Furuno FR1500 Mk3 User Manual download pdf (Page 61)

-2.2-

2.2 Criteria of Tracking

A target measuring 800 m or more in the radial or circumferential direction is regarded as a

landmass and not acquired or tracked. Echoes smaller than 800 m are regarded as targets to

be tracked.

The FURUNO ARPA ATA video processor detects targets in the midst of noise and

discriminates radar echoes on the basis of their size. Target whose echo measurements are

greater than those of the largest ship in range or tangential extent are usually land and are

displayed only as normal radar video. All smaller ship-sized echoes which are less than this

dimension are further analyzed and regarded as ships and displayed as small circles

superimposed over the video echo.

When a target is first displayed, it is shown as having zero true speed but develops a course

vector as more information is collected. In accordance with the International Marine

Organization Automatic Radar Plotting Aid (IMO ARPA) requirements, an indication of the

motion trend should be available within 20 scans of antenna and full vector accuracy within 60

scans. The FURUNO ARPAs/ATAs comply with these requirements.

Acquisition and tracking

A target which is hit by 5 consecutive radar pulses is detected as a radar echo. Auto acquisition is

not defined in paints but in time, which should be less than 3 seconds of initial stage.

Manual acquisition is done by designating a detected echo with the trackball. Automatic acquisition

is done in the acquisition areas when a target is detected 5-7 times continuously depending upon

the congestion. Tracking is achieved when the target is clearly distinguishable on the display for 5

consecutive or alternate paints out of 10 consecutive scans whether acquired automatically or

manually. Required tracking facilities are available within 0.1-32 nm on range scales including 3, 6,

12 nm; full plotting information is available within one scan when the range scale has been changed.

Targets not detected in 5 consecutive scans become “lost targets”.

Quantization

The entire picture is converted to a digital from called “Quantified Video”. A sweep range is divided

into small segments and each range element is “1” if there is radar echo return above a threshold

level, or “0” if there is no return.

The digital radar signal is then analyzed by a ship-sized echo discriminator. As the antenna scans, if

there are 5 consecutive radar pulses with 1’s indicating an echo presence at the exact same range,

a target “start” is initiated. Since receiver noise is random, it is not three bang correlated, and it is

filtered out and not classified as an echo.

The same is true of radar interference. Electronic circuits track both the closet and most distant

edges of the echo. At the end of the scanning of the echo, the discriminator indicates the measured

maximum range extent and total angular extent subtended by the echo. If the echo is larger than a

ship-sized echo in range extent and/or angular width, adjusted as a function of range, it is declared

to be a coastline and the closet edge is put into memory as a map of the area.

This land outline is used to inhibit further acquisition and tracking of ship sized echoes beyond the

closest coast outline. 5 consecutive scans of coastal outline are retained in memory to allow for

signal variation. All smaller echoes are declared to be ship sized and the middle of the leading edge

is used to provide precise range and bearing coordinates of each echo on every scan. This

range/bearing data is matched to previous data and analyzed from scan-to-scan for consistency.

When it is determined to be as consistent as a real target, automatic acquisition occurs and tracking

is initiated.

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Furuno FR1500 Mk3 User Manual Page 61

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