Navigation

2.1 Introduction

To be able to fly from point A to point B it is important for the flight crew to know where they are and where they’re headed. 
Before radar, air traffic control was dependent on pilot position reports via radio. Today, most of the time, we have radar, which makes it possible to closely track the position of aircraft. 
Good knowledge about navigation and navigation aids is important for air traffic controllers and therefore we will start with a basic review of this area.


2.2 Position Reference System [C]

Positions on the earth are often given as coordinates in a coordinate system consisting of two parts, latitude and longitude. 
Latitude is the coordinate giving the position in north-south direction and longitude is the coordinate giving the position in west-east direction. The earth is divided into 360 parallels of longitude or meridians. The reference or zero-meridian is located at a longitude equal to the position of Greenwich in the UK. 
The longitude is expressed as the number of longitudinal degrees east (E) or west (W) of the reference meridian. In north-south direction the earth is divided into 180 parallels of latitude. The reference parallel of latitude is the equator. The latitude is expressed as the number of latitudinal degrees north (N) or south (S) of the equator. 
To be able to define positions more accurately the smaller units minutes and seconds have been introduced. These units are based on the 1/60-system, which means one degree of latitude or longitude is equal to 60 minutes and one minute is equal to 60 seconds.

  • Position coordinates: N59°02′12″ W032°39′55″

N590212 is the latitude and means that the position is 59 degrees, 2 minutes and 12 seconds north of the equator. W0323955 is the longitude and means that the position is 32 degrees, 39 minutes and 55 seconds west of the zero meridian. 
One minute of latitude is equal to the distance of one nautical mile (nm), which is equal to 1852 meters. 
One minute of longitude is not equal to the same distance everywhere on earth because the circumference of the earth is varying with latitude.

 

2.3 Loxodrom and Great Circles [C+]

A Great circle is a circle on the surface of a sphere that has the same circumference as the sphere, and devides the sphere into two equal hemispheres. It is the largest circle that can be drawn on a given sphere.

The Equator is one example of a great circle as are all meridians. 
In navigation, a loxodrome (or rhumb line) is a line crossing all meridians at the same angle, i.e. a path of constant bearing. 
If you follow a given compass-bearing on Earth, (having taken into account magnetic deviation) you will be following a rhumb line, which spirals from one pole to the other, with the exception of 90 and 270 degrees, lines of constant latitude, e.g. the equator. 
Near the poles, they are close to being logarithmic spirals, so they wind round each pole an infinite number of times but reach the pole in a finite distance. 
The pole-to-pole length of a rhumb line is (assuming a perfect sphere) the length of the meridian divided by the cosine of the bearing away from true north.


2.4 Directions [S]


All directions in aviation are expressed with the 360-degree system. 
The horizon is divided into 360 equal parts and one revolution equals 360 degrees. 
A direction of due north means a direction of 360, due east direction 090 and so on.

In aviation there are two (2) basic definitions of directions. Heading and track. 
Heading is defined as the direction where the aircraft nose points (the longitudinal axis of the aircraft). 
When adding the effect of wind the direction of the path of the aircraft over the ground will be slightly different then the aircraft heading and that is called track.
 
 

2.5 Variation [C]

All directions will be given relative to the North Pole. The magnetic North Pole is not located on the same position as the true North Pole and this results in two possible references of directions. If the direction is given with reference to magnetic north it is expressed in degrees magnetic, and if the direction is given with reference to true north it is expressed in degrees true.

The heading indicator in the aircraft will show the direction relative to magnetic north and consequently all headings assigned to aircraft should be in degrees magnetic.

 

2.6 Speed [S]

In aviation speed is normally measured in knots, which are defined as nm/hour. 100 knots equals 185 km/h.

There are different ways to measure speed.

 

2.6.1 Speed for the pilot [C]

The conventional airspeed indicator depends on the effect of air being forced into a small tube mounted on the outside of the aircraft (pitot tube). This airspeed indication is affected by the density of the air, which changes with altitude and ambient air temperature. As the aircraft climbs, the air becomes less dense, and the airspeed indicator shows a lower speed than the aircraft is actually moving through the air. 
There are other ways of measuring the speed, that aren’t affected by the temperature and air-pressure, such as GPS. 
Pilots rely on indicated airspeed to control the aircraft, because it is a true representation of how the aircraft will behave in respect to stall speeds and other known aircraft characteristics. 

 

2.6.2 Indicated Air Speed (IAS) [S+]

IAS is the speed that the pilot can read in the cockpit and speed restrictions issued by air traffic control is normally given in IAS in the lower airspace (see Mach below). 
IAS is based on measurements of how many particles of air that hit the aircraft in a given period of time. The faster the aircraft flies and the higher the density of the air is, the more particles will hit the aircraft. 
An aircraft keeping the same groundspeed (see below) will get a lower IAS when the aircraft climbs.


