Nautical Instruments Flashcards
Layout of magnetic liquid compass
1: glass cover
2: fitting for azimuth instrument
3: gasket
4: expansion ring
5: center of rotation (line through taps)
6: float (loading pivot pin with ≈ 100 g)
7: pivot pin (iridium)
8: ring magnet
9: compass card with gradation
10: lubber line
11: jewel/safire
12: bottom cover
13: filler plug
14: bridge for pivot pin
15: lead ring
floating:
- buoyancy
- damping
Earth magnetic field:
Components
Vectors:
T: direction and strength of earth magnetic field
V: vertical component
H: horizontal component
i: inclination
What is the lubber line?
Reference mark on the inside of the compass bowl
Errors that may impair the reading of a magnetic compass?
- Parallax (lubber line close to compass card)
- Collimation (misalignement of compass card and ring magnet)
Requriements for a magnetic liquid compass?
- Stable reading
- Sensitive
How to achieve stable reading of a magnetic liquid compass?
=> compass should not be brought out of balance easily:
1. strong (ring) magnet with poles far apart
2. different rolling periods of compass and ship
3. small friction between pivot pin and jewel
4. compass pivot must be in intersection of cardanic axes
5. spacing between compass card and housing
6. weight on pivot pin ≈ 100g
Types of magnetism on board?
Permanent: high tensile keeps magnetism => doesn’t change with course
Induced: milder steel sensitive to earth magnetic field => changes with heading
Directive force on a magnetic compass
H: horizontal component of earth magnetic field
S: horizontal component of entire disturbing field on board
=> H’ in direction of Nc (compass north)
=> angle H - H’: deviation
H/H’ ≈ 1
high altitudes: H smaller ; S constant => deviation ↑
Compensating a compass
- magnets:
P (fore/aft)
Q (transverse)
r (vertical) - weak iron:
spheres
flinders bar
Reason:
deviation < 5 degree
H’/H ≈ 1
Dutch ships: every 2 years, unless it can be demonstrated in compass book that continuously < 5 deg and ≈ 1
Fluxgate compass: principle
iron core(s) with 2 coils around it excitation snd sensing coils => the overlaying of the induced magnetic field over the earth’s magnetic field leads to a small current in the sensing coils, which can be read out.
Advantages magnetic compass?
+ independent of electricity
+ no electronics
+ problems easy visible
+ long MTBF
Disadvantages magnetic compass?
- shows Nc => calculation to Nt necessary
- directive forces decreases dramatically due to inclination in higher latitudes
- can not be placed everywhere (steel vessel; engine; …)
Different ways to take a bearing?
- hands
- shadow pin
- notch and wire visor
- PELORUS
- Thomson instrument
Different logs?
- towing log (Cherub log)
- pressure log
- EM log (electromagnetic)
- doppler log
- sailing vessel log
- hand log (Dutchman’s log)
Towing log:
principle and pro/cons
rotation of log rotator is transferred via a line to the clock => distance
+ cheap
+ no electircity
+ easy to repair
- inaccurate at low/high speeds
- only usable in open water
- vulnerable (seaweed, shartks)
- distance measured => speed to be calculated
- distance in water
Pressure log:
principle and pro/cons
measuring pressure differential between dynamic and static pressure in a tube => the pressure differential deflecting a membrane
static tube: ensuring similar pressure both sides of the membrane, if not making way; compensating for draft change
+ very precise
+ can give both speed and distance
- inaccurate at low speed (low pressure differential)
- restless when ship pitching
- vulnerable in shallow waters
- speed in water (not over ground)
- measures only speed ahead (not astern)
EM log:
principle and pros/cons
in a probe, a magnetic field that is continuously switching poles is induced by vertical coils connected to AC
=> this magnetic field separates the ions in the water, which create a voltage differential measured by pick-ups (voltage differential is directly proportional to speed through water)
+ precise for all speeds
+ can measure forward and aft speed
- measures speed through water
- depends on AC electricity source
Doppler log:
principle and pros/cons
Principle: frequency shift when source and receiver of sound move closer - apart
300 kHz; 60 degree
when transmitting, the receiver is blocked
doppler shift = differential between transmitted and received frequency is measured (almost linear dependency between delta and speed)
Bottom track: upto 200 m depth
Water track: when the dopplerlog loses bottom contact, it will automatically switch to water track, where the water in 10-15m can also reflect the ultrasonic pulses. For this to happen, the receiver is also blocked shortly after receipt of the echo.
