GPS technology for vehicle testing

Defining velocity

Not all GPS engines are created equally…Most GPS engines measure velocity extremely accurately. In fact, this is the most accurate parameter a GPS engine outputs; many good measurements come from velocity. This is because velocity measurement is made using the Doppler shift in the signals coming from the satellites. If you are tracking seven satellites, then this is like having seven police radar guns aimed at you: the measurement is made by taking them all into account.

However, other crucial factors involved in producing useful velocity measurements for vehicle testing are sample rate, frequency response, latency and noise.

Sample rate

The number of samples produced each second is of primary importance in vehicle testing. The most usual update rate for GPS is once a second, which is fine for navigation, but it is not much use if you are trying to measure speed, braking distance, acceleration, lap-time or track position. The minimum for these sorts of tests is 5Hz to see any level of detail, and 20Hz is normally considered the minimum entry point for a decent GPS vehicle testing solution. The top end GPS systems can update 100 times a second, and these are suitable for specialized applications such as brake testing and high speed dynamic manoeuvres.

Latency

This is the delay between the change in vehicle motion, and the output from the GPS changing, and is completely different from the frequency response. The latency is only important if you are measuring other parameters at the same time, or if you want to use the output of the GPS engine to control something.

Velocity comparison between a consumer 1Hz GPS engine (blue) and a survey grade 20Hz engine (red)

Frequency response

This is how fast the velocity measurement can react to a change in vehicle motion, for example, during the onset of an ABS assisted stop, the rate of change of acceleration (jerk rate) is very high, and to measure distance and speed accurately, the GPS has to be able to track these changes very accurately.

To show an example, here are three brake stops which were carried out using a low, medium and high frequency response setting on the GPS engine. The blue trace is the reference (IMU corrected) vehicle speed, and the red trace is the GPS measured speed. You can see as the frequency response gets higher, the over/undershoot of the velocity gets smaller.

High frequency response

Medium frequency response

Low frequency response

Noise

The faster you sample the velocity, the more noise will appear on the signal. There is a very careful balance between filtering the velocity to remove noise, and introducing a poor frequency response. By incorporating an IMU and Kalman filter, you get the best of both worlds, an improved frequency response with less noise.

Blue trace is the IMU filtered data, the red is the original data

Defining position

The best way to show the positional accuracy of GPS engines is to do a scatter plot. This is a long term test with a survey grade antenna, mounted high enough to get a completely clear view of the sky, in a fixed position. The standard way of defining accuracy in GPS engines is to use this scatter plot to work out the 95% CEP value. X 95% CEP means that 95% of the results of the scatter plot will fall within a circle of X meters.

Commercially available GPS engines

The standard type of GPS engine used in Sat Nav devices and Mobile Phones is very small and very cheap. Coupled with a very simple and small antenna, these devices update their speed and position once a second and have an accuracy of around 3-5m 95% CEP. However, even at this low end of the market, the velocity is usually still fairly accurate at around 0.2-0.5 km/h. Whilst an update rate of once a second (1Hz) is not enough for any kind of high speed vehicle analysis, we have optimized the settings in one of the best commercial grade receivers to output data at 10Hz.

Survey grade

Survey grade GPS engines use much higher quality components than commercial GPS engines, employ very powerful processors and use patented techniques to get higher positional accuracy and high speed updates. They are a lot larger than the commercially available GPS engines, and can cost up to 100 times more. The 24hr position scatter plot on the left shows a commercial engine in red, an un-adied survey grade in blue, aided by SBAS corrections in green, aided by a 20cm base station in purple, and you can just make out an RTK 2cm aided system in yellow!

You may ask what is special about the GPS engines we use in our VBOX products? The answer is that we have not only selected the best GPS engines for different products, but we have also worked with the manufacturers of these GPS engines to optimize them for our particular requirements, so you can be sure that you are getting the very best performance that is available.