Simrad ARGUS FMCW 3G, User Manual

Page: 92 / 150

90 |
Tracking | Argus Radar Operator Manual
Tracking
General information
Radar target tracking facility
The radar system includes a facility for acquiring and automatically tracking radar echoes.
Each radar target center is used to estimate the relative speed and direction for anti-collision
purposes.
Tracking error sources and effects
There are some factors that can generate errors and confusion in tracking and/or reduction of
target detection capability to the radar operator. Such factors are:
•
Sea, Rain, Snow and Low Clouds Returns
•
Radar Interference
• Sidelobe Echoes
• Blind Sectors
• Low Signal to Noise Ratio and Signal to Clutter Ratio.
Sea, Rain, Snow and Low Clouds Returns
Sea returns are caused by the r eflection of the radar electromagnetic wave by the surface ir-
regularities of the sea and by the water droplet layer existing in the vicinity of the sea surface.
They are displayed as a bright circular area (sea clutter) around the PPI centre having a radius
that, depending on the sea state and the height of the antenna, can be up to 6-7 NM.
Rain, Snow and low Clouds Returns have tw o main effects on the radar electromagnetic wave:
• Attenuation
• Masking effect.
The first effect reduces both the power of the direct signal hitting a target and the signal
returning to the antenna after reflection from the target. The second effect is due to the elec-
tromagnetic wave reflected by rain drops and ice crystals and appears as a bright area (rain
clutter).
Radar echoes in sea or rain clutter areas are practically more or less masked by the clutter.
In order to minimise this problem, the operator must adjust the manual sea/rain controls or
select the automatic sea/rain controls.
Tracked targets entering these areas could not work properly. Generally the effects of such
errors appear as continuous big changes of the target course and speed vector. Sometimes
the symbol of a target that has been acquired at high speed can slip away from the real target
position after a certain time and this event may generate the loss of the target (reported by a
lost target alarm).
These errors, in not very extreme weather conditions, can normally be avoided or at least mi-
nimised by the operator with proper adjustments of sea/rain controls. Refer to “Rain” on page
66 and “Sea” on page 67.
Radar interference
Occasionally, another radar operating in the same frequency band can generate interfer-
ence. Normally this appears on the radar screen as a series of spirals. The main characteristic
of the interference is that it is not sweep to sweep correlated. For this reason this should not
generate errors in the tracking process due to the sweep to sweep correlation performed
by the video extractor. In very rare cases, when the interference falls on the tracked target, it
can cause a deformation of the size of the echo and consequently a small displacement of its
centre of gravity; this can generate a momentary small error in the course and speed values.
Sidelobe echoes
Radar antennas have a radiation pattern consisting of a main lobe and several very small sid-
elobes. Most of the energy transmitted by the radar is radiated and received back on the main
lobe, and a very small part on the sidelobes. This problem has no effect in case of distant or
small targets, but the returns from a large target at short range (less than 3 NM) can generate,
on both sides of the main echo and at the same range, arcs or series of small echoes.
5