Vera Blair
Professor E. Yontz
MLIS 7000
6 November 2002
AIRCRAFT INSTRUMENT NAVIGATION
There are trillions of bits of molecules and wavelength in the universe,
some of, which are processable by humans through their five senses and
some of which are perceptible only by man-made instruments
such as radio receivers. Scientists are much concerned with the collection
of information and its utility and applicability to everyday life. With
the exponential development of technical advances, intense debates are
occurring as to just what is data, and at what point does it become information.
According to Lester (6), information transfer occurs every day in a multitude
of channels. There are an increasing number of sending and receiving devices
such as television, telephones, and computers, making global dissemination
of information instantaneous. I thought it would be interesting to trace
the exchange of information communication technology of aircraft navigation
instrumentation.
The concept of navigation has been around ever since human beings have
made purposeful efforts to get from one place to another and find their
way back. Navigation of any sort requires a transfer of information
of some kind, from a sender of a signal to a receiver. On land, navigation
was relatively easy…there were landmarks, trees, mountains, terrain and
lakes and rivers that people could use to navigate. Over-water and
out of sight of land navigation was more difficult. The Phoenicians
used the stars for guidance. The signal from the stars was always
there, but it needed to be transferred to the user. This was possible
only on clear nights, limiting the use of the signal.
With the advent of aircraft came a more serious need to find ways to navigate.
The first aircraft, of course, were slow and carried little fuel, limiting
their range. Aircraft could and did use stellar navigation, but again,
they could only do so on a clear night. The basic navigation done
by pilots was “dead reckoning.” This is still used today as a backup
navigation tool. The term “dead reckoning” comes from the word “deduced.”
Pilots who fly in clear weather still use basic pilotage, looking out of
the window and comparing terrain features with a chart.
The first real instrument used by pilots to navigate was the magnetic compass.
The only signal required was that of the magnetic pole of the earth and
the only receiver was the mechanical compass inside the airplane, a simple
transfer of information. It is a self-contained unit and does
not require electricity or any other outside power. While the compass is,
in principle, an extremely simple instrument…. The pointer always points
to the North…right? The interpretation of the compass in the airplane is
very complicated and required a full understanding of errors inherent in
the system.
The geographic north and south poles, or true north and true south, are
quite different form the magnetic poles of north and south. Charts
are oriented to true north and south, and the compass reads magnetic north
and south. If a pilot followed the compass without understanding
this error, called “magnetic variation,” he would soon be in quite a different
location from where he wished to be. The amount of variation differs
depending on where on the earth the aircraft is located. Variation
is also called the angular difference between the true and magnetic poles
(Jeppesen 3-19).
The modern aircraft has many metallic components and instruments run by
and electrical systems, which also produce errors in the compass indication,
called “deviation” (Jeppesen 3-20). Since the compass rotates within
a fluid-filled container, when the airplane is accelerated or decelerated,
the compass also produces errors, since the needle tries to stay oriented
toward the north at all times. There are also turning errors inherent in
the compass, depending what hemisphere the aircraft is positioned in.
In the Northern Hemisphere, on a heading of north, the compass initially
indicates a turn to the south. This compass is still used in modern
sophisticated airplanes and today modern backups are part of standard equipment,
but the basic principle of this very simple piece of equipment must still
be understood, and the sole interpretation between the communication of
the signal from the earth and the receiving compass in the aircraft still
rests between the pilot’s ears.
Dead reckoning in its most basic sense refers to calculating the aircraft’s
position from known data and extrapolating to find the next desired point
on the chart. A line is drawn on the chart from where you are to
wherever you wish to navigate. All available parameters are
used: aircraft magnetic course, magnetic heading, airspeed, ground speed,
estimated time enroute, estimated fuel burn estimated time of arrival as
well as wind speed and direction. None of these parameters are precise:
first of all the wind speed can never be predicted with accuracy, and neither
can airspeed and groundspeed, and if the pilot miscalculates his fuel burn,
he will make an unscheduled stop short of his destination. The wonderful
thing is, however, that this method of navigation requires no ground-based
signal except the terrain itself, therefore has no noise interference.
It costs nothing and is accurate enough for Visual Flight Rules, or VFR.
The first attempts at navigation, which required both a ground-based signal
and a signal receiver in the aircraft, were homing beacons. Airways
of navigational charts were each given a color and a number. According
to Hiltz, the quadrant on the chart that was oriented toward magnetic north
was the N quadrant and the alternate was the A quadrant. The pilot
heard the Morse code for these two letters in his headset. When the two
separate signals merged into one, the pilot was on the airway. The
skill here was to fly slightly to the right of the airway to always be
able to hear the Morse code of one of the signals, assuring proper course
guidance. As the pilot neared the station, the signal became louder.
