Advances in navigational radio receivers installed in aircraft, the development of aeronautical charts which show the exact location of ground transmitting stations and their frequencies, along with refined flight deck instrumentation make it possible for pilots to navigate with precision to almost any point desired. Although precision in navigation is obtainable through the proper use of this equipment, beginning pilots should use this equipment to supplement navigation by visual reference to the ground (pilotage).

There are four radio navigation systems available for use for VFR navigation:

• VHF Omnidirectional Range (VOR)
• Nondirectional Radio Beacon (NDB)
• Long Range Navigation (LORAN-C)
• Global Positioning System (GPS)

15.10.1 Very High Frequency (VHF) Omnidirectional Range (VOR) #

The VOR system is present in three slightly different navigation aids (NAVAIDs): VOR, VOR/DME, and VORTAC. By itself it is known as a VOR, and it provides magnetic bearing information to and from the station. When DME is also installed with a VOR, the NAVAID is referred to as a VOR/DME. When military tactical air navigation (TACAN) equipment is installed with a VOR, the NAVAID is known as a VORTAC. DME is always an integral part of a VORTAC. Regardless of the type of NAVAID utilized, the VOR indicator behaves the same.

The prefix “omni-” means all, and an omnidirectional range is a VHF radio transmitting ground station that projects straight line courses (radials) from the station in all directions. From a top view, it can be visualized as being similar to the spokes from the hub of a wheel.

The course or radials projected from the station are referenced to magnetic north. Therefore, a radial is defined as a line of magnetic bearing extending outward from the VOR station. Radials are identified by numbers beginning with 001, which is 1° east of magnetic north, and progress in sequence through all the degrees of a circle until reaching 360.

VOR ground stations transmit within a VHF frequency band of 108.0–117.95 MHz. Because the equipment is VHF, the signals transmitted are subject to line-of-sight restrictions. Therefore, its range varies in direct proportion to the altitude of receiving equipment. Generally, the reception range of the signals at an altitude of 1,000 feet above ground level (AGL) is about 40 to 45 miles. This distance increases with altitude. [Figure 15-28]

VORs and VORTACs are classed according to operational use. There are three classes: T (Terminal), L (Low altitude), and H (High altitude). The normal usable range varies by class and altitude (for example, Terminal facilities are usable to about 25 NM at 12,000 feet and below; Low altitude facilities to about 40 NM below 18,000 feet; High altitude facilities to greater distances at higher altitudes).

The useful range of certain facilities may be less than these values; for further information concerning these restrictions, refer to the Communication/NAVAID Remarks in the A/FD.

The accuracy of course alignment of VOR radials is considered to be excellent, generally within plus or minus 1°. However, certain parts of the VOR receiver equipment deteriorate, and this affects its accuracy, particularly at great distances from the VOR station. VOR accuracy checks are not a regulatory requirement for VFR flight; however, to assure accuracy of the equipment, these checks should be accomplished frequently. The following means are provided for pilots to check VOR accuracy: an FAA VOR test facility (VOT); certified airborne checkpoints; and certified ground checkpoints located on airport surfaces.

If an aircraft has two VOR receivers installed, a dual VOR receiver check can be made by tuning both receivers to the same VOR ground facility; the maximum permissible variation between the two indicated bearings is 4 degrees. As a guide to assure acceptable accuracy, the required IFR tolerances can be used—±4° for ground checks and ±6° for airborne checks.

The VOR transmitting station can be positively identified by its Morse code identification or by a recorded voice identification which states the name of the station followed by “VOR.” If the VOR is out of service for maintenance, the coded identification is removed and not transmitted. VOR receivers are designed with an alarm flag to indicate when signal strength is inadequate to operate the navigational equipment.

15.10.2 Using the VOR #

For VOR radio navigation, there are two components required: ground transmitter and aircraft receiving equipment. The ground transmitter is located at a specific position on the ground and transmits on an assigned frequency. The aircraft equipment includes a receiver with a tuning device and a VOR or omninavigation instrument. The navigation instrument could be a course deviation indicator (CDI), horizontal situation indicator (HSI), or a radio magnetic indicator (RMI). Each of these instruments indicates the course to the tuned VOR.

15.10.3 Course Deviation Indicator (CDI) #

The CDI is found in most training aircraft. It consists of (1) an omnibearing selector (OBS) sometimes referred to as the course selector, (2) a CDI needle (Left-Right Needle), and (3) a TO/FROM indicator.

The course selector is an azimuth dial that can be rotated to select a desired radial or to determine the radial over which the aircraft is flying.

