Electronic Navigation Aids

What is AIS?

The automatic identification system, or AIS, transmits a ship’s position so that other ships are aware of its position. The International Maritime Organization and other management bodies require large ships, including many commercial fishing vessels, to broadcast their position with AIS in order to avoid collisions.

 

AIS Class A and Class B Transponders

AIS Types. Class A: Mandated for all vessels 300 GT and above engaged on international voyages as well as all passenger ships. Class B: Provides limited functionality and is intended for non-SOLAS vessels. Primarily used for vessels such as pleasure crafts

161.975 MHz

AIS operates principally on two dedicated frequencies or VHF channels: AIS 1: Works on 161.975 MHz- Channel 87B (Simplex, for ship to ship) AIS 2: 162.025 MHz- Channel 88B (Duplex for ship to shore)

• Exchange of short text (safety related) Messages
• Operates in the VHF maritime band on two designated channels (2250 x 2 slots / min)
• High capacity - over 2000 reports / min
• Advantages over radar: The AIS is able to detect ships within VHF/FM range around bends and behind islands, if the landmasses are not too high
• A typical value between ships is to be expected about 20 to 30 nautical miles depending on antenna height

around ten years ago various organizations discovered that, much to everyone's surprise, these short range signals could in fact be picked up from above the Earth's atmosphere.

This was not expected as the maximum horizontal range at sea level is around 50 nautical miles (74 kms), yet these same signals could be received on the ISS, 400 kilometers up.

Type of messages

• Static
• Dynamic
• Voyage related
• Safety and Security

If the waves are transmitted directly towards the seabed, perpendicular to the keel, there will be no Doppler shift. This is because there will be nor relative approach between the source and reflecting surface, hence the transmitted and received frequency will be the same.

This can also be proved mathematically with cos 90° = 0 in the above equation "f_r will be equal to "f_t

This is also referred to as bottom track or ground track. The transmitted pulse with its limitation in power and can reach up to a depth of usually 200 meters. Beyond this depth, the echoes from the seabed become very weak and hence the strength to calculate the Doppler shift. Interestingly, echoes are also available from water layers between 10 and 30 meters below the keel. When the ship moves at her usual full-speed it carried some mass of surrounding water with it and thereby providing a distinct interface at a depth of about 20 meters between moving and still water. This interface acts as an echoing surface of the acoustic waves. These echoes are weak since the echoing surface is liquid, but stronger than the echoes coming from the depths of over 200 meters and enough to measure Doppler Shift. The speed obtained from this layer is speed over water i.e. to say it affected by set and drift of the prevailing current. The equipment automatically changes over to water track when the echoes from the sea-bed are not strong enough.

Athwart ship's Speed (port- starboard)

Doppler log on ground track mode can provide athwart ship speed as well and for this purpose a similar Janus configuration is used on the port and starboard sides. The athwart ship speed is calculated in the similar manner as the forward and aft speed.

Electromagnetic Log

This type of log can give only speed through water and is greatly affected by the current flowing under the ship.

The induced e.m.f. and hence the speed indication will vary with the conductivity of the water.

This e.m.f. is picked up by 2 electrodes.

This induced e.m.f. is very small hence amplification is required.

The amplified signal thus drives the mechanism which is connected to indicator. Hence, the induced e.m.f. which is directly proportional to the velocity is finally displayed on the indicator.

The induced e.m.f. is directly proportional to the velocity.

Velocity when integrated with time gives distance

An Electromagnet consisting of a coil carrying alternative Current (A.C.) generates a vertical magnetic field in the water around the probe.

The induced e.m.f. 'E' is given by the following:

E = F × L × V

Where F = magnetic field

L = the length of the conductor

V = the velocity of the conductor through the magnetic field.

The electromagnetic speed log is based is upon the induction law, which states that if a conductor moves across a magnetic field, an electro motive force (e.m.f.) is set up in the conductor.

Alternatively, the e.m.f. will also be induced if the conductor remains stationary and the magnetic field is moved with respect to it.

