Chilbolton 94 GHz Galileo radar data

Robin Hogan
11 April 2002

Introduction

This dataset consists of effective radar reflectivity factor (Z) measured by the 94 GHz Galileo cloud radar situated at Chilbolton, England (51.1445°N, 1.4370°W). The instrument is a bistatic system that operates continuously round the clock in a vertically-pointing configuration. It was developed for the European Space Agency by Officine Galileo, the Rutherford Appleton Laboratory and the University of Reading.

This document is a revision of the original documentation of 26 February 2001, and describes the recalibrated data that was released to BADC in spring 2002. If you have used the reflectivity values of the original Galileo data for anything quantitative then you should definitely download the newer data, as there was a steady and significant loss of sensitivity during the 18 month duration of this dataset, that we were previously unaware of.

The characteristics of the radar are as follows:

_________________________________________________

Frequency:                              94.00 GHz
Antenna diameter:                       0.6 m
Peak power:                             1.6 kW
Pulse width:                            0.5 µs
Pulse repetition frequency (PRF):       6250 Hz
System noise figure:                    10 dB
Beamwidth:                              0.5°
Range resolution:                       60 m
_________________________________________________

History of operations

The radar was initially operated in summer 1996 for a few months, but a problem with sensitivity was found. This was rectified when the radar was Dopplerised, and from September 1999 until February 1999 the instrument was operated from the `Receive Cabin' at Chilbolton. It was then mounted on the side of the main 25 m antenna at Chilbolton to permit scanning with the 3-GHz radar, although did not operate round-the-clock until the end of April 1999. On 17 October 2000 it was removed from the side of the main dish to solve the problem of gaps in the data whenever the 25 m antenna was scanning through precipitation. During this time the output power from the instrument dropped steadily; the calibration section below discusses the implications of this for sensitivity and calibration. Doppler velocities measured during this time were unreliable and so have not been included in the dataset. The data described here are from 1 May 1999 until 17 October 2000.

From January 2001 to March 2002 the radar was operated from the ground at Chilbolton and a new data acquisition system installed which permitted measurement of the mean Doppler velocity and Doppler spectral width. Operations from the ground meant that the data stream was not interrupted every time the 3-GHz radar was scanning, but unfortunately it also meant that direct cross-calibration with the 3-GHz radar was not possible. Nonetheless, it is hoped that these data can be released on BADC in the future.

Processing that has been applied to the data

The following stages of processing have been applied to the raw data (in this order):

True linear averaging of the original 1-s data to 30 s.
Rejection of rays recorded when the radar was pointing more than 5° from zenith (i.e. when the 3-GHz CAMRa radar on to which the 94 GHz radar was attached was scanning through precipitation).
Rejection of `glitched' rays when the data acquisition system got out of sync with the transmitter.
Calculation of the noise level by looking at the assumed cloud-free gates between 13 and 14.5 km. This is stored in the nez variable.
Subtraction of the noise component from the measured echo power, and thresholding of the resulting signal.
Clean-up of speckle noise by rejecting cloudy pixels or pairs of cloudy pixels with cloud-free pixels above and below them.
Range correction of the data to account for the inverse square law and any range offset.
Application of calibration figure (to both Zh and nez).
Spring 2002: Recalibration based on numerous calibration events with the 3-GHz radar.

The result of this processing is that the minimum-detectable signal at a range of 1 km was around -50 dBZ in May 1999, increasing to around -38 dBZ in October 2000. The sensitivity decreases with range according to the inverse square law; therefore the minimum-detectable signal at 10 km is 100 times (20 dB) higher than at 1 km. The `chil2nc' program was used to process the data and convert it to NetCDF; source code for the latest version may be downloaded from http://www.met.rdg.ac.uk/radar/software.html.

Data format

The data is provided as daily NetCDF files containing the following variables:

frequency: The frequency of the radar in GHz (94.00).
latitude: The latitude of Chilbolton in degrees north (51.1445).
longitude: The longitude of Chilbolton in degrees east (-1.4370).
altitude: The altitude of the radar antenna above mean sea level in metres (90 m when the radar was on the side of the main CAMRa dish).
time: A vector containing the centred time of each ray of data, in decimal hours UTC.
range: A vector containing the centred range from the antenna, in km, of each range gate. Note that to get height above mean sea level you should add the altitude to this value.
nez: A vector containing the `noise-equivalent radar reflectivity factor' at 1 km in dBZ (i.e. the level of the thermal and instrumental noise normalised to a range of 1 km), after calibration. It is calculated for each ray using the range-gates between 13 and 14.5 km which are assumed to be cloud free. It increases through the dataset due to the steady loss of instrument sensitivity, but can also vary on short timescales by as much as 2 dB when low clouds pass overhead because of the change in sky brightness temperature at 94 GHz.
Zh: An array containing effective radar reflectivity factor, in dBZ. A value of -999 dBZ indicates that no signal was detected. The comments attribute contains the following string: Calibration convention: in the absence of attenuation, a cloud at 273 K containing one million 100-micron droplets per cubic metre will have a reflectivity of 0 dBZ at all frequencies. In addition to the usual sources of attenuation at 94 GHz (liquid water clouds, water vapour, oxygen, melting snow and rain), it has been found that when the radomes covering the antennae of the radar become wet, they strongly attenuate the beam, typically by more than 10 dB. Reflectivity values measured at all altitudes during rain (and shortly afterwards) should therefore not be trusted.

