TABLE OF CONTENTS
Introduction to GNSS
What is GNSS?
Global Navigation Satellite System (GNSS) refers to satellite-based navigation systems that provide precise geo-spatial positioning on or near the Earth’s surface. While many people use the term GPS to describe these systems, GPS specifically refers to the United States’ NAVSTAR Global Positioning System.
Types of GNSS
(source, United Nations Office for Outer Space Affairs)
Figure 1. Types of GNSS.
At present, there are three fully-operational GNSS:
- Navigation Satellite Timing and Ranging | Global Positioning System (NAVSTAR GPS; United States)
- Global Orbiting Navigation Satellite System (GLONASS; Russia)
- COMPASS/Bei Dou (China)
Regional Navigation Systems
Besides global systems, there are four developing global regional satellite navigation systems, namely:
- European Satellite Navigation System (GALILEO; Europe)
- Indian Regional Navigation Satellite System (IRNSS; India)
- Quasi-Zenith Satellite System (QZSS; Japan)
These regional systems typically use geo-stationary satellites, which provide coverage in specific regions for specific purposes, unlike the global constellations of orbiting satellites used by GPS and GLONASS.
Evolution of GNSS
The early systems, known as GNSS-1, such as NAVSTAR GPS and GLONASS, were primarily designed for military applications and were later adapted for civilian use. The widespread adoption of GPS by civilian users over the past 10-15 years has led to the development of new generation systems, called GNSS-2, which focus more on civilian needs.
New Generation GNSS
The new-generation Global Navigation Satellite Systems (GNSS), known as GNSS-2, were introduced to enhance civilian applications by offering increased accuracy and more robust signals. This generation of GNSS includes the European Union's Galileo system, which became operational in 2016, and China's BeiDou system, which completed its global coverage in 2020 (Qi et al, 2024).
Components of GNSS
All GNSS share three main components:
- Space Segment: The satellites in orbit.
- Control Segment: Ground stations that monitor and maintain the satellites.
- User Segment: The receivers used by end-users to calculate their location using satellite signals.
Focus on GPS for Agriculture
For farmers and other civilian users, the choice lies in selecting the appropriate receiver within the user segment. They cannot influence the space or control segments. This module will focus primarily on the GPS system, given its prevalence and relevance in agricultural applications.
How does GPS work?
Figure 2. Explanation of how GPS works.
Most GPS receivers will not give a reading unless four satellites are being simultaneously tracked.
This type of GPS position determination is known as ‘code-phase’ operation because the ranging code being carried on the L1-band radio wave is used. At present, code-phase operation is restricted to using only the L1 band unless you have a military decoder that can read the P code (which is on both radio waves). Calculating travel times using only information from the code on a single radio wave restricts the degree of accuracy that can be achieved. This is due to: the relatively long cycle time of the code transmission (1/1000th of a second), errors introduced by location and time-variable delays on signal transmission by the Earth’s atmosphere (see GPS errors). Using information from codes on two frequencies can eliminate this error.
Carrier phase operation is a way of improving the accuracy of time, and therefore position measurement, without a military decoder. For this, the actual radio wave or ‘carrier’ is used to improve the time measurement process. If two carriers are monitored the transmission delay errors can be removed. Using the carrier in the timing process improves time measurement because the carrier is cycling 1000 times faster than the code it carries, so the smallest unit of time in the measurement process is 1000 times smaller. Monitoring the difference in time measurements calculated from two carriers of different frequency enables the atmospheric delay to be determined.
GPS Errors
The accuracy of a GPS receiver depends on many factors, not just how it operates. For high-accuracy tasks like autosteer, it's critical to understand what can affect signal quality. Besides the method used to process signals (like C/A code, P code, or carrier phase), the receiver's accuracy can be influenced by various error sources. The most significant error occurs when there is a loss of line of sight between the receiver and the satellites. This is why GPS works less effectively indoors or under thick foliage. It's especially important to keep this in mind when using autosteer in fields with trees that might block the signal.
