  We present one of the best sampled early time light curves of a gamma-ray
burst (GRB) at radio wavelengths. Using the Arcminute Mircrokelvin Imager (AMI)
we observed GRB 130427A at the central frequency of 15.7 GHz between 0.36 and
59.32 days post-burst. These results yield one of the earliest radio detections
of a GRB and demonstrate a clear rise in flux less than one day after the
gamma-ray trigger followed by a rapid decline. This early time radio emission
probably originates in the GRB reverse shock so our AMI light curve reveals the
first ever confirmed detection of a reverse shock peak in the radio domain. At
later times (about 3.2 days post-burst) the rate of decline decreases,
indicating that the forward shock component has begun to dominate the
light-curve. Comparisons of the AMI light curve with modelling conducted by
Perley et al. show that the most likely explanation of the early time 15.7 GHz
peak is caused by the self-absorption turn-over frequency, rather than the peak
frequency, of the reverse shock moving through the observing bands.
