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A
Absolute Positioning
A positioning mode in which a position is identified with respect to a well-defined coordinate system, commonly a geocentric system. It is in contrast to relative positioning. (A geocentric system is one whose point of origin coincides with the center of mass of the earth).
Accuracy
Accuracy of GPS fixes are expressed in terms of statistical probability which can be confusing and frustrating for people – many of whom would prefer to talk in facts. A GPS receiver doesn't know the actual position it is trying to measure so it does its best in terms of its specifications, averaging and other techniques to arrive at its best estimate. Manufacturers then measure their receiver's performance and express the accuracy on product and spec sheets in probability terms. See also CEP and RMS.
Acquisition of Satellites
This is time it takes a GPS receiver from turn-on to find and receive GPS satellite signals, extract the information required and compute a positional fix. Acquisition times are different for cold starts, warm starts and hot starts. Re-acquisition times are often given to specify the time it will take a receiver lock back on to the satellite signals after an interruption. This is likely to be less than 2 seconds. A-GPS is a technique designed to provide ephemeris information to the receiver via other networks so that the receiver has advance information that tells it where to look for satellites.
Active Antennas
Active antennas have a built-in amplifier powered from the receiver via the central core of the coax cable to generate signal gain. They provide sufficient gain to overcome coax cable losses.
Almanac
A data file that contains orbit information on all satellites, clock corrections, and atmospheric delay parameters. It is transmitted by a GPS satellite to a GPS receiver, where it facilitates satellite signal acquisition within GPS receivers. Almanac data must be acquired before GPS navigation can begin.
Antenna Detection and Protection Circuit
This receiver feature ensures that the receiver continuously monitors the condition of the antenna system, recognizes a failure if one occurs and reports it.
Anywhere-fix
An anywhere fix is the ability of a GPS receiver to start position calculations without being given an approximate location that would help narrow down the search for satellites.
Assisted GPS
Assisted Global Positioning System is where current ephemeris data from another source (eg a network) is received by a GPS receiver to speed up the process of acquiring position.
Attenuation
GPS Signals lose strength (attenuation) as they are conveyed down a cable. Attenuation (signal loss) can be countered by active antennas (preamp included) or in-line amplifiers while providing the proper signal level to the GPS receiver. For longer cable runs an in-line amplifier can also be used with an active antenna.
Attributes
In both GIS and GPS jargon, attributes are values or descriptive characteristics of any feature being fixed. Position itself is not usually described as an attribute although it is.
Availability (Satellite availability)
The number of hours per day that a particular location has sufficient satellites (above the specified elevation angle and less than the specified PDOP value) to make a GPS position fix is known as satellite availability.
B
Backward Compatibility
A general and relative technology term that describes the ability of a new/changed model of a product to fit into the software, communications and sometimes hardware systems that previous models could use - all without much modification. The absence of backward compatibility sets up a discontinuity that could result in various components being unusable.
Bandpass filters
This is a filtering technique that improves a receiver's immunity to other RF signals in a hostile RF jamming environment. The effect of bandpass filters is to enhance performance meaning that the GPS receiver is not wasting effort deciphering irrelevant signals.
Base Station (GPS Base Station)
It describes a static receiver that is set up at a known location that collects data in a form that can be used for correcting the accuracy of positions taken by another GPS. The base station records time stamped error parameters for each visible satellite that is able to be applied to the observations made by the other receiver using special software – or in real time. The accuracy of the non-static receiver(s) is enhanced by the process called differential correction. Base stations or reference stations are also used for cm accurate kinematic surveying techniques.
C
C/A code
The standard (Clear/Acquisition) GPS code; also known as the "civilian code" or S-code.
Clock bias
The difference between the clock's indicated time and true universal time – normally in UTC.
Circular Error of Probability (CEP).
CEP is a measure of horizontal accuracy (2D) often used for a GPS receiver.
