The page contains a list of definitions for some terms that are used in this website.
Inertial Navigation Systems An Inertial Navigation System uses one or more Inertial Sensors (gyro or accelerometers) to track your position as you move. The most comprehensive form of Inertial Navigation Systems is a Six-Axis Inertial Navigation System. Since our universe has three dimensions and three rotations, a six-axis system can measure all forms of motion correctly. A system with less than six inertial sensors does not work under all conditions. The RT3000 is a Six-Axis Inertial Navigation System.
GPS The Global Positioning System, or GPS, is a collection of satellites that orbit the earth. Each satellite transmits a signal and, using the signal from four or more satellites, a GPS receiver can compute its position, velocity and time. GPS is capable of providing a highly accurate, continuous global navigation independent of other positioning aids. GPS provides 24-hour, all weather, worldwide coverage. The system uses the NAVSTAR (NAVigation Satellite Timing And Ranging) satellites that consists of at least 24 operational satellites. These satellites are arranged in six orbital planes at 55 degrees to the equator. The orbital period of the satellites is approximately 12 hours and they orbit at an altitude of about 20000km. They provide GPS receivers with at least six satellites in view at all times. A minimum of four satellites are required to allow the GPS card to compute its current latitude, longitude, altitude (with reference to mean seal level) and time. The GPS satellite signal identifies the satellite and provides the positioning, timing, ranging data, satellite status and the corrected ephemeris (exact orbit information) of the satellite for the users. The satellites can be identified either by the Space Vehicle Number (SVN) or the Pseudorandom Code Number (PRN). The GPS satellites currently transmit on two L-band frequencies; several other frequencies are planned for the future. The first frequency (known as L1) is centred at 1575MHz and the second frequency (known as L2) is centred at 1227MHz. The L1 carrier is modulated by the C/A (Course Acquisition) and the P code (precision), which is encrypted for military and other authorized users. The L2 carrier is modulated only with the P code and is for military (and other authorized users) only. Only carrier-phase information from L2 can be measured by civilian GPS receivers. Five base stations and three up-loading stations are located around the earth. The base stations track and monitor the satellites via the signals that they transmit. These signals are passed to a master control station where the ephemeris is re-computed. The ephemeris is the exact orbit and timing information for each satellite. The resulting ephemeris is transmitted back to the satellites via the uploading stations. It can then be used by GPS receivers go give the best possible measurements.
Heading Heading is the angle where your vehicle is pointing compared to the angle of True North. Heading can also be measured to Magnetic North or Grid North. The exact definition of Heading means that you cannot use your velocity vector to compute it. The angle from the velocity vector should be called the Track Angle or the Course Over Ground. Many GPS products incorrectly use the term Heading to mean Track Angle.
Coarse Acquisition GPS satellites transmit their information on two main frequencies. Additional frequencies are being added to new satellites for the future. The main frequencies that civilian GPS receivers can use are L1 and L2. The Coarse Acquisition Code, or C/A code, is one of the signals broadcast on the L1 carrier. This code is only broadcast on the L1 carrier. The other signals on the L1 carrier are the Precise Code, or P code, and the navigation message. The C/A code is used by civilian receivers to locate position. The code is used to determine pseudorange (the apparent distance to the satellite), which is then used by the GPS receiver to determine position. The C/A code is coarse compared to the P-code. The P code is more precise, but it is encrypted into the Y code (by something called Anti-Spoofing), and it cannot be decoded without having a key to the encryption. This is not available to civilian users. The P code is ten times as fast, which means it can determine the pseudorange ten times more accurately. However, it is much more difficult to search, which is why even the military needs the C/A code. The C/A and the P codes are pseudo random number (PRN) codes. This means that they have the characteristics of random noise. However, they are not random; they are very precisely defined. Out of all the possible random sequences, they have been very carefully selected so that they are as orthogonal to each other as possible. Orthogonal codes can operate at the same frequency and cause the minimum amount of interference. Up to 37 sequences have been defined for the C/A code. Each satellite has its own different C/A code sequence. The sequence is repeatedly broadcaster over and over again. The timing of the sequence is critical. It is this timing that allows the local GPS receiver to measure the pseudorange accurately. The C/A code sequence affects the phase of the L1 carrier wave. Conceptually the in-phase L1 carrier can be thought of as a zero and the reverse