Interpreting Housekeeping data (TN-002)
In addition to the time-series and product data, the radar also records so-called "Housekeeping" data, which describes the current operating state of the radar. It may be used to determine, among other things, the current transmitter power, radar PRF and so on.
Housekeeping data types
This is a list of available housekeeping data types
- Radar Information
- Scan Segment
- Processing Mode Information
- Transmitter Information
- Transmitter Power Update
- Transmitter Pulse Sample
- Radar Event Notification
- Calibration Information
The housekeeping data may appear at any location within the data stream, there is no predefined order. Processing software is expected to assume some sensible default conditions, then update them when the appropriate housekeeping data is seen.
The following sections describe the important housekeeping structures, with a link to the appropriate documentation page for further details.
Radar Information
Radar Information describes those aspects of the radar that are not expected to change over the course of the data collection. This includes such items as the antenna gain, radar location and beam width. Details
Scan Segment
A scan segment defines the properties of a radar scan. This may consist of one or more sweeps of the antenna. An antenna sweep can be a 360 degree rotation of the antenna in PPI mode, a sector of the complete 360 degree rotation, or a cut of an RHI. In each case, the signal processor emits a scan segment describing the scan. The actual start of the scan is indicated by a Radar Event Notification. Details
Processing Mode Information
This structure defines the state assumed by the signal processor and describes aspects of the radar that are "dynamic", such as the PRF, radar polarization state, integration method and so on. Details
Transmitter Information
As the name implies, this structure defines the current operating condition of the radar transmitter. Some of this information is duplicated in the processing mode information structure. Details
Transmitter Power Update
This structure is emitted each time the radar measures the transmitter power. In the normal operating mode, the transmitter power is sampled every two seconds. The measured transmitter powers are used to update the radar constants for the V and H polarization modes. Details
Transmitter Pulse Sample
The Transmitter Pulse Sample contains a sample of the transmitter pulse, sampled at a rate of 40 MHz. This may be used to estimate the spectral properties of the transmitter pulse. The transmitter pulse samples are taken along with the power measurements. Details
Radar Event Notification
"Events" encountered by the radar are indicated by the event notification structure. Examples of events include the starting and stopping of scan volumes and sweeps. Typically, time-series files are partitioned by sweep, and product data files are partitioned by volume, so these messages are not normally required when analyzing saved data files. Real-time processors can use it to determine the state of the antenna scan. Details
Calibration Information
The calibration information is emitted by the signal processor when it detects any change in the radar state that affects the calibration variables. Thus, it is not found in time-series data files, which are recorded prior to signal processing. Details
Computing Calibration terms
In order to maintain proper gain calibration, the radar constantly samples the transmitter power, and makes these measurements available to the signal processing software. Thus, a single "radar constant" no longer applies. This section describes how to compute the radar calibration constants from the available data.
Radar Gain Constants
Parts of the radar hardware that do not change are lumped together into the base_radar_constant found in the Radar Information structure. This includes the system gain of all components before the radar calibration plane, excluding the antenna. The antenna gain is also obtained from the Radar Information structure.
The radar receiver gain is obtained from periodic calibrations that the radar performs. The calibration results are stored in calibration files, along with the receiver noise floor and other parameters. The transmitter power is obtained from the Transmitter Power Update structure.
From this information, the V and H receiver constants may be computed as
z_con_h = base_radar_constant - tx_power_h_dbm - 2 * ant_gain_h_db - rx_gain_h_db z_con_v = base_radar_constant - tx_power_v_dbm - 2 * ant_gain_v_db - rx_gain_v_db
The constants may be re-computed each time a new transmitter power update is received.
Differential Gain Constant
The differential gain constant is used to adjust the measured , to compensate for system drifts. The bias is adjusted each time a transmitter power update is obtained, using the following expressions:
zdr_bias_vhs = zdr_cal_base_vhs_db + (tx_power_v_dbm - tx_power_h_dbm) + (rx_gain_v_db - rx_gain_h_db) zdr_bias_vh = zdr_cal_base_vh_db + (tx_power_v_dbm - tx_power_h_dbm) + (rx_gain_v_db - rx_gain_h_db)
The zdr_cal_base_vh_db and zdr_cal_base_vhs_db variables are obtained from the Radar Information structure, these are system dependent parameters obtained during radar calibration. The resulting zdr_bias_vh and zdr_bias_vhs constants are added to the raw obtained from the spectral moments of the time-series data.
Nyquist Velocity
The Nyquist velocity, or unambiguous velocity may be obtained as follows
= wavelength_cm * / prt_usec
Here, wavelength_cm is obtained from the Radar Information structure and prt_usec is obtained from the Processing Mode Information structure.
