Navigation Aids

Navigation Aids

The principles of air navigation are the same for all aircraft, big or small. Air navigation involves successfully piloting an aircraft from place to place without getting lost, breaking the laws applying to aircraft, or endangering the safety of those on board or on the ground.

Air navigation differs from the navigation of surface craft in several ways: Aircraft travel at relatively high speeds, leaving less time to calculate their position en route. Aircraft normally cannot stop in mid-air to ascertain their position at leisure. Aircraft are safety-limited by the amount of fuel they can carry; a surface vehicle can usually get lost, run out of fuel, then simply await rescue. There is no in-flight rescue for most aircraft. And collisions with obstructions are usually fatal. Therefore, constant awareness of position is critical for aircraft pilots.

 Navigation Techniques: 

     Good pilots use all means available to help navigate. Many GA aircraft are fitted with a variety of radio navigation aids, such as Automatic direction finder (ADF) or non-directional beacons (NDBs), VHF Omni directional range (VOR) and satellites. 

    

Automatic Direction Finder (ADF) or non-directional Beacons (NDBs):

    ADF uses NDBs on the ground to drive a display which shows the direction of the beacon from the aircraft. The pilot may use this bearing to draw a line on the map to show the bearing from the beacon. By using a second beacon, two lines may be drawn to locate the aircraft at the intersection of the lines. This is called a cross-cut.  Alternatively, if the track takes the flight directly overhead a beacn, the pilot can use the ADF instrument to maintain heading relative to the beacon, though "following the needle" is bad practice, especially in the presence of a strong cross wind - the pilot's actual track will spiral in towards the beacon, not what was intended. NDBs also can give erroneous readings because they use very long wavelengths, which are easily bent and reflected by ground features and the atmosphere. NDBs continue to be used as a common form of navigation in some countries with relatively few navigational aids. In Pakistan NDB operates at 190 ± 525 KHz. 

                 

  

                                                                                                                                  

VHF Omni Directional Range (VOR): 

VOR is a more sophisticated system, and is still the primary air navigation system established for aircraft flying under IFR in those countries with many navigational aids. In this system, a beacon emits a specially modulated signal which consists of two sine waves which are out of phase. The phase difference corresponds to the actual bearing relative to true north that the receiver is from the station. The upshot is that the receiver can determine with certainty the exact bearing from the station. Again, a cross-cut is used to pinpoint the location. Many VOR stations also have additional equipment called DME ( distance measuring equipment) which will allow a suitable receiver to determine the exact distance from the station. Together with the bearing, this allows an exact position to be determined from a single beacon alone. For convenience, some VOR stations also transmit local weather information which the pilot can listen in to, perhaps generated by an Automated Surface Observing System.

There are two main types of VORs in operation (i.e. two types of actual ground installation) but the aircraft receiver is the same for both. The receiver is unaware which type of ground station is in use - it experiences the same effects from both. It's the method of creation that differs. 

Conventional VOR ( C VOR ) : 

The C VOR employs a rotating directional antenna. Consider for a moment a directional antenna which has a transmission pattern of one broad peak and one broad null in the horizontal plane. If we were to feed this antenna with a VHF carrier and also rotate the antenna at 30 revs/second (1800RPM) - think of how an AM receiver would view this rotation. An AM receiver would see carrier amplitude modulated by a sine wave of 30Hz - the phase of which would be determined by the receiver's position around the station. A practical C VOR doesn't actually spin the aerial at 1800 RPM (although the earliest ones did) - it uses electronic switching of an aerial array to achieve the effect. Additionally, the reference signal is transmitted by FM modulating it onto a 9960 Hz sub carrier (deviation +/-480Hz). The reference signal provides the receiver its comparison to station north. The receiver compares the AM 30Hz variable phase signal recovered with the decoded 30Hz reference signal from the FM sub carrier and determines the radial position from North on which the receiver lies.

  

Doppler VOR ( D VOR ) : 

The D VOR is a later and improved design of VOR which suffers less from sitting errors. The C VOR requires a clear area of at least 1500 ft in radius. The D VOR is more practical in crowded areas or where there are tall buildings. However, it's a big structure around 100ft in diameter. The D VOR reverses the usages of the two 30Hz signals. However, by also reversing the direction of its rotating variable signal it produces exactly the same result in the receiver. The receiver has no "knowledge" that it's a D VOR as opposed to C VOR it's receiving and operates as normal. In the D VOR the main VHF carrier is AM modulated at 30 Hz providing the Reference signal. This is transmitted from a central omnidirectional antenna and has the same phase all around the VOR for any receiver.