What is a GNSS/GPS base station?

Over the past thirty years, Global Navigation Satellite Systems (GNSS) have had an enormous impact on the way we live. From powering everything from satnav systems to logistics and transportation planning, disaster warning systems to commercial drone usage.

Needless to say, surveyors and location professionals were quick to see the potential benefits of GNSS. By offering a rapid and persistent source of positioning data anywhere on earth, GNSS constellations, including GPS, revolutionised surveying workflows, significantly reducing the time and resources needed for a range of common surveying tasks.

Nevertheless, most commercially available GNSS systems typically only achieve an accuracy of 2-3 metres, even in perfect conditions. This is far from sufficient for surveying applications. To deliver the accuracy needed, GNSS surveyors had to develop methods that would offer additional precision, and foremost among them was the pairing of a mobile GNSS receiver (rover) with a static base station. This differential approach is at the heart of high-accuracy modern GNSS surveying.

A GNSS base station is a stationary receiver set up at a precisely known location that collects satellite signals to calculate errors in GNSS data. It provides real-time correction information to mobile GNSS receivers (rovers), ensuring centimetre-level accuracy for surveying and mapping applications by accounting for signal distortions caused by atmospheric interference, satellite geometry, and multi-path errors.

As covered in our post “Is GNSS better than GPS?”, GNSS applications rely on various constellations of orbiting satellites that are constantly transmitting signals. The Global Positioning System (GPS) is the most well-known of these constellations, but there are many others now active, including the EU’s Galileo and Russia’s GLONASS.

In everyday usage, a GNSS receiver on the ground picks up these signals and calculates its position based on the signal arrival times. However, this process has inherent limitations that affect its accuracy. GNSS signals can be affected by various factors, introducing errors into your positioning data. Here are some of the main culprits:

• Atmospheric interference. The Earth’s atmosphere can delay and distort GNSS signals. These variations can introduce errors of several metres in your final position calculations.
• Satellite geometry. The number of satellites a GNSS receiver can “see” and their geometric arrangement in the sky significantly impact accuracy. Limited satellite visibility, especially in areas with obstructions like tall buildings or dense vegetation, can lead to positioning errors.
• Multipath interference. GNSS signals travel in a straight line from the satellites to the receiver. However, reflections from surrounding objects like buildings, trees, or even the ground itself can create secondary, weaker signals. The receiver might mistake these reflected signals for the direct path, leading to miscalculated distances and inaccurate positioning.

Unfortunately, standalone GNSS receivers only have access to the raw signals from the satellites. As a result, they can’t fully account for these location-specific errors. In many applications, this isn’t a major issue. For instance, accuracy to within a metre or two is sufficient for location-based smartphone services. In surveying, however, this is a major issue as centimetre-level accuracy is often required.

That’s where differential GNSS comes into play. This describes a range of related techniques used to achieve the high accuracy required for surveying. The core principle of differential GNSS is to leverage a reference location with a precisely known position. This reference point acts as a benchmark for correcting the errors affecting GNSS signals in a specific area.

A GNSS receiver set up at a static reference point is known as a base station. By using a base station, GNSS positioning data can achieve the level of accuracy demanded by surveying applications. Let’s look more closely at how it works in practice.

A base station is a stationary GNSS receiver set up at a precise location to calculate errors affecting GNSS signals in the area. It achieves this by continuously monitoring available GNSS satellites and comparing its calculated position against its known coordinates. Then, it transmits the calculated error corrections to mobile GNSS receivers (rovers), which can then apply these to their own positioning data.

This can be achieved in two main ways:

• Post-Processing Kinematic (PPK). This approach involves collecting GNSS data for an extended period at both the base station and the rover. Once you’re done with your field work, specialised software then post-processes this data together with the base station’s precisely known position. This offers high levels of accuracy (often millimetre-level), but requires additional post-processing steps.
• Real-Time Kinematic (RTK). In this approach, the base station transmits real-time error corrections to the rover using radio or a mobile network. This allows the rover to adjust its position calculations on-the-fly and achieve centimetre-level accuracy in real-time. RTK is ideal for applications where immediate, high-accuracy data is essential.

While RTK has advantages in terms of speed, there are some potential drawbacks. Unlike PPK, it relies on a continuous connection between the base station and the rover. If this is disrupted, either through physical obstructions or interference, it can slow down your surveying workflow or introduce inaccuracies into your data.

When choosing a GNSS survey base station, several essential features contribute to its overall performance, accuracy, and reliability. Let’s look at some of the most important considerations:

• High-precision GNSS receiver and antenna. The heart of a base station is a high-quality GNSS receiver capable of tracking signals from multiple constellations (GPS, GLONASS, Galileo, BeiDou, etc.) across a wide range of channels. A multi-channel receiver offers enhanced signal reception and processing, crucial for maximising accuracy.
• Rugged and weatherproof design. Base stations are often deployed outdoors for extended periods. Look for robust construction with weatherproofing to withstand various environmental conditions like rain, dust, and temperature extremes. Consult the model’s IP rating to see how well-protected it is against dust and moisture – two common hazards on job sites.
• Range of communication options. Base stations typically offer various communication methods to transmit corrections to rovers in the field, including UHF radio, cellular modems, Bluetooth, and sometimes WiFi. The right choice depends on the desired range and specific application, but the more options you have, the better.
• Long-lasting power supply. Base stations need a reliable power source. Depending on your setup, this might be an internal battery with long runtimes or external battery options.
• Easy-to-use software. Look for intuitive software that simplifies base station configuration, data monitoring, and troubleshooting. This will save you time and ensure a smooth workflow.

If you’re looking for a GNSS receiver that meets and, in fact, exceeds all these requirements, take a look at the Trimble R750. This powerful model is highly efficient and accurate in even the harshest conditions, and is well-suited to both RTK and PPK survey workflows. Its modular design means that it can be used as both a base station and a rover, and the combination of a 4G LTE modem and 2 Watt UHF transceiver ensures it can handle both short- and long-range communication with ease.

Or, if you want to benefit from a truly cutting-edge solution, the Trimble R12i is an excellent choice. It incorporates Trimble’s ProPoint positioning engine, capable of rapidly integrating multiple sources of positioning data for maximum accuracy, as well as Trimble Inertial Platform (TIP) technology for calibration-free tilt correction. And you needn’t worry about the risks of taking such an advanced device on more challenging job sites – it offers military-spec protection against the elements, with an impressive IP67 rating for dust and water resistance.

While these GNSS receivers are both more than capable of functioning as base stations, they also support more advanced features that can be used to cut out the base station entirely. In particular, both can connect to virtual reference station (VRS) services.

Rapid technology advancements in surveying are constantly offering new solutions and workflow optimisations. A virtual reference station (VRS) utilises a network of continually operating reference stations (CORS) to transmit position correction data directly to your rover, typically through a mobile connection. This can serve as a convenient alternative to the classic base+rover approach to GNSS surveys.

For a more detailed look at how VRS works, as well as an overview of alternatives such as Trimble’s RTX service, read our post “What is VRS?”.

VRS solutions mean there’s no need for you to set up a base station, nor do you need to worry about keeping your rover sufficiently close to maintain a connection. As a result, you’re not only saving time on the initial setup but avoiding the need to repeat the process if you’re undertaking a larger project. And all this without sacrificing on accuracy!

Trimble VRS Now is a great example of just how sophisticated and robust VRS systems have become. With access to a CORS network that covers large parts of North America, Europe, Australia and New Zealand, you’ll be able to enjoy sub-centimetre-level accuracy across a wide range of locations.

VRS solutions are an exciting advance in surveying technology, supporting a faster and more efficient approach to GNSS surveys. However, a VRS solution isn’t always the best option – nor is it without its own potential pitfalls. It’s important to consider the benefits and downsides of both a base station and a VRS before deciding on which to choose.

Generally speaking, a VRS service will be a quicker approach, as you won’t have to go through the process of setting up a base station. However, you should also consider:

Potential limitations of base stations

Nevertheless, a base station setup poses some potential challenges that a VRS system can help you avoid, including:

• Setup errors. When setting up a base station, even the smallest errors in establishing the base’s location can compromise the accuracy of your rover data. In a worst-case scenario, it can lead to an entire day’s work being wasted.
• Operational range. For an RTK survey, the base station needs to be able to send correction data in real-time to the rover. As a result, you will need to maintain a continuous connection between base and rover. That means you will be limited in terms of distance, as well as being required to avoid obstructions that might block the signal.
• Lack of redundancy. If your base station fails due to a mechanical issue, you may find yourself facing substantial delays in your surveying work. Because it leverages a large network of reference stations, a VRS service isn’t subject to single points of failure.

Ultimately, which of the two approaches is better suited for your needs will depend on a number of factors, including the location of the job site and the equipment you have access to. Nonetheless, VRS services are continuing to improve at pace. As coverage extends and connection issues become less pressing, exploring a VRS option is an increasingly worthwhile choice.

Base stations have long been a core feature of GNSS surveying. By supporting a reliable and efficient differential GNSS method, they have enabled surveyors to achieve the necessary accuracy. And while new services such as VRS have come to supplement or even replace the base station as a core part of differential GNSS surveying, there are still plenty of occasions where the traditional base-and-rover setup maintains its advantages.

Whatever your preferred approach to GNSS surveying, SEP can provide the equipment and accessories you need to deliver accurate and reliable results at pace. We stock everything from GNSS receivers and data collectors to powerful surveying software for your post-processing needs.

And with over 35 years of experience supporting surveyors, engineers and other location professionals to tackle a diverse range of surveying challenges. Browse our well-stocked shop or contact us to discuss your needs with a member of our expert team.