Servers by jDrones

RCTM Message Bandwidth Requirements

Hello All,

We do a lot of mapping and have several drones with RTK enabled GNSS receivers. We’ve come full circle twice on what we prefer; RTK or PPK. Our current preference is RTK. If you have access to CORS, it is fast and easy, requiring little extra time or effort. The big limitation in the hills of WV is internet access. Due to the mountainous terrain and rural environment, we have large swaths of land with no cellular access, especially in the coal fields were we do most of our flying.

I don’t think something like weBoost from Wilson Amplifies will work consistently and reliably. So I am spec’ing out a satellite based internet access system. To ensure I purchase the right equipment, I need to know the bandwidth requirements for RCTM messages. If anyone can give me an approximate bandwidth, I’d be grateful.

Thank you,
Dave

Wouldn’t it be better to get a base and connect to it when internet is not available?
I am in a similar situation and never had an internet shortage so far but i am thinking about buying an f9p based base station.

Not for us and our workflow. We sometimes fly six sites a day, not many close enough to use the same base position. The other challenge is establishing and maintaining a known point at each site. Very hard to do on mine sites. And then communicating the exact location of the known point to team members would be difficult. Time is another big consideration. I don’t want to spend the time to set up and tear down a base station 3-5 times in one day. We have an adequate CORs network in WV. Seems like the fastest and simplest (perhaps not the cheapest) thing to is get an auto aligning satellite antenna that gets us sufficient bandwidth. You can get one from Hughes for 5k, which is less than many base station receivers.

On a side note, I have been experimenting with the Ublox F9 for several months. While my analysis hasn’t been rigorous, I’ve been pleased with its performance.

Dave

Found an interesting power point (if there is such a thing) about compact RTK, which is an experimental concept that attempts to reduce RTK degradartion in high latency data links.

In that the author says:

“Conventional RTK requires approx. 1500 bps (RTCM v3.0 MT1004, 9 satellites)”

The satelitte based data link I am looking at has lactency between 800ms and 1200 ms. Not sure how that affects RTK fix quality.

The bandwidth for RTCM message broadcast from a base station will vary depending on the specific messages and the frequency at which they are sent, number of satellites in view, the constellations being tracked and the frequencies used. We track four constellations, dual frequency and the bandwidth from our own base station is ~3-4kb/s. From the local CORS site the data rate is ~7kbps.

Thanks for the reply. What frequency?

Any idea how 1 second of latency will affect the accuracy of the rover?

Thanks,

Dave

The CORS sites broadcast at 1 Hz and the latency is some value less than 1 sec. Obviously for maximum accuracy the fresher the data the better, but I do not believe that the latency is too critical. By default, the receivers we use will maintain an rtk fix for 30 seconds after loosing corrections. The error will grow progressively as the data ages but most of what you are correcting (atmospheric effects) do not fluctuate significantly in a short time span. I would not be concerned with 1 second latency. We could configure our own base station to broadcast RTCM data at rates higher than 1Hz but the extra bandwidth would limit our radio range and there would be no perceivable gain - especially for vehicles as large as ours.

Thanks, I appreciate your thoughts

Servers by jDrones