Sending Assisted GNSS (A-GNSS) to Bangle 2 GPS (AT6558)

It's not wrong to try and see what else you can get out of your device, but it's also important to temper your expectations. The documentation we have covers an entire class of devices with wildly different feature sets, and pricing to match. You get what you pay for, and SMA isn't going to spend extra on a chip with features they aren't intending to use or advertise. Heck, they probably designed the watch around the absolute cheapest option they had access to.

So, some of the commands you're aware of are for features that simply aren't present in the AT6558 - RINEX is one of those things that a manufacturer is likely to charge a lot more for, as that's sought after for more expensive devices in narrow markets with much higher margins on them than the consumer GPS market, like professional survey equipment. That said, the datasheet does claim the AT6558 is suitable for DGPS use, so shrug

And some commands are intended to give options to the hardware designer. There is no secret second IMU in the AT6558. Not even the more expensive chips have one. What they do have are spare pins on their IC packages that they can cheaply repurpose as a GPIO or serial bus expander. If you look at the datasheet you'll see several reconfigurable GPIO pins, including some that can be assigned as an I2c or SPI master. You can connect a separate device to these pins and then communicate with it via the GPS, which is useful if you've run out of pins on your primary microcontroller. This is explicitly called out in the datasheet in section 5.7 SPI - "SPI master device interface is used for connecting the SPI interface devices, such as MEMS sensors, FLASH, etc..". RXM-SENSOR is a convenience function for controlling an external IMU on this second bus - but since we already have an IMU directly connected to the nRF52 in the watch, this is unnecessary and therefore won't be connected to anything useful.