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Almond grower Andrew Sharpe was keen to add some precision to the spray setup on his tractors at Sharpe Almonds. His farming operation is relatively small, so all equipment upgrades had to be kept within a fairly modest budget.

This article documents the process of integrating an Emlid Reach RS2+ to his setup in order to provided precision positioning through a combination of GNSS reception and RTK corrections.

Background – DGPS spray control

“I use automatic rate control on our farm sprayer setups. Over the years I’ve added different equipment to our tractors, however, some of the gear stopped performing as intended due to changes in Australia’s differential beacon network.

This article details the benefits and challenges of the equipment we’ve adopted – and the current solution which involved integrating an Emlid Reach RS2+ GNSS receiver with our Topcon X20 spray control unit.

Our initial setup

Around 2012 we acquired a secondhand KEE Technologies / Topcon X15 Touch Screen Guidance Computer with a spray program & a spray interface. This unit provides Guidance by using a GPS signal, inputted into one of four serial Ports on the rear of the unit via a NMEA GGA & VTG Sentence.

Originally the unit performed quite well using a Trimble 132 “The Choice” Receiver – providing 12 Channel GPS reception & the choice of free-to-air DGPS (Differential GPS) VHF Beacon or an optional commercial Omnistar subscription-based DGPS Correction.

As we had an AMSA (Australian Maritime Safety Authority) DGPS Correction Beacon service reference station approximately 150 km away, we used the option to provide us with their free-to-air DGPS VHF signal which worked very well – even though we are working amongst an orchard of Almond Trees. What we found was that with the Trimble GPS we were able to locate our spray start/stop points without too much trouble.

Back in the days prior to adding GPS gear, we opted to use the turning brake of the tractor to provide a ‘skid mark’ on the ground to indicate where the spray tank ran out.

When the Trimble GPS Receiver was receiving the AMSA Beacon correction, the lines where the tractor was recorded stayed reasonably straight. However whenever the AMSA Beacon dropped out (which happened occasionally due to maintenance) the GPS derived tractor position would sometimes cross from one machine guidance lines into the neighbouring line (row of trees) & back again.

Given that the tractor spray program was running Auto Section Control & Limit to Boundary, the spray control would switch off every time the GPS lines would cross from one row into the neighbouring row, which makes my spray job harder.

At this point, the TopCon X15 interpreted the GPS location as indicating that this particular area of the orchard had already been sprayed. This did not happen very often, as the AMSA beacon was generally quite reliable.

GPS Spray control – without beacon corrections

This all changed around 1st July 2020 when AMSA discontinued the DGPS service and the local DGPS beacon was switched off permanently. AMSA simply stated that “it is no longer required to support safe navigation in Australian waters”.

I now had a problem – using the Receiver with no DGPS correction meant that in certain places my virtual row drawn on the screen would frequently tend to drift sideways into adjoining rows. I had to Override the Auto Section Control or switch off the sprayer whenever I struck one of these spots – which was a bit frustrating.

I decided to look for alternative solutions. As we are not a big user of GPS – compared with broadacre farmers –  I tried to look for inexpensive options. I quickly decided against upgrading the Trimble 132 to Omnistar subscriptions as these seemed expensive for what I was looking for.

I then started looking into other options and explore what was available.

My key selection criteria was focussed on solutions that were:

  1. Inexpensive – not wishing to pay a fortune either to buy outright or ongoing expenses
  2. Reasonably Compact – as we are in trees, a roof mount antenna is prone to damage. In the case of the Trimble 132 – the antenna was able to be placed under a PVC irrigation pipe end cap – so something similar in size to the Trimble 132 Antenna would be ideal
  3. Accurate – a receiver that receives more than just the same 12 satellite channels which the Trimble 132 was limited to and also offers inexpensive DGPS options.
  4. Simple – possibly something all contained in the one unit.

Trimble 252 with RTK

I already had a Trimble 252 on RTK – using a dealer based network which we pay a subscription to – this was used primarily for Auto Steer Functions in our broadacre cropping at another property. Things against the Trimble 252 was primarily its size – It would not fit under the end cap – so protecting it might have been an issue. I could not find anything that easily could be used to protect it from tree strike. The Trimble 252 also was limited to 12 Channels – so satellite reception would have been similar to the 132, but did have the added benefit of RTK corrections from a base station.

