  To exploit the full potential of radio measurements of cosmic-ray air showers
at MHz frequencies, a detector timing synchronization within 1 ns is needed.
Large distributed radio detector arrays such as the Auger Engineering Radio
Array (AERA) rely on timing via the Global Positioning System (GPS) for the
synchronization of individual detector station clocks. Unfortunately, GPS
timing is expected to have an accuracy no better than about 5 ns. In practice,
in particular in AERA, the GPS clocks exhibit drifts on the order of tens of
ns. We developed a technique to correct for the GPS drifts, and an independent
method is used for cross-checks that indeed we reach nanosecond-scale timing
accuracy by this correction. First, we operate a "beacon transmitter" which
emits defined sine waves detected by AERA antennas recorded within the physics
data. The relative phasing of these sine waves can be used to correct for GPS
clock drifts. In addition to this, we observe radio pulses emitted by
commercial airplanes, the position of which we determine in real time from
Automatic Dependent Surveillance Broadcasts intercepted with a software-defined
radio. From the known source location and the measured arrival times of the
pulses we determine relative timing offsets between radio detector stations. We
demonstrate with a combined analysis that the two methods give a consistent
timing calibration with an accuracy of 2 ns or better. Consequently, the beacon
method alone can be used in the future to continuously determine and correct
for GPS clock drifts in each individual event measured by AERA.
