here are some of the best notes i have come across,you can mail me any new notes if you think can be added to this list.This note is not for commercial use and posted cause i was told it`s free for use,anyone claiming copyright can ask me to remove them.
COURSE RECORDER
(1)No lubrication required on account of simple mechanism.
(2)Every watch the officer is to check whether the correct course is being recorded & pens are full of ink.
(3)Course recorder clock always to be set on GMT.
(4)To be switched off in port.
(5)Date, Time and Place important. Start up should be noted.
(6)Should not run out of paper.
DECCA
Ranges vary due to atmospheric changes, 400 miles during daytime & 250 miles by night.
Chains: each chain has one master & two or more slave stations. Chains - Zones - Lanes
Green Zone - 18 Green Lanes. Red Zone - 24 Red Lanes Purple Zone - 30 Purple Lanes.
Frequencies: Fundamental Frequency is 14.0466 KHz. (F).
Green: 9F Red: 8F Purple: 5F Master: 4F
Phase difference between received signals is measured.
Frequency = 3*10^8 / Wavelength = C / Wavelength = F.
Decca Operations:
�Compare Deccameter readings & check whether Deccometre / LI Display difference is < 0.5.
�Lane identification sequency flashed thrice every minute.
�Check master reading is 23.8, 23.9, 00.0, 00.1 or 00.2. If master is not press LI Zero & wait for next sequence.
ERRORS:
There are two kinds of errors:
�Fixed or Systematic:
1.Land Effect & 2.Magnetic Anomalies.
The speed of emergent wave changes when crossing over from land to sea.If decca signal comes over land its phase too will be affected.Magnetic anomalies too affect the signals for a particular region.These are corrected by marine data sheets.
MARINE DATA SHEETS:These sheets provide the basic operating information that the decca navigator requires. Additional data sheets are issued from time to time & are grouped in the following order:
1)Receiver Operating Instructions.
2)Accuracy (Fixed Errors).
3)Accuracy(Variable Errors).
4)Operational Data.
5)General Information.
6)Additional Information.in ALRS Vol. 5.
Since the latices on the chart are computer generated the corrections are given in the form of sheets.
�If the error is circled it is to be subtracted.
�Errors are provided in the data sheets for each chain & centilane.
VARIABLE OR RANDOM ERRORS:
These errors change with season, time, the day, etc. & cannot be applied.
User has to be aware that they exist & and errors are to be allowed for.
DIRECTION FINDER
The bellini tosi loops are connected to the field coils in the receiver & the search coils within the field coils are turned to determine the direction of the signal. This arrangement is called RADIO GONIOMETER.
SENSING
In order to decide which of the two possible bearings is correct a short vertical ariel called the sense ariel may be used in conjunction with the loops. The phase of the current induced in the rotating loop depends on the direction of the loop in relation to the radio waves.
Reason for two loops in bellini tosi antenna is that an ac current in a coil produces an alternating magnetic field inside the coil windings which is proportional to the current.
�This field may be represented by a vector with the length proportional to the strength of the field / current in the direction of the field.
�Within the field coils two fields equally strong are generated simultaneously.
�The two fields combine to form a field whose direction is the same as the lines of force.
ERRORS IN D/F
1)LOOP SIGHTING:
a)QUADRANTAL:Caused due to re-radiated signals from the ship's hull which tends to effect the radio signalscoming from the vessel's bows & quarters. Maximum error is at 45 degrees port / starboard bows / quarters & minimum at ford & aft.
b)SEMI-CIRCULAR ERROR:Caused due to the re-radiated signals from vertical conductors of ship. eg. masts, posts, derricks.
2)ENVIRONMENTAL CAUSES:
a)LAND & COAST EFFECT:This is due to refraction at the land & sea faces. Errors in bearings will be large if bearing line & coast line make a small angle. Marine radio beacons are preferred.
b)NIGHT EFFECT:When a signal is transmitted from a station some of it travels along the surface of the earth - ground waves & some bounces off the ionosphere - sky wave.The sky wave reception causes interference hence take signals < 25 miles only at night.
3)HALF COVERGENCY:
The radio wave follows a great circle path so the bearing observed is a great circle bearing.
Formula: 1/2 convergency = 1/2 d'long * sin m'lat
�Applied where d'long is more than 4 degrees.
�Always towards the Equator.
(CONVERT RELATIVE TO TRUE BEARING).
ECHO SOUNDER.
Speed of sound in water is 1500 mts/sec.
1)CONTROLS:
�Range Switch: to vary the range from lower to higher.
�Scale Selector: feet, fathoms & metresselector for scale.
�Fix Marker:this button when pressed will make the stylus mark the paper.keeps a record of event or time.
�Paper Speed Control: to select fast or slow speed.
�Draft Setting: used to set zero line according to the draft of the vessel.
�Gain or Sensitivity: to make the echoes darker on the paper in case they appear weak.
�Dimmer: adjust the illumination of light.
2)ERRORS:
�PROPAGATION ERROR:The velocity of sound increases when temperature, pressure & salinity increase. due to high salinity in red sea, depths to be increased by 5%.