+ accurate at low speeds
+ in bottom track: SOG
+ wash does not affect reading
- minimum UKC 1 m
- air under transducer interferes with signal
Echo sounder:
Calculation
s = v * t
D = 1/2 * s
Ultrasound in water
Frequency used: 150-200 kHz
Speed: ~ 1,500 m/s
Depends on:
* Temperature
* Salinity
* (Pressure)
But max deviation: ~ 4%
Going into cold FW: careful!
Impact on ultrasonic waves in water
- Refraction: speed is different in layers of different density
- Reflection: e.g. boundary reflection between different layers
- Absorption: energy loss as waves promulgate through the water column
Echo sounder echo:
strength depends on
- water depth
- soil
- angle between sound beam and bottom
Echo Sounder:
flow diagram
transmitter = loudspeaker
receiver = microphone
oscillator sends 0 to CPU
Oscillator: Piezo (quarz) or artificial chrystal
Echo sounders:
misreadings
Pythagoras error (if transmitter/receiver separate)
Multiple (layered) echos: returning signal is reflected against hull and bounces back => turn down gain
Fish, airbubbles, layers of great temperature/salinity difference
Zero adjustment (UKC, water depth)
Echosounder:
Range Switch
Range, where the depth is expected.
Always start with lowest range
Echosounder:
Gain Switch
Adjust to have only clearest echo recorded.
Double echos (reflected) : 2nd (deeper) bottom echo will disappear if gain is tuned down a bit
Echosounder:
Zero Adjustment / Draft Setting
Used to add either the draft from the sensor to the keel to show UKC or add the draft from waterline to sensor to show water depth
Echosounder:
Pulse length
Duration between leading and tailing edge of the impulse
=> determines minimum draft that can be measured:
D = 1/2 S with S = v* t
em waves
300,000 km/s = 300 m / microsecond
Reflection time: time between transmission/reception of echo at 1 NM
wavelength:
X (3cm = 9,000 MHz) : smaller horizontal beam angle with same size scanner; sharper image
S (10cm = 3,000 MHz) wave length
-> by design; not changeable
Choosing radar range:
Considerations
Purpose (fog, coastal navigation)
Circumstances (coastal navigation, narrow channels)
Radar
Presentation
Relative:
- not stabilized (HU) ~ Pelorus (true course + clockwise angle
- stabilized: course up (CU) or north up (NU)
True Motion
Radar:
Azimuthal magnification
Horizontal beam angle (= beamlets = time base) 0.5-2 degrees (depending on quality of scanner)
=> echos will be widened by 1/2 horizontal beam angle on either side
=> dots appear as arcs!
=> total angle magnification is 1/2 beam angle either side = 1 beam angle
=> take bearing through cente of echo
Radar:
Resolution in bearing
Minimum angle between 2 objects in same distance from scanner to give 2 separate echoes
angle between 2 objects <, >, = horizontal beam angle / time base
Radar:
Resolution in range
minimum distance between 2 objects in same bearing, so we can still see 2 separate echoes
Depends on pulse length
=> pulse length = pulse time * 300 m/microsecond
…. e.g.: 30 m (for pulse time = 0.1 microseconds)
=> IF distance < half pulse length = 15 m => return signals of 30 m length overlap
Radar:
Blind Distance
Depends on:
- height of scanner
- vertical beam angle
Radar:
Sea Clutter
Increase the gain at Amplifier depending on time =distance from scanner:
=> Swept Gain
Radar:
Blind Sectors
Locate:
On short range without sea clutter
=> if waves => easily detectable
=> note clockwise angle bearing
Could move scanner higher, but
- greater sea echoes
- larger blind distance
- more energy loss from scanner to display
Radar:
False echoes
- side lobes (< 1nm distance from object) => leads to arcs around actual echo
- multiple reflections (like2nd depth of echosounder)
- reflections from objects on own ship (slightly larger distance, different angle)
- false echo from other time base (2nd trace)
Radar:
Adjusting
- STDBY => ON
- Brilliance
- Gain
- Tune
- FRM / VRM (fixed/variable range marker)
- Sea Clutter (if needed)
- Rain Clutter (if needed)
Radar:
Take bearing
GNSS systems
Global Navigation Satellite Systems
- GPS
- Beidou
- Glonass
- Galileo
GPS:
system components
- Space segment
- Control segment
- User segment
GNSS principle
GNSS receiver measures runtime needed for a signal from satellite to receiver
=> radius = sphere around each satellite
=> satellite clock and receiver clock are not synchronized = clock error
=> but because satellite atomic clocks are kept synchronized by Master Control Station the distance errors are similar for all satellites
24 satellites @ 20 km height with 12 hrs circulation period
=> need 4 satellites for 4 unknowns (lat, long; alt; clock error)
=> or 3 for ships (alt known)
MPP: Most Probable Position
GNSS
datum transformation
100 different ellipsoids, to match the configuration of earth in different areas
GNSS and chart must use same reference ellipsoid!