Most usually CEP is stated as [radial] distance from a true position inside which 50% of observations are predicted to fall and conversely outside of which the other 50% will fall. Some manufactures provide a 95% CEP as well. CEP does not directly correlate to sigma of RMS but a CEP of 95% is roughly 2RMS.
Coarse Acquisition (C/A)
C/A Code is a spread spectrum direct sequence code that is used primarily by commercial GPS receivers to determine the range to the transmitting GPS satellite.
Cold start
Time required for the first fix from first turn on of the receiver. It is sometimes called the TTFF (time to first fix) and is an important specification for many applications. The ephemeris information held by the receiver will be out of date so it has to download this data and use it.
Colossus RF ASIC™
It is a Trimble product. Colossus is tightly integrated single chip double-conversion radio receiver with pseudo-baseband 1 and Q outputs. It has a low noise amplifier, programmable frequency synthesizer for two clock frequencies (12.5 or 13Mhz), two built in base band samplers, and power down circuitry. It has an on-chip voltage controlled oscillator (VCO) that eliminates the need for external VCO components. The Colossus uses just 20mW of power.
Constellation
It is a configuration of satellites and can refer to either the specific set of satellites used in calculating positions or all the satellites visible to a GPS receiver at one time.
Covert Antennas
Covert is a convenient term to describe applications that require an antenna that achieves good satellite visibility – but where the antenna itself should not be seen. Covert antennas can be incorporated into an external structure (car roof, specially constructed enclosures etc) subject to various design considerations (see our technical documents).
Continuous Tracking Sensitivity
Continuous tracking is a measure of GPS receiver performance and specifies the lower sensitivity threshold at which a receiver will continue to be able to track the satellites it must have for position fixing.
Values for most manufacturers are usually at somewhere between -152dBm and -160dBm (don't be confused with values expressed as dBW which are an additional -30dBm
Cycle Track
This a an operational mode for Lassen GPS that enables satellite tracking but cycles power off in other parts of the receiver to enable a reduction in power consumption. Cycle track typically provides output data every few seconds
D
Datum
A datum is a reference surface (sometimes described as a surface of equipotential but in GPS terms it is also a mathematical model chosen to generally (WG84) – or specifically in terms of a region- best approximate the earth's sea level surface. To make global calculations a geometrical shape is required but because of anomalies in the earth's surface gravity it can't be used. An artificial best-fit mathematical datum is therefore used instead.
dBW vs dBm
The relationship between these two is a constant of 30dBm. So a sensitivity of -185dBW is the same as a sensitivity of -155dBm. Make sure you incorporate the difference in specifications and compare apples with apples– some manufacturers use one and some use the other.
dBW
The Decibel watt or dBW is a unit for the measurement of the strength of a signal expressed in decibels Relative to one watt.
dBm
dBm is an abbreviation for the power ratio in decibel (dB) of the measured power referenced to one milliwatt (mW).
Differential GPS (DGPS)
A technique to improve GPS accuracy that uses pseudo range errors measured by a GPS base station at a known location to improve the measurements made by other GPS receivers within the same general geographic area. The Trimble Lassen iQ for instance provides this accuracy enhancing option. The DGPS corrections can be applied in real time by transmitting the corrections directly to the receiver or it can be carried out at a later date (by processing the two data files together)
Digital Signal Processor (DSP)
The Digital Signal Processor is a general purpose processor that can be used in lieu of a custom GPS chip to provide a true software GPS receiver. This allows for extremely flexible software since the receiver is no longer restricted by hardware design. Trimble uses this in their Resolution T.
Digital Signal Processors are software only GPS. Software receivers are said to be capable of replacing the current ASIC architectures with a flexible and up-gradable design that is cost effective for multi-system, multi-mode, and multi-sensor solutions.
State-of-the-art software receivers are available today for high-end users and offer significant new and innovative features. SDR techniques continue to evolve with GPS and Galileo, using ever more powerful microprocessors and DSPs with inevitable mass market implementation.
