4.
VOR Receiver Check

4.1 Periodic VOR receiver calibration is most important. If a receiver’s Automatic Gain Control or modulation circuit deteriorates, it is possible for it to display acceptable accuracy and sensitivity close into the VOR or VOT and display out.of.tolerance readings when located at greater distances where weaker signal areas exist. The likelihood of this deterioration varies between receivers, and is generally considered a function of time. The best assurance of having an accurate receiver is periodic calibration. Yearly intervals are recommended at which time an authorized repair facility should recalibrate the receiver to the manufacturer’s specifications.
4.2 14 CFR Section 91.171 provides for certain VOR equipment accuracy checks prior to flight under IFR. To comply with this requirement and to ensure satisfactory operation of the airborne system, the FAA has provided pilots with the following means of checking VOR receiver accuracy:
4.2.1 FAA VOR test facility (VOT) or a radiated test signal from an appropriately rated radio repair station.
4.2.2 Certified airborne check points.
4.2.3 Certified check points on the airport surface.
4.3 The FAA VOT transmits a test signal which provides a convenient means to determine the operational status and accuracy of a VOR receiver while on the ground where a VOT is located. The airborne use of VOT is permitted; however, its use is strictly limited to those areas/altitudes specifically authorized in the Airport/Facility Directory or appropriate supplement. To use the VOT service, tune in the VOT frequency on your VOR receiver. With the CDI centered, the omni.bearing selector should read 0 with the to/from indicator showing “from,” or the omni.bearing selector should read 180 with the to/from indicator showing “to.” Should the VOR receiver operate a Radio Magnetic Indicator (RMI), it will indicate 180 on any OBS setting. Two means of identification are used. One is a series of dots, and the other is a continuous tone. Information concerning an individual test signal can be obtained from the local FSS.
4.4 A radiated VOR test signal from an appropriately rated radio repair station serves the same purpose as an FAA VOR signal and the check is made in much the same manner as a VOT with the following differences:
4.4.1 The frequency normally approved by the FCC is 108.0 MHz.
4.4.2 Repair stations are not permitted to radiate the VOR test signal continuously, consequently the owner/operator must make arrangements with the repair station to have the test signal transmitted. This service is not provided by all radio repair stations. The aircraft owner or operator must determine which repair station in the local area provides this service. A representative of the repair station must make an entry into the aircraft logbook or other permanent record certifying to the radial accuracy and the date of transmission. The owner/operator or representa-tive of the repair station may accomplish the necessary checks in the aircraft and make a logbook entry stating the results. It is necessary to verify which test radial is being transmitted and whether you should get a “to” or “from” indication.

4.5 Airborne and ground check points consist of certified radials that should be received at specific points on the airport surface, or over specific landmarks while airborne in the immediate vicinity of the airport.
4.5.1 Should an error in excess of plus or minus 4 degrees be indicated through use of a ground check, or plus or minus 6 degrees using the airborne check, IFR flight shall not be attempted without first correcting the source of the error.
CAUTION. No correction other than the “correction card” figures supplied by the manufacturer should be applied in making these VOR receiver checks.
4.5.2 Locations of airborne check points, ground check points and VOTs are published in the A/FD.
4.5.3
If a dual system VOR (units independent of each other except for the antenna) is installed in the aircraft, one system may be checked against the other. Turn both systems to the same VOR ground facility and note the indicated bearing to that station. The maximum permissible variations between the two indicated bearings is 4 degrees.

5.
Distance Measuring Equipment (DME)

5.1 In the operation of DME, paired pulses at a specific spacing are sent out from the aircraft (this is the interrogation) and are received at the ground station. The ground station (transponder) then transmits paired pulses back to the aircraft at the same pulse spacing but on a different frequency. The time required for the round trip of this signal exchange is measured in the airborne DME unit and is translated into distance (nautical miles (NM)) from the aircraft to the ground station.
　
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