Decoding Scan Listings

S-Pol IHOP_2002, Oklahoma Panhandle
May/Jun 2002

The scan listings provide a description of the radar antenna motion while sampling a 3-Dimensional portion of the atmosphere. To sample in 3-D, the antenna moves in both azimuth and elevation, with the most typical scanning involving holding one angle fixed for a period, while the other angle changes; then, the fixed angle is changed or incremented and the process repeats (an exception is the co-plane scan, where azimuth and elevation change simultaneously). The NCAR scan listings are grouped by scan volume representation, within which there are multiple sweeps. A sweep is defined as an antenna motion where one angle remains fixed and the other varies (in the case of the COP scan, the co-plane angle is the fixed angle for a sweep, with the azimuth and elevation changing according to a known formula).

The scan listings for this project were compiled from the real-time S-Pol data stream. Scan delineation for this data stream can be slightly different from the post-processed data stream, with the most significant differences being in the scan start/end times and start/end angles of the sweeps (times of a given scan/sweep may differ by one or two seconds and several tens of degrees).

Scanlists contain the following information:

First Line of a volume grouping

Scan volume number, V:: sequentially assigned by the processing software; will generally differ from the volume number assigned in post processing
Scan volume start date
Scan volume start and end times
Radar name
Scan type:
- SUR, or "surveillance": antenna does a full 360° rotation in azimuth, while maintaining a fixed elevation angle
- PPI, or "plan position indicator": antenna does a partial rotation in azimuth (either counter-clockwise or clockwise), while maintaining a fixed elevation angle
- RHI, or "range-height indicator": antenna maintains fixed azimuth, while moving up or down in elevation
- IDL, or "idle": antenna is not moving
- CAL, or "calibration": antenna is either idle, typically pointing at "blue sky", or is tracking the sun for a solar calibration; in practice, the tape writing system is usually out-of-the-loop for these operations, so the CAL scan type seldom appears in the scan listing.
- VER, or "vertical pointing": antenna is pointed vertical or near-vertical, but generally rotates in azimuth; these are special scans to aid in determining Zdr bias
- COP, or "co-plane": used only when performing coordinated dual-Doppler scans, where each radar scans along a tilted plane that passes through the baseline joining the radars; both the azimuth and elevation vary in concert for these scans. Co-plane scans sample only half of a hemisphere, on either one side or the other of the baseline between two radars.
Range to center of first gate, G1, in meters
Pulse Repetition Frequency, prf: number of radar pulses sent in one second
Number of pulses averaged to produce a radar beam, hits
Number of megabytes written by the data system since the last re-initialization of tape recording, MB

Second Line of a volume grouping

Clutter Filter status at the start of this volume, Clt (due to limited implementation of this feature, there is no simple way to tell if the filter status changes within the volume; review low-level scans to determine if filter status changes)
variables present in this data stream, Fields; post processing Fields will usually vary from those listed in the real-time scan listings
Number and spacing of range gates, Gates @; spacing in meters

Third Line of a volume grouping

This line contains headings for the various columns that appear in the suceeding "scan" lines. Each scan line starts with the scan number (sequential, starting when tape recording was re-initiated), and the start/end of a single scan (or sweep).

fmt: apparently always null (or blank)
Fxd: the nominal (or target) value of the angle held fixed during a given sweep; the fixed angle may be either the azimuth or elevation, depending upon the scan type (e.g., fixed angle is elevation for PPI and SUR scans, azimuth for RHI).
Sector: the rotational limits of the varying angle of the sweep; the limits on the sector also imply the direction of the scan (CW or CCW, UP or DOWN)
AvR: average rotation rate; not a reliable number, since it is estimated from the start and end limits of the sweep, combined with the time for sweep completion (antenna turn-around time can have a big impact on this value)
Pct: percentage of dropped beams
Bms: total number of beams/rays in a sweep
Recs: a cumulative number indicating the starting beam of the sweep; the beam counter is periodically reset.
MB: sweep position, in Megabytes, from the start-up of the data archiving (initiallization of the archiving process)

Succeeding Lines of a volume grouping

One line per sweep or scan. Can be as many as 64 sweeps in a given volume grouping. Each line contains start and stop times, as well as all the information described by the heading line.