very easy to identify, making NDB monitoring an active niche within the DXing hobby. In North America, the NDB band is from 190 to 435 kHz and from 510 to 530 kHz. In Europe, there is a longwave broadcasting band from 150 to 280 kHz, so the European NDB band is from 280 kHz to 530 kHz with a gap between 495 and 505 kHz because 500 kHz was the international maritime distress (emergency) frequency. The beacons that transmit between 510 kHz and 530 kHz can sometimes be heard on AM radios that can tune below the beginning of the medium wave (MW) broadcast band. However, reception of NDBs generally requires a radio receiver that can receive frequencies below 530 kHz. Often "general coverage" shortwave radios receive all frequencies from 150 kHz to 30 MHz, and so can tune to the frequencies of NDBs. Specialized techniques (receiver preselectors, noise limiters and filters) are required for the reception of very weak signals from remote beacons. The best time to hear NDBs that are very far away is the last three hours before sunrise. Reception of NDBs is also usually best during the fall and winter because during the spring and summer, there is more atmospheric noise on the LF and MF bands. Beacon closures As the adoption of satellite navigation systems such as GPS progressed, several countries began to decommission beacon installations such as NDBs and VOR. The policy has caused controversy in the aviation industry. Airservices Australia began shutting down a number of ground-based navigation aids in May 2016, including NDBs, VORs and DMEs. In the United States as of 2017, there were more than 1,300 NDBs, of which fewer than 300 were owned by the Federal Government. The FAA had begun decommissioning stand-alone NDBs. As of April 2018, the FAA had disabled 23 ground-based navaids including NDBs, and plans to shut down more than 300 by 2025. The FAA has no sustaining or acquisition system for NDBs and plans to phase out the existing NDBs through attrition, citing decreased pilot reliance on NDBs as more pilots use VOR and GPS navigation. See also Cardioid Differential Global Positioning System (DGPS) Electric beacon Instrument flight rules (IFR) Transponder landing system (TLS) References Further reading International Civil Aviation Organization (2000). Annex 10 — Aeronautical Telecommunications, Vol. I (Radio Navigation Aids) (5th ed.). U.S. Federal Aviation Administration (2004). Aeronautical Information Manual, § 1-1-2.