  A fundamental task in wireless communication is Channel Estimation: Compute
the channel parameters a signal undergoes while traveling from a transmitter to
a receiver. In the case of delay-Doppler channel, a widely used method is the
Matched Filter algorithm. It uses a pseudo-random sequence of length N, and, in
case of non-trivial relative velocity between transmitter and receiver, its
computational complexity is O(N^{2}log(N)). In this paper we introduce a novel
approach of designing sequences that allow faster channel estimation. Using
group representation techniques we construct sequences, which enable us to
introduce a new algorithm, called the flag method, that significantly improves
the matched filter algorithm. The flag method finds the channel parameters in
O(mNlog(N)) operations, for channel of sparsity m. We discuss applications of
the flag method to GPS, radar system, and mobile communication as well.
