Figure 16-35. Flying an RNAV course.
While the capabilities and methods of operation of VOR/DME RNAV units differ, there are basic principles of operation that are common to all. Pilots are urged to study the manufacturer’s operating guide and receive instruction prior to the use of VOR/DME RNAV or any unfamiliar navigational system. Operational information and limitations should also be sought from placards and the supplement section of the AFM/POH.
VOR/DME-based RNAV units operate in at least three modes: VOR, en route, and approach. A fourth mode, VOR Parallel, may also be found on some models. The units need both VOR and DME signals to operate in any RNAV mode. If the NAVAID selected is a VOR without DME, RNAV mode will not function.
In the VOR (or non-RNAV) mode, the unit simply functions as a VOR receiver with DME capability. [Figure 16-36] The unit’s display on the VOR indicator is conventional in all respects. For operation on established airways or any other ordinary VOR navigation, the VOR mode is used.
To utilize the unit’s RNAV capability, the pilot selects and establishes a waypoint or a series of waypoints to define a course. A VORTAC (or VOR/DME) needs to be selected as a NAVAID, since both radial and distance signals are available from these stations. To establish a waypoint, a point somewhere within the service range of a VORTAC is defined on the basis of radial and distance. Once the waypoint is entered into the unit and the RNAV en route mode is selected, the CDI displays course guidance to the waypoint, not the original VORTAC. DME also displays distance to the waypoint. Many units have the capability to store several waypoints, allowing them to be programmed prior to flight, if desired, and called up in flight.
RNAV waypoints are entered into the unit in magnetic bearings (radials) of degrees and tenths (i.e., 275.5°) and distances in NM and tenths (i.e., 25.2 NM). When plotting RNAV waypoints on an aeronautical chart, pilots find it difficult to measure to that level of accuracy, and in practical application, it is rarely necessary. A number of flight planning publications publish airport coordinates and waypoints with this precision and the unit accepts those figures. There is a subtle but important difference in CDI operation and display in the RNAV modes.
Figure 16-36. RNAV controls.
In the RNAV modes, course deviation is displayed in terms of linear deviation. In the RNAV en route mode, maximum deflection of the CDI typically represents 5 NM on either side of the selected course without regard to distance from the waypoint. In the RNAV approach mode, maximum deflection of the CDI typically represents 1¼ NM on either side of the selected course. There is no increase in CDI sensitivity as the aircraft approaches a waypoint in RNAV mode.
The RNAV approach mode is used for instrument approaches. Its narrow scale width (¼ of the en route mode) permits very precise tracking to or from the selected waypoint. In VFR cross-country navigation, tracking a course in the approach mode is not desirable because it requires a great deal of attention and soon becomes tedious.
A fourth, lesser-used mode on some units is the VOR Parallel mode. This permits the CDI to display linear (not angular) deviation as the aircraft tracks to and from VORTACs. It derives its name from permitting the pilot to offset (or parallel) a selected course or airway at a fixed distance of the pilot’s choosing, if desired. The VOR parallel mode has the same effect as placing a waypoint directly over an existing VORTAC. Some pilots select the VOR parallel mode when utilizing the navigation (NAV) tracking function of their autopilot for smoother course following near the VORTAC.
Navigating an aircraft with VOR/DME-based RNAV can be confusing, and it is essential that the pilot become familiar with the equipment installed. It is not unknown for pilots to operate inadvertently in one of the RNAV modes when the operation was not intended, by overlooking switch positions or annunciators. The reverse has also occurred with a pilot neglecting to place the unit into one of the RNAV modes by overlooking switch positions or annunciators. As always, the prudent pilot is not only familiar with the equipment used, but never places complete reliance in just one method of navigation when others are available for cross-check.
Automatic Direction Finder (ADF)
Many general aviation-type aircraft are equipped with ADF radio receiving equipment. To navigate using the ADF, the pilot tunes the receiving equipment to a ground station known as a nondirectional radio beacon (NDB). The NDB stations normally operate in a low or medium frequency band of 200 to 415 kHz. The frequencies are readily available on aeronautical charts or in the Chart Supplement U.S.
All radio beacons, except compass locators, transmit a continuous three-letter identification in code, except during voice transmissions. A compass locator, which is associated with an instrument landing system, transmits a two-letter identification.
Standard broadcast stations can also be used in conjunction with ADF. Positive identification of all radio stations is extremely important and this is particularly true when using standard broadcast stations for navigation.
NDBs have one advantage over the VOR in that low or medium frequencies are not affected by line-of-sight. The signals follow the curvature of the Earth; therefore, if the aircraft is within the range of the station, the signals can be received regardless of altitude.
The following table gives the class of NDB stations, their power, and their usable range:
NONDIRECTIONAL RADIO BEACON (NDB)
(Usable radius distances for all altitudes)
Class
Power (Watts)
Distance (Miles)
Compass Locator
Under 25
15
MH
Under 50
25
H
50–1999
*50
HH
2000 or more
75
*Service range of individual facilities may be less than 50 miles.