�AERATION:Caused by bubbles in the water due to:
a)v/l going astern.
b)moving rudder hard over.
c)air pockets left over by bad weather
d)breaking water over shoal. air bubbles reflect the sound waves resulting in false echoes.
�MULTIPLE ECHOES:In shallow waters echoes may be received from the first, second & third reflections from the sea bed.Correct echo is the first one, second & third to be ignored.
�PYTHAGORUS ERROR:This error occurs if the v/l has two transducers, one a transmitter & the other a receiver.Distance travelled by sound is more than actual depth of water below keel.More pronounced in shallow waters.
�FALSE BOTTOM ECHOES: This occurs in deep waters when an echo may be received after the stylus has completed one rotation & begun another.
G.P.S.
SPACE SEGMENT:
�Satellites are to be operated in 6 orbital planes in very high orbits, approximately 20,200 kms above the earths surface.
�Four satellites are located in each plane having one spare in every alternate orbit. the configuration being 21+3 satellites. the orbital planes are inclined at 55 degs to the equator.
�The orbital period of these satellites is just under 718 minutes resulting in the satellites passing over the same ground point each day, excepting the fact that they are four minutes earlier (give or take 1.7 secs).
�The orbit design was developed to guarantee that atleast four satelites are alkways in view at every point on the earths surface 24 hours a day.
GROUND SEGMENT:
�The ground or control segment refers to the ground based element of a gps system which manages the performance of the satellites �This is through orbital tracking, clock monitoring and therefore fundamentally is responsible for the daily control of the system.
�The control segment of the navstar system consists of three main types of operational facilities.
�The master control station situated at colorado springs is responsible for overall satellite control, navigation performance estimation and ephemeris production.
�Four further sites at hawaii, ascension islands, diego garcia and kwajalein alongside the master are operated as monitor stations for tracking the satellites and collecting range data to produce informationfor ephemeris (orbit) modelling.
�Uplink antennas to transmit navigation data and commands to the satellites have also be hoisted from such stations.
�The uplink frequency is centered on 1783.74 Mhz, with downlink frequency of 2227.5 Mhz.
USER SEGMENT:
�The user segment mainly consists of the gps antenna and receiver, additional features however would include a differential gps antenna and various interfaces into systems.
�From noise to signal:the first task for the gps receiver is to get enough signals from the satellite transmissions into the receiver itself.
�It is usually achieved with a pre-amplifier/head amplifier in the antenna unit to boost the signal before sending it down the cable.
�Antennas are designad to receive all signals within the relevant band.the gps signals are very weak and indistinguishable from the background noise at first and second glance.
�These signals are also spread over a 20 Mhz band-width centered around the l1 frequency of 1575.42 Mhz. this has the same effect as transmitting a much more powerful signal and also allows much more information to be incorporated into the transmissions.
�From signal to numbers: most signal processing tasks are now undertaken in micro-processors, but to allow this the signal must be converted from analogue to digital form.
�From numbers to code: the gps receiver will now have at this point the ability to sample the substantially altered frequency of the satellites. the primary task of the receivers is to measure ranges to the satellites.this is achieved by attempting to identify the code transmissions superimposed on to the signals.
�Each satellite transmits a unique code, or technique known as code division multiple access. to identify the code the receiver has to produce an exact replica of the satellite code sequence and to mach the two together.
�From numbers to phase: to make use of this code information to produce ranges and thus position the receiver, also needs another set of information, the satellite ephemeris.
�The ephemeris being modulated over the carrier frequency and needs also to be extracted.
�From measurement to position: once the receiver has produced the necessary information in terms of measured pseudo range and navigation data, this is then poassed to a dedicated microprocessor dealing with the position computation and, usually, the user interface as well.
THE PSEUDO- RANGE:
The pseudo- range is a measure of distance from the receiver to the satellite, usually expressed in mts. The term pseudo is used because the range is contaminated. For time to be accurately measured between thetwo sites the clocks must be accurately synchronised. The clocks between the satellites are synchronised, so the ranges measured between them would actuallyu be true ranges. But the receiver clock is not synchronised to the satellites which gives an error which can be resolved mathematically, hence the term pseudo-range. The pseudo-range cannot be converted to a true range without other sets of information, namely ranges to three other satellites and accurate knowledge of all the satellites positions in space (and time).
COMPUTATION OF POSITION:
The ability to measure pseudo range and knowledge of the satellites position at all times, the GPS receiver now has enough information to calculate a position. For every computation of position there are,then four unknowns called x,y,z and t (for time). But by this point the receiver has also managed to acquire a whole series of knowns namely, pseudo ranges to the satellites and the position of those satellites. As long as the receiver can measure as many ranges to the satellites as there are unknowns then position can be calculated quite simply through a series of four simultaneous equations. This is a mathematical technique that uses a combination of known quantities to calculate a combination of unknown quantities, but it does require symmetry in its equation forms- basically the same number or more knowns to unknowns.