Dilution of Precision (DOP)
An indicator of how good the satellite geometry of a particular constellation will be in determining a position. Positions tagged with a higher DOP value generally constitute poorer measurement results than those tagged with lower value.
Disciplined Clock
The disciplined clock is one in which GPS with its long term accuracy and stability is used to discipline (calibrate) a quartz `oscillator which has high short term accuracy b but not the same long term stability.
GPS keeps long-term wander in check and disciplines the inexpensive ovenized oscillator to remove small biases in the frequency. It is then able to maintain its frequency for extended periods of time even during a GPS outage. (see Holdover)
Direct Chipset Integration
Direct integration is where individual GPS chip components (parts of Standalone GPS Module) are used in a host system.
The Colossus and IO chips along with the Trimble First Software are all available for direct integration into OEM products. The advantages are flexibility and cost savings.
Doppler Effect or Shift.
The apparent shift in the frequency of a received radio signal due to the relative motion of the signal source (transmitter) and receiver.
Doppler-aiding
A signal processing strategy that uses a measured doppler shift to help the receiver smoothly track the GPS signal. This allows more precise velocity and position measurement
E
Ephemeris
A description of the path of a celestial body indexed by time (from the Latin word, ephemeris, meaning diary). The navigation message from each GPS satellite includes a predicted ephemeris for the orbit of that satellite valid for the current hour. The ephemeris is repeated every 30 seconds and is in the form of a set of 16 parameters with corrections that account for the irregularities in the orbit caused by the earth gravitational field and other forces.
Ephemeris Errors
Errors which originate in the ephemeris data transmitted by a GPS satellite. Ephemeris errors are removed by differential correction.
E911
In the US the E-911 edict from the FCC (Federal Communications Commission) is the main Location-Based Services (LBS) driver. This edict said that mobile 911 caller location needed to be established for 67 percent of all calls to within 50 meters and 95 percent of calls to within 150 meters for wireless hand phone-based solutions technologies that can be used to locate a caller.
F
Fix
A colloquial and familiar term used when a GPS receiver has been able to receive and decode GPS satellite RF signals and calculate a geographical position. It means that the GPS has calculated a single position in terms of a minimum of latitude, longitude (or grid position), altitude, time and date.
Failure Rates (refer MTBF)
FreqOut
FreqOut™ (Frequency Output) architecture incorporated in Trimble timing products to produce atomic-clock performance for quartz prices.
Flash memory
Flash memory is a form of non-volatile computer memory that can be electrically erased and reprogrammed. Unlike EEPROM, it is erased and programmed in blocks consisting of multiple locations (in early flash the entire chip had to be erased at once). Flash memory costs far less than EEPROM and therefore has become the dominant technology wherever a significant amount of non-volatile, solid-state storage is needed.
FirstGPS®
Trimble patented architecture that consists primarily of two integrated circuits (ICs) and the FirstGPS® Software. The space and power benefits that it supplies occur both in 'measurement platform' products and as a complete PVT solution. First stands for Flexible Integration of Real Time Software Tasks. The FirstGPS software is able to leverage the host CPU and memory to calculate PVT positions. Its design enables it to tolerate time delays up to 5 seconds between interrupt services from the host PC (250 times longer than previously) which is a very useful level of flexibility.
G
Geodetic surveys
Surveys carried out on a global and regional basis to establish control networks (comprised of reference or control points) that then provide a primary basis for accurate local land mapping.
GPS Time
GPS, Global Positioning System time, is the atomic time scale implemented by the atomic clocks in the GPS ground control stations and the GPS satellites themselves. GPS time was zero at 0h 6-Jan-1980 and since it is not modified by leap seconds GPS is now ahead of UTC by approximately 13 seconds.
Gain
In electronics language, the normal use of the word gain is to specify the ratio of the signal output of a system to the signal input where it is increased through the use of an in-line amplifier. Gain is normally measured in dB and a gain of ten says that either the voltage or power is increased by a factor of ten. Gain is a way of overcoming signal attenuation in a cable.
