Fast Switching

Fast switching phase calibration is a method of reducing phase variations arising in the troposphere at high frequencies. Fast switching phase calibration allows for diffraction limited imaging of faint sources on long baselines at high frequencies.

1. The basics
2. How to determine the best cycle time
3. How to do it in OBSERVE
4. The Atmospheric Phase Interferometer
5. Spectral line issues

The Basics

Due to variations in the water vapor content of the troposphere, there will be an additional source of phase noise when observing at high frequencies. Tropospheric phase fluctuations can be characterised by a "root phase structure function", corresponding to the rms phase variations as a function of baseline length:

where is in cm and b_eff is in km. The value of K varies according to the weather, with K = 20 under good weather conditions at the VLA site (typical fall and winter nights), K = 30 under average weather conditions at the VLA (fall and winter days, summer nights), and K > 60 under poor weather conditions (summer days). The exponent n = 0.7 for baselines shorter than about 1 km, and n = 0.33 for baselines longer than 1 km. The characteristic timescale for these variations is simply the baseline length divided by the wind speed in the turbulent layer, typically about 10 m/s.

There are currently three ways to deal with those phase fluctuations, and on this page we will discuss method number 3:

1. If your source is stronger than 100 mJy, you can apply self-calibration.
2. If your target source contains a maser source (e.g. SiO), you may want to exploit the technique of self-calibration by monitoring the atmospheric phase fluctuations using the maser in one IF and applying the solution to data taken in the other IF.
3. If your source is weak and you observe in A or B array, you can use the method of fast switching. This is just calibration of your phases using an external calibrator, however the cycle time is short enough to 'stop' the phase fluctuations. Normally, there is a 20 seconds overhead each time the VLA slews to a new source. The fast switching mode at the VLA has been introduced to avoid those extra 20 seconds, and thus allows a much shorter cycle time.

It has been shown that fast switching phase calibration at the VLA will stop tropospheric phase fluctuations at an effective baseline:

where t_cyc is the calibration cycle time and v_a is the velocity of the winds aloft (typically 10 m/s at the VLA site). Hence, fast switching will be effective for cycle times shorter than the baseline crossing time of the troposphere b/v_a. Currently fast switching is considered useful in A and B arrays, but may also be useful at more compact configurations if your sources are at low elevation.

How to determine a suitable cycle-time

In order to determine your cycle time, you need to decide how large phase fluctuations are acceptable in your observations - note that a 90 degrees phase RMS will easily wipe out a source. The table below shows a typical RMS phase in degrees for the VLA at 7 and 13 mm after calibration. These values are derived with the Site Test Interferometer. Measurements of phase were made by observing a geostationary satellite beacon (at 50 deg elevation) at 11.3 GHz with two 1.8 m satellite dishes separated by 300 m. The results were then scaled to 7 mm and 1.3 cm. (To estimate the RMS phase noise at 1.3 cm, simply double the cycle time between phase calibrator and source from Q band.)

RMS phase (in degrees) for the VLA at 7mm and 1.3cm after calibration under median conditions

7 mm	tcycle=2 min	tcycle=5 min	tcycle=10 min	1.3 cm	tcycle=4 min	tcycle=10 min	tcycle=20 min
Month	Day .. Night	Day .. Night	Day .. Night		Day .. Night	Day .. Night	Day .. Night
Jan	25.5 .. 18.8	47.1 .. 34.6	74.7 .. 54.9		25.5 .. 18.8	47.1 .. 34.6	74.7 .. 54.9
Feb	25.0 .. 15.1	46.1 .. 27.8	73.2 .. 44.2		25.0 .. 15.1	46.1 .. 27.8	73.2 .. 44.2
Mar	30.2 .. 20.9	55.7 .. 38.4	88.4 .. 61.0		30.2 .. 20.9	55.7 .. 38.4	88.4 .. 61.0
Apr	45.4 .. 24.5	83.5 .. 45.1	132.6 .. 71.6		45.4 .. 24.5	83.5 .. 45.1	132.6 .. 71.6
May	38.1 .. 21.9	70.1 .. 40.3	111.3 .. 64.0		38.1 .. 21.9	70.1 .. 40.3	111.3 .. 64.0
Jun	39.1 .. 20.3	72.0 .. 37.5	114.3 .. 59.5		39.1 .. 20.3	72.0 .. 37.5	114.3 .. 59.5
Jul	41.7 .. 27.6	76.8 .. 59.0	122.0 .. 80.8		39.1 .. 20.3	72.0 .. 37.5	114.3 .. 59.5
Aug	51.1 .. 29.7	94.1 .. 54.7	149.4 .. 86.9		51.1 .. 29.7	94.1 .. 54.7	149.4 .. 86.9
Sep	62.0 .. 34.9	114.3 .. 64.3	181.4 .. 102.1		62.0 .. 34.9	114.3 .. 64.3	181.4 .. 102.1
Oct	51.6 .. 30.8	95.1 .. 56.7	150.9 .. 89.9		51.6 .. 30.8	95.1 .. 56.7	150.9 .. 89.9
Nov	32.3 .. 29.7	59.5 .. 54.7	94.5 .. 86.9		32.3 .. 29.7	59.5 .. 54.7	94.5 .. 86.9
Dec	22.4 .. 15.6	41.3 .. 28.8	65.5 .. 45.7		22.4 .. 15.6	41.3 .. 28.8	65.5 .. 45.7

Thus decreasing your fast-switching cycle time from 5 minutes to 2 minutes under median conditions will improve the final, measured rms phase noise in your map by about a factor of 2. The minimum allowed time per source is 20 seconds, implying a minimum total cycle time of 40 seconds.