When the course selector is rotated, it moves the CDI or needle to indicate the position of the radial relative to the aircraft. If the course selector is rotated until the deviation needle is centered, the radial (magnetic course “FROM” the station) or its reciprocal (magnetic course “TO” the station) can be determined. The course deviation needle also moves to the right or left if the aircraft is flown or drifting away from the radial which is set in the course selector. If the flag displays a “TO,” the course shown on the course selector must be flown to the station. [Figure 15-29] If “FROM” is displayed and the course shown is followed, the aircraft is flown away from the station.

15.10.4 Horizontal Situation Indicator (HSI) #

The HSI is a direction indicator that uses the output from a flux valve to drive the compass card. The HSI [Figure 15-30] combines the magnetic compass with navigation signals and a glideslope, giving the pilot an indication of the location of the aircraft with relationship to the chosen course or radial. The course deviation bar operates with a VOR/Localizer (VOR/LOC) or GPS navigation receiver to indicate left or right deviations from the course selected with the course select pointer.

The desired course is selected by rotating the course select pointer, in relation to the compass card, by means of the course select knob. The HSI has a fixed aircraft symbol and the course deviation bar displays the aircraft’s position relative to the selected course. The TO/FROM indicator is a triangular pointer.

When the NAV warning flag appears it indicates no reliable signal is being received.

The glideslope pointer indicates the relation of the aircraft to the glideslope.

15.10.5 Radio Magnetic Indicator (RMI) #

The RMI [Figure 15-31] is a navigational aid providing aircraft magnetic or directional gyro heading and VOR, GPS, and automatic direction finder (ADF) bearing information. The RMI consists of a compass card, a heading index, two bearing pointers, and pointer function switches. The two pointers are driven by any two combinations of a GPS, an ADF, and/or a VOR. The pilot has the ability to select the navigation aid to be indicated. The pointer indicates course to the selected NAVAID or waypoint.

15.10.6 Tracking With VOR #

The following describes a step-by-step procedure to use when tracking to and from a VOR station using a CDI. [Figure 15-32]

First, tune the VOR receiver to the frequency of the selected VOR station and check the identifiers to verify that the desired VOR is being received. As soon as the VOR is properly tuned, the course deviation needle deflects either left or right. Then, rotate the azimuth dial to the course selector until the course deviation needle centers and the TO-FROM indicator indicates “TO.” If the needle centers with a “FROM” indication, the azimuth should be rotated 180°. Now, turn the aircraft to the heading indicated on the VOR azimuth dial. If a heading is maintained with a crosswind, the aircraft drifts off course and the VOR course deviation needle gradually moves in the direction of the desired radial.

To return to the desired radial, the aircraft heading must be altered toward the needle. As the aircraft returns to the desired track, the deviation needle slowly returns to center; when centered, the aircraft is on the desired radial and a wind drift correction must be established by holding a heading that keeps the needle centered.

As the VOR station is passed, the course deviation needle fluctuates, then settles down, and the “TO” indication changes to “FROM.”

Generally, the same techniques apply when tracking outbound as those used for tracking inbound.

15.10.7 Tips on Using the VOR #

• Positively identify the station by its code or voice identification.
• Keep in mind that VOR signals are “line-of-sight.” A weak signal or no signal at all is received if the aircraft is too low or too far from the station.
• When navigating to a station, determine the inbound radial and use this radial. If the aircraft drifts, fly a heading to re-intercept the course, then apply a correction to compensate for wind drift.
• If minor needle fluctuations occur, avoid changing headings immediately. Wait momentarily to see if the needle recenters; if it does not, then correct.
• When flying “TO” a station, always fly the selected course with a “TO” indication. When flying “FROM” a station, always fly the selected course with a “FROM” indication. If this is not done, the action of the course deviation needle is reversed.
• When navigating using the VOR it is important to fly headings that maintain or re-intercept the course. Just turning toward the needle will cause overshooting the radial and flying an S turn to the left and right of course.

15.10.8 Time and Distance Check From a Station #

To compute time and distance from a station, first turn the aircraft to place the bearing pointer on the nearest 90° index. Note the time and maintain heading. When the bearing pointer has moved 10°, note the elapsed time in seconds and apply the appropriate formulas. [Figure 15-33] The time from station may be calculated by a short method: if the elapsed time for a 10° bearing change is noted in seconds, the time from the station in minutes is determined by moving the decimal point one place. Thus, if 75 seconds are required to fly a 10° bearing change, the aircraft is 7.5 minutes from the station.

The distance from the station is computed by multiplying TAS or GS (in miles per minute) by the previously determined time in minutes. For example, if the aircraft is 7.5 minutes from station, flying at a TAS of 120 knots or 2 NM per minute, the distance from station is 15 NM (7.5 × 2 = 15).

