Navigation Systems-Transcrição
Navigation Systems-Transcrição
Navigation Systems-Transcrição
Aside from pilotage and dead reckoning, other forms of navigation are also available to
pilots through the use of electronic navigational aids, or NAV aids for short. These
systems transmit signals to aircraft through radio waves and tell pilots where they are
and where to go. Before we get into what these systems are and how they work, we
first need to review radio waves and antennas.
Radio waves are a type of electromagnetic radiation. Artificially generated radio waves
are used for fixed and mobile communication, broadcasting, radar, computer networks,
and of course, navigation. These artificially generated radio waves are created from
antennas. Antennas convert the electric current of a signal into a radio wave so it can
travel through space to a receiving antenna, which then converts it back into an electric
current to be used by a receiver.
The antennas on an airplane are all different sizes and shapes. This is because they
receive or transmit different types of radio waves. There are three different types of
waves, each with different characteristics. They are ground waves, sky waves, and
space waves. Ground waves are lower frequency waves that travel close to the surface
of the Earth and will in fact follow the curvature of the Earth.
The lower the frequency of a wave, the further the signal will be able to travel. These
ground waves travel reliably and predictably along the same route day after day, not
being influenced by outside factors. Sky waves are higher frequency waves that can
also travel for long distances, but instead of following the curvature of the earth, the
waves are refracted or bent by the ionosphere and sent back down to earth.
Using sky waves, high frequency radios can send messages across oceans using only
50 to 100 watts of power. Space waves consist of very high frequency waves or higher
that neither bend nor refract. These waves travel in a straight line passing through the
ionosphere and allow navigation from space. Most major navigation systems these
days operate with signals broadcasting as space waves.
Objects between the transmitter and receiver may reflect and block the signal. This
means that the waves have to have a line of sight between the two for the signal to be
received. For aircraft, the airplane has to be within line of sight of the navigational aid in
order for the system to work. One of the oldest types of Navid still in use today is called
the Non directional radio beacon or NDB.
While NDB's are not as common in the United States as they used to be, they are still
used in other countries around the world and NDB is simply just a groundbased AM
radio transmitter that transmits radio waves in all directions. In the United States, these
NDB's operate on the frequency range of 190 to 535 kHz. Because NDB's operate in
this low to medium frequency band, they are not subjected to the line of sight
limitations of space waves.
To navigate via NDB's, pilots need to have installed in their aircraft and automatic
Direction Finder, or ADF. The face of an ADF contains a needle that points to the
relative bearing of the NDB. The relative bearing is the number of degrees, measured
clockwise, between the aircraft's heading and the direction from which the bearing is
taken from. You can use this simple formula to calculate the magnetic bearing to the
station.
Magnetic heading plus relative bearing equals magnetic bearing. For example, if your
airplane is flying a heading of 030 and the ADF is indicating a relative bearing of 120,
then that means that the NDB is at a relative bearing of 150 degrees. If you wanted to
fly towards the NDB, 150 would be the initial heading to turn to.
When flying directly toward the NDB, the needle will look like this, pointed straight up at
the NDB station. Once crossing over it, the needle will reverse direction, but still point
at the NDB as you fly away from it. It is possible to track both towards and away from
an NDB station. This sounds really easy right? Just keep the needle straight up and
you'll fly right towards the station. Well, in a no wind situation that would probably be
just fine.
However, most of the time there is in fact wind. If a pilot were to keep the needle
straight up on a windy day as they were navigating, they'd be doing a procedure called
a homing. Homing is not a recommended procedure to follow, as you would not be
flying in a straight line. Instead of homing, tracking should be used to fly from station to
station in a straight line. Tracking involves compensating for the wind by turning slightly
into the wind and thereby staying on course.
When you are on course and tracking to the station, the airplane's wind correction
angle should equal the number of degrees the ADF is deflected from. Straight up Ndb's
can be spotted on sectional charts with this magenta colored symbol. In the vicinity of
the symbol you will find a box containing the name of the NDB, frequency ID and
associated Morse code.
Before you can actually navigate via an NDB, you need to tune and identify the desired
station. Tuning in a station is pretty easy, just look up the frequency on your chart and
enter it into your receiver. After it's entered, we need to confirm that we are receiving
the correct station and that it is operational. As part of an NDB's transmission, they'll
send out their ID in Morse code format. We the pilots, must listen to the Morse code.
And verify that it matches what's printed on our chart. After we've identified the station,
we can use it for navigation. However, since there is no flag on the instrument to advise
us whether or not it is operating properly, we must continue to monitor the Morse code
for as long as we intend on using the station. Luckily, we can turn down the volume so
it's not as obnoxious. There are four different classes of NDB's.
They all operate on the same principles, but the different classes contrast and how far
away their signal can be reached. The weakest of all NDB's is the Compass Locator.
This low powered NDB uses less than 25 watts of power, giving it a range of only 15
nautical miles. The other three classes are labeled as Medium, High, High and High
High, with each offering progressively larger ranges.
To receive a signal from an NDB the aircraft's ADF is able to determine the relative
bearing from the aircraft to the NDB station, This is accomplished through the use of
two antennas on board the aircraft, one being the loop antenna, the other being the
sense antenna. The loop antenna is a directional antenna containing 2 or more
stationary loops of wire looking at just one loop. If radio waves hit the loop in any
direction other than directly perpendicular, a voltage will be induced over the antenna.
By using multiple loops oriented in different headings, the system can deduce, down to
two possible headings, that the signal is coming from both 180 degrees apart. To
remove this ambiguity, the sense antenna is also used. This antenna, which is more or
less just a straight wire, looks at the electrical field of the signal receiving an identical
signal from all directions.
Looking at the phase of the signal and not the amplitude, the ADF receiver compares
the sense antenna signal with the loop antenna and is able to remove the ambiguity
and deduce the relative bearing of the NDB station. Now before you start relying on
Ndbs for navigation, you should be aware of its limitations. The first of many errors is
called the thunderstorm effect.
During a thunderstorm, the ADF needle will be temporarily deflected towards the
lightning strikes instead of the NDB. Next is the night effect, where NDB signals can be
refracted by the ionosphere and return as sky waves. This effect is largest during the
dawn and dusk hours. This can cause interference with distant NDB stations.
Mountains can also have an effect on the NDB signal as they can reflect the NDB
signal.
Finally, there's the coastal effect. As the airplane is flying across a coastline, the ADF
needle will bend slightly towards the coastline when crossing it at an angle. All of these
errors result in erroneous bearing information which affects the ADF needle. Since the
pilot has to monitor the NDB Morse code. Hearing any static on that frequency along
with the ADF needle acting erratically are two indicators that there may be an error and
what you are receiving.
While Indy bees are a dying technology, our next Navid is still very much alive and
much more common in the National Airspace System. This type of Navid is called a
Very High Frequency Omnidirectional range, also known as a VOR.
Sistemas de Navegação (Navigation Systems - YouTube)