Angle Scanning Versus Frequency Sampling

This chapter is a digression from the several previous retrieval chapters. It deals with hardware design choices for an MTP. During the "early days" of MTP work, which consisted of more ground-based observing than airborne observing, it became evident that since all retrieval procedures need a good quality calibration the MTP design should rely upon as much angle scanning as possible. As the JPL emphasis shifted to airborne MTPs it slowly became evident that angle scanning was no more prefereable than frequency sampling from the standpoint of post-observation calibration. That is this chapter's "story."

In the previous chapters it was assumed that different altitudes were "sampled" by observing at a selection of elevation angles with an MTP having a small number of frequencies, such as 2 or 3 frequencies.
Frequency sampling is an alternative strategy for achieving information from a wide range of altitudes.

During the "early days" of ground-based microwave remote sensing of atmospheric temperature profiles (the 1970s and 1980s) two groups were active, a JPL group (led by Bruce Gary) and a NOAA Wave Propogation Labortory group (led by Ed Westwater). The JPL group favored angle scanning and the NOAA group favored frequency sampling. An argument can be made for each philosophy. Angle scanning allows for the observation of a wide range of applicable altitudes with a minimum of hardware, but it assumes that temperature isopleths are flat and horizontal;
frequency sampling makes no assumptions about temperature isopleths and it requires no moving parts. But frequency sampling requires a lot more calibration than can be reasonably performed (at least that's my position).

With angle scanning it is possible to inter-calibrate frequency channels simply by comparing observables at elevation angles that have the same applicable altitude. Provided isopleths are flat and horizontal this inter-channel calibration should allow for the adjustment of one or the other channel to achieve internal consistency. For ground-based systems an average comparison during several days of observations could be used to provide additional assurance that the isopleths, on average, were horizontal. For airborne MTPs the solution is simpler: merely compare channels while flying at different azimuths, or make the comparisons when outside air temperature is not changing. Both methods assure that the airborne MTP inter-channel calibration is unaffected by non-horizontal isopleths.

The dynamic range of applicable altitudes that can be achieved is limited by the antenna's beamwidth. A narrow beam (such as 5 degrees) allows for a greater dynamic range than a wide beam (such as 10 degrees). This is due to the fact that the lowest applicable altitude besides the horizon view is about a beamwidth above the horizon times the sine of that angle times the applicable range.

Inter-channel calibration of an angle-scanning MTP usually requires interpolation between observables. Note that sine all channels share the same set of viewing angles (since they use the same horn antenna). It makes sense to choose a set of viewng angles that provide uniformly spaced "logarithm of applicable altitudes" (to minimize observable redundancy), and it is unlikely that two frequencies will have the same applicable height for any pair of viewing angles. Therefore, inter-channel calibration usually requires an interpolation between observables that straddle the applicable altitude of the other channel. As a practical metter this is not a problem.

For an airborne MTP an alternative procedure for achieving calibration of each channel has become common.
The airborne MTP requires a fairing and a window with substantial strength, and high density polyethylene is normally used (because of it's low dielectric properties). For each channel absorptions and reflections are likely to be different for different viewing angles. This is partly due to the ray path through the window being slightly different for each viewing angle since the window has to conform to the shape of the fairing and the fairing shape is usually not a section of a circle for the entire range of viewing angles. Another effect could be more important, and that's differences in dirtiness of the window material between locations. It is common practice to use a stiff bristle brush to clean the outside of the fairing before each flight in order to minimize changes in absorption or reflection at differenet locations on the window. (The window is grooved on the outside and inside surfaces to provide a "lens coating" effect to minimize reflections, and these grooves are better sites for dirt accumulation than a smooth surface.) There is a lingering concern that the directional coupler may not prevent LO singal from reaching the horn where it would then reach the window and be partially reflected back into the horn to create a standing wave radiometer output level offset. This offset would vary with viewing angle since the distance between the horn and the window is not exactly the same for each angle. If this leaking LO standing wave offset is present it could cause an additional component of required angle dependent calibration.

For these various reasons it is unwise to attempt inter-channel calibration using an airborne MTP. Instead, each channel ant each viewing angle is calibrated by comparing TB observables with predicted ones based on RAOBs. Whenever the MTP flies close to a RAOB site this "window correction table" (WCT) can be computed. Typically 5 to 10 such WCTs are required to achieve sufficient accuracy. A median at each element of the WCT ensemble is better than an average, since temporal and spatial interpolations of RAOB T(z) profiles can sometimes be innacurate.

As the previous two paragraphs should have made clear, calibration of the airborne MTP is no simpler using angle scanning than frequency sampling. Thus, it should be left to the hardware engineer whether to use one or the other, or how to partition reliance upon the two approaches. The engineer's goal should be to maximize the range of applicable altitude sampling in the shortest time for completing an observing cycle. So far the airborne MTPs that JPL has flown have included 2 or 3 frequencies that are selected for each of 10 elevation angles.

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