A special meeting for those interested in Water Vapor Radiometry (WVR) was held on Monday, 7 June 1999 to review progress in the field and to identify missing elements for an overall implementation strategy for ALMA, at the Imaging 99 conference in Tucson. To the best of my recollection, the following people were present: Mel Wright, Bob Sault, Peter Napier, Bryan Butler, Darrel Emerson, Graham Moorey, John Payne, Jeff Mangum, Mark Holdaway, Simon Radford, Al Wootten, Min Yun, Johannes Staguhn, John Richer, Lee Mundy, Dave Woody, Martina Wiedner, Jack Welch, Douglas Bock, Bob Brown, Stephane Guilloteau, Paul Ho, Masato Ishiguro, Larry D'Addario, Ryohei Kawabe, Morita, and Matsuo.
Woody reviewed the status of WVR at OVRO. The current system measures and corrects path length variation to provide residuals down to about 200 microns in virtually all weather excluding rain or snow. The spectral line time resolution used is 2s, and 10s is used for continuum data. As an example, WVR-corrected interferometry towards 0309+411, with 3C84 used as a calibrator, was reported over a time period covering the approach of a weather front. Before front passage, excess path was about 500 microns, increasing to many mm after front passage. The uncorrected image showed a featureless blob containing 13 mJy, while after correction a fairly crisp image contained the expected 34 mJy. The system will be available for application to all data in the next year, though some improvement in algorithms will be needed--for instance, a discriminator to judge whether correction provides improvement on a particular baseline is not present. In a separate conversation, Guilloteau mentioned that the IRAM software employs a discriminator, which requires improvement on a calibrator observations before application of the correction to the source data. At IRAM, data is collected in both corrected and uncorrected states while at OVRO the two datasets exist side-by-side for later correction. There was some discussion of 183 GHz, a major worry being the much broader bandwidth necessary for sampling of the line profile. Plambeck mentioned that one could still employ 22 GHz on ALMA antennas by cutting a hole in the front panel and using a beam offset 10' of so near the center blocked region of the surface.
Staguhn reported on the BIMA/U. Md. backend effort. In the newly constructed correlator, 16 channels cover the 22 GHz, folded into 7 GHz of bandwidth. The goal is to match the observed lineshape to expected changes as a function of altitude to measure the height of the turbulent layer. A goal is to achieve 35 micron residuals. Eventually, cooled amplifiers will be used to reach this goal.
Plambeck suggested building a correlation radiometer, combining sky and reference load signal to get the difference in power between the sky and the load. Costs and manpower are a worry but a really good instrument should be built to discover the limits of the technique. Such a device might be employed at BIMA. Guilloteau mentioned a Zurich source for 22 GHz amplifiers.
Dave Woody mentioned measurements at OVRO aimed at determining the structure of the water vapor--correlation of the WVR observations on differing baselines gives the velocity of the phase screen and pointing beams to the same point at 2 km height and correlating can give the height of the layer, though some attempts were not clear on the structure.
Butler mentioned the VLA system, essentially a clone of the OVRO system with channels near hinge points and 5/6 s dumptime, a cooled system with stability about 10x worse than the OVRO system.
Staguhn mentioned that the present spectrometer bandwidth is too small for the 183 GHz line, which would need more like 20 GHz. The effects of liquid water were discussed, noting that it is more prevalent at lower sites than some of the higher sites considered for future interferometers.
Bob Sault discussed the AT system, which will be used for the AT's 3.5mm system. It is similar to the OVRO system, with several channels. Modeling will make the system robust to errors. Results compared to radiosonde and model atmospheres suggest errors in the latter. A single layer will be modeled. There is a two year timescale to implementation of this system.
Guilloteau discussed the IRAM system. For the last four years, continuum radiometery has been employed at 220 GHz and 3mm simultaneously, which corrects to 50 micron path length accuracy in the absence of liquid water. Problems include the loss of phase when moving to calibrator--the correlation can be improved however. On hot days, the model overestimates the correction by perhaps 30%, a problem believed to originate in convection disturbing atmospheric layers. A line radiometer is planned for the 22 GHz line with the aim of recovering usable observing time in summer as much as possible. The 183 GHz line is usable at PdBI in the winter. The planned system uses off the shelf components to achieve good stability in a three channel system. Although the first system will be at ambient temperature, it will probably be cooled eventually. No interference has been noted at the site yet. The line of sight will be through the secondary with some underillumination. Two online datasets will be available, one with corrections and one without. The continuum system is used the 70% of the time during which no liquid water is present, with very good results. Cirrus has no influence on the system.
Ishiguro described a 22 GHz radiometer using a single channel to compare with round trip HALCA measurements. This year an 8 channel filter with two at the hinge point will be finished for the 10m antenna near Usuda(?).
Morita described a 220 GHz differential radiometer system at two of the NMA antennas which can be used with fast switching to improve phase measurements. Implementation of a larger system is still under consideration.
Wiedner described her measurements at 183 GHz using a WVR system on the CSO-JCMT interferometer. At 1.5 mm of water, the 183 GHz line core becomes self-absorbed in the atmosphere. The wings may show less optical depth, however. A total of three channels including +/- 7.5 GHz off center are used. In practice, use of one channel only seems to do better than combining channels. on this 164m baseline. The interferometer is infrequently operated, and has been plagued by poor weather. The operating system of one of the antennas limits sampling to 10s intervals. At 350 GHz several experiments were described. On a bad day, data were improved to 113 microns excess path length. The scale factor is determined by analyzing the brightness temperature of the sky, and the 225 GHz optical depth from the NRAO tipper. At best, an improvement to 38 microns was noted (over 5 minutes of data). The importance of good calibration was stressed, and needing to know accurately where the filters fall on the line. A cooled system should be capable of better measurements. The sensitivity in the 183 GHz line wings is such that they should provide a good probe of the water vapor column up to total columns on the order of 10mm, according to Butler; hence the VLA site should produce useful data in this band.
Radford reported on an early look at 183 GHz data from Chajnantor, from Delgado and Otarola.
Several persons noted that contamination by ground pickup contaminated the total power effect. For the strong 183 GHz line, this effect may be less important. More modeling is needed. FTS data could be included in a model to get a better handle on the altitude and amount of water vapor. Richer described a possible use of 2-axis drive on the 183 GHz mirrors to provide some 2d mapping capability.
It was mentioned that Chris Wilson is building a third 183 GHz WVR for use on the SMA, for inclusion with the JCMT and CSO in interferometry. By good fortune, she was at Hale Pohaku where I saw her a few days afterward. She informed me that the system was coming along, and quoted a cost of $67K Canadian for the radiometer, a copy of the Wiedner device. She also mentioned efforts be a Canadian researcher to develop a similar system using near-IR water lines for a similar end. She will determine this person's name, as he will probably be present at the URSI meeting.
It was decided that much more data was needed at 183 GHz before implementation on the ALMA. However, a WVR should definitely be included in the WBS. IRAM will look into further implementation of a system at PdBI.