Using calibration files with Time Series data (TN-001)


Calibration files are saved each time the radar performs a self-calibration. These files are required when processing time series data in order to obtain the receiver gain, and to perform noise subtraction. This article describes a typical calibration file, and how to use it.

Radar Constant

The CSU-CHILL radar measures the transmitted power on each channel periodically, and uses this to adjust the radar constant. In addition, the receiver gain is measured at least once a day, and is also used to adjust the radar constant. Thus, the radar constant is not really a constant. In order to simplify matters, a constant is stored to account for all non-changing parameters. This is embedded within the radar information (radar_info) packet usually included at the beginning of every volume file. This is referred to as the "base radar constant" (). Also included in this structure is the antenna gain(, ), which is specified separately since this is re-calibrated based on solar calibrations.

The transmitter power information is embedded within the recorded data files in a transmitter info (xmit_info) packet. Power readings (, ) indicated are in dBm. Currently, transmitter power is measured once every 2 seconds.

The receiver gain (, ) is included in the data files within the calibration terms structure (cal_terms), it may also be obtained from the calibration files described in the section below.

Given the base constant, transmitter power, antenna gain and receiver gain, the radar constant may be computed as:

The "-40" term arises since CHILL has historically used a range correction equation in which range is expressed in kilometers, and this is reflected in the base constant. When using the more common range correction in meters, a factor of 100 must be added.

Typical values

For rough calculations when the exact values of the constants are not available, the values given in the table below may be used. These are based on the values recorded by the radar in November 2009.

Constant Value

File structure of a typical calibration file

A calibration file is a flat ASCII file, containing several name-value pairs. Each name is a word with no white space within it, and is separated from the value by an '=' sign. An example is shown below:

noise_v_rx_1 = 40.647709
noise_h_rx_2 = 42.922058
noise_v_rx_2 = 41.802731
noise_h_rx_1 = 40.799435
ldr_bias_h_db = -0.453558
ldr_bias_v_db = -1.560466
gain_v_rx_1_db = 127.485725
gain_v_rx_2_db = 126.864494
gain_h_rx_1_db = 128.118271
gain_h_rx_2_db = 127.607368
zdr_cal_base_vhs = 1.250000
zdr_cal_base_vh = 1.100000
sun_pwr_v_rx_1_db = 24.056929
sun_pwr_h_rx_2_db = 23.603369
sun_pwr_v_rx_2_db = 23.007452
sun_pwr_h_rx_1_db = 24.567917

Background information on the CHILL receiver design

The noise, sun power and gain parameters have four variants, measured with the V and H LNAs, and two different receive paths. This is due to the transfer switch used in the receiver RF section introducing two independent paths for each channel from the antenna to the receivers.

Receiver Block Diagram

Assume we are looking at signals from the V antenna port and LNA. The signal travels through the power limiter, image rejection filter and LNA. The position of the transfer switch depends on the polarization state of the transmitter.

Polarization Switch Position
Simultaneous or single-polarization Off
Alternating, Vertical On
Alternating, Horizontal Off

In simultaneous or single-polarization mode, the switch is turned off and the digitizer channels perform their indicated task.

When the transmitter is in alternating mode, it rapidly switches between horizontal and vertical polarization modes, and the transfer switch also switches between channels. Thus, the digitizer's "V" channel becomes the cross-polar signal digitizer and the "H" channel becomes the co-polar signal digitizer. For clarity, we will refer to the digitizer's "V" channel as channel 1, and "H" channel as channel 2. Thus, in order to compute the absolute received power, the receiver gain to use depends on the transmit mode, which decides the pairing of the LNA and digitizer channel.

In order to obtain a noise floor for all four possible paths, the transfer switch is rapidly toggled on and off on a pulse-to-pulse basis when receiver calibrations are performed. Which noise floor to use when performing noise subtraction also depends on the signal path.

Parameters measured during the calibration

During the routine radar calibrations, the radar measures the receiver noise floor by pointing the antenna at a clear area of the sky (the so-called "Blue Sky" reading), and averages thousands of samples from the four receive channels. The average readings are stored in the noise_* variables as linear values (no dB conversion).

Next, the antenna tracks the sun, and several thousand samples of the solar radiation are captured, and the average values are stored in the sun_pwr_* variables.

Finally, the receiver injects a signal at a known power into the receiver reference plane. The received power is noted, and is used to obtain the receiver gain, the calibration constant needed to convert the received power into an absolute power reading. These are stored in the gain_*_db variables. The receiver gain is stored in dB.

The ldr_bias* variables are estimated by the signal processor from the solar radiometric noise. When correcting LDR from the V channel, subtract ldr_bias_v_db from the measured power ratio, and subtract ldr_bias_h_db when correcting LDR from the H channel. This is true both in single-polarization as well as alternating mode.

The zdr_cal_base_vh term is added to the measured ratio of to to obtain in alternating polarization mode. For simultaneous mode, the zdr_cal_base_vhs term is used instead.

Gain and Noise Floor measurements to use

This section describes the circumstances in which to use the different calibration variables.

Single-polarization V-only mode

Channel Gain Noise Floor
Vertical Co-polar gain_v_rx_1_db noise_v_rx_1
Horizontal Co-polar N/A N/A
Vertical Cross-polar gain_h_rx_2_db noise_h_rx_2
Horizontal Cross-polar N/A N/A

Single-polarization H-only mode

Channel Gain Noise Floor
Vertical Co-polar N/A N/A
Horizontal Co-polar gain_h_rx_2_db noise_h_rx_2
Vertical Cross-polar N/A N/A
Horizontal Cross-polar gain_v_rx_1_db noise_v_rx_1

Dual-polarization Simultaneous-transmit mode

Channel Gain Noise Floor
Vertical Co-polar gain_v_rx_1_db noise_v_rx_1
Horizontal Co-polar gain_h_rx_2_db noise_h_rx_2
Vertical Cross-polar N/A N/A
Horizontal Cross-polar N/A N/A

Dual-polarization Alternating-transmit mode

Channel Gain Noise Floor
Vertical Co-polar gain_v_rx_2_db noise_v_rx_2
Horizontal Co-polar gain_h_rx_2_db noise_h_rx_2
Vertical Cross-polar gain_v_rx_1_db noise_v_rx_1
Horizontal Cross-polar gain_h_rx_1_db noise_h_rx_1