Antenna height as well as frequency or wavelength governs the number of lobes in the interference pattern. The number of the lobes and the fineness of the interference pattern increase with antenna height. Increased antenna height as well as increases in frequency tends to lower the lobes of the interference pattern.

The pitch and roll of the ship radiating do not affect the structure of the interference pattern.

DIFFRACTION

Diffraction is the bending of a wave as it passes an obstruction. Because of diffraction there is some illumination of the region behind an obstruction or target by the radar beam. Diffraction effects are greater at the lower frequencies. Thus, the radar beam of a lower frequency radar tends to illuminate more of the shadow region behind an obstruction than the beam of a radar of higher frequency or shorter wavelength.

ATTENUATION

Attenuation is the scattering and absorption of the energy in the radar beam as it passes through the atmosphere. It causes a decrease in echo strength. Attenuation is greater at the higher frequencies or shorter wavelengths.

ECHO CHARACTERISTICS

While reflected echoes are much weaker than the transmitted pulses, the characteristics of their return to the source are similar to the characteristics of propagation. The strengths of these echoes are dependent upon the amount of transmitted energy striking the targets and the size and reflecting properties of the targets.

The radar wave in its simplest form may be described as a plane, transverse, linearly polarized, electromagnetic wave.

The classification PLANE WAVE refers to the shape of the wavefront or equiphase surface, i.e. the locus of points at which the fields are in the same phase of their cycle of variation. Being perpendicular to the equiphase surface, the direction of propagation would vary from point to point if the surface were not plane.

The classification TRANSVERSE refers to the plane tangent to the equiphase surface being perpendicular to the direction of propagation. With both the electric-and magnetic-field vectors lying in the transverse plane, the radar wave may be described as completely transverse as a TRANSVERSEELECTROMAGNETIC WAVE. With only one of the field vectors lying in the transverse plane, the wave is either a TRANSVERSEELECTRIC or TRANSVERSE-MAGNETIC WAVE. Either of these waves can be transmitted in a waveguide.

The classification LINEARLY POLARIZED indicates that the components of the electric and magnetic fields in the transverse plane do not change in direction from point to point or with time. The direction of polarization is the direction in space of the electric-field vector.

An ELLIPTICALLY POLARIZED WAVE can be obtained by combining vertically and horizontally polarized waves. With difference in the phases of the variations of the electric-field intensity in the two waves, the direction of the resultant electric field will vary with time and distance. Therefore, in any transverse plane, the head of the resultant electric-field vector will follow an elliptical path. If the component waves are of equal magnitude and 90° out of phase, the ellipse will become a circle, and the resultant wave will be CIRCULARLY POLARIZED.

receiver. This error causes the indicated ranges to be greater than their true values.

A device called a trigger delay circuit is used to eliminate the fixed error. By this means the trigger pulse to the indicator can be delayed a small amount. Such a delay results in the sweep starting at the instant an echo would return to the indicator from a flat plate right at the antenna-not at the instant that the pulse is generated in the transmitter.

Line Voltage

Accuracy of range measurement depends on the constancy of the line voltage supplied to the radar equipment. If supply voltage varies from its nominal value, ranges indicated on the radar may be unreliable. This fluctuation usually happens only momentarily, however, and after a short wait ranges normally

are accurate.

Frequency Drift

Errors in ranging also can be caused by slight variations in the frequency of the oscillator used to divide the sweep (time base) into equal range intervals. If such a frequency error exists, the ranges read from the radar generally are in error by some small percentage of the range.

To reduce range errors caused by frequency drift, precision oscillators in radars usually are placed in a constant temperature oven. The oven is always heated, so there is no drift of range

The range to a target can be measured most accurately on the PPI when the leading edge of its pip just touches a fixed range ring. The accuracy of this measurement is dependent upon the maximum range of the scale in use. Representative maximum error in the calibration of the fixed range rings is 75 yards or 1-1/2 percent of the maximum range of the range scale in use, whichever is greater. With the indicator set on the 6-mile range scale, the error in the range of a pip just touching a range ring may be about 180 yards or about 0.1 nautical mile because of calibration error alone when the range calibration is within ac ceptable limits.

On some PPI's range can only be estimated by reference to the fixed range rings. When the pip lies between the range rings, the estimate is usually in error by two to three percent of the maximum range of the range scale setting plus any error in the calibration of the range rings.

Radar indicators usually have a variable range marker (VRM) or adjustable range ring which is the normal means for range measurements. With the VRM calibrated with respect to the fixed range rings within a tolerance of 1 percent of the maximum range of the scale in use, ranges as measured by the VRM may be in error by as much as 2-1/2 percent of the maximum range of the scale in use. With the indicator set on the 8-mile range scale, the error in a range as measured by the VRM may be in error by as much as 0.2 nautical mile.

Pip and VRM Alignment

The accuracy of measuring ranges with the VRM is dependent upon the ability of the radar observer to align the VRM with