Glossary

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Baud Rate

The speed at which the data of each message is known as the Baud Rate. The default rate on a VBOX is 500Kbytes/second. This also happens to a commonly used rate by most vehicle manufacturers. The Baud rate of a device that you wish to connect to a CAN bus must be the same as the data on the CAN bus.

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CAN

CAN stands for Control Area Network. It is a form of multiplexed wiring designed by Bosch and allows the linking of a number of control systems together, normally in a vehicle, so that they can share information.

In the past it would have been necessary to have at least one wire for every signal on a vehicle making wiring looms bulky and expensive. CAN bus multiplexing allows a large number of signals to be transferred digitally using only a pair of twisted wires.

Sharing of information also reduces the number of sensors that are needed. For example, the engine controller has its own sensor to monitor coolant temperature. Using CAN it can periodically broadcast the temperature reading so that the information is available to any other systems on the car that are interested. One such system might be the instrument cluster, which would
use the information to drive its temperature gauge.

See also CAN Frames.

CAN Frames

CAN transfers signals as packets of data sometimes referred to as 'Frames'. Each frame consists of an identifying number (called, appropriately, an IDENTIFIER) and a group of data bytes. A maximum of 8 bytes can be attached to each identifier.

Standard CAN Identifier

A standard CAN Identifier is 11 bits long. Eleven bits gives a possible 4096 different identifiers although in practice few vehicle manufacturers use more than about 20. Because a CAN frame can have up to 8 data bytes, it is common that a number of signals are attached to each identifier. An example CAN frame is shown below.

Although the physical aspect of CAN is standardised, the way in which vehicle manufacturers assign identifiers to signals on a CAN bus varies from one manufacturer to another. This means that just because identifier xyz on one brand of vehicle contains an RPM signal it probably won't on another. A typical CAN frame on a vehicle might look like this:

A frame such as this might be transmitted periodically by the engine controller. In this example the first 2 data bytes are used to represent RPM as a 16bit number. The 3rd byte contains the value of the throttle position sensor. The 4th byte is engine coolant temperature. The 5th byte contains flags for binary information such as switch inputs. In the example, bytes 6,7 and 8 are not used. If a control unit connected to the CAN bus in our example needs to know engine RPM then it only needs to listen out for a message with an identifier of 432 and extract the first two bytes.

Extended CAN Indentifiers

The only major difference to frames with standard identifiers is that extended frames use 29bits for their identifier to give a wider number of possible values. The J1939 standard for trucks uses extended CAN identifiers and clearly specifies which signals are attached to which identifier. The CAN01 automatically receives extended and standard type identifiers by default.

Baud Rate

The speed at which the data of each message is known as the Baud Rate. The default rate on a VBOX is 500Kbytes/second. This also happens to a commonly used rate by most vehicle manufacturers. The Baud rate of a device that you wish to connect to a CAN bus must be the same as the data on the CAN bus.

Data Format

The data contained within a CAN frame can be in two different formats, Motorola and Intel.

Motorola:
16-bit integer 1234 hex (4660 decimal) is stored in memory as...

32-bit integer 12345678 hex (305,419,896 decimal) is stored as...

Intel:
16bit integer 1234 hex (4660 decimal) is stored in memory as...

32-bit integer 12345678 hex (305,419,896 decimal) is stored as...

The following screen shot is taken from the Channel setup screen of a Vehicle Can Interface channel on a VBOXIII.

You can clearly see the structure of the eight data bytes and the fact that this particular channel labelled Sats is only 8 bits in length and occupies the first data byte.

CEP

CEP means Circle Error Probable. CEP is an indicator of the delivery accuracy of a weapons system, used as a factor in determining probable damage to a target. It is defined as the radius of a circle into which a missile, bomb, or projectile will land at least half the time.

For example, an ICBM warhead with a CEP of 100 meters will impact within 100 meters of the target point in at least 50 percent of all attempts.

95% CEP means 95% of the time the position readings will within a circle of diameter 3M of the true position. This error is due to the changing state of the ionosphere, constantly changing the time taken for the satellite signals to reach the earth.