We had also installed a Trimble SNB900 Repeater station to repeat the signal from the Dealer Network through our property providing a fairly reliable reception of the RTK base via a 900 MHz Signal. All up the Trimble 252 would have been a relatively good & cheap option if it wasn’t for its size. We were already paying a yearly subscription for the RTK corrections and if needed, we could move our SNB 900 repeater to other locations on our property if we suffered signal drop outs. It was currently located in a prime location to serve our broadacre farming activities, with power available to keep it running 24/7.

Trimble 332

I had purchased a Trimble 332 ‘Ultimate Choice’ Receiver secondhand at some point prior to the AMSA Beacon signal being switched off. This receiver had extra options compared with the Trimble 132 being:

  1. WAAS / EGNOS correction Via Satellite
  2. RTK correction via radio
  3. Omnistar VBS-XP/HP correction via Satellite

The issues in this case were that – WAAS or EGNOS are free SBAS type corrections – only available in the United States or Europe. Although the Australian SouthPAN SBAS correction service has been announced, I am unsure if old equipment is compatible or if companies will release updates for out of date equipment to use this technology. If this was available for a receiver such as the Trimble 332 it may been a viable option for DGPS correction.

Another issue with RTK via the 332 was the need for the RTK feature to be ‘Unlocked’ by the manufacturer.  I would have to purchase an unlock code from Trimble to upgrade to the Omnistar HP signal & another Unlock to RTK. I would also have to purchase a radio to receive the RTK correction signal. In summary, the SBAS option would be ok if Trimble would release an update for the Trimble 332 Receiver to receive the Australian SBAS signal – but I would have to test in the field to see how the reception was is in our situation.

Omnistar costs

Once again the issue with Omnistar was the expensive subscription costs. I haven’t looked any further into this as to take advantage of the HP signal I would have to purchase an unlock code to upgrade to this technology.

Topcon SGR-1 considerations

I purchased a Topcon SGR-1 Receiver second-hand. The only DGPS correction this receiver offers is an SBAS reception. WAAS, EGNOS & the Japanese SBAS system.

Once again this is similar to the Trimble issue – it won’t receive the northern hemisphere SBAS systems. If Topcon had released an upgrade to allow it to work on the Australian system this would be good. The SGR-1 is also an antenna & receiver all in one – making it better than the Trimble 132 / 332 Units as far as being a compact all in one unit – not creating any extra clutter in the tractor cabin. This only required a cable to connect from the receiver to the screen in the tractor.

It receives both GPS & GLONASS Satellites – which makes its reception better than the Trimble 132 or Trimble 332 when they are operating autonomously – without any DGPS correction. This could effectively seeing twice as many satellites as the Trimble units. Testing the Topcon unit did prove a theory of mine to a degree.

Downfalls being that it only offered differential correction via free to air SBAS reception where available and once again Omnistar – which I did not wish to pursue.

While I was relatively happy with the Topcon SGR-1 it still did tend to drift on occasion into adjacent rows particularly in one location – the same location where the Trimble receivers drifted once the AMSA beacon was switched off.

Emlid GNSS receivers

I was talking with an old friend – Alistair Hart, from Mangoesmapping – about my issues since the shutdown of the AMSA service . He mentioned that he was a dealer for EMLID Receivers & maybe these would offer a solution.

After initial Discussion it was concluded that the Emlid receivers would not work in their current configuration (at the time) as the Topcon X15 / Topcon X20 screens received the old NMEA sentences with GP headers & the Emlid Receivers only put out the very latest GN Sentences which were too advanced for our equipment.

After several Months I had almost forgotten about the Emlid gear when Alistair phoned me back to say that the Emlid Engineers had now modified the software to allow the receivers to NMEA sentences packages to be configured by end users, and support GP headers, which was compatible with my old spray control equipment.

Wiring the Emlid RS2+ and the Topcon controller

I was starting with an RS2+ and a power/data cable to hook up to the receiver’s RS-232 port.

One end plugged into the Emlid & the other had bare cables. After finding the cable colour codes on the Emlid docs site I set about making an interface box to connect the cable to my Topcon Unit.