The SW Conductor moving horizontally through this magnetic field has an electromotive force induced into it proportional to the speed of the vessel.

In the EM log the 'F' and 'L' are maintained constant, therefore the induced e.m.f. is directly proportional to the velocity 'V',

which is the velocity of the vessel through the water.

The speed output from an EM log depends upon the water flow by way of the sensors.

the European Union's Galileo

the United States' Global Positioning System (GPS)

GLONASS, which stands for "Global Navigation Satellite System," is a satellite-based navigation system developed and operated by the Russian Federation. and other similar navigation systems like and

GLONASS was initially developed by the Soviet Union and became fully operational in the 1990s. It was designed to provide accurate positioning, navigation, and timing information for a wide range of applications, including civilian, commercial, and military purposes. The system consists of a constellation of satellites in orbit around the Earth, ground control stations, and user equipment.

Pseudorange corrections are adjustments

Pseudorandom (PRC) Code

Precision: The P code provides a higher level of accuracy compared to the C/A code. This is achieved by using longer codes and more complex modulation techniques.

Anti-Jamming: The P code is designed to be resistant to jamming and spoofing, making it more difficult for adversaries to interfere with military GPS operations.

Selective Availability: In the past, the U.S. military intentionally degraded the accuracy of the GPS signals available to civilian users. This practice was known as "Selective Availability." The P code was not affected by this degradation, ensuring that the military still had access to accurate positioning

Encrypted: The P code is encrypted, meaning that unauthorized users cannot easily decode and utilize the signal. This enhances the security of military operations that rely on GPS.

Each S.Vs' Transmitting Frequency ( 2 Bands )

L1 Band : 1575.42 MHz  λ= 19 cm)

Carries both ( C/A) Code & (P) Code

 

L2 Band : 1227.60 MHz  λ= 24.5 cm )Carries only P Code

Superimposed on both the C/A and P codes is the Navigation message

Dual—channel receivers (tuning to both L1 and L2) are able to use both frequencies to correct effects of ionospheric refraction.

Horizontal Dilution of Precision (HDOP):

HDOP is a measure of the geometric quality of the satellite configuration in relation to the receiver's position on the Earth's surface. It indicates the precision or accuracy of the horizontal position (latitude and longitude) calculated by the GPS or GNSS receiver. Lower HDOP values indicate better satellite geometry and typically result in more accurate horizontal positioning.

Vertical Dilution of Precision (VDOP):

VDOP is similar to HDOP but specifically refers to the vertical position (altitude) accuracy. It measures how well the satellites are distributed in the sky in relation to the receiver's position for determining altitude. Lower VDOP values indicate better satellite geometry for accurate altitude calculation.

1. Rudder Control>>sensitivity .. responsiveness ... determining the amount of Rudder angle needed to correct the deviation from the desired course e.g. 2 Deg for 2 deg off-course.

2. Counter Rudder>>For instance; Judging the gradual shift from 090 on the compass to 088, with a 2-degree deviation, requiring a 4-degree adjustment in helm angle. Setting counter rudder is to enable the ship's prompt return to the intended course.

3. Rate of Turn>>When turning, the rudder will move as much as it takes to attain the required turn rate without exceeding the set value.

4. weather or yaw>>any alteration in heading caused by wind, resulting in the vessel veering off course. Change in heading means that it requires rudder input for correction. The differentiating factor is the 'time delay'. If the ship doesn't realign within 2 seconds, it's categorized as Course Changing and rudder needs to be used

5. Permanent» Permanent rudder setting involves a temporary adjustment of the midship position away from the rudder's midship position. This helps maintain a steady course for the interim period.

6. Rudder limit>>Rudder can be set at a limit angle e.g. @ 10 deg. Rudder will never go beyond 10 deg. when correcting the off course. That setting will cause rate of turn to reach back to set course. Auto pilot may also be set to use own programming itself.

7. Off course alarm>>audible signal on setting when the course is deviated.