How to know what accuracy of the GPS receiver do you really need?
If you are considering to purchase a farm GPS system, you will confront with the notion of accuracy. There is a clear relationship between the precision of GPS receiver and how expensive it is. What level of GPS accuracy is sufficient for you? Is submeter accuracy enough or should you go for RTK? These are the questions we try to answer in this article.
I have 10-year experience in agricultural technologies. Our mobile app – FieldBee from eFarmer has been downloaded by more than 120,000 farmers from around the world. I love to meet and communicate with our customers. Asking how they use the app. Therefore, all the following information and recommendations are based on my experience as a technical engineer and the experience of our community of farmers that are using our tractor navigation application.
What does “accuracy” mean
First, let’s deal with the very concept of “accuracy”. GPS accuracy can be absolute and relative.
Absolute accuracy means that if you specify your location, you will be able to return to this point in a day, month, year, with the level of accuracy that is listed in the technical characteristics of the receiver, for example – 4cm – 50cm – 1m. This is why absolute accuracy characteristics explain the applicability of a GPS receiver for geodesy work.
Relative or pass-2-pass accuracy – shows how position deviates from a specific location if one comes back to it in 10, 20, 40 min. This is what we need in farming when accuracy is important during the whole day and allows you to make parallel rows in the field.
Why does GPS accuracy Vary?
Now when we have clarified the concept of the accuracy itself, let’s look at what does it depend upon. We can outline to categories of factors:
- Receiver type: GPS of your smartphone, outdoor GPS tracker, an external smart antenna, RTK external antenna.
- Signal quality: obstacles, accessibility of corrections (DGPS), proper installation of the antenna.
Types of GPS devices for farmers
GPS of Your Smartphone or Tablet
- Absolute accuracy: 2 – 20 meters;
- Pass-2-pass accuracy: 1 – 3 meters;
- Popular models: Samsung Galaxy S, Samsung Tab.
For area measurement. It is possible to use your smartphone’s or tablet’s built in GPS receiver for measuring the area of a field. If you hold a tablet or a smartphone in your arms in the open air you can achieve the optimal level of reception of satellite signal.
For parallel driving. Pass to pass accuracy of the built in GPS is not suitable for reliable tractor driving as you will see your position jumping around. Although our customers use it sometimes when driving in Skip and Fill driving pattern and for tracking covered area.
For field notes. Suitable for soil sampling, to mark places of loading and unloading of materials, points of entry into the field.
External GPS Tracker
- Absolute accuracy: 2 – 20 meters
- Pass-2-pass accuracy: 50 cm to 2 meters
- Popular models: Garmin GLO, Quartz 818
This type of devices has the same accuracy as the built-in receiver of your smartphone/tablet. But it benefits from the fact that you can install it in a more suitable location and get better reception. For example, on tractor roof, hood or on a mast.
For area measurement. With a tracker, you can measure field area more accurately. It is important to understand that this type of measurement can be used for mapping fields, but can’t be used for the exact calculations or precise position on the map because of low absolute accuracy. (eg your field boundary may be shifted to a road).
For parallel driving. From experience of farmers in our community, you can use it for spraying operations with implement width of 11 meters and more. But work at night and fog is now possible.
For field notes. To indicate the position of a stop / pause of work, points of entry into the field, mark obstacles.
Transport operations. The tracker is suitable for tracking routes such as field – warehouse.
Smart Antenna
- Absolute accuracy: 1.5 meters
- Pass-2-pass accuracy: 20 – 50 cm
- Popular models: FieldBee
These receivers deliver relatively high accuracy of 20 – 50 cm, which will enable you to perform operations and measurements with higher precision than before.
Measurement of field boundaries. Due to the high relative accuracy, you can measure the area of the field with high precision, but because of the low absolute accuracy, you cannot use it for compliance with government regulations and resolving issues with your neighbours for example.
For parallel driving. Particularly comfortable to work on spraying operations without tramlines, because it is possible to achieve minimal overlap between passes. It already allows you to work in fog and at night.
For field notes. You can use such GPS receivers to mark different spots with geo-location in the field, such as for example end/start points, location of your scouting photos, obstacles, entry point to the field and storage, garage, home etc. It can help to find bearings in the field, it can be particularly useful if your apprentice/contractor is going to drive the field for the first time, or if the orientation is obstructed by the crop.
Transport operations. Suitable for route tracking. eg. Field-Warehouse-Elevator
RTK system for farmers
- Absolute accuracy: 0-4 cm
- Pass-2-pass accuracy: 0-4 cm
- Popular models: FieldBee GNSS RTK system
Measurement of field boundaries. With RTK system, you will be able to measure areas with 0-4 cm accuracy.
For parallel driving. RTK system gives you the highest possible accuracy. And you can perform all the field operations for vegetables, fruits and other crops. This level of precision enables precise auto steer and autopilots.
With such accuracy there are, of course, additional sources of error, such as the cabin shaking may result in a deviation of 10-50 cm. Tilt compensation systems are designed to solve this problem. For example FieldBee auto steering system has built in terrain compensation.
Also important to understand that in order to receive the correction signal system should be connected to the Internet or be within the coverage of the radio signal (in case of FieldBee RTK system the distance form receiver to the base station should be 2 km in radio mode or 10 km via Internet).
For field notes. You can definitely make all the notes with the very high precision.
Transportation. RTK accuracy enables you to track the route with centimeter precision. However, it is unlikely such precision is needed on the road, it’s like a microscope to hammer nails. But it can be suitable for truck and tractor operations performed near combine.
Signal Quality
We have discussed all the various types of receivers available on the market for farmers and their projected accuracy. Now it is very important to understand that the GPS receiver is alike FM radio receiver. But unlike sound, GPS receiver converts radio signal to location coordinates.
The noise in a radio receiver is the same as the scattering of location points in determining GPS position. Therefore, the quality of signal can significantly affect the accuracy of the GPS system. And it depends upon:
- Proper antenna installation;
- Obstacles and weather conditions.
To achieve high-quality signal, GPS antenna should be installed on the roof of the cabin in open air. The best position is the intersection of the rear axis of the wheels and the axis of movement.
Also, the following factors can lower the accuracy of the signal: – bad weather conditions, trees, buildings, high-voltage lines and other obstacles. Receivers that support multiple positioning systems (Galileo, GPS, GLONASS and BeiDou) are more stable.
This overview should help you understand that all receivers can be used for certain types of operations. But in order to use the system for all types of field operations and to get maximum benefits it is better to go for RTK system, aseptically now when it became more affordable.
The biggest advantages of FieldBee equipment that it was created with help of farmers to make it available to all the farmers in the world. It is cost effective and highly accurate. No need to make a trade-off between the accuracy and the price. We give 2 years warranty on our products and 30 day money back guaranty.
If you read our blogs – please let us know what questions do you have regarding GPS systems for farmers. We will be glad to share our knowledge with you.
