It looks like it is no longer possible to access the raw GPS measurements on the newest version of the u-blox M8N receiver. Access to these raw measurements on the M8N has always been through debug messages not officially supported by u-blox. Last year, when they migrated from the 2.01 version of firmware to the 3.01, version they scrambled the output of these messages so they were no longer readable by RTKLIB.
Until recently though, the units they were shipping still had an older 2.01 version of ROM. With these units it is possible to downgrade the firmware to 2.01 using the instructions on their website. With the older firmware loaded, the receivers revert to their previous behavior and the debug messages are no longer scrambled.
Apparently their newest units are shipping with a 3.01 version of ROM and this ROM is not compatible with the older 2.01 version of firmware. If you attempt to load the older firmware it will appear to succeed but will still be running the newer code.
You can see what version of ROM and firmware your receiver is running using the UBX-MON-VER message from the u-center console. The example below shows the message output for one of the newer modules with the 3.01 ROM after attempting to download the older firmware. I believe the firmware listed under “Extension(s)” is the ROM version and the firmware listed under “Software Version” is the version of firmware loaded to flash. In this case you can see that the ROM is version 3.01 and that the flash is still running version 3.01 even though it was attempted to load the 2.01 firmware.
In an older version of the M8N module, the ROM code listed under “Extension(s)” would have been 2.01 and the firmware listed under “Software Version” could be either 2.01 or 3.01 depending on how old the module was and what firmware had been downloaded to it.
There are a few more details about the issue on the u-blox forum in this thread. Thanks to Marco for making me aware of the issue and Clive and Helge for providing a detailed explanation of what is going on.
If you are using the u-blox M8T, and not the M8N, then you will be using the officially supported raw measurement messages and would normally not care about access to the debug messages. The only exception I know of is that the resolution of the SNR measurements are 0.2 dB in the debug messages and 1.0 in the official messages. I have not confirmed that the debug messages on the 3.01 M8T firmware are scrambled but it is likely that they are.
[Note 6/25/17: A couple of readers have pointed out that this is not the whole story. It would have been more correct to say that the newest M8N modules are not usable with the publicly available versions of u-blox firmware and RTKLIB. It turns out that u-blox did not use a particularly sophisticated method to scramble the debug messages and there are now several modified versions of u-blox firmware and RTKLIB floating around that have been hacked to unscramble the messages. I don’t want to get into the question of ethics or legality of using these codes but just say that I personally am less comfortable using the debug messages in the modules where u-blox has made an obvious attempt to prevent this and have avoided any use of them at least for the time being.]