A signal strength at the antenna of -155dBm subject to gain of 10dB will become -135dBm and much easier to process as a result of being stronger.
GIS (Geographic Information System)
A computer based system that is capable of collecting, managing and analyzing geographic spatial data. This capability includes storing and utilizing maps, displaying the results of data queries and conducting spatial analysis.
Global Navigation Satellite System (GLONASS)
This is the Russian counterpart to GPS. GLONASS provides worldwide coverage, however, its accuracy performance is optimized for the northern latitudes and is specified as identical to that of GPS SPS.
Geoid.
It is the equipotential surface of the earth and the gravity field which coincides closely with mean sea level. The geoid is the primary heighting reference surface for surveying and of course for calculating drainage works - but is not much use for GPS because it can't be represent mathematically. GPS receivers use spheroidal models as datums and in particular the standard one of WG84.
Geometric Dilution of Precision (GDOP).
The effects of the combined errors of four variables (latitude, longitude, altitude, and time) on the accuracy of a three-dimensional fix.
Global Positioning System (GPS)
A system for providing precise 3D location which is based on data transmitted from a constellation of 24 active satellites, operated by the US Government. The term GNSS is now more commonly used to include the combination of several Nationally controlled GPS eg Glonass, Navstar and Galileo.
GLONASS
A Russian alternative to Navstar the US GPS System. Not yet fully operational. The satellites transmit on a different radio frequency.
Galileo
The European Union has designed a third GPS system that is predicted to be operational before 2010. Each GPS system will form part of the GNSS system in future.
GNSS
GNSS stands for Global Navigation Satellite System and is a voluntary federation of more than 200 worldwide agencies service committed to providing the highest quality data and products as the standard for Global Navigation Satellite Systems (GNSS). currently the IGS includes two GNSS, GPS and the Russian GLONASS, and intends to incorporate future GNSS like Galileo along with other components to improve accuracy.
Grid
Also "Coordinate System". A coordinate system is a representation of the earth on a flat surface, using parallel lines as a basis for position measurements.
GPS Week
Incremental number of weeks, starting at 0 hours UTC on the date January 6, 1980. For instance week 1393 is the week of 18-25th September 2006.
Ground plane
A ground plane is a surface used in conjunction with a GPS antenna to reduce multipath signals. The purpose is to remove the non-line of sight reflected signals that will have take taken longer to reach the antenna and as a result provide an erroneous signal that degrades the accuracy of positional fix accuracy by the receiver.
GPS Clock
A GPS Clock is a combination of a GPS receiver and a very high-quality and stable oscillator. GPS is used to discipline (calibrate) the oscillator to remove small biases in the frequency. The GPS Clock can synchronize both system timing and transceiver frequency and is virtually fail-safe. It generates timing signals, and never needs to be re-calibrated.
GPS Technology Licences
These are software licences entitling an integrator to use the actual GPS algorithms for digital signal processing. They are part of the real IP of a GPS company. An integrator would load the software onto his processors and no hardware is required.
GPS Chipsets
GPS modules are made up of chips (ICs) that provide navigation calculations, communications and software for processing the PVT. Chipsets are these component parts. An integrator can incorporate chips sets rather than modules to save space, power and cost. The host's processor would be used to perform the calculations.
GPS Modules
A GPS Module is a self contained GPS receiver. It has a navigation platform, communications and software. It is able to perform all function from receiving satellite signals from the antenna through to providing a host computer with a PVT.
H
HS GPS (High sensitivity GPS)
High Sensitivity receivers use extra correlators and digital signal processing to search for GPS signals very quickly. This results in very fast times to first fix when the signals are at their normal levels, for example outdoors. When GPS signals are weak, for example indoors, the extra processing power can be used for weak signals to the point where they can be used to provide a position or timing solution. By the time the signals arrive at the user's receiver they are typically as weak as –160dBW (-130dBm). Outdoors GPS signals are typically around the -155dBW (-125dBm). level.
