To M8N or not

[Pedals2Paddles]

That sounds reasonable.

 

[pyrate]

From the times that I have talked with them, and seeing VU interact publicly. seems they go out of their way to be helpful
Wish more companies would do that.

 

[fpvsteve]

Long story: (to skip, jump below to short story)

So I'm very much an "if it ain't not broke, don't fix it" type, and while I've never had a single issue with Solo's GPS, I find myself contemplating replacing it with an M8N. Here's why: When I first got the Solo, I wouldn't take off unless I had at least 8-9 satellites. I can fly just fine in stabilize/manual, but given the cost of the Solo, I wanted the added safety of a solid GPS lock, and I was happy to wait as long as it took to acquire one.

But before long, there was inevitably times when no amount of waiting was going to result in more than 5 or 6 satellites, and I cautiously got comfortable taking off with that few, even though I viewed this as exactly the type of letting down your guard that'll end up biting you in the rear sooner or later. Again, I'm fine flying in manual, I'm not afraid of doing so, but a good part of the attraction to a "smart" drone is the "smarts", which are all pretty much dependent on a solid GPS lock. So getting used to flying without a dependable GPS lock is somewhat defeating the whole point of having spent so much on a "smart" drone.

So, I'm planning to build another camera quad and ordered an M8N GPS from Drotek for it, and got the idea to test it against some others - the stock Solo 7N and a M8N from RTFQ. I hooked them each to a PX4 flight controller, placed them all in the exact same location, and tested them all within the span of an hour, so the conditions were pretty much identical.

Here's the results:

View attachment 3002

Solo's 7N never saw more than 4 satellites and the lowest hdop recorded was 1.98. Looking at the map in Mission Planner, the drift wasn't too bad, it reliably stayed within 20 feet over the course of about 15 minutes. Most worrying though was that if anything got anywhere near the GPS, it'd drop all satellites immediately. I couldn't even slightly preposition it, holding it only from the sides, not covering the ceramic antenna, without it loosing a fix entirely.

Next up was a Mini M8N from RTFQ. It quickly got 8 to 9 satellites in the exact same spot the 7N couldn't get more than 4, and before long it was bouncing between 13 to 15 with an HDOP of .71. Looking at the map, it did drift more than the Solo's, about 25 feet over 15 minutes. I could touch/preposition the GPS and it might drop a satellite or two, but still had plenty in reserve. Never dropped a lock.

Last was the M8N from Drotek. Like the RTFQ, it was quickly up to 8 to 9 satellites, saw 16 at max but spent most of the time with 13 to 15 and a had an HDOP of .69 at it's lowest, but averaged around .75. Like the RTFQ, I could touch/move the GPS without loosing more than 1-2 satellites and never lost a lock. Over the 15 minutes, it drifted less than 15 feet.

I've read that the number of satellites and HDOP aren't the only determiners of position data quality, and this would seem to bear that out. The RTFQ M8N had way more satellites and way lower HDOP, but drifted more than the Solo's 7N. It's lock was more resilient, but drift is drift. The Drotek though seems to offer both a far more resilient lock and lower drift. I know people are putting the RTFQ GPS into Solos, but I'm just not comfortable trusting $1500+ of quad and camera to a bargain basement GPS. But the Droteks have a much better reputation for quality, and I'm seriously considering swapping it for the Solo's 7N.

Short Story:

I tested the Solo's stock GPS against M8Ns from RTFQ and Drotek and found the RTFQ unit to be better in some ways, worse in others, but the Drotek seemed to be better all around. Why would I not want to replace my Solo's GPS with this? Are there any other testing criteria that I could use to determine if the Drotek would ultimately be a worst performer? Just saying "It's not all about satellite numbers and HDOP" is as dubious as saying it is - if it's not, then what does determine the best performance and how can I test for that?

