Area Navigation (RNAV)
Introduction
Area Navigation at first were simple mechanical devices which altered the display from the radio navigation aids to simulate the repositioning of the particular aids on the the aircraft's planned track. The pilot could then fly to these 'offset' navigation aids. Electronic computation made such devices lighter, and expanded the possibilities of the techniques. Crew could produce a flight plan which did not have to pass over any radio aids at all and the navigation computer would guide them along.
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Eventually, the computers in an aircraft developed to a stage when they could produce the aircraft's performance parameters. They could then calculated ideal vertical flight path for climb and descent to arrive over places at particular attitudes.
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The Flight Management System (FMS), coupled to the autopilot, can now fly the whole route which the crew have planned in three dimensions. Even the fourth dimension time, can be controlled by the FMS, as airspeed adjustments can be computed to make required times at certain positions.
VOR/DME & RNAV
A simple Area Navigation uses the received signals from VOR and DME stations with a dedicated computers and a CDI or HSI to guide the pilot along his planned route in two dimensions.
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The crew selects a series of 'waypoints' which are their planned turning points or timing points, these waypoints are called as ' phantom station'.
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To define these phantom stations, the crew must calculate or copy the range and bearing of each waypoints from one or more pairs of VOR or DME stations. The computer then calculates and displays directions on the CDI or HSI to the pilot to keep the aircraft on track between the last way point and the next, as if the phantoms where actual VOR/DME stations.
TRADITIONAL VS RNAV NAVIGATION
RNAV Inputs
The inputs are from VOR's and DME's, usually two of each, ADF is not regarded as accurate enough to provide a suitable input to the RNAV computer.
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The most accurate guidance is provided when the computer is able to fix the aircraft's position using two or more DME ranges. This system is called as 'rho-rho'.
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Less accurate is a fix using a VOR radial and a DME range, this system is called as 'rho-theta' fix.
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Least accurate, but acceptable, is from two VOR stations, called as 'theta-theta'.
RNAV
Display
The CDI or HSI displays lateral displacement (distance to one side) from planned track. When used in this fashion, the display is slightly changed from the basic VOR display.
IN RNAV mode, the horizontal dots on the instrument face represents not an angular displacement from the VOR radial, but a distance to either side of the track. Each dot represents one nautical mile displacement.
Control Unit
A Control Display Unit (CDU) incorporates the selectors for the basic system. The pilot must be select the range and bearing of the phantom waypoints from each of the inputs on the CDU. The unit in turn will display a digital readout of these ranges and bearing , and in addition usually of the track and range to the selected waypoints.
A switch provides the facility to prepare the next waypoint while still providing directions to the present one.
CONTROL AND DISPLAY UNIT (CDU)
Benefits of RNAV
When using RNAV equipment, aircrafts are not constrained to follow the airways systems. Consequently, there are several advantages to be gained by operators of such aircraft. They can plan to fly more direct routes, thereby saving fuel, cost and environmental damages. Fast aircrafts may be able to overtake slower ones laterally by changing routes, allowing a smooth flow traffic.
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Aircraft departing from aerodromes can be routed away from the previous traffic which may be slower or have a lesser rate of climb by directing them to phantom station waypoints.
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Aircrafts holding before descent can hold at phantom station at their ideal descent points rather than congested VOR beacons closer to their destination which may involve additional air miles in the descent.
Accuracy, Reliability and Coverage
Because the system uses actual VOR and DME stations, the accuracy, reliability and coverage depends on that of the selected real stations. The range of those primary aids defines the coverage available. The position accuracy depends not only on the individual accuracies of the basic aids, but also by the geometric positions of the stations themselves.
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Accuracy is also affected by the fact that the DME ranges, which are most accurate and preferred inputs, are slant ranges, and the equipment is not normally set up to make the pythagoras calculations.
Advance RNAV Systems
The availability of other inputs, and the improvements in computer processing power, lead to the development of more sophisticated RNAV systems. At the same time, the increased reliability of these other inputs, and of aircraft instruments, have allowed pilots and controllers to reduce separation between aircrafts in flight with as much safely as before, or more.
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That increased the instruments reliability, a consequence of fitting Air Data Computers (ADC), can be coupled with the performance information in the FMS to allow the RNAV equipments to produce directional in the certial plane (V-NAV) as well as the horizontal, or lateral (L-NAV).
Advance Inputs
Any accurate navigation aid can provide input to a RNAV system. However the main improvements came with the inputs from one more more Inertial Navigation System (INS). This gives RNAV system a reliable input even outside the range of DME or VOR stations.
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Omega was a prime input for a while, Loran-C may be used currently like VOR and DME. Doppler is available for use as a self contained aid with the potential to replace one or more of the Inertial Navigation System, and GPS provides a major inputs to most systems.
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Air data inputs are also required by modern system to provide the V-NAV commands. From the Air Data Computer (ADC) comes Mach number, airspeed, temperature, pressure altitude and vertical speed information. These allow the performance parameters to be calculated, and can be compared with the position calculations to provide display of drift and wind velocity if desired.
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Self contained on-board systems, such as Inertial Navigation System are triplicated in most airlines, to increase the accuracy by averaging out three positions. Triplication also provides back up in the event of failure of one or two of them. In addition, the outputs from each INS can be compared by a computer, and if one is different from the rest the computers can discard its information automatically and display a failure signal to to pilot.
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The errors of the self contained systems, such as INS or Doppler, are proportional to the distance travelled . In contrast, the error of radio navigation aids such as DME and VOR depends mainly on the range from the transmitting station. These different type of inputs with their different error characteristics complement each other.
Self-contained systems are said to have a very good 'short term stability', in that their errors are small immediately after switching them on. However, they have poor 'long term stability', because their errors increases with time and the become progressively more inaccurate.
