The S-Pol Ka-band calibration for RICO is uncertain, but it is fairly certain that the calibration varies with time. Noise power has been examined previously to establish variations in receiver gain over time (spot checks several times per operation day), but no two-way (transmit/receive) calibration information is readily available. A decision was therefore made to examine power return from a nearby free-standing radio tower, and determine if such signal would serve as a calibration proxy.
Results from the tower proxy are presented. The tower seems to serve as some sort of indicator of Ka calibration stability, but results indicate that the Ka tower return varies erractically over time periods as short as three minutes. Excursions in the tower return can be as small as +/- 1 dB for multi-hour periods, but can be greater at times (on the order of +/- 2 to 3 dB), and often show sudden, large negative excurions that may last for a few data points, or only a single point. The excursions in the horizontal power do not often match excursions in vertical power, and expected changes in Ka Zdr during periods of excursions are not always evident (ZDR_VH_K rarely shows aggregate changes of 3 dB during periods of noisy tower cals; see data of 11-Jan-05, near 13:20 for a case of changing ZDR_VH_K). The issue to resolve is whether the tower returns serve as a good indicatior of moment-to-moment Ka calibration.
At a minimum, it seems that a steady run of maximum returned tower power can be used to track Ka calibration stability on a day-to-day, or perhaps an hour-to-hour basis. Additionally, if the S-band reflectivity is considered stable throughout the project, and we can determine Ka/S-band reflectivity offset for a few stable periods of tower return, we can then directly relate the changing tower return to a modified Ka/S-band reflectivity offset for large periods of the project, without doing daily cloud echo comparisions. [Note: proper consideration must be given to discontinuities in the system, such as the magnetron change out of 14-Dec, and the code change of 26-Dec that attempt to re-adjust the Ka HH/VV gate match.]
There was a single, highly visible tower located on Barbuda at a distance of about 2.40 km, azimuth 14.4° from the radar. Since there is some blockage by shrubs at the lowest routine scan elevation, it was decided to use either the 1.3° or 1.5° elevation scan (these scans are mixed in the plots, with no indication of when which angle was used). The tower was routinely visible at these tilts when the S-PolKa PPI sector was centered to the north, a condition that existed for about 2/3 of the RICO project.
There are obvious issues associated with ensuring that estimates of tower return were consistent with time. Since S-Pol does not do beam "indexing", and beam spacing was on the order of 0.8°, it might seem that a high degree of variability would be introduced in the tower return estimate. This is not the case: the Ka system is extremely insensitive to ground clutter, with Ka clutter about 40 dB below that seen at S-band; the tower signal did not saturate the Ka receiver (actually, any single pulse may have saturated, but we do not have time series data available to analyze this!), and antenna sidelobes were not a concern; additionally, the Ka receiver performs pulse averaging in linear (as opposed to logarithmic) space. Under these circumstances, as long as all the energy returned from the tower is properly and completely summed, results should be consistent. [A further note is available.]
Meteorological phenomena will still introduce inconsistencies, as will changes in refractive index, alterations to the tower, or, perhaps, small inconsistencies in elevation angle. An important consideration is whether the Ka tx/rx dish is wet.
Summing of energy for the six or seven radar beams surrounding the tower was done for every available 1.3°/1.5° scan for all periods of Ka operations. Special care was taken to center the set of six beams so the energy of the end beams was at least 20 dB below the peak energy, and preferably 30 dB below. The 30dB requirement was met most of the time; the 20dB condition seemed sufficient to remove any appreciable variation in the estimate, and points were not excluded based on the less restrictive condition. Typically, tower scans were available every three minutes.
Results are presented in graphical form (tables will be made available at a later time).
Please especially note that plots are provided separately for the single gate at 2.25 km, and for the gate at 2.40 km. Be sure and read the legends on the plots!
