Is it possible to use the Facet’s USB connector for the telemetry radio link for the base-rover RTCM data?
The JST connectors and cables are fragile. The 4-pin to 6-pin JST cable is not a common item like a 6-pin to 6-pin JST cable. I’ve had to frankenstein a few, either in the field or when Sparkfun.com was out of stock.
I’ve purchased different models of 900Mhz telemetry radios to use with my two Facets in a base-rover setup and the radio manufacturers all seem to make USB cables for their radios.
What you’re asking is for the RTK Facet to behave as a USB host: the Facet would need to act as a computer to enumerate the host. A few years ago this was next to impossible, now it’s extremely difficult. If we moved to a platform that was much beefier than the ESP32, such as a RasPi or other, then yes, it would be possible. But I don’t think we’re headed in the SBC direction.
I think the solution is to embed a radio in the unit itself. This is doable, but it (as you know) takes time.
As some know, I’ve become a fan of the RFD900x radios for my Facet base-rover RTK system. If properly powered they are rated for 1W, a nice bump up over the 100mW Holybros. It definitely helps with the range, though at that frequency a lot of the range issues are due to terrain and other obstacles.
I frankensteined a cable, that is I took apart two other cables and put the parts back together. The result
is an 8-way header socket on one end and a 4-pin JST on the other. The 4-pin JST connector on the Facet is not standard in the RC/drone community, this is why I had to make a cable. I dissected a Sparkfun cable for the 4-pin JST shell, and, if I recall correctly, dissected a RFD-900x-to-6-pin JST cable for the wire, JST ‘pins’, and the 8-way header socket. The RFD900x cables with 6-pin JST connectors are commercially available premade so it’s a quick pin (insert) swap. Just don’t mess up the pinout; don’t want to let the magic smoke out!!
The Facet radio ports aren’t, by my calculations, rated to supply enough current to run the RFD900x at full power, I run them at 0.6W. Maybe someday I’ll make a cable to power the RFD900x separately. If I recall correctly, the Facet radio port doesn’t output a full 5V unless the Facet is getting external (charging) power on the USB-C, so I power the base so the radio gets full voltage. I’m working on getting a full 5V to the RFD900x on the rover. My understanding is the RFD900x needs both 5V and more current than the radio port will provide to output full RF power. I’ve assumed the protocol used by the 900 MHz telemetry radios works bidirectionally and thus both the rover and base radios need to be powered symmetrically for full range. This is unlike 450MHz surveyor RTK radios with a 35W base transmitter and a 1W radio on the rover.
The range difference between the 0.1W Holybro and the RFD900x was definitely noticeable and helpful, but it won’t get you around big obstacles (hills, valleys, etc). It’s not like using a 35W 450MHz surveyor radio.
The RFD900x radios support a multipoint mesh firmware that I am experimenting with. That may allow a third RFD900x to be a repeater between the base and rover. That should get me around hills.
Wish the Facets had the RFD900x radios built in to them. I do mean to try squeezing one in there someday. The RF connectors are physically problematic.
I do mean to try squeezing one in there someday. The RF connectors are physically problematic.