Conventional GPS receivers integrate the received GPS signals for the same amount of time as the duration of a complete C/A code cycle which is 1ms. This results in the ability to acquire and track signals down to around the -160dBW level. High Sensitivity GPS receivers are able to integrate the incoming signals for up to 1,000 times longer than this and therefore acquire signals up to 1,000 times weaker. A good High Sensitivity GPS receiver can acquire signals down to -185dBW (-155dBm), and tracking can be continued down to levels approaching -190dBW (-160dBm).
High Sensitivity GPS can provide positioning in many but not all indoor locations. Signals are either heavily attenuated by the building materials or reflected in as multipath. Given that High Sensitivity GPS receivers may be more sensitive this is sufficient to track through 3 layers of dry bricks or up to 20cm of steel reinforced concrete for example.
High sensitivity receivers
High-sensitivity GPS receivers are those that claim sensitivities in the -155dBm to -165 dBm range. There are many factors which appear to place both theoretical and practical limits on receiver sensitivity without other adverse effects - like losses in accuracy through multipath signals being included in the positional calculation.
Hard mount Antennas
These antennas typically have a screw thread at the bottom that enable them to be fitted to a metal or other surface and secured with a nut underneath. They suit non-covert, high impact and permanent applications.
Holdover
GPS signals can be lost/interrupted by a receiver for various reasons, such as lightning strikes and damage due to maintenance activities or vandalism. When this happens the amount of time that a GPS timing receiver can continue to operate inside its accuracy specifications before the drift accumulates is called the holdover period. It is dependent on the oscillator's quality and the quality of the algorithms that control the software.
I
Interference and strong-signal susceptibility
When receiver sensitivity is increased, susceptibility to various forms of interference increases. Although not always a problem with receivers of normal sensitivity, where receivers have high sensitivity, measures must be taken to prevent erroneous acquisition of lower-level PN code correlations from undesired signals.
Indoor GPS
This is a term used to describe the broadening of the GPS receiver's range to include signal tracking inside a building. With newer receivers and massively-parallel correlation, a GPS receiver can accumulate many copies of the encoded GPS signal – remembering that the code repeats itself identically every millisecond – so, with enough correlators, a GPS receiver can accumulate a complete copy of the code every millisecond, a thousand copies every second, and so on. In this way the GPS receiver can acquire the encoded signals even when indoors, where the signal is weaker than outdoors. A-GPS becomes a necessity (to provide the equivalent data) for indoor GPS use.
In-line amplifier
An in-line amplifier is a small amplifier inserted into an antenna cable used to generate signal gain. An antenna cable plugs into the in-line antenna and another cable conveys the enhanced signal from output plug to the GPS receiver. Power is normally delivered via the cable and the in-line amplifier is more usually placed nearer the antenna so that it is the signal with the highest signal-to-noise ratio itself that is amplified. An in-line amplifier can also be used with an active antenna.
J
Jam Resistant Antennas
In harsh RF environments (RF interference on or near the frequency being used) weak GPS signals can be overridden – or jammed. Jam resistant antennas are those with filtering abilities that remove the peripheral frequencies and allow the GPS signals to be used for their intended purpose.
K
Kalman Filter.
A Kalman filter software program estimates the time error, frequency error, frequency drift and Keplerian orbit parameters for each of the satellites and its operating clock. It is an optimum mathematical procedure for recursively estimating dynamically changing parameters, such as the position and velocity of a vessel, from noise- contaminated observations.
L
Latency
Latency refers to delays in signal transmission. Any transmission that is not instantaneous has a level of latency. The amount of latency is important because 'old' data can lose its relevance and result in calculation errors or in some cases unusable data because the time window has passed. Lag is a u synonym for latency.
