In April 1978, a complication arose when no commands were transmitted to "Voyager 2" for a period of time, causing the spacecraft to switch from its primary radio receiver to its backup receiver. Sometime afterwards, the primary receiver failed altogether. The backup receiver was functional, but a failed capacitor in the receiver meant that it could only receive transmissions that were sent at a precise frequency, and this frequency would be affected by the Earth's rotation (due to the Doppler effect) and the onboard receiver's temperature, among other things. For each subsequent transmission to "Voyager 2", it was necessary for engineers to calculate the specific frequency for the signal so that it could be received by the spacecraft.