Build RTK Radio module for Topcon AGI4

Hi all,

I have a question about a GPS reciever that is currently in my tractor (bought it included).

The reciever is a TopCon AGI4 , without RTK radio module.

So the reciever only has an accuracy of about 20cm . I have been looking for info about the
radio module and obviously it communicates with the reciever trough a serial port.

Would it not be possible to build my own radio module with a NTRIP reciever and send the
data over serial to the reciever?

Why ? original radio costs 7000€ . No thanks. :smile:

anyone has any experience ? Thanks allot !

AGI-4 has an external radio port but it would not give RTK accuracy if the original radio was not present. I’m sure they have some security built in but never seen the serial communications between the main unit and the radio logged and analysed. No idea how well it is protected.

The original radio however is nothing like 7000 €. It is expensive as it sort of includes the RTK unlock. Perhaps some 3000 € for the modem and you need the gyro unit too if not already present.

Hi there,

this price comes from Topcon itself (i got a quote last year) .

The gyro is indeed already present. I’ve been looking for a second hand RTK radio for a while but they aren’t sold seperatly that quick i suppose. Only option is to sell my agi4 and get a new one with RTK included. that way i’m in the ‘normal’ range of prices.

thanks for your reply.

Have you asked for an AGCO spare part? My understanding is that the modem part is compatible with the AGCO branded Topcon AGI-4 variants (Fendt VarioGuide and MF/Valtra/Challenger C3000) even if the receiver itself is not (because of a customer SW variant).

EDIT: your link points to the AGI-3 receiver. The modems are different on the two.

A bit more information on this :smile:

It seems the AGI4 (yes i was wrong linking to the AGI3 docs) has the option to connect it to an external modem. In the cab of the tractor is a 9 pin serial port where you can connect an external modem.

I’m going to keep digging into the possibility to build my own NTRIP reciever that send the serial data
to the AGI4. Not so much information to be found about this.

The 9 pin DB connector at the cabin roof is not for an external modem. The actual AGI-4 receiver has two big connectors, the unused one has pins for the external modem. From the VarioTerminal menus you can select radio, internal NTRIP modem or the external modem port.

Are you sure about that?

Have a look at page 139-140 of the varioguide manual.

With this information i think the connected device sends serial data over this connector to the reciever.

Nevertheless, if it needs to be connected to the reciever it self, so be it. Just trying to figure out how.

Those manual pages describe how to configure the portable base with VarioGuide. You connect the portable base with a null-modem cable, let the two communicate and then you remove the cable. After that RTK correction data will be transferred via the internal RF modem (Satel) within the AGI-4 receiver.

This configuration can be done manually too. I’m using both the internal NTRIP modem (Motorola) as well as the RF modem to connect to my private base. Need to handle the configuration manually as the base has another Satel modem, no portable base.

Out of curiosity, why are we discussing this at the AgOpenGPS forum? Is your intention to use AgOpenGPS with the Fendt VarioGuide receiver? Sort of easy to feed AOG from the Fendt serial port but I don’t see the point when the tractor is equipped with a factory built auto-steer system.

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Hi, let me clear that one out :

I’m pretty new to all this GPS stuff, i’m a software developer next to beeing a farmer , but this
is a total new playground.

I live in Belgium, where we have the totally free RTK signal named Flepos. My biggest question is,
is there a way to get this signal into my AGI4 reciever ? I think AOG has lots explained in this area.
Why not getting an original GSM radio module for the AGI4 ? It’s just too darn expensive. There have
to be other ways. But now that i’ve read somewhere that this radio contains an unlocking file for the
AGI4 (that has special AGCO firmware) i doubt i would ever get it to work.

So anything that clears out what and how things work is very highly appreciated !
Thanks allot,

So you have a Fendt from a few years back before they changed from Topcon hardware to Novatel/Trimble. The tractor is auto-steer capable with a VarioGuide Precision receiver (Omnistar HP supported and the gyro unit included). A part time farmer driving a Fendt, very much like myself but why do I see a much older face in the mirror. :cowboy_hat_face:

I’m aware of Flepos but I’m not familiar with the details. This Fendt GNSS receiver would need older dual constellation RTCM messages like 1004 for GPS and 1012 for GLONASS. Flepos very likely has an option for this correction data content rather than sending the new multi-constellation messages. With the OEM modem you could easily have RTK accuracy supported.

My price information for that AGI-4 RTK modem is from about 2017 where my notes say 2400 Euro (I believe 0% VAT). As much as I’d prefer to promote AOG, at your starting point I’d say the OEM RTK modem is the only option that makes sense. Just need to find it at the correct price.

