<P> While pilots study these effects during initial training, trying to compensate for them in flight is very difficult; instead, pilots generally simply choose a heading that seems to average out any fluctuations . </P> <P> Radio - navigation aids must keep a certain degree of accuracy, given by international standards, FAA, ICAO, etc.; to assure this is the case, Flight inspection organizations periodically check critical parameters with properly equipped aircraft to calibrate and certify NDB precision . The ICAO minimum accuracy for NDBs is ± 5 ° </P> <P> Besides their use in aircraft navigation, NDBs are also popular with long - distance radio enthusiasts ("DXers"). Because NDBs are generally low - power (usually 25 watts, some can be up to 5 kW), they normally cannot be heard over long distances, but favorable conditions in the ionosphere can allow NDB signals to travel much farther than normal . Because of this, radio DXers interested in picking up distant signals enjoy listening to faraway NDBs . Also, since the band allocated to NDBs is free of broadcast stations and their associated interference, and because most NDBs do little more than transmit their Morse Code callsign, they are very easy to identify, making NDB monitoring an active niche within the DXing hobby . </P> <P> 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 . </P>

What important considerations do non directional beacons present