Wrapping it up, as I said in the beginning, I've never had a problem with Solo's GPS, other than the low average number of satellites it picks up, it's always worked fine. But this year, I'm planning to film some rock climbers, which means I'll be flying in close to a solid rock face, which means a good portion of the sly will be entirely blocked. Given the low number of satellites Solo's GPS typically sees, blocking so much of the sky is a pretty good way to end up with a dropped GPS fix, hence my interest in the M8N to get a more resilient signal.

Thanks for sharing. I can confirm your test results.
I also use the Drotek unit with resulted in a significant better accuracy in flight and radically reduced fix times compared to stock.
With Galileo enabled in parallel to GPS and Glonass I achieved up to 23 sats at HDOP up to 0.8. (built in Solo)

My advice to anyone who wants to swap their GPS: go for high quality boards, the cheap China stuff will underperform compared to the stock GPS.
Do a proper shielding, best is to replace the copper foil and to ground it.

 

[cheebs]

I have a Solo that will be out of warranty soon, so can begin modding without too much worry of voidingit. I have a coupe of Drotek chips on the way, @Ian [P13] would you make me a mounting kit like you do for the RTFQ chip, in exchange for one of the Drotek chips? (to use as a model and as payment)

 

[User Name]

Chris Anderson I've noticed on Twitter in the past gets a bit over excited about GPS firmware tweaks, there's only so much that can be done in the firmware for the GPS itself and I'm pretty sure there's not much more that can be done now.

Yea, I'd agree with that assessment, if some magic improvement was possible... why wouldn't it have already been implemented. Only so much you can improve hardware with software. But it should be right around the corner, so I don't mind waiting just to see.

My advice to anyone who wants to swap their GPS: go for high quality boards, the cheap China stuff will underperform compared to the stock GPS.
Do a proper shielding, best is to replace the copper foil and to ground it.

I'll still wait to see what the next firmware does, the trees are still leafless and ugly from winter anyway, so it's still not quite flying season anyway. But short of some unlikely magical improvement, I'll most likely throw the M8N in and see how it performs. I use them on all my other quads, even the cheap ones have been essentially 100% reliable, the the testing I did showed the Drotek to be better still.

 

[pyrate]

the other day mine was not locked before I got ready to go, walked over and rotated it 90 degrees and it locked about 30 seconds later

prolly just a fluke

 

[Vegasrobbi]

I feel software improvements have made Solo's GPS a non issue for me. I purchased my Solo in the dark days of version 1. whatever. I remember waiting over 5 minutes+ to get lock, getting frustrated and packing it up. Like many I installed the GPS isolation mod with additional copper sheet. Wanting to believe the GPS mod helped, it didn't help much IMO.

A few updates later I don't think about GPS anymore. I power on the controller, then Solo and by the time I get the app connected GPS is usually ready to go. I normally get 9-10 satellites by takeoff and about 12-13 in flight, hdop is usually 1.4-1.7.

On more than one occasion out in the desert flying low to the horizon I lost visual sight of Solo, once over 4,000 feet out. I could have FPV'd or flew the app back but hitting RTH brought Solo back to within 3 feet of home virtually every time. I'd say Solo holds position as well if not a little better than my P3P.

In December I sent Solo back to 3DR for a bad FC. 3DR so graciously removed my GPS mod including the copper sheet and placed it in the bottom of the box. My GPS works better now than ever and good enough that I have built confidence in it's abilities.

I think there is more to Solo's GPS than just the GPS unit, and while I would welcome improved hardware it would have to be more than a small incremental improvement for me to change what I have. Even if there are little gains in software left it's still good enough IMO.

 

[Jubalr]

My experience mirrors Robbi's except that even though my Solo is from the first shipment, I never had 5 min waits for a lock. But it was longer than today with the updates we have had. I power the controller then the Solo, and before I can get the app synced and running, Solo is waiting on me.