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Differential GPS (DGPS)

Differential GPS is a method of improving the accuracy of the Lat, Long and Height positional data measured by the GPS engine. A GPS engine is placed in a fixed position and left to work out its current position. By telling the GPS engine that it is not moving, it can monitor the signals coming from each satellite and work out the corrections which need to be applied to each satellite in order to remove the effects of delays in the signal travelling through the ionosphere. These corrections are then broadcast to a moving GPS receiver.

There are two different types of DGPS correction with varying degrees of accuracy improvement available to VBOX's.

See also WAAS/EGNOS and Local Base Station.

Doppler Effect / Shift

The Doppler effect (or Doppler shift), named after Austrian physicist Christian Doppler who proposed it in 1842, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from an observer. The received frequency is higher (compared to the emitted frequency) during the approach, it is identical at the instant of passing by, and it is lower during the recession.

The relative increase in frequency can be explained as follows. As the source of frequency emission (train for eg.) moves toward the observer, there forms a compression for the frequency between the source and the observer reducing the length of the waves which in effect increase the frequency and in turn the pitch.

For waves that propagate in a medium, such as sound waves, the velocity of the observer and of the source are relative to the medium in which the waves are transmitted. The total Doppler effect may therefore result from motion of the source, motion of the observer, or motion of the medium. Each of these effects is analyzed separately. For waves which do not require a medium, such as light or gravity in general relativity, only the relative difference in velocity between the observer and the source needs to be considered.

For more details see Doppler Effect on Wikipedia.

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EGNOS/WAAS/MSAS

EGNOS = European Geostationary Navigation Overlay System / WAAS = Wide Area Augmentation System (US) / MSAS = Multi-functional Satellite Augmentation System (Japanese)

These DGPS systems consist of a network of fixed Base stations that transmit DGPS corrections up to a geostationary satellite. This satellite then transmits these corrections back to earth over a wide area. All geostationary satellites by definition have to sit over the equator, so to receive these corrections, you have to have a good view to this part of the sky. The collective name for these systems is a Satellite Based Augmentation System or SBAS for short.

The signal from the geostationary satellite is broadcast on the same frequency as the normal GPS signals, so they can be picked up using a normal GPS antenna alongside the GPS messages. If you are in view of this geostationary satellite and your GPS receiver is DGPS enabled then it will be able to receive the correctional information and benefit from improved positional accuracy.

Typical improvements in accuracy are around 1M 95% CEP for lat/long and 2M 95% CEP for height using the VBOX IISX. The VBOX3i gives around 1.4m 95% CEP with SBAS.

Electronic Stability Control (ESC)

Electronic stability control is a technology that improves the safety of a vehicles handling by detecting and preventing skids and slides, helping the driver maintain control of the vehicle. This technology is applied through a computerised system.

US organization National Highway Traffic Safety Association, NHTSA, estimates ESC will reduce single-vehicle crashes of passenger cars by up to 59%, with a much greater reduction of rollover crashes. NHTSA also estimates ESC would save 5,300 to 9,600 lives and prevent 156,000 to 238,000 injuries in all types of crashes annually once all light vehicles on the road are equipped with ESC.

Extended CAN Indentifiers

See CAN Frames.

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FMVSS126

FMVSS126 stands for Federal Motor Vehicle Safety Standard #126 and is a ruling requiring the inclusion of Electronic Stability Control (ESC) on all new vehicles. It has been developed with the goal of reducing the serious risk of vehicle rollover accidents.

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GLONASS

GLONASS (Russian: ГЛОНАСС, abbreviation of ГЛОбальная НАвигационная Спутниковая Система; tr.: GLObal'naya NAvigatsionnaya Sputnikovaya Sistema; "GLObal NAvigation Satellite System" in English) is a radio-based satellite navigation system, developed by the former Soviet Union and now operated for the Russian government by the Russian Space Forces. It is an alternative and complementary to the United States' Global Positioning System (GPS), the Chinese Compass navigation system, and the planned Galileo positioning system of the European Union (EU).

Development on the GLONASS began in 1976, with a goal of global coverage by 1991. Beginning on 12 October 1982, numerous rocket launches added satellites to the system until the constellation was completed in 1995. Following completion, the system rapidly fell into disrepair with the collapse of the Russian economy. Beginning in 2001, Russia committed to restoring the system and by September 2010 it is now fully restored (24 of 24 satellites are operational).