2.6.3 True Air Speed (TAS) [S+]

True airspeed is the speed at which the aircraft is moving through the air. It has no relation to the wind. In some aircraft, a true airspeed indicator is provided along with the conventional "indicated airspeed" gauge. 
Most pilots estimate their true airspeed prior to a flight, and calculate the actual true airspeed during the flight. This data is needed for flight planning purposes. It is used along with wind data to arrive at, you guessed it, ground speed. 
A thumb of rule, which can be used to obtain TAS is to increase IAS by 2% for every 1000’ of increase in altitude.

 

2.6.4 Ground Speed (GS) [S]

While true airspeed is the speed at which an aircraft moves through the air irrespective of the wind, ground speed is the speed the aircraft is moving over the ground. 
If an aircraft is flying at a TAS of 250 knots with a 30 knot tailwind the GS will be 280 knots. 
That said, just remember that the speed you see on your radar is ground speed, and the speed the pilot normally sees is indicated airspeed. Your speed will usually show a higher value than the pilot's, depending on his altitude, unless he has a strong headwind. 


2.6.5 Mach (M) [S]

In the upper airspace Mach is normally used to express speeds.

Mach is a quotient of the local speed of sound and Mach 1.0 is equal to the speed of sound.


2.7 Wind [S]

The wind direction is given in degrees just like it is given for aircraft direction. 
The direction of wind is always given as the direction from where the wind comes. 

In a weather report for an airport (METAR) the wind direction and strength is given as for example 18003KT, where 180 is the direction (south) and 03 is the strength in knots.


2.8 Time [S+]

When giving time in aviation, it is important to define what time you refer to since an aircraft often flies trough many time zones. Hence time is always given in UTC (Universal Time Co-ordinated) in aviation. 

The big difference between UTC and other time format is that UTC doesn’t change with DST (Daylight Saving Time or summertime). Adding the letter ”Z”, which is pronounced ”Zulu”, marks times given in UTC.

  • WET (Western European Time) = UTC during winter
  • CET (Central European Time) = UTC+1 hour during winter
  • WET = UTC+1 hour during DST
  • CET = UTC+2 hours during DST

 

2.8.1 Date and reading time [S+]

Dates can also be added in the format 211020Z, which means time 10:20 UTC on the 21:st.

In radio communication time is often given with only the number of minutes, for example time 14:54 is expressed as “time 54”. Time 16:00 is expressed as “on the hour” and 11:30 can be expressed either “time 30” or “on the hour and a half”.

 

2.9 Navigational aids [S]

Navigation aids are used by flight crew to navigate between different positions on the earth and may consist of transmitters on the ground, receivers in aircraft and most recently also satellites.

The crew navigates between different navigation aids and points called VOR, NDB and intersections. With a joint name these are called waypoints.
 
2.9.1  Non Directional Beacon - NDB  [S+]
 
NDB is a radio beacon transmitting non-directional radio signals. 
The pilot can determine the direction or “bearing” to the beacon with an instrument called ADF indicator.
NDBs have a maximum range of about 50-100 miles, depending on equipment. NDBs with three letters have a longer range then NDBs with two letters.

 

2.9.1.1 More on NDB [C+]

The NDB operates between 200 and 1750 kHz, but in Europe most frequencies are between 255 and 455.

The ADF receiver can be used when line-of-sight transmission becomes unreliable, or when there is no VOR equipment on the ground or in the aircraft. 
It is used as a means of identifying positions, receiving low and medium frequency voice communications, homing, tracking, and for navigation on instrument approach procedures. 
The low/medium frequency navigation stations used by ADF include Non-Directional Beacons, ILS radio beacon locators, and commercial broadcast stations. 
The ILS radio beacon is a beacon which is placed at the same position as the outer marker of an ILS system (or replaces the OM).

 

2.9.2 Very high freq. Omni-directional Radio range – VOR [S+] 

A VOR is transmitting directional radio signals and is more accurate then a NDB.
With the navigation instruments the pilot can intercept and fly with a specified direction towards or from the beacon. These directions are called radials. If you fly due south from a VOR, you fly on radial 180. If you are due south of the VOR and point the aircraft towards the VOR (north) you are still on radial 180, but flying on heading 360. 
If you fly towards the VOR it is common to say you are flying on a radial inbound it, and if you fly away from it you are flying on a radial outbound it. 
The maximum range of a VOR is about 100-200 miles.


2.9.2.1 More on VOR [C+]

VHF Omni-directional Range (VOR) operates between 108 and 117.950 MHz.