One of the disadvantages that should be considered when using low frequency (LF) for navigation is that LF signals are very susceptible to electrical disturbances, such as lightning. These disturbances create excessive static, needle deviations, and signal fades. There may be interference from distant stations. Pilots should know the conditions under which these disturbances can occur so they can be more alert to possible interference when using the ADF.
Basically, the ADF aircraft equipment consists of a tuner, which is used to set the desired station frequency, and the navigational display.
The navigational display consists of a dial upon which the azimuth is printed and a needle which rotates around the dial and points to the station to which the receiver is tuned.
Some of the ADF dials can be rotated to align the azimuth with the aircraft heading; others are fixed with 0° representing the nose of the aircraft and 180° representing the tail. Only the fixed azimuth dial is discussed in this handbook. [Figure 16-37]
Figure 16-38 illustrates terms that are used with the ADF and should be understood by the pilot.
To determine the magnetic bearing “FROM” the station, 180° is added to or subtracted from the magnetic bearing to the station. This is the reciprocal bearing and is used when plotting position fixes.
Figure 16-37. ADF with fixed azimuth and magnetic compass.
Figure 16-38. ADF terms.
Keep in mind that the needle of fixed azimuth points to the station in relation to the nose of the aircraft. If the needle is deflected 30° to the left for a relative bearing of 330°, this means that the station is located 30° left. If the aircraft is turned left 30°, the needle moves to the right 30° and indicates a relative bearing of 0° meaning that the aircraft is pointing toward the station. If the pilot continues flight toward the station keeping the needle on 0°, the procedure is called homing to the station. If a crosswind exists, the ADF needle continues to drift away from zero. To keep the needle on zero, the aircraft must be turned slightly resulting in a curved flight path to the station. Homing to the station is a common procedure but may result in drifting downwind, thus lengthening the distance to t
he station.
Tracking to the station requires correcting for wind drift and results in maintaining flight along a straight track or bearing to the station. When the wind drift correction is established, the ADF needle indicates the amount of correction to the right or left. For instance, if the magnetic bearing to the station is 340°, a correction for a left crosswind would result in a magnetic heading of 330°, and the ADF needle would indicate 10° to the right or a relative bearing of 010°. [Figure 16-39]
When tracking away from the station, wind corrections are made similar to tracking to the station, but the ADF needle points toward the tail of the aircraft or the 180° position on the azimuth dial. Attempting to keep the ADF needle on the 180° position during winds results in the aircraft flying a curved flight leading further and further from the desired track. When tracking outbound, corrections for wind should be made in the direction opposite of that in which the needle is pointing.
Although the ADF is not as popular as the VOR for radio navigation, with proper precautions and intelligent use, the ADF can be a valuable aid to navigation.
Global Positioning System
The GPS is a satellite-based radio navigation system. Its RNAV guidance is worldwide in scope. There are no symbols for GPS on aeronautical charts as it is a space-based system with global coverage. Development of the system is underway so that GPS is capable of providing the primary means of electronic navigation. Portable and yoke-mounted units are proving to be very popular in addition to those permanently installed in the aircraft. Extensive navigation databases are common features in aircraft GPS receivers.
Figure 16-39. ADF tracking.
The GPS is a satellite radio navigation and time dissemination system developed and operated by the U.S. Department of Defense (DOD). Civilian interface and GPS system status is available from the U.S. Coast Guard.
It is not necessary to understand the technical aspects of GPS operation to use it in VFR/IFR navigation. It does differ significantly from conventional, ground-based electronic navigation and awareness of those differences is important. Awareness of equipment approvals and limitations is critical to the safety of flight.
The GPS navigation system broadcasts a signal that is used by receivers to determine precise position anywhere in the world. The receiver tracks multiple satellites and determines a pseudorange measurement to determine the user location. A minimum of four satellites is necessary to establish an accurate three-dimensional position. The Department of Defense (DOD) is responsible for operating the GPS satellite constellation and monitors the GPS satellites to ensure proper operation.
The status of a GPS satellite is broadcast as part of the data message transmitted by the satellite. GPS status information is also available from the U.S. Coast Guard navigation information service at (703) 313-5907 or online at www.navcen.uscg.gov. Additionally, satellite status is available through the NOTAM system.
The GPS receiver verifies the integrity (usability) of the signals received from the GPS constellation through receiver autonomous integrity monitoring (RAIM) to determine if a satellite is providing corrupted information. At least one satellite, in addition to those required for navigation, must be in view for the receiver to perform the RAIM function; thus, RAIM needs a minimum of five satellites in view or four satellites and a barometric altimeter (baro-aiding) to detect an integrity anomaly. For receivers capable of doing so, RAIM needs six satellites in view (or five satellites with baro-aiding) to isolate the corrupt satellite signal and remove it from the navigation solution. Baro-aiding is a method of augmenting the GPS integrity solution by using a nonsatellite input source. GPS derived altitude should not be relied upon to determine aircraft altitude since the vertical error can be quite large and no integrity is provided. To ensure that baro-aiding is available, the current altimeter setting must be entered into the receiver as described in the operating manual.