phase carrier can be thought of as a one. Each zero or one (in-phase or reverse-phase) of the carrier wave is called a chip. The C/A code is 1023 chips long and it broadcast at 1.023 Mega-chips per second. Therefore, the C/A code repeats every millisecond. The broadcast travels at the speed of light, so each chip is about 293 metres long and the whole sequence is about 300 km. There are 1540 cycles of the L1 carrier-wave for each chip and the frequency of the L1 carrier wave is 1.575GHz. Good GPS receivers can usually measure the pseudorange to about 1m or better. Several satellites averaged together can give differentially corrected positioning to 50cm or better. The GPS receiver can also measure the angle of the carrier wave, called the carrier phase. This can be measured to better than 1mm. On L2 civilian GPS receivers can only measure the carrier phase of the carrier wave. The actual information on the L2 frequency is P code; it is encrypted and only available to military users. Using the carrier phase of the L2 signal and all the information on the L1 signal it is possible for a differential GPS receiver to achieve accuracies of 1cm or better.
CAN, Controller Area Network A Controller Area Network, or CAN bus, is a network designed for sending many signals, like voltages, over a few wires. A CAN bus can support at least 2048 different signals using the standard identifiers, or over 500 million signals using the extended identifiers. At top speed a CAN bus can update nearly 100,000 signals with 16-bit resolution. The CAN bus guarantees to transmit the highest priority signals first, lower priority messages wait until the CAN bus is available. One main advantage of the CAN bus over other serial formats, like Ethernet or RS232, is that the technique for transmitting the signals is defined. No software is required to decode the signals on the CAN bus, so all devices on the CAN bus can send and receive without worrying about how the data is encoded. The way that the CAN bus encodes the data makes it ideal for real-time signals, like voltages from an ADC, but it is not suitable for sending an email or transferring a file. It is possible to put many nodes on a CAN bus. Each node is a device that can send or receiver a signal (or both). Each node has a CAN bus controller that monitors the CAN bus for signals that it wants to receive and drives the CAN bus with a signal that it wants to send. A CAN bus uses two wires to transfer the data between the nodes. These are normally referred to as CAN High and CAN Low. One of the wires goes to the CAN High pins of all the nodes and the other wire goes to the CAN Low pins of all the nodes. The two wires must form a twisted pair and should have an impedance of 120R. The wires should be terminated at each end using a 120R resistor. In practice it is common for only one 120R resistor to be fitted to short CAN busses. Although technically a ground is not required, many CAN bus drivers do not have a large enough common-mode voltage range. When two devices have a large variation in voltage between them then the CAN bus can fail to work. It is always best to connect the ground between the two systems. Some CAN bus devices are isolated, meaning that their ground and the CAN bus ground are separate. This is not strictly necessary for short CAN busses but can be useful for longer ones. The signals on the CAN bus are encoded in to messages. If the analogy of the signal is a wire, the analogy of the message is a cable containing several wires; the analogy of the CAN bus is a big cable containing lots of small cables (that contain several wires). Each message had an identifier. The analogy of the identifier is a marking on the “message cable”, like marking the cable with a number. The signals inside the message can range from 1-bit all the way to 64-bits. A message cannot be longer than 64-bits, so no signal can have more than 64-bits of precision. If the signals are 16-bits then 4 signals can be carried by the message. Similarly, if the signals are 8-bits then 8 signals can be carried by the message. Signals of different precisions can be assembled in to one message too, so long as the message is not longer than 64-bits. Also, the length of a message is always a multiple of 8-bits. Specifying a signal on the CAN bus requires several bits of information: * Whether the message identifier is standard (11-bits) or extended (29-bits) * The message identifier itself * The Start Bit of the signal in the message * The Length or End Bit of the signal in the message * The encoding of the Signal in the message (signed integer, unsigned integer are most common). It is also customary to provide the scaling and offset of the encoding so that the signal can be scaled into its proper units. A CAN DBC file, or CAN Database is a common format used to hold all the information on a CAN bus. If there are 200 or more signals on a CAN bus that you are interested in then it will take a very long time to enter them all in to your acquisition system. A company called Vector designed a database format that is very commonly used to specify all the messages and signals. Most data acquisition systems can use load Vector’s CAN DBC format. If your sensor comes with a CAN DBC file then you will not need to load all the messages and signals yourself. The CAN DBC files are available for the RT3000.
