I am running an u-Blox 8 based receiver from navilock (NL-8012U) with gpsd 3.16. While the receiver worked at first try fine out of the box I investigated a little issue setting the GPS's cycle time (gpsctl -c). Looking inti the code I found that the u-blox driver mixes endianess. Sending any changed value caused the receiver to respond very slowly. I attached a patch that should fix that issue. I also added a change that allows the receiver to accept rates up to 20Hz as this is specified in the documentation for the receiver as limit. I also changed the ACK/NACK prints as I found it very unhandy to reflect the ACK/NACK message class and IDs rather than the message the ACK/NACK refers to. Perhaps that would also be a valuable change to add.

Little things I investigated are related to documentation of gpsctl: It says that the -c option specifies the cycle time in seconds while I investigated that it takes the update rate. So giving a higher value increases the update rate for me. Also, the -c and -T  options are missing in the SYNOPSIS of the man page.