Basics of Radio Waves and Doppler Effect

Decimetric waves are radio waves with a frequency range of 300 to 3000 MHz. 1 GHz (Gigahertz) equals 1000 MHz (Megahertz).

The Doppler effect causes a change in the frequency of a signal due to the relative motion between the source and the receiver. This effect is exploited in Doppler navigation systems to determine aircraft ground speed and drift. In this case, the aircraft is moving towards the receiver, so the received frequency will be higher than the transmitted frequency.

The effective length of a half-wave dipole antenna is approximately half the wavelength of the signal it is designed to transmit. Lambda (wavelength) is the distance travelled by the signal in one cycle of its propagation. Speed of light (c) = 3 x 10^8 m / s.

A broad bandwidth allows for the transmission of a wider range of frequencies, which can be useful for carrying more information or for more accurate positioning.

Radio waves are reflected by metallic objects that are comparable in size to the wavelength of the signal.

Surface attenuation refers to the loss of signal strength as radio waves travel over the surface of the Earth. This attenuation is greater for longer wavelengths.

Sky waves travel through the ionosphere and can propagate long distances. Ground waves travel along the surface of the Earth and have a shorter range. Duct propagation occurs when radio waves are trapped between layers in the atmosphere and can propagate long distances.

ILS is an instrument landing system that uses radio waves to provide pilots with precise guidance for landing. The ILS frequency is within the VHF band. VHF is 30 to 300 MHz.

The gain of an antenna is a measure of its ability to focus power in a specific direction. It is a measure of its ability to focus electromagnetic radiation in a given direction. The HIGHER the gain, the more power is delivered over a smaller area.

VHF frequencies are generally less prone to static interference than lower frequencies. VHF is 30 to 300 MHz. HF is 3 to 30 MHz. LF is 30 to 300 kHz.

The VHF band is broadly 30 MHz to 300 MHz, with short wavelengths (100m to 10m).

The frequency and wavelength of a radio wave are inversely proportional. The speed of light is constant (c). c = f x λ.

LF radio waves travel farther during the day than MF radio waves; MF radio waves are more susceptible to atmospheric attenuation. LF is 30 to 300 kHz. MF is 300 kHz to 3 MHz.

As the frequency of a transmitter increases, the range of the ground wave decreases. This is due to the higher frequencies being more susceptible to attenuation by the Earth's surface.

The Doppler effect causes a change in the frequency of a signal due to the relative motion between the source and the receiver. Doppler navigation systems use this effect to determine aircraft ground speed and drift.

In Doppler navigation, sea bias error occurs due to changes in the Doppler shift over water. This is NOT corrected by the land/sea switch.

A long period in memory can reduce the accuracy of any Doppler navigation system. This is because the inherent drift in the system accumulates over time.

Doppler Navigation systems use frequencies of 8.8 kHz and 13.3 kHz.

The 4-beam moving Janus array measures the Doppler shift from each of the beams. The array rotates until the beams with the smallest Doppler shift are aligned with the aircraft's centerline.

Radio altimeters are active radar-based devices that measure the distance between an aircraft and the ground. They typically operate in the range of 4.2 to 4.4 GHz (4.2 to 4.4 x 109 Hz). They use the principle of measuring the time taken for the radar pulse to travel to the ground and back.

Coastal refraction occurs when radio waves from an NDB cross from land to sea. The waves bend towards the normal as they slow down.

Night effect is a phenomenon that affects the accuracy of NDB bearings. The D layer of the ionosphere, which is present only at night, reflects NDB signals, causing errors in the bearings received by the aircraft.

ADF is a navigation system that uses radio waves to determine the direction of an NDB. It uses a loop antenna to determine the null point of the NDB signal. The null is aligned with the NDB when the loop is rotated to a maximum signal strength.

The BFO is the beat-frequency oscillator. It is used in an ADF receiver to detect the 1020 Hz modulation signal from an A1A NDB. The signal will be received in the LOOP position on the ADF and allows the pilot to determine the signal direction for the ADF to follow.

Station interference occurs when a second NDB signal is in range of the aircraft and is received simultaneously by the ADF. This can cause errors in the ADF bearing.

The D layer of the ionosphere does NOT affect the accuracy of NDB bearings at any time. The D layer is a region of the ionosphere that is below the E-layer and the F-layer.

A VOR is a radio navigation beacon that transmits a signal that can be used by aircraft to determine their bearing from the beacon. VOR beacons use a rotating limacon signal pattern. This creates the VOR signal.

A VOR beacon operates within the VHF band. VHF is 30 to 300 MHz.

The TO/FROM indicator shows whether the aircraft is heading towards or away from the VOR,

The Declared Operational Coverage (DOC) of a VOR is the area where the accuracy of the VOR signal is guaranteed. The DOC is defined in terms of both distance from the beacon and altitude. The signal will not be accurate outside the DOC.

Scalloping is a type of error that can occur when using VOR navigation. It results in a rapid fluctuation in the bearing indication. Scalloping occurs when the signal path is distorted, due for example to reflections from buildings or mountains. This is another challenge in VHF band signal propagation.

A Doppler VOR reverses the reference and variable signals compared to a conventional VOR. This can lead to a reversal of the TO/FROM indications when using a Doppler VOR, as the signal phase difference is the same but the reference and variphase signals are flipped.