All of the components were easily sourced from the nearest Jaycar store – these being:

  • a serial cable
  • a terminal Block
  • a project Box
  • a 2 pin connector compatible with the output from the Topcon Unit &
  • some 2 core wire suitable to power the Emlid Unit.

I connected the cable between the Emlid RS2+ and the Topcon screen via a junction box. Once I changed the Emlid unit to transmit GP sentences it showed on the screen that it was receiving a signal.

The only other change needed was to tell the Emlid unit to turn on when power is supplied from the Topcon Unit and turn off when the Topcon Unit Powers down. This was all easily configured within the Emlid Flow App from my mobile phone.

Testing GNSS reception

With the receiver all wired up, I did some testing. Straight away I was very impressed – going from a maximum of 9 satellites ever seen visible on the Trimble 132 & 252 to a maximum of 16 seen on the Topcon SGR – I was now seeing approximately 30 satellites with the Emlid, which reflected the ability of the RS2+ to receive signals from GPS, Glonass, Beidou and Galileo constellations. I also noticed the Topcon screen was displaying its height up to 3 decimal places rather than a maximum of 2 with the other receivers. The only drawback was still the lack of signal corrections.

Receiving corrections on the Emlid

In order to apply RTK corrections to the GNSS receiver, I had a few options.

A local base station could be created by installing a second Emlid Reach receiver to act as a base reference station & send the correction via LoRa (915MHz) radio.

NTRIP corrections could be received by the Emlid Reach via an Internet connection. The receiver allows users to install a SIM card directly in the receiver, allowing the receiver to connect to the Internet via a network provider like Telstra. NTRIP corrections are available either through a paid subscription service such as HxGN Smartnet or for free via the Australian Government AUSCORS correction service.

To date I have only used the free NTRIP option as this seemed to work extremely well in our area – being only 30 km From Adelaide. I found very few areas where the mobile signal dropped out – indicated on the Topcon screen showing I had no Differential. Most of the time the signal seemed to show RTK Float rather than RTK – although when working I don’t know the difference unless I click on the GPS Icon or download a map of the spray coverage & check the layer in the map as to what level of correction was operating.

Local corrections via LoRA

The next step is to set up a second Emlid Reach receiver to act as a base station. Corrections will be sent to the receiver on the tractor via LoRa. The base station allows for an external 6dBi LoRa antenna to be connected. The plan is to mount this on top of a shed to see how LoRa compares to NTRIP In my application.

Overall I was very impressed with the performance of the Emlid Reach RS2+ receiver. It ticked quite a few boxes for me as a replacement for my older unit:

  1. Compact – it fitted under my PVC end cap.
  2. Accurate – the receiver provided a very accurate signal – showing very little shift – even without a corrections.
  3. Did not add any clutter to the interior of the tractor cab – the unit is self contained.
  4. Easy to program – I just connected to the GNSS receiver via a smartphone app. No complicated menus to navigate through as is the case with the Trimble units.
  5. Corrections – Has a couple of options to access RTK via NTRIP and LoRa
  6. Multi-use – I can take the unit off the tractor & use it as a compact surveying tool to locate assets such as valves and pipes in our orchard for future reference.

12 months of testing

Based on my use of the last 12 months I would say the unit is an excellent replacement for someone that wants an inexpensive solution to integrate with if legacy units such as Topcon / Kee Technologies X10, X15 and X20 units without the need to have to spend big $$ on newer units.

The only issue I found was a warning that appears with regard to Autosteer. The warning was ” Steering No Valid GGA Input”. Eventually I solved the problem with this. The Emlid was sending too much information into the X20. Disabling all satellites except GPS & Glonass constellations got rid of this warning. This took me back to the 15 or so satellites. As I don’t use Auto Steer on that tractor that is not really an issue. I enabled all the other satellite constellations to maximise the consistency of my satellite based positioning.

SBAS via Satellite

The only other issue I have is that the Emlid doesn’t offer SBAS free to air correction via Satellite*, I think it would be a great option to have included!”

Andrew Sharpe,
Sharpe Almonds, South Australia


*Note: SBAS is now available in Australia and New Zealand via a service called SouthPAN and offers sub-metre levels of accuracy. Emlid engineers are aware of the SouthPAN service but are focussed on delivering corrections via NTRIP and LoRA which offer centimetre levels of accuracy.

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