GPS

GPS stands for Global Positioning System, a satellite based navigation system made up of a network of satellites placed in orbit by the U.S. Department of Defence.

There are currently 24 US Military owned satellites orbiting the earth on 20Km orbits, giving 100% world coverage. The free signals from these satellites can be used in different ways to determine a number of measured parameters of movement and position.

To measure the Longitude, Latitude and height, the GPS receivers measure the different delays in the signals coming from 4 or more satellites. The distance to each satellite is calculated and then using triangulation, the 3D position of the GPS antenna is calculated.

The GPS engines used in VBOX's also calculate the Doppler shift in the carrier frequency of each satellite transmission to build an accurate measurement of the speed of the GPS antenna in the X, Y and Z planes. X is north/south, Y is east/west and Z is vertical velocity.

The X and Y velocities are combined to give 'course over ground' speed and heading data.

The velocity and heading channels are the accurate channels from which most VBOX data is derived: velocity, heading, accelerations, radius of turn, deviation, braking distance etc.

The less accurate positional channels, longitude, latitude and height are only used for plotting the vehicle path.

GPS Accuracy

GPS Position relies on precise measurements of the distance from the receiver to the satellite, and therefore suffers from numerous effects which can reduce the quality of the signal. These include atmospheric effects which delay the signals from the satellites and reflections from nearby objects such as buildings which introduce multipath, again adding to the length of the signal.

Luckily however, GPS Velocity can be measured using a different method which measures the change in signal from the satellite, or Doppler Effect. By measuring this change, the errors which normally affect GPS have very little influence over the quality of the signal, and the resultant velocity measurement is phenomenally accurate. Another great aspect of GPS is that all satellites have atomic clocks on-board, and by utilising this signal, the timing remains stable to within less than a millionth of a second.

Therefore, instead of using position to measure distance, the accurate Doppler derived velocity is integrated using the precise time signal to derive distance. The result of the combination of these two is an extraordinarily accurate distance measurement.

GPS Latency

Centimeter-accuracy GPS positions are always available to the user with a delay called latency.

This latency has several origins. One of them is of course the calculation of position data itself, but we must also consider the time for transmission to the user, usually by a serial link, and of course the correction
data transmission which occurs between the fixed and moving receivers, via a radio serial link.

In other words, GPS latency is the time between the measurement of velocity, and when the VBOX reports this velocity.

The latency depends on the baudrate of this link, which is adjusted according to the power of the emitter and the distance between the two receivers, but it also depends on the number of visible satellites (since a correction message is sent for each of them). As a result, the total latency can be as large as two seconds.

GPS latency is only an important factor when using the VBOX velocity output compared with any external signals such as a brake trigger. If you are using the VBOX with the supplied VBOX Tools software, or the multifunction display, then the latency is taken into account and does not affect the accuracy in any way.

If you are taking the CAN output of a VBOX or the raw data from a logged file, then you will need to be aware of the latency, and take this into account.

Latencies:
VBOXIII 12.5ms
VBOXIIS 55ms
VBOXIIDCF 35ms

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Inertial Measurement Unit (IMU)

An Inertial Measurement Unit contains three angular rate sensors or gyroscopes and three linear accelerometers. The sensors are normally arranged at 90 degrees to each other, so they can measure the three directions of our 3D universe.

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Kalman Filter

A Kalman Filter is a very effective type of filter that, in the case of VBOX's, compares the positional and the velocity data as part of the smoothing routine. I.e. if the velocity exhibits a jump in speed over a short time yet the positional data does not corroborate this jump in speed, then the jump will be reduced and smoothed accordingly. This applies in the same manner for a jump in position that is not backed up by a jump in speed.

Live Kalman Filtering
Kalman Filtering that is applied live by a VBOX (where available), like any live filtering process, will not only smooth the data but will delay transients and sudden velocity changes in the data. This should therefore be used with caution, and only in cases where a live smoothed velocity or position is required from the VBOX.

Post processing Kalman Filtering
A Kalman filter that is applied to logged data has the benefit of being able to smooth individual data samples with respect to the values before and after the smoothed point. This has the major benefit of having virtually no effect on the latency of transients and velocity changes. This type of smoothing is available under 'Tools' in the VBOX Tools software.

