  The secondary modulation with the NeumannHoffman code increases the
possibility of bit sign transition. Unlike other GNSS signals, there is no
pilot component for synchronization in BeiDou B1/B3 signals, which increases
the complexity in acquisition. A previous study has shown that the delay and
multiplication (DAM) method is able to eliminate the bit sign transition
problem, but it only applies to pretty strong signals. In this paper, a
DAM-based BeiDou signal acquisition approach, called variable length data
accumulation (VLDA), is proposed to acquire weak satellite signals. Firstly,
the performance of DAM method versus the different delays is analyzed. The DAM
operation not only eliminates bit sign transition, but it also increases noise
power. Secondly, long-term signal is periodically accumulated to improve signal
intensity in order to acquire weak signals. While considering the Doppler
frequency shift of ranging codes, the signal length must be compensated before
accumulating long-term signal. Finally, the fast-Fourier-transform based
parallel code phase algorithm are used for acquisition. The simulation results
indicate that the proposed VLDA method has better acquisition sensitivity than
traditional non-coherent integration method under the same calculation amount.
The VLDA method only requires approximately 27.5% of calculations to achieve
the same acquisition sensitivity (35 dBHz). What is more, the actual
experimental results verify the feasibility of the VLDA method. It can be
concluded that the proposed approach is an effective and feasible method for
solving the bit sign transition problem.