RAIM messages vary somewhat between receivers; however, generally there are two types. One type indicates that there are not enough satellites available to provide RAIM integrity monitoring and another type indicates that the RAIM integrity monitor has detected a potential error that exceeds the limit for the current phase of flight. Without RAIM capability, the pilot has no assurance of the accuracy of the GPS position.
Selective Availability
Selective Availability (SA) is a method by which the accuracy of GPS is intentionally degraded. This feature is designed to deny hostile use of precise GPS positioning data. SA was discontinued on May 1, 2000, but many GPS receivers are designed to assume that SA is still active.
The baseline GPS satellite constellation consists of 24 satellites positioned in six earth-centered orbital planes with four operation satellites and a spare satellite slot in each orbital plane. The system can support a constellation of up to thirty satellites in orbit. The orbital period of a GPS satellite is one-half of a sidereal day or 11 hours 58 minutes. The orbits are nearly circular and equally spaced about the equator at a 60-degree separation with an inclination of 55 degrees relative to the equator. The orbital radius (i.e. distance from the center of mass of the earth to the satellite) is approximately 26,600 km.
With the baseline satellite constellation, users with a clear view of the sky have a minimum of four satellites in view. It is more likely that a user would see six to eight satellites. The satellites broadcast ranging signals and navigation data allowing users to measure their pseudoranges in order to estimate their position, velocity and time, in a passive, listen-only mode. The receiver uses data from a minimum of four satellites above the mask angle (the lowest angle above the horizon at which a receiver can use a satellite). The exact number of satellites operating at any one particular time varies depending on the number of satellite outages and operational spares in orbit. For current status of the GPS constellation, please visit http://tycho.usno.navy.mil/gpscurr.html. [Figure 16-40]
VFR Use of GPS
GPS navigation has become a great asset to VFR pilots providing increased navigation capability and enhanced situational awareness while reducing operating costs due to greater ease in flying direct routes. While GPS has many benefits to the VFR pilot, care must be exercised to ensure that system capabilities are not exceeded.
Types of receivers used for GPS navigation under VFR are varied from a full IFR installation being used to support a VFR flight to a VFR only installation (in either a VFR or IFR capable aircraft) to a hand-held receiver. The limitations of each type of receiver installation or use must be understood by the pilot to avoid misusing navigation information. In all cases, VFR pilots should never rely solely on one system of navigation. GPS navigation must be integrated with other forms of electronic navigation, as well as pilotage and dead reckoning. Only through the integration of these techniques can the VFR pilot ensure accuracy in navigation. Some critical concerns in VFR use of GPS include RAIM capability, database currency, and antenna location.
Figure 16-40. Satellite constellation.
RAIM Capability
Many VFR GPS receivers and all hand-held units are not equipped with RAIM alerting capability. Loss of the required number of satellites in view, or the detection of a position error, cannot be displayed to the pilot by such receivers. In receivers with no RAIM capability, no alert would be provided to the pilot that the navigation solution had deteriorated and an undetected navigation error could occur. A systematic cross-check with other navigation techniques would identify this failure and prevent a serious deviation.
In many receivers, an updatable database is used for navigation fixes, airports, and instrument procedures. These databases must be maintained to the current update for IFR operation, but no such requirement exists for VFR use. However, in many cases, the database drives a moving map display that indicates Special Use Airspace and the various classes of airspace in addition to other operational information. Without a current database, the moving map display may be outdated and offer erroneous information to VFR pilots wishing to f
ly around critical airspace areas, such as a Restricted Area or a Class B airspace segment. Numerous pilots have ventured into airspace they were trying to avoid by using an outdated database. If there is not a current database in the receiver, disregard the moving map display when making critical navigation decisions.
In addition, waypoints are added, removed, relocated, or renamed as required to meet operational needs. When using GPS to navigate relative to a named fix, a current database must be used to properly locate a named waypoint. Without the update, it is the pilot’s responsibility to verify the waypoint location referencing to an official current source, such as the Chart Supplement U.S., sectional chart, or en route chart.
In many VFR installations of GPS receivers, antenna location is more a matter of convenience than performance. In IFR installations, care is exercised to ensure that an adequate clear view is provided for the antenna to communicate with satellites. If an alternate location is used, some portion of the aircraft may block the view of the antenna increasing the possibility of losing navigation signal.
This is especially true in the case of hand-held receivers. The use of hand-held receivers for VFR operations is a growing trend, especially among rental pilots. Typically, suction cups are used to place the GPS antennas on the inside of aircraft windows. While this method has great utility, the antenna location is limited by aircraft structure for optimal reception of available satellites. Consequently, signal loss may occur in certain situations where aircraft-satellite geometry causes a loss of navigation signal. These losses, coupled with a lack of RAIM capability, could present erroneous position and navigation information with no warning to the pilot.
Pilot's Handbook of Aeronautical Knowledge (Federal Aviation Administration) Page 70