The range of a VHF-signal can be calculated using the formula [1.25 x √ (height of the transmitter in feet) + 1.25 x √ (height of the receiver in NM)]. The signal is weakened by several factors such as terrain and different weather-phenomena. 
The VOR works on a "light-tower" principle. Imagine that it has a rotating light bundle, and a steady 360 degree light. The light bundle swings around one time per minute (1 RPM = 6 degrees/sec). If the light bundle hits the magnetic North, the steady light flashes once. Take a stopwatch and start timing when the steady light flashes, stop timing when the light bundle hits you.
In practice, the steady 360 degree light is actually a steady 30 Hz signal on the VOR, and is modulated (pasted) on the VOR frequency. The rotating bundle is also a 30 Hz signal. The receiver will compare both signals and determine the difference in phase, and in this way the position relative to the VOR is determined. 
A VOR usually also includes a Morse Ident, and sometimes there is also a VOICE channel (ATIS) pasted on the VOR frequency.
A VOR is often accompanied by a DME. 
The accuracy of a VOR is about 2 degrees.

 

2.9.3 Distance Measuring Equipment – DME [S+] 

DME is another type of beacon with which the range between the aircraft and the transmitter can be measured and presented to the pilot.
A DME does not make it possible to see the direction from the aircraft to the DME station, only the range.

             

2.9.3.1 More on DME [C+]

Distance Measuring Equipment (DME) operates between 962 and 1213 MHz.

The measuring is initiated from the aircraft. The interrogator sends out two pulses which are received by the DME station. After a 50 microsecond delay, the ground station sends a pulse back, which is 63 MHz higher or lower than the original frequency. The further the DME station is away, the longer the pulse needs to travel. By timing the time difference between sending and receiving the signal (minus the 50 microsecond delay) you can determine the distance. 
This is the direct distance from the aircraft to the ground station, and not the distance over the ground. If an aircraft fly at 10 km overhead a VOR/DME, the DME will read 5.5 nm!
A lot of VOR frequencies are coupled with a DME frequency. Every VOR frequency has a fixed DME frequency.

 

2.9.4 Intersection / Fix [S+] 

An intersection or fix is not a ground-based navigation aid, but only a position on the surface of the earth defined with the position reference system. 
As an intersection is not a transmitter it cannot be flown to with conventional navigation instruments. To fly to an intersection the aircraft has to be RNAV (area navigation) equipped. 
RNAV means that the aircraft will calculate its position and the direction to the next waypoint by means of different ground based navigation aids (VOR/DME) as well as by positions from GPS and INS/IRS.

 

2.9.5 Global Positioning System – GPS [S+]

GPS is a satellite positioning system developed by the United States Department of Defense (DOD) for use on land, sea and in the air.
It will likely be the major component of the ICAO - designated GNSS - Global Navigation Satellite System. 
The full GPS constellation has 24 operational satellites to provide continuous, highly accurate three-dimensional position information globally.

 

2.9.5.1 More on GPS [C+]

GPS is operating in 1,900 NM orbits, each satellite continuously transmits signals on 1227.6 and 1575.42 MHz.

The GPS receiver automatically selects the signals from four or more satellites to calculate a three-dimensional position, velocity and time. 
Using the un-encrypted coarse acquisition navigational signal (C/A code) which will be available to all civil users, system accuracy will be at least 100 meters horizontally and 140 metres vertically, 95% of the time. 
Unlike ground based navigation systems, GPS provides global coverage with virtually no signal inaccuracies associated with propagation in the earth's atmosphere. Signal masking can occur with mountainous terrain, man-made structures and with poor antenna location on the aircraft.


2.9.6 Inerital Navigation System (INS) [C+] 

Inertial Navigation Systems (INS) are completely self-contained and independent of ground based navigation aids. After being supplied with initial position information, it is capable of updating with accurate displays of position, attitude, and heading. It can calculate the track and distance between two points, display cross error, provide ETAs, ground speed and wind information. It can also provide guidance and steering information for the pilot instruments. 

The system consists of the inertial platform, interior accelerometers and a computer. The platform, which senses the movement of the aircraft over the ground, contains two gyroscopes. These maintain their orientation in space while the accelerometers sense all direction changes and rate of movement. The information from the accelerometers and gyroscopes is sent to the computer, which corrects the track to allow for such factors as the rotation of the earth, the drift of the aircraft, speed, and rate of turn.
The aircraft's attitude instruments may also be linked to the inertial platform.

The accuracy of the INS is dependent on the accuracy of the initial position information programmed into the system. Therefore, system alignment before flight is very important. Accuracy is very high initially following alignment, and decays with time at the rate of about 1-2 NM per hour. Position updates can be accomplished in flight using ground based references with manual input or by automatic update using multiple DME or VOR inputs.
 