THE DILUTION OF PRECISION:
This is one of the primary quality control indicators available to the user of the GPS receiver. It indicates the geometrical relationship of the satellites. The dilution of precision was the mechanism used by the GPS system planners to identify the best orbit geometry of the satellites to provide the best user geometries. On its own the DOP figure is only a qualitative figure with the prescribed numbers not being of any specific units. the most complete dop is the Geometric Dilution Of Precision (GDOP). This is the factor used to design the orbital arrangement of the satellites. gdop brings together the four unknowns of the system. The most frequently used dop's are PDOP ( position dilution of precision) and HDOP ( horizontal dilution of precision). PDOP is used by those interested in three dimentional positioning (lat/long/ht). HDOP is for two- dimensional positioning. For most purposes HDOP is used by the marine community. DOP figures are actually used by a navigator in the following way.
If a composite pseudo-range measurement error of 10 mts. (95% probability) is assumed for the system, then by multiplying this figure by the relevent dop value, eg. 3.0, gives an overall position accuracy of 30 mts. DOP figures are therefore only relative numbers, with smaller dop's giving better accuracies. The final constellation is designed to guarantee pdop figures of better than 12.0 all over the earth's surface, though figures of better than 7.0 are to be generally expected.
For marine applications HDOP figures are more useful & the design figure of 3.0 is to be expected for the majority of coverage.
ERRORS OF THE SYSTEM.
1)GDOP - If the configuration of the available satellites is not suitable, accuracy of the fix is affected.
2)IONOSPHERIC & TROPOSPHERIC DELAYS OF SIGNALS - While ultra high frequencies are used for transmission, yet the ionosphere & troposphere refract the signals causing delays which lead to inaccuracy of positions. These errors can be reasonably predicted & are fed into a mathematical model built into the software of the receiver.
3) SATELLITE CLOCK ERROR - Any errors in the clocks on board the satellites will lead to errors in time measurement.
4) USER CLOCK ERROR - Same as above. This is eliminated by using an extra satellite equation.
5) DEVIATION OF SATELLITES FROM THEIR PREDICTED ORBITS - Since the position of the satellites needs to be known very accurately, it is obvious that if the satellite deviates from its orbit, for any reason, then inaccuracy in fixes will result.
6) RECEIVER ERRORS - These are small & are caused due to internal noise, computational errors, etc.
THE DILUTION OF PRECISION.
| GDOP | GEOMETRIC DILUTION OF PRECISION INTEGRATES X,Y,Z, AND TIME. |
| PDOP | POSITION DILUTION OF PRECISION INTEGRATES X,Y,Z, THREE DIMENTIONAL POSITIONS. |
| HDOP | HORIZONTAL DILUTION OF PRECISION. TWO DIMENTIONAL MARINE POSITIONING. |
| EDOP | EASTING DILUTION OF PRECISION |
| NDOP | NORTHING DILUTION OF PRECISION, X AND Y POSITIONING SEPERATED. |
| TDOP | TIME DILUTION OF PRECISION FOR TIME TRANSFER USERS. |
The satellite geometry is presented to the user by a factor known as the dilution of precision. These figures are used to assess the potential positioning quality of a certain satellite constellation and to help provide realistic quality control information. The procedure used to define these values is quite complicated, but it relates the difference in three dimentions of the user to all the concidered satellites in a geometrical sense. The resultant dop figure then suggests the amplification of pseudo-range measurement error into user positioning error. Different DOP's are used depending on the type of position calculated. HDOP for a two dimentional fix and PDOP for a three dimentional fix.
The DOP figures are used by the navigator in the following way. If a composite pseudo-range measurement error of 10 metres (95% probablity) is assumed for the system, then multiplying this figure by the relevant dop value, eg. 3.0, gives an overall positioning accuracy of thirty metres. dop figures are therefore only relative numbers, with smaller dop's giving better accuracy. the probablity figures detailed in the circular brackets are indications of the confidence in the position accuracy.
LOGS
ELECTROMAGNETIC LOG:
In a magnetic field created by a solenoid, the magnetic flux lines extend out in the water. Salt water being a conductor flowing past the solenoid creates an emf. This is picked up by a sensor & a pre-calibrated voltmetre.
The voltage induced is proportional to the speed of the water.
DOPPLER EFFECT:
The doppler frequency shift is the difference between the received frequency which can be measured & the transmitted frequency which is known.
� THERE ARE 2 TYPES OF LOGS;
1. CONTINUOUS WAVE (CW) &
2. PULSE TYPE.
The pulse type is better than the cw. type because there is a signal leakage from the transmitter to the reeciver which interferes with the received signal (feedback).
�It uses a higher frequency than the echo sounder as the smaller wavelength makes a concentrated beam possible hence, less loss of energy to water, stronger echoes & smaller area of transducer required. Also diffused reflection is aquired by the high frequency which is neccesary because of oblique incidence of the transmitted pulse.
Also you can download these files which will help you during orals
| FLAGS | flags.zip |
| LIGHTS | lights.zip |
| MORSE | morse.zip |
| RULES | rules.zip |
| SOLAS | Solas.zip |
| SOUND | Sounds.zip |
Copyright � 2000-2002 Rautelas. All rights reserved.