Marine Electronic Navigation

Stephen F. Appleyard

• Verlag: Routledge
• Jahr: 2006
• ISBN:
  9781134963096
• Auflage: 2
• Thema: Commerce
• Thema: Commerce Général

Aclassic reference text for both nautical students and for all who have a professional involvement with marine electronic navigation systems, this second edition has been substantially enlarged to include all of the electronic systems now encountered by navigation / communication personnel.

1 Radiation and Propagation

An understanding of the behaviour of radio waves during propagation from their source to point of reception is fundamental to an understanding of all radio navigation systems. It is particularly relevant to an appreciation of their limitations, since propagational effects are invariably the major source of error. (A more detailed treatment of this topic is included in chapter 19.)

1.1 Electromagnetic Waves

Radio waves form a specific part of the total spectrum of electromagnetic radiation of which light is also a part. An electromagnetic wave can be considered as an oscillating electric force travelling through space, and inseparably accompanied by an oscillating magnetic force in a plane at right angles to it (Figure 1.1). The plane of the electric field in space provides the basis of defining the wave’spolarization. A wave with a vertical electric field is said to be vertically polarized.

Figure 1.1 The electromagnetic wave, illustrating the relationship between electric and magnetic fields, and direction of propagation.

The relationship of radio waves with the rest of the spectrum of electromagnetic radiation is illustrated in Figure 1.2. Radio waves are usually specified in terms of their frequency (f), which is related to wavelength (λ) by the expression:

Figure 1.2 The spectrum of electromagnetic radiation.

where c is the velocity of electromagnetic radiation in a vacuum (free space) and has been determined as being 299, 792 km/s, although the approximation of 3 × 10⁵ km/s is often used in equation [1.1].

The unit of frequency is the Hertz (Hz), and the radio wave part of the electromagnetic spectrum extends from 3 × 10³ Hz to 30 × 10⁹ Hz, although these are not rigidly defined limits. Since very large numbers are involved the following prefixes are assigned:

Figure 1.3 illustrates the position of the different radio navigation aids in the radio wave spectrum and relates them with other sources of radio waves.

Figure 1.3 The radio wave spectrum.

1.2 Propagation

The behaviour of radio waves and the influences which affect them during their passage from transmitter to receiver are dependent upon the frequency of the wave. All radio waves within a given frequency band will have the same propagational characteristics irrespective of their use, and so the following descriptions of propagation apply equally to other radio signals on the same or adjacent frequencies.

When energy is radiated from an omnidirectional transmitting antenna some energy will travel away from the earth, and some will travel away from the antenna remaining (initially) parallel with the ground. In explaining the mechanisms of propagation these two directions are considered separately, and are termed ‘skywave’ and ‘groundwave’ respctively. The relative importance of the skywaves and groundwaves depends upon many factors, which include the frequency of the transmission, the time of day, and the distance between transmitter and receiver.

1.2.1 Groundwaves

The groundwave can be subdivided into two components, the ‘space wave’ and the ‘surface wave’. The space wave can be further divided into the ‘direct wave’ and the ‘ground reflected wave’. These latter two waves illustrated by Figure 1.4 are of little significance in the various radio navigation systems described in this book, since their range is short and in many cases the two waves cancel at the receiver.

Figure 1.4 The two components of groundwave propagation, space wave and surface wave. Space wave has two components, (a) direct wave, and (b) ground reflected wave.

Of more significance is the surface wave, since in this case the earth’s surface and the lower atmosphere influence the wave in such a way as to cause it to follow the curvature of the earth. Since energy is transferred from this wave to the ground, the distance over which the wave can propagate depends upon the frequency of the transmission, and the properties of the ground over which the wave passes. The distance over which a surface wave can travel before suffering unacceptable attenuation varies from only hundreds of feet to many thousands of miles.

At low and medium frequencies, horizontally polarized surface waves suffer much greater attenuation than vertically polarized surface waves. In this frequency range therefore, antennas are designed to transmit and receive vertically polarized waves.

Since the space wave does not play any significant part at the frequencies of the radio navigation aids described in this book, the general expression groundwave is used throughout to mean surface wave.

1.2.2 Skywaves

It may be thought that waves travelling away from the surface of the earth would be lost into space and thus play no further part. This is by no means always the case since around the earth is the ionosphere, a belt of ionized gases which extends from approximately thirty miles to several hundreds of miles from the earth’s surface. The effect of the ionosphere is to cause waves of certain frequencies to refract, and ultimately reflect back to the surface.

1.3 The Ionosphere

During day-time the densest region of ionization exists between altitudes from sixty to six hundred miles. Throughout this region there are several layers in which the ionization density is at a maximum, known as the D-, E- and F-layers (Figure 1.5a). During the day, the F-layer splits into two layers and these are designated F1 and F2. The density of ionization of these layers depends upon many factors including time of day, season, latitude, and the phase of the eleven-year sunspot cycle. During night-time, all layers of the ionosphere slowly de-ionize. In particular the D-region quickly disappears in the absence of the sun and quickly ionizes shortly after the following sunrise (Figure 1.5b).

Figure 1.5 The ionosphere: (a) day-time; (b) night-time.

During both day- and night-time the ionosphere has the effect of refracting the radio waves which pass through it. The amount by which the waves are refracted is dependent upon the density of the ionization of each of the layers, and on the frequency of the radio waves. In general, as the frequency decreases the amount of refraction increases, until the point is reached where the wave isactually reflected back from the ionosphere (Figure 1.6). Since the density of the ionosphere varies daily and seasonally, a radio wave of a given frequency may be reflected at some times and not at others.

Figure 1.6 Ionospheric refraction and reflection of radio waves, frequency f₁ > f₂ and f₂ > f₃.

The skywave returning to earth provides signal reception at a distant point from the transmitter, termed the skip distance (Figure 1.6). Skywaves can undergo two or more reflections. When radio signals are used for communication, the presence of the reflected skywave is of great value since it makes communication over many thousands of miles possible, far beyond the range of the groundwave signal. Communication frequencies are therefore often chosen to make optimum use of skywave signals.

The opposite to this is generally the case for radio navigation systems, since these rely upon a precise knowledge of the propagation times of the signal from transmitter to receiver. For communication purposes accurate ionospheric predictions can be made relating to the presence of skywave reflections for given frequencies, but it is the precise time delays caused by the ionosphere which are difficult to predict with precision. In certain cases skywaves are used to give extended operation, Loran C being one example, but the positional accuracies are considerably reduced from those normally achieved with groundwaves. Usually the presence of a skywave is a matter of nuisance, and in extreme cases the skywave interferes with the groundwave to cause a system to become unusable.

Transmissions from Loran C (100 kHz), Decca (70–130 kHz) and direction-finding (DF) beacons (up to 350 kHz) behave in a similar manner. During day-time the ionized D-region attenuates the skywave both before and after it is reflected by the E-region. The skip distance falls within the groundwave, but the skywave has been attenuated sufficiently to prevent serious interference with the groundwave.

At night the D-region de-ionizes and the attenuation of the skywave is now less. Reflections occur from both the E- and F- layers, with some signals returning within the groundwave and some beyond. The precise effect of these sky waves on the performa...