The Global Positioning System (GPS) is a satellite-based navigation system that obtains location information anywhere on or near the earth. The system works in all weather conditions as long as there is an obstructed line of sight to four or more space satellites. There are multiple GPS receiver manufacturers nowadays, but no single receiver is perfect. Every system has inherent errors that designers and developers must understand and account for. That is where error analysis and testing come to play their role. Testing is important for the improvement of the navigation filtering algorithms in all GPS-enabled devices. This process is integral in creating an end product that can provide accurate position and navigation data.
A GPS simulator is a tool used by manufacturers to analyze and test the common inherent errors that may affect a GPS-enabled device’s signal acquisition and performance in varying environments. The simulator mimics the same type of radio signals emitted by the satellites themselves. It allows developers to understand the errors that receivers may have as if they are really outside, provide a fix for them, and ultimately create a more reliable end product for users.
What are the different inherent system errors in GPS systems today?
Space Segment Errors
The two common sources for space segment errors are from the satellite clocks used by the system and from the satellite’s positioning. The clocks in the GPS satellites are usually very accurate. However, there are times when they can be inaccurate. Unfortunately, any small inaccuracy results in a significant error in the position calculation of the receivers. For instance, 10 nanoseconds of clock inaccuracy results in approximately three meters of location error. Similarly, the orbits where these satellites travel in are usually very precise. But like the satellite clocks, these orbits can sometimes vary a small amount. Also, like the satellite clocks, any small variation in the orbit results in a significant error in the position calculated.
User Segment Errors
The user’s GPS receiver is the user segment of GPS. Consumer-grade GPS receivers consist of a radio receiver, an internal antenna, and a digital clock that receives GPS signals to provide accurate coordinates for the receiver. The typical errors in this segment are usually caused by fundamental design flaws that result in incorrect compensation for tropospheric and ionospheric delays. Troposphere and ionosphere are different layers in the atmosphere of the earth. Variations or delays in these layers are caused by multiple factors, including changing humidity, temperature, atmospheric pressure, and even electrically charged particles called ions.
These errors occur when a GPS signal is reflected off an object, such as a building, to the GPS antenna. If a receiver receives both the direct and a reflected signal, rather than only the direct one, it will use both of these signals. The delayed signal causes the receiver to receive an inaccurate position calculation.
The use of location-based systems has reached unprecedented levels. GPS is used in many household devices, in malls, in vehicles and even in the microchips in our pets. However, just like any system, these come with inherent errors that manufacturers and designers are constantly working on. Understanding the different errors that may cause inaccuracy on a GPS device is important for developers to ultimately fix these errors and provide a more reliable system for end users.