Where possible, it is advised to log the raw data with as little smoothing as possible, and then apply postprocessing smoothing as required. If smoothing is applied at source, detail can not then be recovered.

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Local Base Station

A Local Base Station is a portable unit that will transmit local DGPS corrections via radio over short distances.

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Photogrammetry

Photogrammetry is the practice of determining the geometric properties of objects from photographic images. Photogrammetry is as old as modern photography and can be dated to the mid-nineteenth century.

In the simplest example, the distance between two points that lie on a plane parallel to the photographic image plane can be determined by measuring their distance on the image, if the scale s of the image is known. This is done by multiplying the measured distance by 1/s.

A more sophisticated technique, called stereophotogrammetry, involves estimating the three-dimensional coordinates of points on an object. These are determined by measurements made in two or more photographic images taken from different positions (see stereoscopy). Common points are identified on each image. A line of sight (or ray) can be constructed from the camera location to the point on the object. It is the intersection of these rays (triangulation) that determines the three-dimensional location of the point. More sophisticated algorithms can exploit other information about the scene that is known a priori, for example symmetries, in some cases allowing reconstructions of 3D coordinates from only one camera position.

For more information see Wikipedia

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RTK

RTK stands for Real Time Kinematic. You can synchronise two GPS engines by comparing the output from each of them, and using the combination of these measurements to calculate the relative positions of each of the antennas. This process is called Real Time Kinematic or RTK.

GPS position calculations are carried out by measuring the distance between the Earth bound receiver and the space based GPS satellites currently in view. Each satellite broadcasts a unique code, the GPS receiver knows these codes, so it can lock onto the transmission from one satellite and compare it with the code coming from another satellite.

By shifting each of these codes in time, the relative distance to each satellite can be estimated. The satellite code is sent in a series of 1's or 0's (bits) at approximately 1000 times a second (1.023Mhz),
and the GPS receiver can line up this code to within 1% of each individual bit. This corresponds to around 3m in error.

A military code is also sent, but this is sent at 10 times the rate of the civilian code, so the accuracy from this (in theory) is 10 times better.

Real Time Kinematic (or RTK) does rely on the relatively slow civilian code to line up the signals, but instead it uses the carrier frequency of the signal, which is over 1000 times faster (1575Mhz). If the receiver successfully lines up the carrier signals between two GPS engines, then the relative accuracy between them can be that much greater.

The civilian code is easy to line up because it contains a sequence of 1's and 0's designed for this purpose. However, every cycle of the carrier signal is similar to the previous one, so they are very difficult to line up.
Using this RTK method two GPS engines can be used to provide accurate positional data.

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Standard CAN Indentifiers

See CAN frames.

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WAAS/EGNOS

WAAS = Wide Area Augmentation System / EGNOS = European Geostationary Navigation Overlay System

These DGPS systems consist of a network of fixed Base stations that transmit DGPS corrections up to a geostationary satellite. This satellite then transmits these corrections back to earth over a wide area. All geostationary satellites by definition have to sit over the equator, so to receive these corrections, you have to have a good view to this part of the sky.

The signal from the geostationary satellite is broadcast on the same frequency as the normal GPS signals, so they can be picked up using a normal GPS antenna alongside the GPS messages. If you are in view of this geostationary satellite and your GPS receiver is DGPS enabled then it will be able to receive the correctional information and benefit from improved positional accuracy.

Typical improvements in accuracy are 1.8M 95% CEP for lat/long and 3M 95% CEP for height.

WAAS is currently available in the US. EGNOS is only in a test phase in Europe (Do not rely on this signal!)

World Geodetic System

The World Geodetic System is a standard for use in cartography, geodesy, and navigation. It comprises a standard coordinate frame for the Earth, a standard spheroidal reference surface (the datum or reference ellipsoid) for raw altitude data, and a gravitational equipotential surface (the geoid) that defines the nominal sea level. The latest revision is WGS 84 (dating from 1984 and last revised in 2004), which will be valid up to about 2010. Earlier schemes included WGS 72, WGS 66, and WGS 60. WGS 84 is the reference coordinate system used by the Global Positioning System.

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