Winter observations are clearly best for high frequencies. Try to schedule observations during the night when phase stability is likely to be better. If you have to observe during the day, then choose a short cycle time to keep the phase rms below 30 degrees (or at worse 40 degrees). If you are doing a detection experiment, try for the best conditions possible.

Note that the amount of effective on-source time depends on the distance, and orientation, of the slew between the source and calibrator. Ensure that you get enough time on source (see below). As an example, in a recent test observation of a source and calibrator separated by 1.8 degrees, a minimum cycle (20 sec for source/20 sec for calibrator) yielded 7 seconds of on-source time for each source. It has been found that a cycle time of about 100 seconds (70 sec on source, 30 sec on calibrator) is adequate under average observing conditions at the VLA, for source-calibrator separations less than 4 degrees or so. In this case the expected residual tropospheric phase fluctuations under good weather conditions at the VLA will have an RMS of about 18 deg at 7mm after calibration for all baselines longer than about 500m, and less than this on shorter baselines.

How to implement fast switching in OBSERVE

Fast switching phase calibration is supported by the on-line system at the VLA, and the mode has been incorporated into the latest OBSERVE program. Following is an example of the implementation of fast switching in an observe file: (for Jobserve details see the Jobserve fast switching page).

1013+248      10 10 00 10 13 53.4344 +24 49 16.359C    XX IRC   0000   1.05   
//DS            10                                                            
1013+248      10 50 00 10 13 53.4344 +24 49 16.359C    QQ   C   0000T        
//DS             1                                                            
//OF   NOD SKY         10 14 47.0654 +23 01 16.568              30  70

In this example the calibrator source is 1013+248 and the target source is 1014+230. The first source card entails a pointing scan at 8 GHz on the calibrator. The second source card indicates the stop time for the fast switching series, and gives the calibrator position, the observing band (Q), the calibrator code (C), the bandwidth code (0000), and turns on the reference pointing (T). Fast switching between the source and the calibrator is delineated in the last card, which is an `off-set' card (//OF) in `NOD SKY' mode, giving the position of the target source, and the amount of time to spend on the calibrator and source (30sec, 70sec), respectively (move time inclusive). For instance, in the above example the total observing time at 43 GHz is 40 minutes, during which time a series of 100 sec observations are made of the source and calibrator, with 30 seconds for the calibrator and 70 seconds for the target source.

The minimum allowed time per source is 20 seconds, implying a minimum total cycle time of 40 seconds. The current OBSERVE program cannot check calibrator-source slew times in the fast switching mode. It is therefore strongly recommended that the observer check the slew time between the source and calibrator using a 'dummy' file in the OBSERVE program with normal source and calibrator cards at a few elevations, to ensure that enough time is being allowed for the source and calibrators in the fast switching file. Note that the move time reported by OBSERVE includes an extra 20s of overhead -- i.e., if OBSERVE claims that the move time is 25 seconds, the actual time spent moving is only 5 seconds. The whole point of fast switching is to avoid this 20 second overhead. However, also note that OBSERVE does not claim to be accurate at the 1 second level...

Due to the short observing cycles, it is recommended to use a 3.3 second averaging time in this mode (subject to the restrictions on minimum integration times for spectral line observations). Off-set pointing is allowed, as is any correlator mode consistent with the integration time and the number of antennas.

For fast switching phase calibration a minimum calibrator flux density of 0.3 Jy is recommended. It is also recommended that every hour or so an observation be made of a stronger calibrator (1 Jy or greater), to determine pointing corrections and to track amplitude variations.

Atmospheric Phase Interferometer

The NRAO has recently installed a Atmospheric Phase Interferometer (API) at the VLA site, and software has been installed for real time monitoring of the phase stability. The API measures continuously the tropospheric contribution to the interferometric phase using an interferometer comprised of two 1.5 m dishes separated by 300 m observing an 11.3 GHz beacon from a geostationary satellite. The primary purpose of the API is to serve as a guide to high frequency observers at the VLA. In particular, the API data are meant to be used to estimate the required calibration cycle times when using fast switching phase calibration, and in the worst case, to indicate to the observer that high frequency observing may not be possible given the weather conditions. Detailed instructions as to the use of the API data can be found on the Atmospheric Phase Interferometer home page.

Spectral Line Observations: Fast Switching & Doppler Tracking

Doppler tracking is only done at the beginning of a scan, as part of the 20-second overhead you're avoiding by using fast switching. So, for non-VLBI spectral line experiments, you should check that the change in frequency from scan to scan is much less than a channel -- don't just blithely set up a one-hour-long fast switching scan! To check on this you can either run dopset, or give a series of short scans in OBSERVE and check to see how the frequency corresponding to a constant velocity changes with time (you may find the Frequency Report helpful in this regard).

Also, be aware that Doppler tracking is performed for the "primary" source (the one given on the first (SU) card) only; the LO settings are then left fixed for the "secondary" source (the one given on the //OF card). Generally this means you will want the source you are actually interested in to be the primary, and the calibrator to be the secondary. As a bonus though, this does mean that your phase calibrator & source are automatically observed with identical LO settings.