Initially, without any navigational aid, flights were limited to daylight flying and only during good weather conditions. In order to navigate from airport to airport outside references, such as roads or railway lines are used as a reference. A line is usually drawn extending from the departure to the destination airport and any prominent landmark is noted. Any deviation from the flight plan, the course is corrected. This type of flying is called Pilotage or Deduced Reckoning. This method however did not take into consideration the crosswind.

A more accurate form is Dead Reckoning. This uses trigonometry and wind forecast to deduce the correct heading for the aircraft to fly to counteract the headwind. This, however, was hampered by the fact that the wind information is a forecast, not the actual wind.

To further help with accuracy in flying, Sectional Charts were developed and produced for the first time by the US government. Road maps were initially used for navigation which proved to be unsuitable as some prominent landmarks were not included.

As advancement in instrumental progresses the use of natural horizon became less dependent and instrumental flying came into existence. The new equipment uses gyroscopic principles and includes the artificial horizon or the attitude indicator, the heading indicator, and the turn coordinator.

The altitude indicator mimics the movement of the horizon, providing pilot with accurate attitude information. The heading indicator indicates the aircraft position and the turn coordinator gives the pilot the rate of turn and its direction.

Even with these precise instruments the pilot still need 15 to 20 miles of visibility to navigate at night.

Early 1920 the US Government introduces the first radio navigation, Four-Course Radio Range. This four course radio range transmit letter "A" and "N" in Morse code for the different sector. If the two transmission overlaps, the dot-dash (A) and the dash-dot (N) , of equal strength, would produce a constant tone in the pilot's headset indicating the aircraft is on the right track. If any deviation occurs in the course, a distinct tone of "A" or "N" would be heard depending on which sector it is on.

The disadvantage in the system is the inability to identify which sector of "A" or "N" it is on. This was later abolish and Marker Beacon was install in its place to provide a more precise navigational aid.

The NDB ( Non Directional Beam) transmits medium frequency band ( 190 ­ 540 kHz ). It can be said that NDb is a Homing Device. By plotting lines of position from two NDMs, the pilot could pin point their direction. Using this information along with the magnetic compass and the NDB receiver, the pilot could the determine the aircraft's bearing from the non directional beacon..

ADF ( Automatic Direction Finder) was a replacement of the manually rotated receiver for the NDB. This device, along with aircraft's heading indicator, the pilot is able to determine the proper heading that would lead to the beacon

VOR is an improvement compared to the A-N method which suffer from reflection. Further improvement is in its infinite number of navigation courses selectable by the pilot instead of just four.

Each VOR frequency is assigned a frequency between 108.10 and 117.9 MHz. Its reception depend on the receiving aircraftıs altitude as the signal is line of sight. Ranges vary from 25 to 200 nautical miles depending on the type of VOR. The VOR transmit two modulated signal, a reference-phase signal and variable-phase signal. The reference-phase signal is designed to use the geographical north as the reference, while the reference-phase signal varies. These two signal are not in phase.

The VOR receiver on board the aircraft measures the phase difference between the two signals to determine the azimuth angle of the aircraft in relations to the VOR transmitter. For example: If the aircraft is directly south of the VOR signal, the receiver would measure a phase difference of 180ƒ. ( See the diagram below). Any lateral deviation from the selected course would be displayed using a vertical pointer known as the course deviation indicator (CDI).

A number of difficulties was encountered. As VHF transmission are line of sight, low- flying aircraft were unable to receive the VOR signal if they were flying below the horizon. Thus the CAA enforced a separation of no farther than 80 nautical miles. This changes was able to accommodate low altitude flying aircraft. Another problem is interference as same frequency would have to be used for such separation. For proper operation of VOR a clear zone of several thousand square feet is required for preventing reflection, distortion or signal blank out.

VOR are divided into 3 categories. These are terminal, low and high-altitude VOR. Terminal VOR (TVOR) are low powered and cover a distance of 25 nautical miles. Low-altitude VOR guarantee interference free reception to aircraft operation up to 40 nautical miles and at an altitude lower than 18,000 feet. This can not be guarantee for aircraft at 40 nautical miles away or above 18,000 feet. High-Altitude VORs are used by aircraft operating between 18,000 and 60,000 feet or 200 nautical miles.

The DME system uses the principle of elapsed time measurement as the basis for distance measurement. The DME system consists of an interrogator located on board the aircraft and a transponder located at the ground station. At regular spaced interval, the interrogator transmits a coded pulse on a frequency of around 1000 Mhz. Once the DME ground base transponder receives this signal, it triggers a reply response to the aircraft transmitting at a different frequency. The elapse time between the interrogation and the receipt of the reply, which is the range, is electronically calculated. Typical time for the signal to travel 1 nautical mile and return is 12.36 millisecond. The distance calculated is the line of sight or slant range distance. This is the distance from the aircraft and the ground station.

Today with the advancement in technology and increasing application in Satellite in navigation, this represents the greatest opportunity to enhance aviation system capacity, efficiency and safety.

The benefit of satellite navigation over those of traditional navigation systems are significant. They achieve greater accuracy than most existing land based systems because the satellite signals do not come from the ground which are prone to ground derived errors.

As Satellite signal is available worldwide, this makes it possible for a goal of single integrated global navigation satellite system (GNSS).

GPS, Global Positioning system, was operational in 1993. It was started and operated by the US Government. The system consists of 24 Satellite covering the whole earth. GPS coupled with Communication Satellite allows for increase in air safety, real time surveillance of aircraft, reduced separation and increases the number of flights on busy transoceanic routes. The FAA, Federal Aviation Authority is committed to making a transition from its ground-based communications, navigation and surveillance system (CNS) to one which will rely primarily on Satellite navigation. However basic GPS does not meet the accuracy critical to the safety of flight, thus WAAS, Wide Area Augmentation System, was developed by the FAA.

This system increases the accuracy to 7 meters vertically and horizontally. Communication Satellite in Geostationary orbit are used to increase the system availability by handling the navigation payloads.

WASS will cover a very large area. The ground station would be linked to form a US WAAS network.. Each of these stations is precisely surveyed and act as a precise reference for the GPS system. Each of these stations would relay the data to the wide area master network where correction information is computed. This is then uplinked to the communication satellite via a ground uplink station. This information is received by the aircraft within the coverage area where precise position is displayed. WAAS provide these benefit to civil aviation.

• Greater runway capability
• Reduced separation minima which allows increased capacity in a given airspace without increasing risk
• More direct en-route flight paths
• New precision approach services
• Reduced and simplified equipment on board aircraft
• Significant cost savings in equipment