Frequently Asked Questions
What is the maximum g-force/speed that can be measured?
The SPEEDBOX is fitted with a 2g accelerometer as standard, but a 6g option is available. The SPEEDBOX-IMU and SPEEDBOX-INS options may measure greater g-forces depending on the exact model selected; please consult the specification table for details. The maximum measurable speed is approximately 1000mph.
How often do you get GPS speed updates?
The GPS system calculates speed every 50ms (20 Hz), however this data is combined with the data from the accelerometers to calculate speed 200 times every second with very high accuracy.
How often do you get GPS position updates?
The GPS system calculates position every 50ms (20Hz).
How accurately is speed measured?
With average GPS reception, speed accuracy of the SPEEDBOX is about 0.05kph (or 0.1% if greater) when travelling at fairly constant speed or during moderate acceleration and braking, and about 0.1kph (or 0.1% if greater) during hard acceleration or braking. The only exception is at very low speeds (under 10kph) where the error increases to about 0.5kph. Do not be fooled by exaggerated claims from other manufacturers… this is as good as it gets. In contrast, a standard wheel speed pickup is only accurate to about 4% at constant speeds, and under hard acceleration or braking the error increases up to around 20%.
How accurately is position measured?
With reasonable GPS reception, positional accuracy is about 3m (CEP). With ideal GPS reception this improves to around 1m.
What happens to the data if you drive under a bridge/tunnel/trees etc?
Because speed is calculated from both the GPS data and accelerometers, even if the GPS data 'disappears' for a number of seconds, you won't be detect any degradation in the quality of the output data. Only if GPS data disappears for an extended time (10+ seconds) will the data start to degrade noticeably due to the slow drift which is inherent in integrated accelerometer calculations. However, if there is a significant gradient change during the GPS outage (for instance a tunnel with a down slope and then up slope) then the speed will be in error, since the accelerometers cannot distinguish change in gradient from acceleration or deceleration.
Where can I buy it?
Check www.race-technology.com for an up-to-date list of stockists and online purchasing
Is it upgradeable?
The SPEEDBOX is upgradeable in a number of ways; please check www.race-technology.com for an up-to-date list of options. Software and firmware updates, including new features, are freely available as we introduce them.
Is it easy to use?
The SPEEDBOX is very simple to install, connect and configure. Full instructions are provided with every unit. With power supply or battery pack connected, the unit automatically starts up and commences measuring and outputting data continuously - no triggering is required. We have also kept the analysis software as simple as possible whilst making it as flexible as we can to ensure that you can do what you need to with it. As with all computer programs, the first time you use it there is a lot to take in - after you've become familiar with it, you will be able to analyse data quickly and efficiently.
What specification of computer is required?
As with most programs, the analysis software will run on just about any PC with Microsoft Windows XP or later - however, the faster the PC, the faster the program will run.
what is the difference between gradient and pitch?
We define gradient as the vertical direction in which the vehicle is moving. This will generally be the same as the slope of the road. We define pitch as the vertical direction in which the vehicle (actually the antenna strip) is pointing. This may not be the same as the slope of the road since the vehicle may pitch during hard braking or acceleration.
Gradient (slope of the road) can be measured using either the SPEEDBOX or the SPEEDBOX-RTK. The SPEEDBOX can be configured to output a gradient message on either CAN, RS232, or on one of the analogue channels. This message contains gradient derived from integrated NED velocities, and is very accurate as long as the vehicle is moving. When the vehicle is stationary the velocities contain only noise, so the gradient is random and undefined.
The SPEEDBOX-RTK can also measure pitch (the angle in which the vehicle is pointing, assuming that the antenna's are mounted in the same plane as the vehicle, or the offset is known and accounted for). By subtracting the gradient from the pitch, the vehicle pitch angle relative to the road can be known. We call this the squat angle, and the SPEEDBOX-RTK can output squat as a message. Pitch is valid both when moving and stationary as long as the system has an RTK lock, squat is only valid when moving since it relies on gradient for it's calculation.
To summarise, to measure the slope of the road use the gradient message, but only when the vehicle is moving.
The following diagram shows the relationship between gradient, pitch and squat angles:
Why does altitude from the SPEEDBOX differ from my consumer GPS?
The SPEEDBOX measures altitude above the WGS84 ellipsoid. This is a globally specified ellipsoid, that is a best fit for the entire Earth, and the standard reference for GPS systems internally.
However, the Earth is not actually a true ellipsiod, but a geoid which has undulations for many reasons, such as the weight of mountain ranges pressing down on the crust. As a result, mean sea level is not consistent thoughout the world, and local mapping agencies, such as the Ordnance Survey in the UK, will reference height to a geoid based on a locally specified mean sea level. This will differ from the WGS84 ellipsoid, and the difference is known as the geoid separation.
Most consumer GPS units output height relative to local mean sea level. The SPEEDBOX outputs height relative to the WGS84 ellipsoid (essentially a best fit to global mean sea level). Hence the two may differ, but neither is incorrect.
Further reading can be found here: http://www.esri.com/news/arcuser/0703/geoid1of3.html