Introduction to GPS Receiver Testing

There is a steady growth in the use of GPS in new and existing markets. Consequently, there is an increasing reliance on GPS technology. With this in mind, it is important for designers, manufacturers and consumers of these products to understand what to expect from such systems. This includes formulating an understanding of the limitations and problems of GPS technologies and how to test them. 

There are a series of fundamental receiver performance parameters applicable to GPS systems and this page highlights these tests 

It demonstrates a GPS RF Simulator is able to generate the conditions required for performing suitable tests. 

You will discover various essential test criteria, and what you should expect to see when undergoing these tests.

For example, short TTFF performance may be vital in automotive applications, but not so important for static position in surveying application. Re-acquisition is probably not a primary consideration for marine applications, where little physical external signal obscuration exists, but is important in automotive applications where tunnels and bridges frequently block signals.

If you'd like to learn more about Spirent's solutions for GPS receiver testing, contact us today.

Complex testing made easy

RF Simulation

An RF Constellation Simulator reproduces the environment of a GPS receiver on a dynamic platform by modelling vehicle and satellite motion, signal characteristics, atmospheric and other effects, causing the receiver to actually navigate according to the parameters of the test scenario.

By its very nature, simulation is a representation of the real world. Simulation though cannot reproduce the full richness of real world conditions in testing. A common misconception is the need to exactly replicate real world conditions for a GPS test to be valid.

However, application of representative effects via simulation is proven (over some 25 years of testing) to exercise receivers and adequately identify their limitations allowing for design centring and optimisation. More importantly, it gives complete repeatability, control and exact knowledge – down to bit level – of the signal stimulating the receiver. This is not possible when using real GPS signals for test purposes.

You should look upon simulator testing as representing the real world, rather than replicating it. Spirent simulators include statistical models enabling simulation of richer multipath environments.

Concept of GNSS simulation using a GSS7000

Fundamental GPS Receiver Tests

1. Cold Start Time to First Fix

TTFF (Time To First Fix) is a measure of how quickly a receiver performs the signal search process. The search process, or ‘signal acquisition’, has two dimensions. The C/A code (The GPS SPS Coarse Acquisition ranging code) dimension associated with the replica PRN (Pseudo-Random Number) code, and the Doppler dimension associated with the carrier.

A Cold Start TTFF is defined as the time between application of power to a receiver and it obtaining the first valid navigational data point, when the following criteria are met:

  • Time unknown
  • Current almanac and ephemeris unknown
  • Position unknown

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2. Warm Start TTFF

A Warm Start TTFF is defined as the time between application of power to a receiver and it obtaining the first valid navigational data point when the following criteria are met:

  • Time is known
  • Almanac is known
  • No ephemeris (or the data is more than 4 hours old)
  • Position within 100km of last fix

3. Hot Start TTFF

A Hot Start TTFF is defined as the time between application of power to a receiver and it obtaining the first valid navigational data point when the following criteria are met:

  • Time is known
  • Almanac is known
  • Ephemeris is known
  • Position within 100km of last fix

Cell Tower

4. Acquisition sensitivity

Acquisition sensitivity is the minimum received power level at which a ‘First Fix’ can occur. The sub-sets of this are separate measurements for each of the cold, warm and hot start-up conditions.


5. Tracking sensitivity

Tracking sensitivity is the minimum power level at which a receiver can continue to maintain lock. The tracking threshold is closely related to measurement errors due to error sources in the receiver’s PLL tracking loops. Phase error, dynamic stress error and thermal noise are the dominant sources of error. Minimising these parameters will enable the receiver to continue to track signals at a much lower power.

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6. Reacquisition Time

Reacquisition time is the time necessary for a receiver to regain its first valid navigational data point after total loss of all received signals.

Fast reacquisition time is important for in-vehicle navigation systems. Consider a car emerging from a tunnel, in which it has lost all satellite signals. Immediately after the tunnel is a junction at which the driver must make an exit. The navigation system needs to be navigating again quickly in order for it to give the correct “Exit Now” instruction.


7. Static Navigation Accuracy

Static Navigation Accuracy is the accuracy to which a receiver can determine its position with respect to a known location. It can be split into three categories:

  • Predictable - The accuracy of a receiver’s position solution with respect to the charted solution. Both the position solution and the chart must be based upon the same geodetic datum. 
  • Repeatable - The accuracy with which a user can return to a position whose coordinates have been measured at a previous time with the same receiver.
  • Relative - The accuracy with which a user can measure position relative to that of another user of the same receiver at the same time.

Satellite

8. Dynamic Navigation Accuracy

Dynamic Navigation Accuracy is the same as Static Navigation Accuracy, except the receiver is undergoing motion in either or all of the three axes of
movement x, y, z


9. Radio Frequency Interference

RFI is defined as any unwanted signal that causes degradation in performance, partial loss, or full loss of navigation. Such signals are often referred to as ‘Jamming’ signals. Jamming, being a widely used term describing a signal that prevents the wanted radio communication from being received.

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Resources

eBook: Choosing a GNSS Simulator
A quick guide to choosing the best simulator and testing approach for GNSS-enabled devices.
eBook: Fundamentals of GNSS Simulation
Understand the basics fast, with our beginner’s guide to the history and future of GNSS technology.

Suitable products for GPS Receiver testing

GSS6300 GPS/GNSS Signal Generator

GSS6300

Accelerate your production testing. The GSS6300 enables fast, accurate testing of positioning capabilities in devices as they come off the production line.

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GSS7000 Multi-GNSS, Multi-Frequency Simulator Series

GSS7000

A flexible platform for GNSS testing, the GSS7000 supports any combination of GPS/SBAS/QZSS, GLONASS, Galileo and BeiDou-2 signals and offers up to 256 channels across 4 frequency bands

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GSS9000 Multi-Constellation Simulator

GSS9000

High-performance multi-frequency RF simulator for R&D testing, offering all signals and codes for GPS, GLONASS, Galileo, BeiDou-2 QZSS, and SBAS


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