The following global attributes are present:

system: A string identifying the instrument that took the data, in this case `Galileo'.
scantype: A string identifying the type of scan that the radar was performing, in this case `Fixed'.
day: The day of the month as a short (two-byte) integer.
month: The month as a short integer.
year: The year as a short integer.
file: The `tape' number of the original Chilbolton-format radar data file from which the data in this NetCDF file was obtained, as a short integer. Where a day of data spans more than one such file, the tape number of the first is given.
raster: The number of the first raster of the first Chilbolton-format data file that was used in producing this data, as a short integer. Chilbolton-format radar files are divided into rasters which, in the case of cloud radar data, are usually one hour long.
options: A string containing the command-line options that were used with the chil2nc program to produce the data. This is useful to see what processing has been done, such as the calibration figure that has been applied in dB (the -Zcal switch).
software_version: The version of chil2nc that was used to produce the data. This field was not implemented in versions of chil2nc earlier than around 0.7.6.

The following global attributes were added when the data were recalibrated:

history: The record of programs that have operated on the data file since its original generation.
command_line: The command line that was used to invoke the program used to perform the recalibration.
comments: This attribute contains the following message: It has been found that the transmitter of the Galileo radar has been losing power steadily since around September 1999. Recalibration of the entire dataset from May 1999 to October 2000 (the parameters Zh and nez) has been performed by comparing with the 3 GHz radar at Chilbolton in 13 scanning events. The 3 GHz radar is itself calibrated to better than 0.5 dB using the method of Goddard et al. (1994, Electronics Letters 30, 166-167). The resulting Galileo calibration is believed to be accurate to around 1.5 dB. It should be noted that this loss of sensitivity means that the fraction of clouds not detected by the radar will increase through the dataset.

Simple programs to read NetCDF files of Chilbolton data into Matlab, IDL and PV-WAVE can be found at http://www.met.rdg.ac.uk/radar/software.html.

Interpretation of radar reflectivity factor and calibration issues

It is taken for granted that if quantitative use is to be made of the Z values then the user has a fairly good understanding of the concept of radar reflectivity and how it is related to the particle size distribution. In this section therefore we concentrate on calibration issues, although the problem of attenuation at 94 GHz is also discussed since it is much stronger than at lower frequencies.

Our approach to calibration of the 94 GHz radar is by reference to the 3 GHz `CAMRa' radar at Chilbolton. CAMRa can be calibrated absolutely by exploiting the non-independence of the radar parameters Z, Z_DR and K_DP in heavy rain, as described by Goddard et al. (1994). However, to compare the two radars directly requires a Rayleigh-scattering target which is both near enough for attenuation to be small at 94 GHz, yet far enough that near-field and ground-clutter effects are small at 3 GHz. We correct for gaseous attenuation at 94 GHz using the temperature and humidity from either the ECMWF or Met Office models, and make the small correction for the near-field effect at 3 GHz using the expression of Sekelsky (2001). The results are expressed in the figure below in terms of the noise-equivalent reflectivity at 1 km (see the definition of nez above):

The error bars indicate the 13 calibration events and their estimated uncertainty, and the numbers indicate the Chilbolton tape number. A steady deterioration in sensitivity is evident, which is due to a loss of transmit power from the tube. Given the uncertainties in the intercomparison of the two radars, the error in the resulting 94-GHz reflectivity factor after calibration is estimated to be around ±1.5 dBZ.

It is important to understand the convention used in the intercalibration of radars of different frequencies because of the temperature dependence of the |K|² parameter of liquid water at millimetre wavelengths. We have calibrated our radars such that Rayleigh-scattering liquid water droplets at 0°C produce the same reflectivity factor at all frequencies. For example, a population of 100µm droplets with a concentration of 10⁶ m^-3 at 0°C would have a Z of 0 dBZ at all frequencies. Hence a radar at frequency f (after calibration and correction for attenuation) will report an effective reflectivity given by

Z_f = Integral from D=0 to D=infinity { (|K_f|²/|K_f,0|²) n(D) D⁶ M_f(D) dD },

where M_f is the Mie/Rayleigh backscatter ratio. |K_f,0|² is the dielectric parameter of liquid water at 0°C, and is 0.93 at 3 GHz, 0.877 at 35 GHz and 0.668 at 94 GHz. Because the |K_f|² of liquid water varies with temperature at 94 GHz, if the example above were repeated at 20°C, a 94 GHz radar using this calibration convention would report a Z of +0.82 dBZ while a 3 GHz radar would still report 0 dBZ. Formulae for the dielectric constants of ice and liquid water can be found in Liebe et al. (1989).

