RACH intercepting

Dear community members, I'm interested in intercepting L band up-link RACH channel I found the BCCH channel of the spot beam I'm currently in then scan the same ARFCN but in uplink I had a cfile and I tried to read it using gr-gmr1 https://github.com/osmocom/osmo-gmr/tree/sylvain/gr/gr-gmr1 in the branch sylvain/gr https://github.com/osmocom/osmo-gmr/tree/sylvain/gr but i have no results , may be I don't know how to use the read utility in sylvain/gr https://github.com/osmocom/osmo-gmr/tree/sylvain/gr I also have another question about the file test.pcap in http://people.osmocom.org/tnt/gmr/ how can I have a file like it and is it l band RACH data or C band.