All this required much skill on the part of pilot and or navigator.
This was used in the 1940’s and was quickly replaced by the VORs, or Very
high Omni-Range instruments operating on a frequency of approximately 108
MHz to 118MHz (Jeppesen 7-3).
The operational principle of the VOR is simple. The ground-based
unit, which looks like a Mexican hat, sends out two signals: a constant
signal sent out in all directions, and another signal that is rotated around
a point. The aircraft receiving equipment looks at the two signals
and interprets the phases of these signals as a point in space where the
airplane is located at the moment. These signals are called radials
from the station (Jeppesen 7-2).
The VOR tells the pilot
only that he is on a specific radial from the station, not the direction
or distance from the station. The unit contains a to/from indicator
that requires the pilot to pay attention whether he is going towards the
station or navigating away from it. Although dead reckoning is used
along with VOR navigation, now there is no need to take into account the
wind direction and speed for the purpose of navigation, although the pilot
still needs to know this to calculate his fuel burn and avoid making an
unscheduled stop.
The information transmitted by the VOR ground equipment is extremely accurate.
The limitations of this type of navigation must be understood, however.
The signal is line-of sight limited, therefore it cannot always be guaranteed,
depending on the altitude of the airplane. Here the signal is quite
clear and accurate, but the receiver is limited. In addition, if
the pilot tunes in the wrong station, which is easy to do, then he receives
a signal from another station and will follow the wrong air route.
The stations have three-letter identifiers, which are in Morse code that
the pilot must identify to avoid this problem.
The VOR receiver also contains a course-deviation indicator, which is a
needle that always points to the center of the air route, and a pilot knows
approximately how far he has strayed from his route. The VOR system is
in principle quite accurate, but still requires pilot interaction and skill
to make good use of it. The signal is line of sight, and the curvature
of the earth, terrain features, and mountains can make part or the entire
signal unusable.
VORs are often combined with distance measuring equipment, or DMEs.
This equipment is another sophistication that lets the pilot know how far
he is from a specific station. Where dead reckoning was an educated
guess as to pilot position, the DME is a precise readout of distance from
or to a particular station. The least accurate units are accurate
to within three percent of total distance: six miles in a range of 200
miles (Clausing 36). A better unit is accurate to within 0.2
nautical miles at any distance, and the best (most expensive) units are
accurate to within 0.1 nautical miles at any distance. VORs and DMEs
are channeled to the same station, and tuning in the VOR automatically
tunes in the DME if the receiver is installed in the aircraft. Now
there are two additional bits of data that are provided to the pilot, depending
on the signal received by the aircraft: the ground speed and the time-to
station. Since rate of change of distance is proportional to groundspeed,
the DME unit times the interval between change in distance to get the time
to station. Since this is constantly measured, it is very accurate.
The unit uses much smaller distances to calculate groundspeed more accurately
than a pilot can do it manually.
There are, however, also limitations to the DME equipment. The ground
unit can not always accommodate all the aircraft that are tuned into it,
thus at busy times around airports, the unit may appear to malfunction,
and will require one user to leave the frequency before another unit can
utilize it.
Since the measurement of the DME is a straight line from the aircraft to
the ground unit, a slant-range error is introduced, which increases with
height above station and nearness to the station. Therefore great
error can be introduced in an aircraft very near to the station.
Slant range error may be disregarded if the airplane is more than a mile
away from the station for every thousand feet of altitude. Here again,
while the electronic information is extremely accurate, the pilot must
interpret the signal.
VOR units have been in existence since the 1950s and have enabled pilots
to fly in instrument conditions (in the clouds). There are specific
Instrument Approaches used by aircraft, which are part of the national
VOR system and guide pilots to within specified distance to and height
above the runway until the pilot is able to complete the landing in visual
conditions.
Whereas the VOR units provide extremely accurate information, the pilot
is still required to interpret much of this information in order to utilize
it effectively. Thus, while the signals have been increasing in accuracy
and precision since the 1950s, the actual information transmitted or communicated
has remained relatively static as long as interpretation by human factors
remains.
The ADF, or Automatic Direction Finder is a piece of aircraft equipment
that is at the same time extremely easy to use. The pointer in the aircraft
always points to the station, and yet it can be very difficult to interpret
at times. It is the oldest form of electronic navigation equipment
still in use at this time (Clausing 78) used mainly in conjunction with
other equipment as and as a backup. The ADF, which is the aircraft-based
part of the unit, consists of an AM receiver, antennas, and circuits that
convert signal strength to directional information when received by the
aircraft unit (Clausing 78). However, as the VOR navigation relies
on line-of-sight, transmission, NDB signals (the ground-based part of the
informational loop) are low-medium frequency (190-535 kHz) allowing the
aircraft to receive a reliable signal at lower altitude and possibly at
greater range (Jeppesen 7-18). Signals are Low-Medium frequency of
190-535 kHz. The ground waves penetrate the earth and the skywaves
are refracted by the ionosphere, giving greater signal strength than the
VORs. The limitations must also be understood, however, in a thunderstorm,
the needle will point preferentially to lightning, exactly where the pilot
does NOT want to go, and there is static induced by precipitation which
also disorients the unit. Note that while electronic information
is becoming more sophisticated, the interpretation of the signals and noise
inherent in the signals has not improved much.