2.9.7 ATS Routes [S+] 

ATS routes are pre-determined routes connecting waypoints to each other, which the aircraft will follow. 
ATS routes are named by a letter followed by two or three numbers. If the route is used in the upper airspace it is also given the prefix “Upper”. For example UN872 (“Upper November 872”). 
Some ATS routes are for aircrafts flying in one direction only.

 

2.9.7.1 More on ATS Routes [C+] 

ICAO states that: “The designation of specific ATS routes within the network should be made so that the majority of recurring flight operations can identify them in flight plans with the least number of designators.”

For routes forming part of the basic ATS route network
  • A, B, G, R – routes which form part of the regional networks of ATS routes and are not area navigation routes.
  • L, M, N, P – area navigation routes (RNAV) which form part of the regional networks of ATS routes.

For routes not forming part of the basic ATS route network

  • H, J, V, W – for routes which are not area navigation routes.
  • Q, T, Y, Z – for area navigation (RNAV) routes.

 

2.9.8 Flight Management System – FMS [C] 

Flight management system (FMS) is the term used to describe an integrated system that uses navigation, atmospheric and fuel flow data from several sensors to provide a centralized control system for flight planning, and flight and fuel management. 

The system processes navigation data to calculate and update a best computed position based on the known system accuracy and reliability of the input sensors. 
This system may also be referred to as a multi-sensor RNAV. 
FMS controls differ widely between aircraft types and manufacturers, but the Typical FMS Control Unit figure, to the right, gives a typical arrangement. 
The heart of any FMS is the navigation computer unit. It contains the micro processor and navigation data base. A typical base contains a regional or worldwide library of navaids, waypoints, airports and airways.
FMS sensor input is supplied from external DME, VOR, air data computer (ADC) and fuel flow sensors. Depending on the capabilities of the navigation sensors, most flight management systems are approved for en route IFR in most classes of RNAV airspace. 


2.10 Navigational Aids Limitations [C+]

Navigational aids can be classified as almost anything, visual or otherwise, as long as it provides an aircraft with positional data.

For our purposes in the VACC, the limitations we are concerned with is the useful range of VOR's, VORTAC's, VOR/DME, and NDB's. VOR's without DME are becoming a rarity, and most navaids, except for NDB's, have distance information available.

Navaids are classified by their useful altitude and distance. This takes into consideration signal strength, their protection from navaids on the same frequency, and other factors. The classes of navaids are depicted on a low altitude chart. The symbology shows the type of navaid, and it can be assumed to be classified as at least "L"-class unless the notation (T) for TVOR appears in the communications box next to the navaid, where name, frequency, and identifier appears. Look at a low altitude chart and try to find a TVOR. The navaids on high altitude charts can be assumed to be "H"-class unless a (L) or (T) appears in the communications box.

The table below shows the useful range of navaids at various altitudes. Naturally, you do not need to commit this to memory, or even keep it for reference, but be aware of the limitations of navigational aids. You are not required to reference the following tables when assigning routes and altitudes, because you are always monitoring the flights on radar. However, the tables may explain why aircraft do not always receive the signal when you expect they should. It may also prevent you from embarrassment when clearing a high altitude aircraft over a TVOR, and wondering why the pilot is too lame to navigate .

VOR/VORTAC/TACAN

ClassAltitudeDistance
T12,000 and below25
LBelow 18,00040
HBelow 14,50040
H14,500 – 17,999100
H18,000 – FL450130
HAbove FL450100

LF/MF Radio Beacon (NDB)

ClassPower (watts)Distance
CLUnder 2515
MHUnder 5025
H50 – 1,99950
HH2,000 or more75

 

2.11 Further reading

If you with to learn more on the topics covered above we list a number of external links which we believe could be of interest to members in general

Time and date:
http://www.timeanddate.com/
GPS - Positioning system:
http://www.colorado.edu/geography/gcraft/notes/gps/gps.html
http://www.gps.oma.be/
Speed:
http://www.womanpilot.com/past%20issue%20pages/2000%20issues/jan%20feb%202000/airspeed.htm
https://ewhdbks.mugu.navy.mil/mach-as.htm
Calculate between IAS, CAS, TAS:
http://www.flightplan.za.net/trueAirspeed.php
NDB:
http://en.wikipedia.org/wiki/Non-directional_beacon
VOR:
http://en.wikipedia.org/wiki/VHF_omnidirectional_range
DME:
http://en.wikipedia.org/wiki/Distance_Measuring_Equipment
Naviagtional Aids:
http://www.centennialofflight.gov/essay/Government_Role/navigation/POL13.htm1