time. Some other method of determining a second measurement was needed.

Transit did this by measuring the signal's Doppler shift. The spacecraft traveled at about 17000 mph, which could increase or decrease the frequency of the received carrier signal by as much as 10 kHz as measured on the ground. While the satellite is approaching the ground station its signals will be shifted up in frequency, and as it recedes they will shift down again. The precise moment when the frequency is exactly equal to the broadcast frequency is when