[greggold]

The Radiomaster and ELRS teams continue to collaborate and are on a roll.  There is now a new version of ELRS Gemini, called Gemini X, even before many people have adopted plain elrs Gemini, and before most people have become accustomed to standard elrs.

In Jan 2024 I wanted to try Gemini and I purchased the BetaFPV SuperG Nano ELRS 2.4G Module for my Radiomaster tx.  I initially flew in gemini switch mode, but now I fly 2 drones in full gemini mode with the purchase of several betaflight superD diversity rxs. I am recording flight logs and will eventually use the data to examine whether gemini elrs is better than standard elrs in terms of avoiding low values of RSSI and link quality and the need for dynamic power jumps. My strong impression so far is that gemini elrs is better than standard elrs. However I mostly compare gemini to setups with elrs rxs that are not full diversity rxs, which is not a fair comparison. One problem I've encountered:  if you fly quads you cannot easily slip in a different rx, you may have to disassemble the drone and connect the rx to the flight controller by soldering the 4 leads.

Gemini involves transmitting on 2 frequencies, using a rx that is really 2 rxs, in the effort to ensure the tx commands have the best possible chance of arriving at the aircraft. The ideas behind gemini X are to take gemini a bit further to include the ability to have one tx emission on 2.4 Ghz, and the other on 900 mhz, and the flexibility to change this to 2  2.4 Ghz emissions (separated a little in frequency), or 2 900 mhz emissions. There are many new modes that are designed to deal with high interference situations, or racing competitions, for long range and other scenarios. 

It seems like too much too soon after the remarkable success of elrs, but the speed of innovation may be difficult to control, either too slow or too fast.

Here are some links to the initial impressions of the usual suspects talking about gemini X.

The best review is by Bardwell: https://www.youtube.com/watch?v=ETBLRYyp7zk

Radiomaster Nomad & DBR4 Receiver - ExpressLRS Gemini X For Under $45
Mads Tech  https://www.youtube.com/watch?v=agHDOIvezI0

Radiomaster ELRS Nomad Gemini Xrossband
Andrew Newton
https://www.youtube.com/watch?v=Dxiz1BkVm3U

RadioMaster NOMAD : ELRS Gemini X is here - any combo of 2.4Ghz and 868/915Mhz using two antennas!
Painless360
https://www.youtube.com/watch?v=g3OgJ8lCoIk

[caffeine]

This is an interesting development, however bear in mind that the 868 to 915MHz frequencies are not legal for use in RC in Australia.

We can use 915 to 928MHz but I don't think the new modules support that.

[greggold]

This is an interesting development, however bear in mind that the 868/915 frequencies are not legal for use in RC in Australia.

We can use 915 to 928MHz but I don't think the new modules support that.

I think 868/915 mhz means that in some areas eg EU you use 868 band and in others eg Aust. you use the 915 band. I expect it is allowed. if you select 915 mhz (called the low band) for both tx emissions, the 2   915 signals may differ in frequency by a small amount.  The above Bardwell video that I just found (and added to the above post) is the best explanation of gemini X, and in particular explains why it would be relevant for someone who one day uses 2.4 Ghz and on another day wants to use the low band, or wants to use the new module with their existing elrs rxs.

[caffeine]

'915MHz' in US use is 902 to 928MHz, so we can't use the lower half of that.

I wonder if there will be a mechanism to select a subset of frequencies.

[caffeine]

I asked on the ELRS discord, and there is apparently an 'AU915' mode, which may restrict it to legal frequencies for us

[greggold]

I asked on the ELRS discord, and there is apparently an 'AU915' mode, which may restrict it to legal frequencies for us

great clarification .... In gemini if you choose 2.4 Ghz band for both tx emissions, the difference between them is 40 mhz, so if that difference is the same for 915 band, then choosing AU915 would mean 915 mhz for one tx emission and 955 for the other - so maybe AU915 would mean 915 and 928.   Bardwell (above link) has made the point on 868/915 mhz band (now referred to as low band in discussion of gemini x) that it has major benefits (greater range, better penetration of obstacles) at very low cost. Its drawbacks include bigger antennas for rxs and txs, and increased latency. However the latency issue may only affect drone racers, who reduce the latency by several millisec by choosing 2.4Ghz over 915mhz. Bardwell says this difference in latency is undetectable by normal rc flyers, and it seems to me to be beyond human physiology to detect such a tiny difference in response time. But top drone racers do amazing feats so maybe they can tell the difference.

I flew on 915mhz with my frsky tx and a module for a while before elrs arrived, and like others found that it completely ended the prcac glitch/interference problems. I will consider it now because my favorite bnf drone - geprc tern - is sold in 2 versions, 2.4 and 900 mhz. When I want to try long range over a river it seems like a good choice.