15.10.9 Course Intercept #

Course interceptions are performed in most phases of instrument navigation. The equipment used varies, but an intercept heading must be flown that results in an angle or rate of intercept sufficient to solve a particular problem. Rate of intercept, seen by the aviator as bearing pointer or HSI movement, is a result of the following factors: the angle at which the aircraft is flown toward a desired course (angle of intercept); true airspeed and wind (GS); and distance from the station.

The angle of intercept is the angle between the heading of the aircraft (intercept heading) and desired course. Controlling this angle by selection/adjustment of the intercept heading is the easiest and most effective way to control course interceptions. The angle of intercept must be greater than the degrees from course, but should not exceed 90°.

When selecting an intercept heading, the key factor is the relationship between distance from the station and degrees from the course. Each degree, or radial, is 1 NM wide at a distance of 60 NM from the station; width increases or decreases in proportion to the 60 NM distance (1 degree is 2 NM wide at 120 NM, and ½ NM wide at 30 NM).

15.10.10 Distance Measuring Equipment (DME) #

Distance measuring equipment (DME) consists of an ultra high frequency (UHF) navigational aid with VOR/DMEs and VORTACs. It measures, in NM, the slant range distance of an aircraft from a VOR/DME or VORTAC.

To utilize DME, the pilot should select, tune, and identify a VORTAC. The DME receiver, utilizing a “paired frequency” concept, automatically selects and tunes the UHF DME frequency associated with the VHF VORTAC frequency selected by the pilot. After a brief pause, the DME display shows the slant range distance to or from the VORTAC. Slant range distance is the direct distance between the aircraft and the VORTAC, and is therefore affected by aircraft altitude. (Station passage directly over a VORTAC from an altitude of 6,076 feet AGL would show approximately 1.0 NM on the DME.) With DME, a pilot may precisely locate the aircraft on a VOR radial.

Most DME receivers also provide GS and time-to-station modes of operation. GS and time-to-station information is only accurate when tracking directly to or from a VORTAC.

15.10.11 VOR/DME RNAV #

Area navigation (RNAV) permits electronic course guidance on any direct route between points established by the pilot. While RNAV is a generic term that applies to a variety of navigational aids, such as LORAN-C, GPS, and others, this section deals with VOR/DME-based RNAV. VOR/DME RNAV is not a separate ground-based NAVAID, but a method of navigation using VOR/DME and VORTAC signals specially processed by the aircraft’s RNAV computer. [Figure 15-34]

In its simplest form, VOR/DME RNAV allows the pilot to electronically move VORTACs around to more convenient locations. Once electronically relocated, they are referred to as waypoints, described as a combination of a selected radial and distance within the service volume of the VORTAC to be used. These waypoints allow a straight course to be flown between almost any origin and destination, without regard to the orientation of VORTACs or the existence of airways.

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.

In the VOR (or non-RNAV) mode, the unit simply functions as a VOR receiver with DME capability. [Figure 15-35]

To utilize the unit’s RNAV capability, the pilot selects and establishes a waypoint or a series of waypoints to define a course. 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, and DME displays distance to the waypoint.

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; in the RNAV approach mode, maximum deflection typically represents 1¼ NM on either side. As always, the prudent pilot never places complete reliance in just one method of navigation when others are available for cross-check.

15.10.12 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. All radio beacons except compass locators transmit a continuous three-letter identification in code except during voice transmissions.

NDBs have one advantage over the VOR: low or medium frequencies are not affected by line-of-sight, so the signals follow the curvature of the Earth and can be received at any altitude within range of the station. NDB stations are classed by power and usable range (for example, a Compass Locator has 25 watts or less and a usable radius of about 15 NM; higher-power stations range further). One disadvantage of low frequency (LF) for navigation is that LF signals are very susceptible to electrical disturbances, such as lightning, which create excessive static, needle deviations, and signal fades.

Basically, the ADF aircraft equipment consists of a tuner, 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. Only the fixed azimuth dial is discussed in this handbook (0° representing the nose of the aircraft, 180° the tail). [Figures 15-36 and 15-37]

The needle of the 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°, 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°, or 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 flightpath to the station. Homing results in drifting downwind, thus lengthening the distance to the station.

Tracking to the station requires correcting for wind drift and results in maintaining flight along a straight track or bearing to the station. [Figure 15-38]

When tracking away from the station, the ADF needle points toward the tail of the aircraft (the 180° position); to correct for wind when tracking outbound, correction should be made in the direction opposite of that in which the needle is pointing.

15.10.13 LORAN-C Navigation #

Long range navigation, version C (LORAN-C) is another form of RNAV, but one that operates from chains of transmitters broadcasting signals in the LF spectrum. Selection of a transmitter chain is either made automatically by the unit, or manually by the pilot using guidance information provided by the manufacturer. LORAN-C is a highly accurate, supplemental form of navigation typically installed as an adjunct to VOR and ADF equipment. Databases of airports, NAVAIDs, and ATC facilities are frequently features of LORAN-C receivers.

