The Racelogic Velocity Box from M-Tech

Non Contact speed and distance measurement using GPS

Hardware Software Accuracy FAQ's Customers News Contacts
Frequently Asked Questions
What is the difference between the VBOX and other competing products ?
  • Removable Compact flash storage gives maximum convenience with up to 1Gbyte.
  • High resolution 24 bit digital output and 16 bit analogue output of velocity.
  • Up to 1000 pulses per metre output available from the digital output.
  • Faster re-acquisition, no loss of data under bridges, no matter what size.
  • The VBOX software has more features than competitive products
  • We offer constant updates via the web.
  • Software customisation available.
  • A high quality of support to the VBOX, as it is designed and manufactured in house, not by a third party.
  • Upgradeability, you will always be offered an upgrade path from your current VBOX to the next level.
  • We use a highly sophisticated Spirent GPS doppler shift simulator to calibrate the VBOX, and can provide a fully traceable calibration certificate. This is the only guaranteed way of developing and calibrating accurate GPS based systems.
  • The velocity output from the VBOX is considerably smoother than most GPS engines due to the fine tuning of the receiver to match vehicle testing situations. This is done using sohpisticated filtering techniques which do not cause any increase in the latency of the signal, (as tested on the doppler simulator).
How does the VBOX still work whilst going under a bridge ?

Amost all GPS systems cannot work under bridges. When a vehicle passes beneath a bridge, the bridge wipes across the antenna's view of the sky over short period of time. This means lock is broken and then re-made on each satellite in view.

However, we have spent considerable time optimising the settings and firmware on the GPS engine we use to regain lock on a satellite instantly it comes back into view, (as long as other satellites are in lock). Therefore, as you pass under the bridge, you lose around 1 or two satellites out of the total, but at all times there still should be enough sky in view to continue calculating velocity and position. The VBOX will work under any bridge, even a large motorway type bridge has enough sky visible to remain locked on to some satellites.

Absolutely no guesswork is involved, no interpolation or heavy filtering is used. You can always see some sky whilst travelling under a bridge, (if you can't you are in a tunnel, where it definately won't work) the trick is to use the satellites that are visible, and seemlessly swap between the ones appearing and the ones disappearing.

We do not use "off the shelf" GPS engines to achieve this high performance re-acquisition. We have refined our GPS engine using our in-house 12 channel GPS doppler simulator, which allows us to develop VBOX under laboratory conditions, honing the accuracy and sharpening the response by tuning the internal parameters in the GPS engine, which just isn't possible if you use the normal road as your test bed.
What is the difference between the VBOX Pro and the VBOX II dgps ?

The VBOX Pro can be used in conditions where you have good satellite visibility, eg. Test tracks, Disused airfields etc. If you have partial obstruction of the sky due to trees, bridges or buildings, then you will need the VBOXII dgps, which works in almost any condition.

The VBOXII also has Compact flash storage, more output connectors, and a high accuracy digital and analogue output of velocity.

I read on your competitor's website that you have had a problem with the digital outputs, is this true, and is this fixed ?

There was a small number of VBOX II's (4 units) that had an instability in the digital output of velocity under certain conditions. This occurred over a short time period, 6 months ago, and was quickly fixed. It in no way affects any of the other 250 VBOX's in circulation, and does not affect any new VBOX's.

Please contact us if you would like a list of the major companies who have come forward to defend our product against this unprovoked attack, and are willing to testify that the VBOX II does not produce the problems they claim.
How does it work?

The VBOX uses a completely new way of measuring velocity. There is a constellation of 24 Satellites used for GPS (Global Positioning Systems) and these satellites are constantly broadcasting a radio signal which is used to determine position and velocity. The service is completely free of charge and works in any country. The VBOX monitors these messages, and by measuring the Doppler Shift* in the carrier signals coming from three or more satellites, it calculates the velocity of the receiver.

Using this method, a very accurate velocity is calculated 20 times a second, and the VBOX data-logs this value along with the position, heading and height of the receiver. The time reference used in all calculations is derived from the on-board Atomic clock signals from the satellites.

To make the VBOX work, all that is required to be fitted to the vehicle is a small magnetically mounted aerial. The VBOX then receives the signals from the satellites and computes velocity and position instantly, and very accurately. As the VBOX has its own battery pack, total fitting to the vehicle only takes a few seconds.

