KUP 4: Ability to determine the ship’s position by use of electronic navigational aids

Learning objective:

The ability to determine a ship’s position using electronic navigational aids is a critical aspect of modern maritime navigation. Electronic navigational aids leverage advanced technology to provide accurate, real-time information about a vessel’s location, speed, and direction, ensuring safe and efficient voyages. The learning objective of this section is to understand how the electronic navigational aids work.

Learning content:

1. Radio direction finder

The method of radio direction finding positioning is similar to that of land-based direction positioning. Due to the limited line of sight and visibility constraints of the ship’s observer, land-based direction positioning has a shorter effective range. However, a ship’s radio direction finder can determine the bearing of a radio station at a greater distance (about 100 nautical miles) and under any visibility conditions. By using the ship’s radio direction finder to simultaneously measure the bearings of signals transmitted by two or more known-position radio stations on the shore, two or more position lines can be obtained, thus determining the ship’s position. This type of radio station dedicated to direction finding is called a radio beacon.

The essence of radio direction finding is to use a directional loop antenna installed on the ship to determine the direction of a working radio beacon. The direction indicated by the direction finder, which is obtained using the loop antenna, has a bidirectional ambiguity. Therefore, to correctly identify the radio beacon’s direction, a non-directional vertical antenna is added to form a composite antenna. This setup solves the bidirectional ambiguity problem of the loop antenna. The operation of identifying the correct direction of the radio beacon is called “sense determination.” The data read from the radio direction finder is the radio bearing reading of the radio beacon. A radio bearing reading of zero indicates the ship’s heading direction. The radio bearing reading Qrr indicated by the ship’s radio direction finder during direction finding does not represent the correct radio bearing Qr of the radio beacon. There is a difference of an angle f between the two, known as the “radio deviation¹“.

2. LORAN C

LORAN C is a low-frequency, pulse-group long-range hyperbolic navigation system developed from Loran-A. It uses pulse envelope matching for coarse measurement and pulse carrier phase matching for precise measurement. This allows the system to obtain the time difference between the pulse signals from the master and secondary stations of the Loran chain, thereby obtaining the hyperbolic lines of position based on the distance difference. The measurement accuracy of pulse carrier phase matching is an order of magnitude higher than that of pulse envelope matching. The Loran-C system operates at a low frequency of 100 kHz, with a long baseline, and employs techniques such as multi-pulse phase coding and correlation reception. Its effective range far exceeds that of the Loran-A system, reaching up to 1000-2000 nautical miles.

There are two types of shipborne Loran-C receivers. The first type is the A/C dual-use receiver, which can receive both Loran-A and Loran-C signals. However, it does not utilize the main advantages of Loran-C’s multi-pulse phase coding, nor does it use correlation reception technology. Instead, it relies on manual pulse phase synchronization, resulting in measurement accuracy being significantly affected by the signal-to-noise ratio (SNR). With an SNR of 5:1, the standard deviation can reach 1-2 microseconds. This type of receiver only provides Loran time difference data and cannot directly display the ship’s position. Users need to use the Loran-C chart or table for positioning based on the time difference data. This type of receiver has mostly been phased out.

The second type is the automatic Loran-C receiver, which employs multi-pulse phase coding, correlation reception technology, and computer technology. It can automatically identify the master and secondary stations, measure the pulse phase difference with high precision, and convert the data into latitude and longitude. Due to the use of correlation reception technology, the automatic Loran-C receiver can detect Loran-C signals buried in noise and can still operate in an environment with an SNR of 1:5, with a standard deviation of only 0.1 microseconds. The operating range of the automatic Loran-C receiver is greater than that of the A/C receiver, increasing from 1200 nautical miles during the day to 2000 nautical miles, and from 700 nautical miles at night to 1000 nautical miles. Currently, most ocean-going ships are equipped with Loran-C receivers¹.

3. Decca

Decca is a low-frequency, phase-difference hyperbolic navigation system. Its positioning principle is essentially the same as that of Loran-C, as both systems determine the ship’s position based on the hyperbolic lines of position created by the distances between the ship and fixed shore stations. The difference between the Decca system and the Loran-C system lies in the method used to measure the distance difference. The Decca system obtains the distance difference by measuring the phase difference of continuous waves that are strictly synchronized and transmitted by two shore stations. The operating frequency of the Decca system is between 70 kHz and 130 kHz, with a reliable working distance of approximately 240 nautical miles. Its positioning accuracy ranges from tens to hundreds of meters, providing high precision. Therefore, Decca is a short-range, high-precision hyperbolic navigation system.

4. GPS

GPS is the abbreviation for Navigation Satellite Timing and Ranging/Global Positioning System, also known as NAVSTAR/GPS. GPS is a radio navigation system based on space satellites. It can provide continuous, high-precision three-dimensional positioning, speed, and time information to users on the sea, on land, in the air, and in space globally and in all weather conditions. The establishment of GPS has brought about profound, epoch-making changes in the positioning and navigation of sea, land, air, and space transport.

GPS determines position by transmitting two types of pseudo-random codes from satellites: the P code and the C/A code. The P code is a precise code, exclusively for military use, highly confidential, with an accuracy of 3 meters (2d rms). The C/A code is a coarse code, available for public and civilian use, with specified accuracies of 100 meters (2d rms) for horizontal position and 157 meters (2d rms) for vertical height. For civilian users, Differential GPS (DGPS) technology can improve positioning accuracy to a few meters¹.

Currently, GPS is the primary navigation system in maritime navigation. GPS receivers are widely equipped on ships, and Differential GPS receivers are also installed on some vessels.

Recommendation:

1. GPS

Reference:

1. Navigation. Ding Yong. Publisher: People’s Communications Press.