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.  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.  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.  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.  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.  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:  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.  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  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.  $^{*}$Service range of individual facilities may be less than  50 miles.  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.  16-29  Automatic Direction Finder (ADF)  NONDIRECTIONAL RADIO BEACON (NDB)  (Usable radius distances for all altitudes)  Power  Distance  Class  (Watts)  (Miles)  Compass Locator  Under 25  15  MH  Under 50  2  5  H  50–1999  $^{* }$5 0  HH  2000 or more  75 