Ehman said in 1994: "We should have seen it again when we looked for it 50 times. Something suggests it was an Earth-sourced signal that simply got reflected off a piece of space debris. He later somewhat recanted his skepticism, after further research showed the unrealistic requirements that a space-borne reflector would need to have to produce the observed signal. The signal's frequency of 1420 MHz is also part of a protected spectrum, a frequency range reserved for astronomical research in which terrestrial transmissions are forbidden, although a 2010 study documented several instances of terrestrial sources either interfering from adjacent frequency bands or illegally transmitting within the spectrum