A great advance in navigation signals occurred with the advent of the LORAN
units, or long-range navigational systems. Whereas the older navigation
units were accurate at relatively short distances, the LORAN units made
the travel especially overwater, feasible and safe. In shorter-route
navigation, the charts treated the earth as if it were flat, since the
curvature made little difference. Now the earth’s curvature begins
to matter, and great circle routes must be flown (or navigated over-water
by ships). The course bearing changes with each line of latitude
and longitude crossed. The LORAN unit automatically changes these
parameters for the pilot. With the advent of this technology arrives
more complicated technical communications data. Whereas for short
distance navigation of up to 50 nautical miles a radio signal and a receiver
were sufficient, now with longer distances crossing over lines of latitude
and longitude a minimum of three fixes must be utilized (Clausing 130).
The master station sends out a signal in the low-frequency band of 100
kHz and compares the time it took for the signal to arrive back at the
aircraft (or boat) and interprets the signal as distance.
Loran units are subject to the same limitations as are all low-frequency
radio communications: they are affected by precipitation-induced static,
which becomes electronic noise and the unit is monitored for Signal-to-Noise
ratio or SNR. The great advantage of the Loran unit is the accuracy:
it is more accurate at 1000 nautical miles than a VOR unit is at 20 nautical
miles (Clausing 140).
According to the Department of Transportation, the Loran system will remain
in the United States until at least the year 2008 or longer. It has
been used for general aviation since it was instituted and is just now
being used more in Europe and Asia. Earlier reports by the Department
of Transportation had claimed that it was in the process of being phased
out with the advent of satellite technology. It would still be needed as
a backup system for GPS (Wilson 37-39).
A very sophisticated navigation system that has been in existence since
the 1970’s is the OMEGA/VLF. Now accuracy is vastly improved through
the use of atomic clocks and computers. While the earlier navigation
systems required minimum amount of training (the ADF simply always pointed
at the station) now there is much more knowledge required by pilots to
make effective use of this new technology. This system is in principle
a fancy Dead Reckoning system that continuously updates its anticipated
point of arrival by its past position information. There is also
the complicating factor that OMEGA utilizes four different frequencies
instead of one. The system is also in effect two complete and complementary
systems - the OMEGA portion for navigation and the VLF portion for communicating
(Clausing 145). The VLF transmitter works in the 10-30 kHz range
and overlap with the audio range. Essentially the transmission is
partially physical transmission of sound waves through the ground and requires
extremely powerful transmitters. The advantage here is the extremely
long-range reception of these signals, up to 11,000 nautical miles.
While the system is usable over much greater distances than previous navigation
systems, it is subject to interference from various sources: charged ionic
particles from solar activity, deflection of these particles by the polar
caps, and, since the signal travels so far, part of the signal may completely
travel around the earth while part of the signal goes only part way, causing
a disturbance in the reception.
The INS or Inertial Navigation System is a unique system, the only system
completely self- contained within the aircraft requiring no ground-based
units and no limit to its technical accuracy. It has obvious military
application since it does not use ground units, which can be destroyed,
or radio units that can be jammed (Clausing 158). The system utilizes
extremely minute changes in speed and direction of aircraft in order to
compute a new position. In this respect it is a very simple system,
and its accuracy is within 0.2 nautical miles per hour of use.
The INS system has been
used since the 1980’s and uses some of the newest laser technology instead
of mechanical gyroscopic instruments.
With the advent of Radar it became possible for pilots to fly safely in
the clouds - IMC or Instrument Meteorological Conditions. Radar is
in principle a simple device that uses microwave energy to find the position
and speed of an aircraft. In its most basic form the radar antenna
sends out pulses of energy, some of which is reflected back from a target
- an aircraft or precipitation - and analyzed for density and velocity
(Jeppesen 2-58). Radar, while at the moment still the backbone of
the Air Traffic Control System, has severe limitations. The ground
based radar unit is the Primary Radar, which sends out pulses of energy
in the form of a rotating beacon. The signals are reflected back
to the receiver unit, which analyzes them. Aircraft distance from
the antenna is calculated by elapsed time between the interrogating signal
and the echo return. The radar signal can be bent by atmospheric
temperature inversions and heavy clouds can cause clutter. There
is also the problem of ground-based clutter that interferes with interpretation
of a radar signal. Sometimes extremely heavy radar returns blank
out the receiver completely and airplanes have flown into heavy precipitation
and thunderstorms while the pilot thought he saw a clear path through the
storms on the radar. Radar Summary Charts have proven to be very
useful for navigation purposes and flight planning. Radar can be used both
in the aircraft to detect weather and on the ground to guide pilots into
the Airline Terminal environment for instrument approaches (Aviation Weather
Services).
