Seascan plugin

Configuration with SeaScanRCU

The Seascan server requires two configuration files to describe the radar used with the Sigma S6 card:  SeaScan0.ini  and SeaScan0.bin.  These files can be created with Rutter's SeaScanRCU utility, and specify the radar pulse modes and digitization modes.  The Seascan server looks for these files in the current working directory when it is started, so it is important to understand what directory this will be.  If you run the Seascan server manually (i.e. from the Windows Start menu or desktop icon), the working directory will be the location of the Seascan.exe program itself, typically:

c:\Program Files\Rutter\Sigma S6\SeaScanRT\

However, the seascan plugin can start/stop the Seascan server automatically when it is loaded, and in this case, the working directory will be the seascan plugin folder, e.g.:

c:\Program Files\radR\plugins\seascan

The seascan plugin provides its own SeaScan0.bin and SeaScan0.ini files and these can be edited with the SeaScanRCU utility.

Configuration with mode tables

The seascan plugin provides another approach to modifying the SeaScan0.bin/ini files:  translation of a digitizing mode table into a .bin file.  To use this approach, do this:

to a file with a different name, e.g. myradar.radar.R 

• copy one of the .radar.R files in the seascan plugin folder, e.g.:
  

  c:\Program Files\radR\plugins\seascan\furuno1954C-BB.radar.R

• use a text editor to modify this new file, using the embedded comments to guide you
• from the seascan plugin menu, choose "Create SeaScan0.bin from radar description file...":
  
  which will prompt you for the name of a radar description file (*.radar.R).  After you select your file, radR will create the new .bin file and prompt you for further action:
  

 Digitizing parameters

The Sigma S6 card maps radar video voltage levels to sample values with a linear map whose parameters are:

• gain: the slope of the voltage to sample value map
• offset: the intercept

Typically, a marine radar uses a logarithmic amplifier, so that the video signal voltage is a linear function of the log of the power received by the antenna.  Thus, Sigma S6 sample values are a linear function of the received power measured in dbm.  For example, with the Furuno 1954C-BB and gain = 255, the relationship is:

     P_in (dbm) = -4.834e01 + 1.738e-02 * sample - 8.020e-01 * offset

The best way to obtain this relationship for your radar is to calibrate the radar + Sigma S6 system with a microwave pulse generator.  However, if you already know the relation between received power and video signal voltage for your radar, then you can combine this with the voltage->sample mapping of the Sigma S6 card to obtain the relationship.  Contact the author of radR for details on this.

Choosing total range and range cell size

The seascan plugin lets you choose from among the digitizing modes specified by the SeaScan0.bin file.

At any time, the modes available to you depend on the PRF (pulse repetition frequency) in which the radar is operating, and the plugin range and resolution submenu will only display available modes. The PRF is detected by the Sigma S6 card and reported to radR.  (For some marine radars, each PRF corresponds to a unique pulse length, but for others the relationship is more complicated.)  The digitizing modes determine how many pulses are digitized per sweep, how many sample for each pulse, and the range covered by each sample (i.e. resolution).  There are some restrictions:

• for a given PRF, the number of pulses digitized per sweep is constant, and must be a multiple of 256, up to 4096
• the number of samples taken per pulse must be a multiple of 256, up to 4096
• the product of sample per pulse and pulses per sweep must be at most 2^22 = 4 * 1024 * 1024
• the total range will be the product of the samples per pulse and the range per sample; range per sample is limited to the choices mentioned in the table at the top of this page

You can create different configuration files to allow for different numbers of pulses to be digitized per sweep at a given PRF, but the SeaScan server must be restarted to switch between such files.  Currently, radR is unable to let you choose which SeaScan0.bin/.ini files the Seascan server should use, so you must manually rename and/or copy different configuration files to the radR/plugins/seascan folder between stopping and restarting the Seascan server, in order to change the number of pulses digitized per sweep.

Range on the radar console vs. in seascan data

The Range setting on your radar console does not directly affect the maximum range or range resolution of data obtained by the seascan plugin, but they are not completely independent either:

• your radar's range setting will typically determine the radar's pulse length and PRF
• for a given PRF, the seascan plugin can digitize the signal in one or more modes as determined by the SeaScan0.bin file, and these modes determine the maximum range and range resolution, regardless of what the maximum range shown on the radar console is
• your radar might use the same PRF for several different range settings, and switching between these range settings on the radar will have no effect on the data obtained by the seascan plugin

The seascan plugin does not ensure that your digitizing settings are sensible.  In particular, it is possible to create modes where the total sampling time for each pulse is longer than the interpulse interval, in which case "pulses" of seascan data will include samples corresponding to more than one physical pulse.  This will result in a bright ring at long range in the radR plot window, and is not recommended for typical use.