Using Emlid Reach + M8N Compass

Hey guys (and this might be a dumb question) but is it possible to use Reach RTK AND the Here M8N compass at the same time? Seems the wiring diagrams permit this BUT not sure if the software allows for it or if it makes any sense. Assuming it’s possible, will having the M8N compass offer any additional redundancy? Seems logical that the answer is yes but just not sure.


Not sure quite what you mean, but I am using a Here M8N as primary and Emlid Reach as secondary GPS, without issues. Compass on the Here is primary, and one of the compasses in the Pixhawk (2.1) is secondary, with the other disabled.

I guess he meant to use the GPS capability of the reach and just the compass of the Here Module without the gps capability of the here gps module.

I think this could work because from my understanding the gps and the compass of such a module is something different and not connected to one another. The only problem i can imagine of is the missing connection to a power source in some ways of wiring the compass

I was asking since I am considering setting up a drone to have the RTK reach as primary compass and the M8N Here as secondary for redundancy (and improved accuracy if having both offers improved accuracy). Basically, just looking for confirmation that this setups is logical.


I’ve written up a Dual-GPS wiki page here:

If you’re going to use two GPSs we generally recommend they be of the same type because the blending or switching relies on the reported accuracies from the GPSs and the numbers from different manufactures may not be perfectly compatible. For example one manufacturer may be overly optimistic about it’s accuracy while another is pessimistic which might lead to favouring one GPS over another when it shouldn’t be. On the other hand, maybe it’s fine :-)… we don’t really have enough practical experience to know.

@rmackay9 I’d strongly caution that using GPS units from different manufacturers puts you firmly into the category of “Here be dragons”. And to a lesser extent even different generation GPS units can result in some oddities (different processing times/lag terms, as well as sometimes changing internal algorithms).

@Stoneman0 mixing a GPS that is attempting to provide an RTK Fix with a 3D/DGPS will result in a lower overall accuracy if you are using mixing, but it will be heavily weighing the RTK Fix unit, and you gain the safety of a fallback device if needed. The caution about different units though remains quite true.

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So…what would you recommend for a redundant system that has a backup GPS or is this even necessary if one is using the reach RTK or Here RTK? :slight_smile:


The go to recommendation is a second reach RTK or Here+ . (You could even configure the second unit to not receive correction data if you wanted to keep it as a normal standalone GPS).

So basically then use the Reach RTK + Second Reach (without corrections) OR Here RTK with here M8M. The point being ok to have two but make sure they are the same company and same iteration for reasons stated prior.

Hardware generations imply you want to stick with that, so Here+ with Here + (M8N vs M8P might be different, but this is probably one of the more similar setups out there that uses technically different hardware).

As far as I know the Reach compass is not available to the Pixhawk as there is no i2c output?

I have two machines with Reach units functioning as secondary GPS. Primary on one machine is an ‘M8N’, not sure if genuine but seems to work perfectly adequately, on the other, with PH2.1, if the Here, which is great.

You can get some strange behaviour from the interaction between GPS, but the Reach always sees fewer sats than the M8N based units. I seldom use RTK navigation, as the advantages in most cases are slim over a decent regular GPS - the Here unit usually gets DGPS (well SBAS) lock fairly quickly, which is pretty good accuracy. RTK navigation I have not got working reliably enough to rely on - useful in situations where the precision/stability is helpful, but you need your eye on the craft in case lock is lost and a large position error appears.So I have always felt that the Reach is functioning as a fall-back unit in case of primary GPS total failure.

I think this has now changed with the advent of GPS fusion, so the advice to operate identical modules is probably sensible.

But, as I said, I have had no issues with the Here/Reach combination. But use instantaneous ambiguity resolution and not fix-and-hold, and for automated missions better to stick to single mode.

No idea on the compass front, the Reach has an imu, but not yet implemented features to make use of it (sadly).

Thanks. :slight_smile: