The first time I heard the term GNSS, I was in a conversation with a professional geologist that suddenly turned into alphabet soup, leaving me extremely confused. Sound familiar? GNSS, GPS, PDOP, GIS. The acronyms come hard and fast and people often expect you to know what they’re talking about without pausing to explain.
For many types of work such as field surveying, archaeology, ecological restoration, and geologic mapping, GNSS has become critical to getting the job done, and we would be lost without it (literally!). But what is it? What does it do? How does it work? Since this technology has become so critical to the work, it’s important for users to understand the answers to these questions at a high level to ensure they’re using it properly and can troubleshoot issues that arise in the field.
In this post I’ll break down what GNSS is, how it works at a high level, how you can get the most out of it for your work, and what related concepts you may encounter as you work with it. I won’t get into the more technical aspects, but if there’s anything else you want to know be sure to reach out and we’ll make a follow-up post.
What is GNSS?
GNSS stands for Global Navigation Satellite System(s). It’s an umbrella term for a type of technology that enables devices to pinpoint their location on Earth by processing signals from satellites in orbit. You’re probably familiar with one of these systems by its name: GPS.
GNSS vs GPS: What’s the difference?
GPS (Global Positioning System) is a specific GNSS that is administered by the United States Space Force, and was the first widely deployed and adopted GNSS. There are a few other systems as of May 2025, including:
- QZSS, administered by the Government of Japan
- Galileo administered by the European Union
- GLONASS administered by Russia’s Roscosmos
- BDS or BeiDou administered by the China National Space Administration
These systems all work roughly the same way and can be used independently or in combination. Using multiple systems, also called constellations, together is likely to increase the accuracy of your calculated position.
How do GPS trackers work?
GPS trackers use a process known as Trilateration. One way to think about this is to imagine that you’ve been dropped off at an unknown location, you have a map of your city, and you’re on the phone with a few of your friends. Each friend in turn tells you where they’re located, and how far they are from you. With this information, you plot their position on the map and draw a circle around them based on how far they are. If you do this with enough of your friends, eventually those circles will intersect at one spot, and you know that’s where you must be located! That’s the general idea with GPS as well.
There are three components to any GNSS, but the two components of the system that are most important to us in the field are the individual satellites in the constellation (our friends, in the example above, called the space segment) and the receivers (us, called the user segment) in our GPS-compatible devices. The third component is called the control segment and is responsible for monitoring and keeping the satellites up-to-date.
Each satellite in the GNSS constellation broadcasts a radio message that includes the time that the satellite generated the message along with some other identifying and positional information for the satellite. When the receiver in our device picks up the radio transmission, it stores the time that the message was received and does some computation to figure out how far the satellite is from the receiver’s current location. Since radio waves travel at roughly the speed of light, and distance traveled can be calculated by distance = speed x time, the device can take the difference between the broadcast and reception times and calculate our distance from the satellite.
One satellite signal isn’t enough to know our position on Earth, though. There are two problems with relying on a single satellite 1) since radio waves spread out from the satellite there are multiple positions that could have the same range at any given time and 2) the satellite has a super-accurate atomic clock whereas our device generally does not, so we can’t directly compare the timestamps.
So how many satellite signals are needed to be reasonably sure of our position? Just four! More signals will help increase the accuracy of the calculation, as well as monitoring multiple radio frequencies or using multiple GNSS constellations, but four signals is enough for a relatively accurate idea of where you are on the planet.
Why do you need four satellites to calculate a GPS position?
The simple explanation is that we need to determine the value of four different unknowns: our latitude, longitude, elevation or altitude, and what time it is. Because we have four variables, we need signals from four satellites in order to evaluate them all. If you’ve studied Algebra, you might recognize this as similar to solving a system of equations with four variables.
You might be wondering: Why do I need to know the time? Doesn’t my cellphone already have a clock? Yes, it does, but the clock in your phone isn’t accurate enough for this purpose. GPS satellites have ultra-precise atomic clocks that are synchronized using the control segment of the system and our devices generally do not. Since the radio frequencies are traveling near the speed of light, even tiny differences on the order of milliseconds can produce large errors in the distances our receiver calculates. Using the satellite system’s internal clock as a control produces far more accurate position calculations.
How accurate is GNSS? What type of device do I need?
Accuracy depends on a number of factors both in the environment and in your equipment. All devices will suffer degraded accuracy if the satellite signals aren’t visible, if you’re in a crowded environment with a lot of obstacles, or even if the satellites you’re receiving signals from are clustered too closely together. That said, here’s a general idea of what kind of accuracy you can expect from certain devices in optimal conditions as of May 2025.
Consumer grade (cellphones, tablets, etc). These can generally fix your location within 5-10 meters. That may be sufficient for a lot of activities such as ecological restoration, geologic mapping, mineral exploration, or certain types of construction.
Professional Grade. If you need to get your location with sub-meter or centimeter accuracy, you’re going to want a professional grade receiver. There’s still a lot of variation within this segment of the market, so you’ll need to research each device’s characteristics. Devices in this segment will use things like multi-frequency or multi-constellation monitoring to enhance accuracy, and others will use differential corrections like RTK (real-time kinematic) positioning to get you centimeter-level accuracy. These devices can be good enough for use in professional surveys, civil construction, etc.
Common Challenges with GNSS in the field, and ways to mitigate them.
Canopy Cover
While a single tree won’t block GNSS signals from your device, the signal power is reduced each time it interfaces with other types of matter, so if you’re mapping in a forest with a dense canopy, you may have trouble getting a strong enough signal to calculate an accurate fix on your location. Another potential source of error in this environment include Multipath errors, where the satellite signal has bounced off of other surfaces before reaching your receiver, making it look like the satellite is further away than it really is.
Tall Land or Structures
Tall land such as mountains or structures like buildings in a city can inhibit your receiver’s ability to fix on a position, or increase the amount of error in the position calculated. This is caused by a few reasons. One issue arises when the land or structure directly blocks satellite signals. GNSS signals below 15 degrees above the horizon are generally filtered out because they can introduce a too much error from the effects of the atmosphere, so when a structure rises up even further and blocks out additional satellites from the field of view, it may result in too few satellite signals being available. They can also introduce multipath errors, especially when surrounded by structures in a city. Using a mounted receiver on a range pole can also assist with mitigating this challenge.
Limited Battery
Calculating accurate positions can be a real battery drainer! Especially if you’re simultaneously mapping, taking photos, or collecting other types of field data. If you don’t need your position at all times, you can turn off GPS monitoring until you need it, but most frequently the best solution to this problem is to bring backup batteries. You can get fairly large, fairly cheap backup batteries and bring them into the field with you for multiple days worth of charge.
One way to mitigate some of these challenges is to use a mounted GNSS receiver on top of a range pole. Elevating the receiver reduces the amount of interference that it’s likely to encounter and increases the likelihood of a strong signal from the satellite. Another solution is to upgrade to a professional-grade GNSS receiver. These receivers use advanced techniques such as multi-frequency monitoring, real-time kinetic positioning, and other differential correction techniques to produce more accurate positions than you’re likely to get from a consumer grade device like a cellphone or tablet. They’ll also have their own dedicated batteries, so you’re able to use your tablet or phone exclusively for other field data collection activities.
Did I miss anything?
GNSS is highly technical and while there is plenty more detail to expand on, this should provide you with a sufficient baseline understanding for your fieldwork. Is there anything else you want to know? Let me know by email: blog@touchgis.app.