Attenuation by both atmospheric gases and liquid water is much stronger at 94 GHz than at lower frequencies. At 10°C, 1013 mb and 100% humidity, the one-way attenuation due to gaseous attenuation at 94 GHz is 0.636 dB km^-1. In summer the typical two-way gaseous attenuation to top-of-atmosphere is 2 dB. If the temperature and humidity profile is known with some degree of accuracy (such as from a model or a radiosonde ascent) then gaseous attenuation can be corrected for. However, when low clouds are present then the liquid water attenuation can easily exceed the gaseous attenuation, and of course the profile of cloud liquid water content is generally far more uncertain. This makes quantitative use of the reflectivity data in ice difficult if there is any low cloud present. At 10°C and 1013 mb, the one-way attenuation of 1 g m^-3 of liquid water is 4.34 dB km^-1. Attenuation by rainfall is even greater, and in moderate and heavy rain can extinguish the signal completely. Wetting of the radomes of the radar also introduces a large attenuation; see point 4 in the next section.

Known problems with the data

Gaps in the data are present due to:
- The CAMRa radar (to which Galileo was attached from February 1999 to October 2000) scanning through precipitation. Cloud data were not recorded whenever the pointing angle was more than 5° from zenith.
- Glitches in the data caused by the data acquisition system losing sync with the transmiter. These events were easy to detect and reject.
A 50 kHz interference was present in the raw data which, if unchecked, resulted in anomalous horizontal `lines' of cloud every 3 km in processed time-height plots of reflectivity. This problem has been tackled in the following ways:
- Characterising the shape of the oscillation in the cloud-free gates at the top of each ray, and then using this knowledge when subtracting the noise from the remainder of the ray.
- Removing isolated cloudy pixels or pairs of pixels from the processed ray.
- Manual removal of any remaining erroneous echos.
Removing this interference has unavoidably compromised the sensitivity to some extent. Also, some anomalous echos may still be present in some of the data, although they are fairly easy to locate subjectively. The removal of pairs of pixels (indicated by the options attribute containing the string `-doubleclean') obviously will remove any genuine cloud that is only two range gates thick.
The reflectivity values in the lowest gates are affected by:
- Ground clutter and leakage of the transmit pulse into the receiver: this appears as a fairly constant return (in time) in the lowest 6 to 8 gates that falls off rapidly with range.
- The near field effect: this appears as decrease in the Z of rain in the lowest 2 or 3 gates. The far-field approximation is generally applicable beyond 2×antenna diameter²/wavelength, which for the Galileo radar is 225 m. A slight error in the range calibration could also have contributed to the apparent reduction of Z in the closest few gates.
- Insects: although the problem is much less at 94 GHz than 35 GHz (due to the fact that insects are usually Mie scatterers), on some hot summer days insects are apparent between dawn and dusk as fairly low Z values up to 3 km. They can be distinguished from cloud using data from the Chilbolton lidar ceilometer.
Wetting of the radomes covering the two antennas of the radar has been found to cause a two-way attenuation of 9-14 dB. The data should therefore not be used quantitatively during rain events, which can be identified using the Chilbolton rain-gauge data also provided by BADC. Other radars at 94 GHz presumably also suffer from this problem.

Conditions of use

If data from the Galileo radar is used in any publication or report then acknowledgement must be given to RCRU at the Rutherford Appleton Laboratory for providing the data. The acknowledgement should be of the form:

We thank the Radiocommunications Research Unit at the Rutherford Appleton Laboratory for providing the 94 GHz Galileo radar data. The Galileo radar was developed for the European Space Agency by Officine Galileo, the Rutherford Appleton Laboratory and the University of Reading, under ESTEC Contract No. 10568/NL/NB.

Who to contact

If you have any problems obtaining the data, please contact the British Atmospheric Data Centre. If you have problems, queries or comments regarding the data themselves that are not covered adequately by this document, or would like to know if any data was recorded on specific dates outside the period available on BADC, please contact Charles Kilburn (C.Kilburn@rl.ac.uk) and Robin Hogan (r.j.hogan@reading.ac.uk). Even if you use the data and have no difficulties at all, we are very interested in knowing the uses to which our cloud radar data is being put, so please contact us!

References

Goddard, J. W. F., J. Tan and M. Thurai, 1994: Technique for calibration of meteorological radars using differential phase. Electronics Letters, 30, 166-167.
Liebe, H. J., T. Manabe and G. A. Hufford, 1989: Millimeter-wave attenuation and delay rates due to fog/cloud conditions. IEEE AP, 37, 1617-1623.
Sekelsky, S. M., 2001: Near-field corrections for meteorological radars. Proc. 30th AMS Conference on Radar Meteorology, Munich, Germany, 32-34.