While coverage is not global, LORAN-C signals are suitable for navigation in all of the conterminous United States, and parts of Canada and Alaska.

LORAN-C absolute accuracy is excellent—position errors are typically less than .25 NM. It operates in a 90–110 kHz frequency range and is based upon measurement of the difference in arrival times of pulses of radio frequency energy emitted by a chain of transmitters hundreds of miles apart. Within any given chain there is a master station, and from three to five secondary stations; LORAN-C units must be able to receive at least a master and two secondary stations to provide navigational information. After the receiver has been turned on, the unit must be initialized before it can be used for navigation; while this can be accomplished in flight, it is preferable to perform this task on the ground.

The LORAN-C signal is subject to degradation from a variety of atmospheric disturbances, and is susceptible to interference from static electricity buildup on the airframe; static wicks and bonding straps should be installed and properly maintained. LORAN-C NOTAMs should be reviewed prior to relying on LORAN-C for navigation.

The prudent pilot never relies solely on one means of navigation when others are available for backup and cross-check.

15.10.14 Global Positioning System (GPS) #

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. The GPS is a satellite radio navigation and time dissemination system developed and operated by the U.S. Department of Defense (DOD).

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 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. GPS-derived altitude should not be relied upon to determine aircraft altitude since the vertical error can be quite large. Without RAIM capability, the pilot has no assurance of the accuracy of the GPS position.

15.10.15 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 GPS constellation of 24 satellites is designed so that a minimum of five satellites are always observable by a user anywhere on earth. 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).

15.10.16 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, to a hand-held receiver. 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.

Some critical concerns in VFR use of GPS include RAIM capability, database currency, and antenna location.

Many VFR GPS receivers and all hand-held units have no RAIM alerting capability, so loss of the required number of satellites in view, or the detection of a position error, cannot be displayed to the pilot. A current database drives the moving map display which indicates Special Use Airspace and the various classes of airspace; without a current database the moving map display may be outdated and offer erroneous information. In many VFR installations, antenna location is more a matter of convenience than performance; if an alternate location is used, some portion of the aircraft may block the view of the antenna, causing a greater opportunity to lose navigation signal. The use of a hand-held GPS for VFR operations is not limited by regulation, but modification of the aircraft, such as installing a panel- or yoke-mounted holder, is governed by 14 CFR part 43.

15.10.17 Tips for Using GPS for VFR Operations #

Always check to see if the unit has RAIM capability. If no RAIM capability exists, be suspicious of a GPS displayed position when any disagreement exists with the position derived from other radio navigation systems, pilotage, or dead reckoning.

Check the currency of the database, if any; if expired, update it, and if an update is not possible, disregard any moving map display of airspace for critical navigation decisions. Be aware that named waypoints may no longer exist or may have been relocated since the database expired.

While a hand-held GPS receiver can provide excellent navigation capability to VFR pilots, be prepared for intermittent loss of navigation signal, possibly with no RAIM warning.

Plan flights carefully before taking off; if navigating to user-defined waypoints, enter them before flight, not on the fly, and verify the planned flight against a current source such as a current sectional chart. Minimize head-down time in the aircraft and keep a sharp lookout for traffic, terrain, and obstacles. Become very familiar with the receiver’s operation, as most receivers are not intuitive.

In summary, be careful not to rely on GPS to solve all VFR navigational problems; only the pilot can navigate the aircraft, and GPS is just one of the pilot’s tools to do the job.

15.10.18 VFR Waypoints #

VFR waypoints provide VFR pilots with a supplementary tool to assist with position awareness while navigating visually in aircraft equipped with area navigation receivers. The uses of VFR waypoints include providing navigational aids for pilots unfamiliar with an area, waypoint definition of existing reporting points, enhanced navigation in and around Class B and Class C airspace, and enhanced navigation around Special Use Airspace. VFR waypoint names (for computer entry and flight plans) consist of five letters beginning with the letters “VP” and are retrievable from navigation databases. The VFR waypoint names are not intended to be pronounceable, and they are not for use in ATC communications. On VFR charts, a stand-alone VFR waypoint is portrayed using the same four-point star symbol used for IFR waypoints. A VFR waypoint collocated with a visual checkpoint on the chart is identified by a small magenta flag symbol and may be used for ATC communications.

When filing VFR flight plans, use the five-letter identifier as a waypoint in the route of flight section if there is an intended course change at that point or if used to describe the planned route of flight.

Any VFR waypoints intended for use during a flight should be loaded into the receiver while on the ground and prior to departure.

Pilots should be especially vigilant for other traffic while operating near VFR waypoints.