Automatic Gain Control and Root Raised Cosine Filter
For better performance we should keep our signal in a constant level regardless of the input signal. For that we will use a Automatic Gain Control, that will do a Software Gain (basically just multiply the signal) that does not change the resolution (so it will not give a better signal) but will sure keep our level constant. We can use the simple AGC block from GNU Radio.
With rate as 10e-3, Reference as 0.5, Gain as 0.5, Max Gain as 4000.
Another step is the RRC Filter (Root Raised Cosine Filter). This is a filter optimized for nPSK modulations and uses as a parameter our symbol rate. The Filter is not very hard to generate (its a FIR with some specific taps), but luckily GNU Radio provide a block for us.
For the parameters we will use 1.25e6 as sample rate, 293883 as Symbol Rate, 0.5 as Alpha and 361 as Num Taps. From these parameters, Alpha and Symbol Rate is provided from the specification. The number of taps you can experiment with, but I found that a good balance between quality vs performance was at 361 taps. After that we should have basically a signal that contains only the BPSK Modulated signal (or noise in the same band). Then we can go to Synchronization and Clock Recovery.