ARPA Notes (old)

Question 1

What are the two common wavelengths?

Answer 1

1. X-band, 3cm, 9,300 to 9,500 Mhz
2. S-Band, 10cm, 2,900 to 3,000 Mhz

Question 2

What are the typical Bandwidths?

Answer 2

1. Horizontal - 0.64 to 2 degrees
2. Vertical - 15 to 30 degrees

Question 3

What are the 3 stages of ARPA?

Answer 3

1. Target Recognition
2. Anticipation
3. Target-Lock (symbol has solid lines)

Question 4

What are ARPA target acquire times?

Answer 4

1. Trend of Motion - 1 minute
2. Full Vector Accuracy - 3 minutes

Question 5

How long is Plot Lag?

Answer 5

Can take 15 seconds to 3 minutes to correct

Question 6

What is the smallest scale for ARPA use?

Answer 6

3nm to 24nm

Question 7

How does ARPA get speed input?

Answer 7

GPS or
Target Based Speed

Question 8

What is GPS accuracy?

Answer 8

Within 10m (40 feet)

Question 9

What is DGPS accuracy?

Answer 9

Within 3m (10 feet)

Question 10

Describe RACONs (RAdar beaCONs)?

Answer 10

-Exhibits a series of dots and dashes
-Represents range and bearing from Racon
-Operates in the X-Band
-Some operate in the S-Band
-Considered short range (6-8 miles)

Question 11

What is the SART range?

Answer 11

8 miles

Question 12

How do you reduce Sea Clutter?

Answer 12

STC - Sensitivity Time Control

Question 13

How do you reduce Rain Clutter?

Answer 13

FTC - Fast Time Constant

Question 14

When operating on Short Range scales:

Answer 14

Pulse Length: Shorter
Pulse Repetition Rate: Higher

Question 15

When operating on Higher Range scales:

Answer 15

Pulse Length: Longer
Pulse Repetition Rate: Lower

Question 16

What is the preferred mode for navigation and collision avoidance?

Answer 16

North-Up Stabilized

Question 17

Speed and heading inputs need to be ________ stabilized to provide the ARPA with speed and course through the water.

Answer 17

Sea.
The use of these devices in a ground stabilized mode for risk of collision could be particularly hazardous in sea areas that experience significant tidal streams and currents.

Question 18

North-Up mode

Answer 18

-Compass stabilized
-Target Pips are at their measured distances in their true directions from own ship.
-North bearing maintained at the top of the screen.
-Heading line changes its direction according to the ship’s heading.

Question 19

Course-Up mode

Answer 19

-Compass stabilized
-Target pips at their measured distances and in their directions relative to the intended course.

Question 20

Head-Up mode

Answer 20

-Un-stabilized
-Target pips at their measured distances and in their directions relative to own ship’s heading.

Question 21

North-Up True Motion

Answer 21

-Ground stabilized
-Own ship and other moving targets move in accordance with their true courses and speed.
-All fixed targets, such as land masses, appear as stationary echoes.

Question 22

What is Bearing Resolution?

Answer 22

A radars ability to differentiate between two targets on the same range.

Question 23

Bearing accuracy is affected by:

Answer 23

-Horizontal beam width (narrow). The greater the beam width, the less accurate.
-Can use lower gain to help.
-It is more accurate in stabilized or North-Up mode.
-Target size.
-Target rate of motion.

Question 24

What is Range Resolution?

Answer 24

A radars ability to differentiate between two targets on the same bearing.

Question 25

Range Resolution determined by:

Answer 25

-Pulse length (shorter)
-Receiver gain
-Range scale
-Cathode Ray Tube spot size

Question 26

Maximum range is determined by:

Answer 26

-Beam width
-Pulse Length (longer)
-Pulse Repetition Rate
-Antenna Rotation
-Target characteristics
-Receiver sensitivity

Radar sets have greater range capabilities while functioning with a longer pulse length where a greater amount of energy is transmitted in each pulse.

While maximum detection range capability is sacrificed when using the shorter pulse length, the advantage is better range accuracy and resolution.

Question 27

Minimum range is affected by:

Answer 27

-Pulse length
-Vertical beam width
-Side-lobe echoes
-Sea return

Question 28

Range accuracy is determined by:

Answer 28

-Range scale
-Plan Position Indicator curvature
-Calibration
-Fixed error
-Line voltage
-Radar scope interpretation

Under the best conditions, the radar range accuracy one could expect is plus or minus 50 yards.

Question 29

What are some Target characteristics?

Answer 29

-size
-shape
-aspect
-height
-composition
-texture

Question 30

What causes attenuation?

Answer 30

-It is the reduction of intensity of the radar wave experienced along its path. It is caused by the absorption and scattering of energy by various forms of precipitation: rain, fog, clouds, hail, snow, and dust.

-The further the radar wave and returning echo must travel through these various mediums, the greater will be the attenuation and subsequent decrease in detection range.

-It is most susceptible to the 3cm X-band