Even if the AGI-4 receiver can be used with an external RTK modem, it would not support RTK if the OEM modem was not present. If they have implemented something like RSA to verify the genuine modem, hacking the system to make it work without the genuine modem would cost more than the OEM modem.

You could try to find a second hand product from or or similar. I saw a complete receiver with a gyro and an RTK modem at 4000 € 0% VAT! The radio modem is different even for different EU countries but if your plan is to use RTK over NTRIP, any AGI-4 modem would be fine.

Long time answer, but still worth mentoining. I did manage to find a second hand RTK for 850€ ! woohoow.


Almost off topic but I was wondering if anyone knows the details of the new Novatel/Trimble approach on Fendts? The Novatel receiver looks very much like a standard Novatel device. Perhaps it would be easier to replace this receiver with an F9P or similar.

I’m not sure if the Novatel receiver has an integrated IMU or if the IMU is within the addtional box that both Novatel and Trimble need in this new configuration.

It just might be less expensive to upgrade to the new approach without purchasing the actual GNSS receiver.

There is an aftermarket modem from DigiFarm but must be subscribed to DigiFarm NTRIP for RTK. I am in the same position. We bought a used Massey 8450 that had a Topcon X-35 console. Between it being traded in and us buying it, the dealer removed the AGI-3 with RTK module and installed an AGI-4 without RTK. Never having dealt with auto steer before we didn’t realize what we were missing. Now that we are thinking of planting with that tractor, adding RTK is more than we had bargained for.

I dont know too much but my version is:

Topcon system the AGI-3/AGI-4 reciver is the navagation controller.

The new Novatel/Trimble approch is they are just standard recivers and then Agco has there addtional box and this is a navagtion controller with an IMU inc with all the other bits too. Fendt just use this navagation controller I think to keep costs down. So then maybe it is possable to talk Trimble serial reciver language to the navagtion controller? You would still need VarioGuide software/unlocks tho to tell the navagation controller what to do.

You must be referring to the Beacon V3 product. The AGI-4 GNSS receiver has a serial port for an external modem. Any simple modem would work, one could use a bluetooth serial adapter (a few Euros) and feed the correction signal from any cellular phone e.g. with the Lance Lefebure app and you would not be restricted to any specific RTK correction signal provider.

Never tried it but I’m sure the external modem is not sufficient. It is the OEM modem that “unlocks” RTK on the AGI-4.

I did not check but I thought the new navigation controller/IMU was the same box for Novatel and Trimble. I would then expect an open interface at least towards the Novatel GNSS receiver, perhaps the same box can do Trimble language towards Trimble and “simple english” towards Novatel. The Novatel option attracts me more because their interface is well documented and could be easier to adapt to.

Trimble at least comes with a specific product name. I assume both are standard Trimble/Novatel HW but possible with some AGCO specific SW?

On the older SCR/S4 series Fendts VarioGuide SW came from the factory and did not need any (paid) unlock. I thought the current unlocks are for the receiver but perhaps not. The Novatel receivers usually work on RTK without any unlocks if the HW is RTK capable. On the tractor one needs an RTK unlock for Novatel too.

The AGI-4 does have a separate port for the modem but unfortunately there is nothing inside our AGI-4. I am not sure if our X-35 console is still “unlocked” for RTK since the previous owner of the tractor had full RTK. I know there is a wire harness on the roof that was not plugged in when we picked up the tractor to try it out before we bought it. We tried to get the original antenna but the guys in the shop had snagged it for someone else and were not giving it back. There are pinout diagrams online but not sure if they will work with generic modems or not. I did see it needs RTCM3 correction data.

The AGI-4 is unlocked for RTK once you have the modem installed. If you remove the modem, it isn’t any more unlocked for RTK. No unlock code needed or paid separately.

Topcon does not work like Trimble or JD. Even the screens have almost all features active from purchase. No unlocks for virtual terminal, section control, rate control, auto-steer etc.


after a long wait the modem arrived, i opened the AGI-4 and found out that they Gyro was not
present also. No option for RTK without the gyro installed. I guess no one knows where i can find
that gyro second hand? new ones are 3500€ excl. vat. nuts.

Hi all,

well in the end i made it work. Found a modem in the UK for 850€ and a IMU also in the UK for 500€,
togheter it makes it work for 1350 which is ok i think.

The system works, only thing that i experience is that the signal hates trees. As soon as i’m driving next to a line of trees the signal is lost pretty quick. Is that even normal?