| Home | Privacy | Contact |

Instrument Flying Handbook
Navigation Systems
Basic Radio Principles

| First | Previous | Next | Last |

Instrument Flying


Table of Contents

Chapter 1. Human Factors
Chapter 2. Aerodynamic Factors
Chapter 3. Flight Instruments
Chapter 4. Section I
Airplane Attitude Instrument
Using Analog Instrumentation
Chapter 4. Section II
Airplane Attitude Instrument
Using an Electronic Flight

Chapter 5. Section I
Airplane Basic
Flight Maneuvers
Using Analog Instrumentation
Chapter 5. Section II
Airplane Basic
Flight Maneuvers
Using an Electronic Flight

Chapter 6. Helicopter
Attitude Instrument Flying

Chapter 7. Navigation Systems
Chapter 8. The National
Airspace System

Chapter 9. The Air Traffic
Control System

Chapter 10. IFR Flight
Chapter 11. Emergency

Basic Radio Principles

A radio wave is an electromagnetic (EM) wave with
frequency characteristics that make it useful. The wave
will travel long distances through space (in or out of the
atmosphere) without losing too much strength. An antenna
is used to convert electric current into a radio wave so it can
travel through space to the receiving antenna, which converts
it back into an electric current for use by a receiver.

How Radio Waves Propagate
All matter has a varying degree of conductivity or resistance
to radio waves. The Earth itself acts as the greatest resistor
to radio waves. Radiated energy that travels near the ground
induces a voltage in the. ground that subtracts energy from the
wave, decreasing the strength of the wave as the distance from
the antenna becomes greater. Trees, buildings, and mineral
deposits affect the strength to varying degrees. Radiated
energy in the upper atmosphere is likewise affected as the
energy of radiation is absorbed by molecules of air, water,
and dust. The characteristics of radio wave propagation vary
according to the signal frequency and the design, use, and
limitations of the equipment.

Ground Wave
A ground wave travels across the surface of the Earth. You
can best imagine a ground wave's path as being in a tunnel
or alley bounded by the surface of the Earth and by the
ionosphere, which keeps the ground wave from going out
into space. Generally, the lower the frequency, the farther
the signal will travel.

Ground waves are usable &r navigation purposes because
they travel reliably and predictably along the same route
day after day, and are not influenced by too many outside
factors. The ground wave frequency range is generally from
the lowest frequencies in the radio range (perhaps as low as
100 Hz) up to approximately 1,000 kHz (1 MHz). Although
there is a ground wave component to frequencies above this,
up to 30 MHz, the ground wave at these higher frequencies
loses strength over very short distances.

Sky Wave
The sky wave, at frequencies of 1 to 30 MHz, is good for
long distances because these frequencies are refracted or
"bent" by the ionosphere, causing the signal to be sent back
to Earth from high in the sky and received great distances
away. [Figure 7-1] Used by high frequency (HF) radios in
aircraft, messages can be sent across oceans using only 50
to 100 watts of power. Frequencies that produce a sky wave
arc not used for navigation because the pathway of the signal
from transmitter to receiver is highly variable. The wave is
"bounced" off of the ionosphere, which is always changing
due to the varying amount of the sun's radiation reaching it
(night/day and seasonal variations, sunspot activity, ate.). The
sky wave is, therefore, unreliable for navigation purposes.

For aeronautical communication purposes, the sky wave
(HF) is about 80 to90 percent reliable. HF is being gradually
replaced by more reliable satellite communication.

Space Wave
When able to pass through the ionosphere, radio waves
of 1.5 MHz and above (all the way up to many GHz), are
considered space waves. Most navigation systems operate
with signals propagating as space waves. Frequencies above
100 MHz have nearly no ground or sky wave components.
They' are space waves, but (except for global positioning
system (GPS)) the navigation signal is used before it reaches
the ionosphere so the effect of the ionosphere, which can
cause some propagation errors, is minimal. GPS errors
caused by passage through the ionosphere are significant
and are corrected for by the GPS receiver system.

Space waves have another characteristic of concern to users.
Space waves reflect off hard objects and may be blocked if
the object is between the transmitter and the receiver. Site
and terrain error, as well as propeller/rotor modulation error
in very high omnidirectional range (VOR) systems is caused
by this bounce. Instrument landing system (ILS) course
distortion is also the result of this phenomenon, which led
to the need for establishment of ILS critical areas.

Ground, Space, and Sky Wave Propagation.
Figure 7.1. Ground, Space, and Sky Wave Propagation.