 

[Maddog]

Oh, no, my tests were just the various GPSs attached to a PX4 flight controller and monitored through Mission Planner. It was just to see, with a 3D position fix, how much the determined GPS location drifted over time. Now Solo uses more than this to maintain a position, but it's a good test of the pure GPS performance. I also subsequently tested an XL Drotek M8N (it's on a 50mm x 50mm board with a larger patch antenna) and I think the drift over 15 minutes never exceeded 8 feet.



How does that compare, if you recall, to the stock Solo GPS? And what motivated you to switch, if I might ask. For me, I'd just like the added assurance that a GPS position fix isn't 1 or 2 satellites away from being lost, and any M8N I've used on other quads regularly gets upwards of 2x the number of satellites that the Solo gets.

Of course, all this being said, I've never had a single GPS glitch with my stock Solo, which is probably why I'm so on the fence when it comes to changing it.

To answer your question, I don't recall how accurate the stock GPS was.
I did the mod because I was routinely waiting 5-10 minutes for a fix. With the M8N I am getting about 8-10 sats in under 2 minutes with an HDOP around 1.5 to start.

In the air it will acquire 20 sats routinely but I haven't noticed how long for that to occur.

Have never lost GPS since the mod.

 

[Eagle Wong]

@Jubalr. Did you put my m8n gps to the solo?

My solo is also the first shipment. It did around 10 mins to flight, even same place at 2nd battery. Replace m8n gps, problem gone.

 

[Jubalr]

@Jubalr. Did you put my m8n gps to the solo?

My solo is also the first shipment. It did around 10 mins to flight, even same place at 2nd battery. Replace m8n gps, problem gone.

Yes, but since I did not have a problem with my GPS, I didn't notice a difference. So I went back to stock unit.

 

[ajdelange]

I have a thought. As a professional land surveyor I use RTK GPS with a base station and rover with a average measurement accuracy of 1/2" in a mile. This system does not depend on only the number of satellites, but the geometry of the satellites (strong or weak triangles). I wonder if solo has that feature built into it. I have had good GPS hover stabilization with my solo with fewer satellites, much better than my DGI. I have had good locks with the RTK with only seven, and no lock with 13 because of the geometry. Just a thought.

I guess I'm a little surprised at how little you know about GPS given that you use it as a professional. You seem to understand the 'strong vs weak' triangles aspect of things. A 'weak' triangle is said to have poor (a high number) Geometric Dilution of Precision (GDOP) and conversely for a 'strong' triangle. The GPS receiver (yours and SOLO's) compute GDOP and SOLO, as it does not use GPS for vertical navigation, reports only the horizontal component of it (HDOP). GDOP, in GPS, depends on the rate of change of the observable (pseudo range i.e. how far away the satellite is) with the thing being measured (position in north, east and up coordinates). The change of pseudorange (in feet) for a 1 foot change in position in a northern direction is computed as is the change for a 1 foot change to the east and a 1 foot change in altitude. The derivatives are in units of feet pseudo range per foot position. DOP is 1 divided by the derivative squared (actually the derivatives go into a matrix and it is the inverse of that matrix times its transpose that is computed) and so it has units feet of position squared per foot pseudorange squared. DOP tells you the contribution of strong and weak triangles (IOW the geometry) on the quality of a fix. There is, of course, another part to the equation and that is how well you can measure pseudorange to the satellite which depends on whether things (trees, buildings, the atmosphere) attenuate the signal, whether there are multipath reflections, how much the ionospheric delay varies etc. The measured pseudoranges are in error expressed in terms of a variance expressed in feet pseudorange squared.

pseudorange squared multiplied by feet of position squared per foot pseudorange squared gives the variance of the estimated position in feet of position squared.

When you use an RTK system accuracy goes way up because you are able to add another measurement and that is carrier cycles count which is much finer than the chip count used in an ordinary system but it requires that you place the base station on a well surveyed monument and measure positions relative to its location. The principles of and math for the GDOP calculations remain the same but now in addition to pseudorange you have a carrier phase measurement. Adding new measurements, be they more GPS pseudoranges, Glonass pseudoranges or carrier phases decrease GDOP.