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.
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.
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 measuring LDR from the V channel, add ldr_bias_v_db, and add ldr_bias_h_db when measuring 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 |