How does GPS actually work?
26 Apr 2011 Jason 2
To most people GPS units are just little black boxes. They have no idea what goes on inside the box or how it works. But they don’t need to – they just need to know how to use the device, similar to driving a car – you don’t need to know how an engine works to drive one.
We’ve provided an overview of how GPS works, with regard to the network and how GPS satellites and GPS units (receivers or PNDs) interact to provide a user with real-time location information, but what goes on inside the “magic box” of a GPS receiver?
But of course we know that these gadgets are not “magic boxes” – there are scientific principles and behind how they operate. Electrical engineers have designed and developed all the electrical components that need to come together to perform the necessary operation of receiving and processing the GPS radio signal.
So for those of us who like to dig a little deeper, here’s an overview of the components that make up a simple GPS unit.
We’ll talk about the two main elements that make up a GPS unit, based on the construction followed by the OpenGPS project, an initiative designed to help familiarize students and interested persons with the inner workings of GPS units.
The two main elements are (1) the chipset and (2) the software that runs on the chipset and performs the signal conversion.
The GPS chipset
The GP2015 is a small-format, triple-conversion, front-end chip (similar in performance to the GP2010 but in a smaller package).
The chip features an on-board analog-to-digital converter or sampler that provides a two-bit quantized output for subsequent signal processing in the digital domain. The GP2021 is a 12-channel correlator chip.
The operating system
Like a lot of electronic devices, GPS units run a type of operating system. The software is generally written in a machine assembly code that can interface directly with the chipset.
While operating systems vary between units, the OpenSource GPS unit uses a C program written in C/C. Initially compiled using Borland versions 4.5 and later, Microsoft Visual C 1.5 was used later on.
The operating system takes care of a range of functions including satellite acquisition and tracking to the decoding of the navigation message and producing raw pseudorange and delta-pseudorange (carrier-phase) measurements.
Normally, low-side mixing is performed by the software to convert the GPS radio signal. It involves mixing an RF signal with a local oscillator (LO) signal of lower frequency to produce a down-converted IF signal. A positive-phase change of the RF signal results in a positive-phase change of the IF.
The signal conversion proceeds through five stages, starting with a 1575.42-MHz GPS signal that is low-side mixed, with a progressive down-conversion with the final output of this process being a final IF signal that has a nominal frequency of fDCO-88540000/63 Hz, where fDCO is the carrier DCO frequency set by the software.
This conversion process produces a signal that can be used by the operating system to calculate accurate location information – the output that a GPS user sees.
While this breakdown of a GPS unit have been greatly simplified hopefully it clears up some of the mystery behind what happens inside a GPS receiver. It should be noted that we’ve kept this discussion generic because GPS hardware and technology continues to evolve, however the main principles of receiving and processing a GPS radio signal remain.
Commercial use for GPS tracking
To learn more about how GPS receivers are being used by commercial fleets check out a selection of case studies.
Categories: Consumer GPS