Difference can be several 100 m
GNSS
risks and errors
- Ionospheric disturbances (solar flares; atmospheric refraction)
- clock errors
- disturbing objects near receiver antenna
- Jamming (system on board goes black) and spoofing (receiver is misled without being able to notice it)
- datum transformation mistakes (ellipsoid)
GNSS
terms to know
COG; SOG
VMG (velocity made good - towards target if tacking)
BRG; DIST; TTG; ETA; XTE
GNSS
achieve higher accuracy
DGPS = landbased => fixed station sends corrections to vessels in 200-500km vicinity => since disabling SA (selective availability) today barely better than common GPS
SBAS=EGNOS=WAAS (Satellite Based Augmented System) = satellite based => 25 ground based monitoring stations send corrections to Master Station and from there to Ground Earth Station, so the final correction comes from a satellite
R95?
95% of time accuracy within 4 m
GNSS:
speed measurement
- calculate speed from 2 positions and time between
- Doppler frequency shift of incoming signal
Ultrasonic sound (depth sounder/doppler log)
Pros / Cons
Pros:
- little impact from ship’s noise
- sharper defined images
- easier to bundle => smaller transducers
Cons:
absorbed more than audible sound
Doppler Log:
Configurations
- Janus: forward/aft to compensate for pitching and trim
- if adding 2 additional transducers => transverse speed can be measured
Sudden, unexpected radar echo
Could be an echo from another time base.
If you suspect a second trace echo, switch to higher range
RACON
RAdarbeaCON
IF RACON is hit by radar transmission, it returns À pulse. Visible as morse code on radar screen if RACON transmits on same frequency as our radar.
Obvious identification in high traffic density.
Typically: 3 cm
SART
Search And Rescue Radar Transponder
Has to be activated
Range:
2 NM at sea level
10 NM at 1 m height
Will give 12 dots on radar screen when hit by radar pulse. Scans wide range of radar frequencies.
NU/CU/HU
heading flash
AIS principles
Working on 2 VHF frequencies (87B; 88B)
4,500 AIS report slots/minute
Not a collision avoidance system <=> only decision support tool
AIS
Objectives and Practice
Objectives:
- improve maritime safety
- protect maritime environment
Practically:
- assist collision avoidance
- enable ports/states to identify ships
AIS
Information groups
- Static data => pre-programmed
-
Voyage data => program pre-voyage
(draft; cargo; destination/ETA; other) use UN/LOCODE : xx xxx>xx xxx - Dynamic data => from sensors
Deviation depends on?
- Course
- distance to magnetic north
- ships S
Magnetic compass:
Directive force
H’ depends on
H and S
Barometers
advantages Ameroid (= disadvantages Mercury)
- easy to read
- less vulnerable (glass, temperature)
- not sensitive to ship’s motion
- can be used as barograph
AIS
pro / con
PRO
- no target swap
- no line of sight required
- no false echoes
- more accurate CPA / TCPA
- detailed info on targets
CON
- dependence on GNSS:
- ionospheric disturbances
- satellite clock error
- jamming/spoofing