Inertial Measurement System An Inertial Measurement System can be anything ranging from a simple accelerometer to a full, 6-axis, Inertial Navigation System. Typically an Inertial Measurement System is different to an Inertial Navigation System because it does not navigate or compute position. An Inertial Measurement System might typically compute orientation, accelerations and angular rates
Inertial Measurement Unit 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.
Multipath Multipath is when the signal from the GPS satellites can be received both directly and as a reflection from a building, trees or other objects. Multipath can cause errors in the GPS measurement and the best performance of GPS is always in an open sky environment, with no reflections from other sources. Common sources that cause a problem are the ground, buildings, trees, bridges, signs, etc. There are several ways that Multipath can be reduced. Placing the antenna directly on a metal surface removes most of the reflected signals from the ground. Having the antenna spaced a few centimetres from a metal surface can substantially increase Multipath. Using a test site that is free from trees, buildings, cliffs, etc. also improves the performance of the GPS measurements. Some antennas (like the GPS-600 and GPS-700 series) include a very effective ground plane. These antennas do not need a metal surface close to the antenna and can be used very effectively above the ground or above a metal surface. The GPS cards in the RT3000 also have very sophisticated algorithms for removing Multipath. Multipath caused when the reflected signal is much longer (e.g. 30m or more) has very little influence on the measurements. But when the multipath comes from a closer object then it can influence the range measurement by as much as 5 metres. Influences on traditional GPS receiver could be as much as 70 metres.
Stabilized Platform A Stabilized Platform normally refers to a system that can measure yaw, pitch and roll. These angles can be used to rotate acceleration measurements so they are in the horizontal plane. Traditional Stabilized Platforms actually held the accelerometers gimbals so that they pointed in horizontal directions regardless of how the vehicle rotated. Modern Stabilised Platforms are generally Strapdown Systems rather than true Platform Systems.
Standard Positioning Service, SPS The term SPS, or Standard Positioning Service, refers to a civilian GPS receiver that has no differential corrections. The receiver is using information broadcast from the GPS (NAVSTAR) satellites alone.
Strapdown Navigator A Strapdown Navigator takes the inertial rotations and inertial accelerations and integrates them to give orientation, velocity and position. The Strapdown navigator also knows about the earth’s shape, gravity, earth rotation. As the vehicle travels across the earth’s surface, the angle of gravity changes; the strapdown navigator corrects the roll and pitch to compensate for this rotation (known as the transport rate).
Strapdown Strapdown is a term used to describe how the accelerometers and angular rate sensors are configured in an Inertial Navigation System or in a Stabilized Platform. When the accelerometers and angular rate sensors are mounted on a fixed, rigid block then the system is called a strapdown system. A computer is used to rotate the measurements made by the accelerometers so that they are in North, East and Down directions. Then the acceleration measurements can be used for navigation. This is different to a platform system where the accelerometers are held in gimbals; the accelerometers are kept so that they point North, East and Down regardless of how the vehicle turns. The angular rate sensors, or gyroscopes are used to keep the platform pointing in the same inertial direction as the vehicle rotates.