*Doppler Shift is the phenomenon whereby a signal frequency changes when the source or receiver are moving relative to each other. This can be observed as a police car goes past, and the note of the siren changes as it passes the observer.

 

How can you prove the accuracy of the system?

There are two ways of investigating the accuracy of GPS systems, either using a GPS doppler simulator, or by in-vehicle testing.

Simulation
We test every VBOX using an expensive GPS Doppler simulator, which simulates the signals from 12 satellites. During this test we simulate various constant speeds, comparing the output of the VBOX with the actual speed. We also simulate 1g, 0.5g and 0.25g brake stops, comparing the real distance with the VBOX reported distance, and the same for acceleration. We simulate a constant radius circle of 10m, 50m, 100m and 250m at varying speeds to test the lateral acceleration measurements. We also simulate an exact 10m, 100m, and 1000m distance test. The tolerances of each VBOX are therefore checked that they fall within specification, and we can then issue a traceable calibration certificate. In all cases so far, the VBOXs which have been calibrated have exceeded our specifications by some margin.

In vehicle testing
Recent tests were performed at Millbrook Proving Grounds on the calibrated mile straights. The tests consisted of driving a vehicle fitted with a VBOX between two markers, which were exactly 1000m apart. Over a series of ten individual runs, the VBOX calculated an average distance which was within 6.5cm of 1000m (0.0065%).

The standard deviation for the 10 tests was only 9.5cm.

In two of the runs, the car accelerated as hard as possible, and then did a full ABS brake stop whilst inbetween the start and finish markers. This made no difference to the reported distance.

Another test was performed whereby two VBOXs were fitted to the same vehicle, and the 1000m tests carried out. Repeatability between the VBOXs for the distance calculation was 0.002%, and the mean velocities were within 0.003kmh of each other.

The VBOX II can take an input from a photoelectric sensor, which allows you to carry out these test at your own track, using reflective strips to mark the start and finish. The speed going into the test can be different from the speed leaving the test, as the VBOX software automatically corrects for the speed latency in the GPS.

This test has been repeated by a number of different vehicle manufacturers and tyre companies, and they all confirm this high level of precision and repeatability.

 
Someone told me the delay in measurement of speed for the VBOX is 400ms, is this true?

No, this is categorically not true and has been quoted by our one of competitors as a reason to buy their speed test equipment. The internal latency of the VBOX is 80ms, which is corrected for when any external triggers are used. (If in doubt, borrow a VBOX and measure it!). This is true for all versions of the VBOX sold past and present.
How do bridges and trees affect the system?

The VBOX II DGPS will not lose lock under bridges, trees or in a built up area. It is therefore suitable for normal road use as well as test tracks.

However, the VBOX Pro will lose lock under bridges and tree cover, and takes about 3-5 seconds to recover. This makes the VBOX Pro unsuitable for normal road use, and should only be used on a test track.
How long does it take to lock onto the satellites?

The satellites are acquired between a minimum of 10 seconds and a maximum of 2 minutes from power up.
How sensitive is the system?

If you move the VBOX aerial with your hand, the speed with which you move the aerial is displayed instantly on a laptop screen, the response is instantaneous, and sensitive right down to 0.1 kmh.

 
Does it require a laptop computer?

The VBOX has 1 hour of internal memory, so a laptop is not required during the test. This allows the VBOX to be used on a motorbike, where the rider can easily carry the equipment in a jacket or backpack. After the test is completed, a laptop is required to download the data and analyse the results. Alternatively, the laptop can be connected at all times during the test, and then the results can be monitored in real time.

There is also the option of a digital display which can be mounted on the dashboard or windscreen and will show vehicle Speed, Distance or Time.

 
What else can be connected to the VBOX?

A brake trigger can be used to signal when the brake pedal was pressed for use in certain brake distance calculations and there is a switch to start/stop logging. There is a choice of a digital or analogue output to allow the VBOX to be connected to a separate data-logger, and information is also transmitted over RS232 and CAN.

A number of different modules can be connected via CAN to measure various analogue and digital inputs. As these modules are linked via CAN, they can be distributed around the test vehicle, minimising the sensor connection distance.

Without a shadow of a doubt, GPS is set to become the new reference by which the performance parameters of a vehicle are determined. All other forms of speed measurement are superseded by this technology in ease of use, and by obviating the need for calibration.

The VBOX encompasses this new technology, and exploits all of the available features in an easy to use, and powerful package.

  

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