- Satellite geometry
Apart from errors in determining the distance between the satellites and receiver, the accuracy of geo-location is also a function of the geometry of the satellites used for geo-location. The optimum geometry is for one satellite to be directly overhead and the other three spread out evenly. As satellites orbit the earth, their geometry relative to a receiver varies and the dilution of position (DOP) errors will vary; this is the main cause of daily variation in the accuracy of geo-location. Receivers with upwards of 12 satellite tracking channels help minimise this effect.
- Satellite errors
Errors in the timing of the onboard atomic clocks or an error in the transmitted data regarding the location of the satellite in space (ephemeris error). - Atmospheric errors
To reach a GPS receiver, the satellite signal needs to pass through the Earth’s atmosphere and, in particular, the ionosphere and troposphere that degrade or slow the signal speed.
- Multipath errors
These are errors caused when the GPS antenna receives signals that have been reflected from a secondary source, such as nearby sheds or silos. This lengthens the travel time, resulting to an error in the distance determination. Multipath errors can be minimised by not placing a fixed base-station antenna near buildings and operating mobile systems away from large structures.
- Receiver errors
The ability of the GPS receiver and associated software to cope with thermal and electronic noise from external sources such as motors, may affect how accurately the receiver can geo-locate itself. The receiver clock, which is not as accurate as the satellite atomic clocks, may also add error.
- Continental drift errors
Continental Drift error in GPS is a significant source of positional inaccuracy due to the Earth's tectonic plate movements. However, through the use of updated reference frames, plate motion models, and continuous monitoring, these errors can be minimized, ensuring that GPS remains a reliable tool for navigation, geolocation, and various scientific applications.
Differential Correction
Differential Correction in GPS is a technique used to enhance the accuracy of GPS data by comparing the positions recorded by a GPS receiver with those of a stationary reference receiver at a known location.
This reference receiver calculates the positional errors by comparing its known location with the location determined by the GPS, then transmits these error corrections to nearby GPS receivers, allowing them to adjust their own readings accordingly. This method significantly reduces errors, providing more precise location data.
GPS Receivers
Stand alone GPS receivers
This type of receiver operates without an external correction. It is commonly used to refer to standard position service (SPS) receivers, which operate by solely using the basic C/A code on the L1-band from the navigation satellites.
Stand alone GPS receivers are the cheapest available, due to the lack of ability to incorporate correction signal or complex circuitry, to use the P code or carrier phase mode of operation. Consequently, stand alone GPS receivers have the lowest positioning accuracy of all the GPS receives on the market. Most SPS receivers contain filtering algorithms to smooth the signal noise when the GPS is moving, making them more accurate when moving and suitable for wide swathing, low-resolution applications such as yield monitoring.
Real-time differential GPS (DGPS) receivers
DGPS receivers require two antennas: one to collect the signal from the GPS satellites and determine position, and another one to receive a signal containing a correction factor, improving the accuracy of the position. Table 1 shows the sources which provide varying levels of accuracy.
Figure 3. Real-time differential signals for GPS receivers.
DGPS tend to cost more than SPS receivers as they require extra components to accept the correction signal and update the position. DGPS receivers can operate via C/A code, carrier phase or a combination of both. The accuracy of DGPS receivers can range from ± 2 millimeters to 2 meters.
Expected accuracy from GPS receivers
Table 1 summarizes the average accuracy obtained using different operational modes of receivers. The achieved accuracy of each receiver depends on the contribution of the errors described at a given place and time.
Table 1. Average accuracy expected from different GPS operational modes.
Receiver accuracy measurement terms
Table 2 lists the standard reporting terms. Using these terms in an example:
A receiver has a circular error probability (CEP) of ± 2 meters.
Interpretation – 50% of the recorded data points will fall within two meters of the real location, while the other 50% will be further away from the real location.
Table 2 includes a number of commonly reported terms which are interchangeable. To address this, a rule of thumb calculator for converting between the different accuracy terms is provided in Table 3. To determine the accuracy, read down the column to the appropriate row for the multiplication factor.
Table 2. Accuracy reporting terms used by receiver manufacturers.
Table 3. Conversion factors to compare accuracy in different terms.
