Multichannel blind deconvolution is the problem of recovering an unknown
signal $f$ and multiple unknown channels $x_i$ from their circular convolution
$y_i=x_i \circledast f$ ($i=1,2,\dots,N$). We consider the case where the
$x_i$'s are sparse, and convolution with $f$ is invertible. Our nonconvex
optimization formulation solves for a filter $h$ on the unit sphere that
produces sparse output $y_i\circledast h$. Under some technical assumptions, we
show that all local minima of the objective function correspond to the inverse
filter of $f$ up to an inherent sign and shift ambiguity, and all saddle points
have strictly negative curvatures. This geometric structure allows successful
recovery of $f$ and $x_i$ using a simple manifold gradient descent (MGD)
algorithm. Our theoretical findings are complemented by numerical experiments,
which demonstrate superior performance of the proposed approach over the
previous methods.