Feedback channel for rover position at 5Hz or more

Q&A forumCategory: QuestionsFeedback channel for rover position at 5Hz or more
nandu asked 4 weeks ago

I need to log the GPS solution (as opposed to raw data) of the rover at 5 Hz or more, preferrably at the base station. The timestamps have to be synchronous with another GPS receiver providing UTC. Operational range – up to 1000m.
What options do I have without having to implement proprietary solutions?
A tutorial describes only one solution with a limitation of the transmission rate to 1 Hz due to low Xbee bandwidth. However, there is an extension to install a second Xbee module. Is it possible to use a second Xbee link (with addional modules on both sides) to transmit NMEA position data to the base station or a stationary computer with an Xbee receiver (e.g. connected via an RS232-to-Everything adapter)?
The Ardusimple shop offers 868 MHz Xbee modules but none for 2.4GHz with support for multiple channels/higher bandwidth. Is any integration of such modules planned?
My fallback scenario would be to use the datalogger module. Would it work with the extension Xbee socket? That is, simpleRTK2B-F9P V3 + Xbee LR + Xbee Shield + Serial Datalogger.

clive1 replied 4 weeks ago

The universe of X-Bee and compatible products is quite large and spans beyond those on the ArduSimple store.
There might not be a zero-effort solution, but you should be able to find a module meeting requirements, and capable of running code/scripts where the NMEA message could be processed into a lower bandwidth message.

nandu replied 4 weeks ago

It is not only about finding a module. It is rather about having an integrated system. I would prefer to not end up with 10 separate items. I am looking for a light-weight RTK solution for a rover or a person – GPS antenna + small RTK box + power pack.
Compatibility is a multi-layered topic. Any hints for sources on compatibility (electrical, mechanical, protocols, software, configuration, etc. ) with Ardusimple?

2 Answers
Ardusimple Staff answered 3 weeks ago

Hi nandu,
yes unfortunately it will not be possible to send both rtk corrections + rover position at 5Hz to the base with standard NMEA messages which are quite long.
What you propose of using 2 radio links, one for correction data and one to send back GGA 5Hz could work. You can do this with the Shield for Second XBee socket yes. You will have to configure the second pair of radios in a different channel, and you can use the miniUSB-to-everything adapter to connect to the PC.
Alternatively if you could have internet in both sites, you could use the WiFi accessory, which integrated NTRIP but also TCP/UDP socket functions. Check the hookup guide to learn more.
We have 2.4GHz module (MR) but the bandwidth is actually very limited when you go far.
Hope I didn’t forget any question, let us know!

clive1 replied 3 weeks ago

I think it might be helpful to diagram the pin connectivity and power budget. Beyond a few hundred mA I’d expect one would need to make alternative arrangements.

The ZED doesn’t provide for any flow-control, so I’d imagine the only pins used are the DIN, DOUT, 3.3V and GND, and perhaps the NRST

2.4 GHz tends to have quite limited range.

Cellular would tend to have the most effective means of reception and transmission , but the lack of flow control could be an issue. Realistically you’d need to be running your own application / stack on the cellular module so you could get your data back to the appropriate server, and deliver RTCM3 data from an NTRIP source, or your own infrastructure.

Off-the-shelf hardware should be available, but would almost certainly need custom configuration or scripts to serve the niche requirements here. Likely to be significant costs in getting others to do that work.

nandu replied 3 weeks ago

Thank you for the infos!

Would a piggy-backed solution fit into a standard Ardusimple case (i.e. simpleRTK + Shield + 2x Xbee MR/LR)?

The 2.4 GHz Xbee modules with PCB antenna by Digi are quite handy. Though, for ranges 500+ m it might be better to use the SMA/u.FL variant with dipole antennas. Unfortunately, these Digi modules cannot be ordered from European distributors (very long lead times). Do you have any plans to offer such modules (XB3-24AST-J with SMA or XB3-24AUT-J with u.FL)? One of the issues with the PCB antenna is that we would need a waterproof case when using the device on a vehicle – this is a show stopper.

In one of the tutorials or posts I read that using UBX format might not be possible on UART1 and UART2 simultaneously. Is there a feature matrix somewhere where I can check which combinations are supported? A use case for such configuration would be receiving RTK from the base station via UART1 and transmitting UBX POSLLH via UART2.

clive1 replied 3 weeks ago

The rule here is that UBX is *not* supported on UART2

nandu answered 3 weeks ago

> The rule here is that UBX is *not* supported on UART2
In the description of simpleRTK2B-F9P V3 it is explicitly stated:
“Selectable UART1 / UART2 connection to the XBee socket, to be able to send UBX protocol over the air.”
So which statement is true?

clive1 replied 3 weeks ago


The V3 ArudSimple board provides a physical switch to connect UART1 or UART2 to the Radio. The GNSS Receiver stills does not support UBX data out of UART2

nandu replied 3 weeks ago

According to the manual for ZED-F9P it supports UBX on both UARTs:
Unless a missed something in fineprint.

clive1 replied 3 weeks ago

Section 5.5 / Table 18 of the R08 document are salient