Agricultural uses for GPS
GNSS is becoming more prevalent in farm management, and NAVSTAR GPS receivers are, at present, the most common PA technology used on-farm due to the tangible benefits (e.g.: guidance, autosteer, control traffic farming – CTF, and tramlining).
In modules G and H, the financial benefits of other PA technologies are mainly site-specific and therefore have a longer return on investment (ROI).
Guidance and autosteer are driving GPS adoption, however, receivers can also be used for a variety of purposes as shown in Table 4.
Table 4. Possible applications for different GPS receiver operation modes in precision agriculture.
Different farm activities call for different levels of positioning accuracy. For example, carrier-phase systems can be used for geo-referencing soil samples, but could also be substituted with cheaper, less accurate systems, especially when only a meter level accuracy is required.
Coordinate systems
A coordinate system refers to the location reference which can be interpreted. It could be as simple as a distance and direction from a point of reference, e.g. corner post. In the case of GNSS receivers, there are two main types of coordinate systems: Geographic and Cartesian. Locations can be converted between the two systems.
- Geographic coordinate system – records locations in latitude and longitude using degrees, minutes and seconds.
- Cartesian coordinate system – takes the globe and flattens it into two dimensions; measurements in Easting and Northings in meters.
Knowing the two coordinate systems, one can deduct that keeping everything in latitude and longitude, i.e. Geographic Coordinate System, simplifies data collection and storage, and minimizes the probability of error being produced. On the other hand, Cartesian coordinates is necessary when data analysis based on distance is required. In practice, consultants need to be aware of how coordinate systems work in their part of the world.
GPS-based vehicle navigation systems
The benefits of GPS-based vehicle navigation systems are the following:
- reduced skip and overlap of inputs;
- improved timeliness;
- reduced driver fatigue;
- modifications to labour requirements;
- reduced compaction;
- improved soil water management;
- increased yield;
- precise seed-bed manipulation (e.g. raised beds);
- inter-row cultivation/spraying; and
- inter-row planting.
Guidance Systems
With this type of system, machinery control remains with the operator. It is the operator’s responsibility to ensure that the machinery is steering in the intended direction. The guidance system uses a signalling device to prompt the driver to maintain a predetermined path. The operator can use personal judgement to override/correct any perceived errors associated with poor GPS positioning. Sub-metre accuracy DGPS is often used for this form of vehicle navigation assistance.
Autosteer Systems
These remove the operator from the majority of steering operations. Most systems require manual assistance at the end of each ‘run’ to direct the vehicle towards the beginning of the next ‘run’. For safety, all autosteer systems have an automatic override system as soon as the operator takes control of the steering wheel. These systems also monitor the quality of the GPS signal and will disengage (or not engage) if the GPS data is not of significant quality. Autosteer systems can be sub-divided into two categories:
Steering Assist
This is an interim level between guidance and integrated autosteer systems. Assisted steering systems have an attachment to the steering wheel that allows the machine to be steered by manipulating the steering wheel. These systems are potentially less accurate than integrated autosteer systems given the nature of the steering control.
Integrated Autosteer
The desired track information is passed directly to the vehicle’s steering system through electronic control of in-line hydraulic valves. These systems require a steering kit to be fitted to each vehicle (unless it has been factory-fitted) to allow communication/response between the autosteer computer and the hydraulic system. Using an RTK GPS with integrated autosteer is considered the most accurate option at the time of writing.
Considerations when buying guidance/autosteer:
- compatibility of systems with existing machinery;
- transferability between machines;
- equipment upgrading and required software;
- availability of after sales support services;
- on-going costs particularly DGPS subscription fees;
- ease of use; and
- diversity of swathing options.
References:
Patrick.Gindler. (n.d.). GNSS. https://www.unoosa.org/oosa/en/ourwork/psa/gnss/gnss.html#:~:text=At%20present%20GNSS%20include%20two,Navigation%20System%20(GALILEO)%20and%20China's
Qi, Y., Yao, Z., & Lu, M. (2024). Dual-assisted high-precision tracking technique for wideband multiplexed signals in new generation GNSS. Satellite Navigation, 5(1). https://doi.org/10.1186/s43020-023-00125-2
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