Parameters included in the plots are:
Plots are presented for the following combination of variables:
Note that S-dBZ is included as a "control", and serves to indicate the general stability we should expect from the tower power/reflectivity estimators. S-dBZ is not equivalent to the Ka parameters, however, in that the S-dBZ is highly affected by sidelobes, and there are pulses included in the integrations that have likely saturated the S-band receiver (receiver saturation would tend to stabilize, or limit, an estimate).
As usual, all results are preliminary.
Two possibilities for large negative excursions in tower return power:
There is relatively little information in the literature concerning the use of hard targets, and particularly random towers, for monitoring of radar calibration. Very little of the available information suggests that a tower calibration proxy has more than a few dB of stability. Rinehart (1978) suggests that hard target observations might have a day-to-day repeatability of around 1 dB (when only the maximum signal returns on a given day are considered); Fabry (2004) mentions concerns with phase cancelling of power, and states that the most significant use of hard targets is in monitoring phase for refractive index studies, as opposed to echo strength; Pellarin et. al. (1999) provide an 80% confidence estimate of about 2.5 to 3.0 dB for the inter-monthly repeatability of reflectivities from non-vegetated mountain sides. No literature was found to support the idea that consistent, short term scans of a nearby tower would have the degree of repeatability seen during the "good" portions of Ka operations. On the contrary, there are issues with refractive index changes, scan repeatability (i.e., exact elevation and scan rate), and signal statistics that would argue for fairly noisy short-term observations.
It should be noted that previous studies were for longer wavelength radars, and not for a Ka system that is extremely insensitive to clutter sidelobe effects. Furthermore, the tower used for the RICO study is perhaps the closest thing to an ideal tower: free-standing (no guy-wires and narrow in azimuthal extent), in the radar far-field, but still relatively close to the radar, visible at a reasonable elevation angle, with the radar beam path to the tower covered by vegetation (likely limiting potential beam multi-path problems).
There's lots of information in the tower data. The best use of the information would be in selecting periods of stable Ka operation (either horizontal or vertical polarization). There are often obvious ramp-up periods shortly after system turn-on when Ka power (particularly HH power) has not reached stable operating conditions; those periods should be avoided for most purposes, or Z_HH_K should be compensated appropriately.
Pending further analyses, it seems safe to not be alarmed by periods of large negative excurions in Ka tower power. If in doubt about a specific period, review the parameter ZDR_VH_K for general stability (bad periods should show sudden, large changes in the gross field of ZDR_VH_K). [Note: ZDR_VH_K is a statistically noisy field, due partly to the unfortunate fact that the range for P_VV_K is offset by about one-half gate from the range of P_HH_K; this situation was only marginally improved with the software change of 26-Dec.]
Maximum tower power return can likely be used to adjust Ka relfectivity on a day-to-day basis. Examples of such adjustments will be developed, although there is no current intention to apply daily adjustments to the distributed data set.
Further analysis is required/desired. Particularly, it is desired to determine if variations in background noise power correlate with changes in returned power from the tower. Secondarily, any relation to "mirage echoes" should be determined. [Note: both of these issues have been addressed in a preliminary fashion, and insight was incorporated into the "Results" section, above; more formalized work-up is pending.]
In a practical way, it would be good to select a stable period of Ka operation, then relate the Ka reflectivities to the S-band reflectivity through cloud echo analyses. One could then determine an emperical "absolute reflectivity" of the tower. A correction could then be applied to Ka data that would also compensate for any variations in Ka magnetron transmit power.
For comparitive purposes, work can be done with data from other S-Pol projects.
This study is continuing
Fabry, F., 2004: Meteorological value of ground target measurements by radar. J. Atmos. Oceanic Technol., 21, 560-573.
Pellarin, T., G. Delrieu, and J.D. Creutin, 1999: Variability of mountain returns during dry-weather conditions: Applications to radar calibration control. Preprints, 29th Conf. on Radar Meteorology, Montreal, QC, Canada, Amer. Meteor. Soc., 772-775.
Rinehart, R.E., 1978: On the use of ground return targets for radar reflectivity factor calibration checks. J. Appl. Meteor.,17, 1342-1350.