The advent of satellite technology has brought the navigation of aircraft
(and lately automobiles) to a degree of accuracy heretofore unimaginable.
The GPS or Global Positioning System is a satellite-based system that uses
a group of 24 satellites permanently positioned around the globe so that
there is overlap in the reception and no blind spots anywhere on earth.
The units are affordable and extremely accurate. Each satellite orbits
the earth twice in 24 hours and transmits a stream of digital data containing
not only the navigational signals but also the status of operation of the
system. A pilot can tell at a glance at his receiver unit, which
satellites are picking up what signal and how strong the signal is (Apollo
Precedus). With the signal from four satellites the unit can calculate
latitude, longitude, and altitude. A pilot receives a continuous
readout of present position, groundspeed, and distance remaining to destination,
heading information, and course-deviation information. Accuracy is
so great that the signal is purposely degraded for civilian use by Selective
Availability, which limits accuracy to within 100 meters. The error
may vary at any time and in any direction (Wilson 37-39). The signal
can also be shut off for civilian use at any moment and the systems remains
vulnerable to jamming and weather interference.
Until the advent of satellite technology the data that was used by aircraft
navigation instruments was in analog format. According to Blair,
the Federal Communications Commission has allocated a bandwidth of 6.5
MHz per CATV channel, for example. That is audio and video for one
station only. A digital format can put up to twelve channels in a
6.5 MHz bandwidth, to be used with a decoder, providing much more communications
capability. A major problem with the advent of cable television
is that the cable TV frequencies overlap with the entire bandwidth of
aircraft communications…VOR, ADF, NDBs and air to air and air to
ground transmissions.
If there is a broken cable for TV, the cable becomes an antenna and depending
upon the amount of power behind the transmission, can interfere with the
communication for the aircraft. Digital information is propagated
in a binary code or unit, reducing signal-to noise ratio and allowing much
clearer reception of the signal. In digital format, the signal is
either completely there or not at all, unlike wave signals, which can be
distorted, making the newer technology more precise and accurate.
The newest technologies, the RNP units (Required Navigational Performance),
have recently been approved for approaches into San Francisco International
Airport. These units are completely self-contained within the aircraft,
and pilots will be able to navigate anywhere in the world without any ground-based
assistance and land in weather impossible to land in up till now (FAA).
Vertical separation has been decreased from 2000 ft to 1000 ft at certain
altitudes and horizontal separation has been decreased to 50 nautical miles
in various oversea routes (FAA AC: No 90-96) allowing more efficient use
of crowded airspace. In reviewing the advance of aviation navigation
capability in the last 100 years it is interesting to note that the role
of the pilot has changed from one of active participation with his aircraft
in the past to one of spectator in the present. The modern aircraft
is capable of flying and landing unassisted, and the pilot is needed in
a supervisory and managerial role to intervene only in case of an emergency.
(A popular pilot adage says that a pilot suffers hours of total boredom
interspersed with moments of abject terror). Airwaves and airspace are
becoming increasingly crowded and general aviation of small, slow, single-engine
aircraft seems to be declining at an alarming rate. There are small
airports closing every week in the United States. It is unfortunate and
ironic that this wonderful new navigation capability is bringing to an
end the romantic era of the individual aviator.
Works Cited
Apollo Precedus User’s
Guide. Oregon: Morrow, Inc., 1997
Aviation Fundamentals.
Colorado: Jeppesen-Sanderson , 1994
Blair, Heath J. Personal
Interview. 2 Nov. 2002
Clausing, Donald J.
The Aviator’s Guide to Modern Navigation. Pennsylvania: McGraw-Hill,
Inc. 1987
Hiltz, David E. Personal
Interview. 3 Nov. 2002
Lester, June and Wallace
Koehler. Information Environment. CD-ROM. New York:
Neal-Schuman, 2002.
United States. Federal
Aviation Administration. Pacific Weather Monitoring Group Report:
AC No.90-96. Washington: GPO, 1996.
United Kingdom. Royal
Institute of Navigation. Federal Aviation Authority Announce
first RNP Approach. Article 39758. United Kingdom: 2002
/wysiwyg://46/http://www.rin.org.uk/poo/
Wilson, JR “The Rebirth
of LORAN.” Interavia 54 (1999): 37-39