source. For example, if you are planning to observe a very bright source at X-band (4cm, ~8 GHz), it may be possible to observe it all the way down to the horizon limits of the stations. However, if you are doing a phase-referenced observation at Q-band (7mm, ~42 GHz), it would be in your best interest to only observe the sources when they have elevations of 20 degrees or higher.

If you have a low-declination source that you would like to observe with the VLBA, it is a very good idea to make sure it will be visible for long enough and at enough antennas to achieve your goals. A good tool to use for this is the JIVE planobs tool.  This tool allows you to plot the elevation of a source as a function of time at each antenna site for many VLBI arrays (VLBA, EVN, LBA, etc.).

To use the JIVE planobs tool to plot the elevation of a source at each of the VLBA stations, follow these steps:

1. Select "Manual Mode"
2. Select your observing band from the drop-down menu
3. Select "Find epoch for given source"
4. Enter the source coordinates
5. In the “Select default VLBI Network(s)” drop-down menu, select “VBLA: Very Long Baseline Array”, and add a check in any additional stations you will use if you plan on proposing for Y1, HSA, or Global VLBI
6. Optionally, you can also specify the percent of time on target, the data rate, and the setup details (the tool is also useful for estimating sensitivity and resolution, if you want to calculate those as well; see the Exposure and Overhead chapter)
7. Click "Compute Observation" (near the top left of the page)
8. After the program is done computing, click on the "Elevations" tab to see the plot of elevation vs. time and the plot of when a source is visible from each station.

Note that the JIVE planobs tool uses 10 degrees elevation as a hard cutoff for when a source is visible from each station.

Another method to estimate the amount of time a source spends above a certain elevation at each station is to build a SCHED .key file for the observation. Then, you can run SCHED and look in the .sum file or use the SCHED plotting tools, especially the “uptime” plot, to get information on the source elevation at each station for each scan. SCHED has information about the horizon limits of each station (in the catalogs/stations_RDBE.dat file), so it does a good job of estimating when a source will be visible at each station.

In addition to the effects on sensitivity and the ease of calibration, it is good to keep in mind that the uv-coverage will be worse when observing low-declination sources. If your science goals include high dynamic range imaging of complex structures, uv-coverage may be a bigger concern than sensitivity. The JIVE planobs tool and the SCHED plotting tools can both generate plots of the estimated uv-coverage for an observation.

Other Special Considerations

There are several other considerations that may impact you observation.

Coordinated Observations

In the event that you need simultaneous observations with the VLBA and another instrument (e.g., you need to know the ratio of flux density from compact components to total flux density), you will need to consider the scheduling constraints of both telescopes.  Typically, it is recommended that you schedule both observations as “fixed date” observations to avoid any uncertainties in observation times that may be introduced by the dynamic scheduler.

Moving Objects

While the VLBA is capable of observing objects moving with respect to the celestial coordinate system (e.g., asteroids, planets, etc.), it is not a default mode.  Special scheduling techniques are required.  If your science target is such an object, you are encouraged to contact the VLBA staff to consult on an observing strategy.  Also, be aware that your object may be in the “near field” and require special techniques for calibration and imaging.

Sun Avoidance

Observations of objects close to the Sun can have extra difficulties in calibration due to increased phase fluctuations and elevated system temperatures.  Unless your science specifically requires pointing at an object near the sun (e.g., testing the effects of General Relativity on the astrometric position of a background quasar), it is strongly recommended that you avoid observing near the Sun.  The lower the observing frequency, the further from the Sun you should point.  The following table contains the recommended minimum angular distance from the Sun to avoid amplitude reduction on the longest VLBA baselines for select observing frequencies:

NOTE: The VLBA scheduling software, SCHED, will warn you if your observations are closer to the Sun than recommended.

Daily UT1-UTC Observations

The VLBA makes daily observations to measure the Earth orientation parameter UT1-UTC for the US Naval Observatory. These observations are centered between 1730 and 1930 UT, and last for no longer than two hours. Two antennas are used for these UT1-UTC observations (usually MK and HN). The necessary antennas may be temporarily removed from an ongoing science observation to complete the UT1-UTC scans. The VLBA Schedulers always attempt to minimize the impact of UT1-UTC scans on science observations.