Inputs from the ground-based radio aids have errors which remain basically constant with time, depending only on the range from the stations. They have poor short term stability, but provided that such inputs can be received from new tations when the aircraft has travelled a long distance, the effectively constant error has not changed, so they have good long term stability.
The accuracy of GPS, especially if updated with differential information and checked with RAIM, makes it the manufacturers preferred external inputs for many moderns RNAV systems.
RNAV System Components
The RNAV System consist of the following components
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Navigation Computer Unit (NCU)
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Flight Data Storage Unit (FDSU)
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Control and Display Unit (CDU)
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Navigation Computer Unit (NCU)
The Navigation Computer Unit (NCU) is a computer which takes the input from navigation aids to provide lateral navigation (L-NAV) directions. It compares all the available inputs in a system called 'hybrid navigation' amd uses a techniques called "Kalman Filtering' to arrive at the most probable present position for the aircraft.
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This Kalman filtering does not just average the various inputs; it gives a calculated 'weighting' to each of them so that the most accurate has more effect on the final position than a less accurate aid.
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Most systems separate the position calculations by the self- contained system inputs from the position calculated by the NCU. On the display of the CDU the pilot sees the two positions and a vector showing the difference between them. He can then manually accept the NCU position to update the self contained position if he desires so.
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The quality and complexity of the Kalman filtering system algorithms have a considerable effect on the accuracy of the final computed position. Even without external inputs with good long term stability, a good filter can use previously found errors in the self contained system to produce an estimated position with high probability of accuracy.
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The NCU compares the computed position with the planned track, and produces an indication of the distance to go to the next waypoint, and the distance off track, as a simple RNAV would do. However the NCU can also produce a L-NAV command to bring the aircraft back on to track, either by the next waypoint or earlier if preferred. This earlier return to track is needed for flight in all airspaces requiring enhanced navigation system.
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The crew may have made the aircraft deviate from the original plan, for example to avoid weather or another aircraft. In this case , the NCU will direct aircraft's return to the original plan either by taking them to the next suitable waypoint or back onto the current track desired. The output from the NCU can also be fed to the autopilot when selected by the pilot.
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Flight Data Storage Unit (FDSU)
The modern system no longer requires pilot to select the navigation inputs he wishes to use. Nor need he dial the range and bearing of his phantom waypoints. The flight data storage unit (FDSU) contains the position and protected range of all navigation aid throughout the world.
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When the pilot selects his phantom waypoint on the CDU, the NCU selects the stations which will give the most accurate fix from the FDSU.
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The FDSu in current system also contains a wealth of information about aerodromes and positions of points on them, Air Traffic Services routes, company routes, Standard Arrival Routes (STARs), and Standard Instrument Departure Procedures (SIDs). It contains information on magnetic variation around the globe, and this can be applied by the NCU when selected by the pilot. the data base of the aerodromes also includes runway threshold and even parking stand positions. This means that the FMS can be set up to the stand position, and automatically updates as the aircraft starts its take-off run.
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Information about aerodromes, reference points, navigation aids and procedure which are required to to updates every 28 days by the help of a computer tape or disk. The next disk is made available early, so that its information can be stored in advance and exchanged for the old information in the FDSU when it becomes effective. On that date the crew will select the new data base from the FMS.
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The FDSU also contains the aircraft's performance details at add to the air data inputs to provide V-NAV guidance.
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Control Display Unit (CDU)
The Control and Display Unit (CDU) of a modern system has the same function as that in the simple RNAV system. It is an interface between the crew and the computer. The display part can give a digital indication of the aircraft's computed position in latitude and longitude. It can also indicate the position of the next selected waypoint, the distance to run to it, the ground speed made good along track, and the estimated time of arrival at the next way point.
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However CDU normally also feeds the navigation display of the EFIS horizontal situation indicator, which leave the CDU screen free to act as a computer control screen.
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When the aircraft is fitted with a Flight Management System (FMS), the NCU forms part of more extensive Flight Management Computer (FMC). The CDU displays a series of pages of information either provided or required by the FMC. To change the information display on one of the data fields on the unit, the pilot writes the details on a 'scratchpad' at the bottom of the CDU screen, then transfers it to the particular field to which it refers by pressing the selection key of that field.
V-NAV
Using the aircraft's performance information stored in the FDSU, possibly as amended by the engine management system, the NCU can calculate the optimum flight level for a particular phase of the flight. The FMS will indicate the best time to change the levels if a cruise flight is not practicable.
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Standard Instrument Departure (SIDs)
The Standard Instrument Departure (SIDs) from aerodromes are published to ensure the smooth flow of departure. SIDs for all aerodromes which the aircraft are likely to use on particular routes are stored in the FDSU. L-NAV direction from the NCU can guide the aircraft along these SID's during its clim, reducing crew workload.
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However, the V-NAV facility allows more sophisticated guidance along the SID. It will direct the crew, or autopilot more likely along ta three dimensional tube in space to not fly over the waypoints which are positioned under the SID path, but also to arrive at each waypoint at the desires altitude, in accordance with ATC clearances.
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Standard Arrival Route(STARs)
Aircraft can be directed to follow Standard Arrival Route (STARs) to guide them on the runway centerline. Again the V-NAV facility can direct the aircraft's descent path to be overhead the waypoints at the cleared altitudes, and on to the centreline at an ideal height.
Autopilot Flight Director System
Since most aircraft are fitted with an autopilot, the NCU can instruct autopilot to obey its commands through an Autopilot Flight Director System (AFDS). The AFDS control the aircraft's primary flight controls to follow L-NAV commands and the engine controls through the auto-throttle to follow V-NAV commands.