For cm-accurate GPS (kinematic) latency is a critical issue because real time data is being exchanged between GPS receivers. Latency in computer, radio and communications networks is what Timing Synchronisation GPS is used to manage.
L-band
The group of radio frequencies extending from 390 MHz to 1550 MHz. The GPS carrier frequencies L1 (1575 MHz) and L2 (1227.6 MHz) are in the L-band.
L1 Frequency Band.
One of the two ultra high radio frequencies transmitted by NAVSTAR GPS satellites. This carrier frequency of 1575.42 MHz transmits both C/A (Standard Positioning Service) and P (Precise Positioning Service) code.
L2 Frequency Band.
One of the two radio frequencies transmitted by NAVSTAR GPS satellites. This carrier frequency of 1227.6 MHz carries the P (Precise Positioning Service) code only.
Location based services
A broad category of emerging business activity in which location (as in a geographical position) is a vital part of the information solution being offered to users.
Local time
Local time is the date/time in terms of UTC for a designated time zone.
Low noise amplifier
A particular type of amplifier always used in GPS antennas designed for amplifying an inbound signal without introducing extraneous signals without allowing out-of-band interference.
M
Mask Angle
It is possible to configure a GPS receiver to ignore (mask) the signals sent from satellites that are not a minimum distance above the horizon in order to reduce the errors associated with signals that travel tangentially to the earth and acquire various distortions owing to the much increased distance run through the atmosphere or through obstructions like buildings, vegetation or topography. The minimum angle above the horizon that is set in the receiver to exclude these low satellites signals is the mask angle.
Measurement Platform
The measurement platform (often a chipset) is one part of a GPS receiver module (eg a GPS standalone module) The GPS measurement data is used by the I/O chip and software in a module – or is passed to the host computer and memory in chipset solutions for the computation of accurate PVT solutions – for instance using the Trimble FirstGPS software.
Multipath
This is the term used to describe interference caused by reflected GPS signals arriving at the receiver by two or more paths, typically as a result of reflection off nearby structures or other reflective surfaces but also from atmosphericducting, ionospheric reflection and refraction, and reflection from terrestrial objects, such as mountains. The GPS has to decipher the confused signals and the position is usually degraded in accuracy.
Multiplexing channel
A channel of a GPS receiver that can be sequenced through a number of satellite signals. Recent GPS receivers employ parallel channels with each dedicated to a separate signal.
Microstrip Antenna.
A type of antenna commonly used with GPS receivers. It is usually constructed of one or more (typically rectangular) elements that are photo etched on one side of double - coated, printed-circuit board. It is often called a patch antenna.
MTBF (Mean time between failures)
this is an extremely important manufacturer's statistic for integrators and developers because of the protection it provides against product failure. Thunderbolt Disciplined Clocks from Trimble for instance have MTBF figures of 200,000 hours. MTBF is calculated to a standard and set of rules and is not arbitrarily set by a manufacturer. Trimble MTBF figures are very high by GPS industry standards.
Multipath Fading
Often in poor signal environments, like indoors, the signal will have large and/or numerous multipath components. This can cause pseudo range biases as well as reduce receiver sensitivity (when phase cancellation of the signal occurs). Multipath fading is a term to describe fluctuations from this source.
N
NAVSTAR
The name given to the US GPS satellite system. NAVSTAR is an acronym for NAVigation Satellite Timing and Ranging. GLONASS and GALILEO will be alternatives. It is part of the GNNS.
Navigation Message
A 37500-bit data message included in the GPS signal. The message, sent at a rate of 50 bits per second, includes the satellite ephemeris, clock data, almanac, and other information about the satellites and their signals
NMEA 0183
A communication protocol used by GPS units and other types of navigation and marine electronics NMEA stands for National Marine Electronics Association and is the association that sets standards for interfacing nautical electronic devices, including Global Positioning System receivers.
Network Time Protocol (NTP)
The Network Time Protocol (NTP) is an Internet Standard Recommended Protocol for communicating the Coordinated Universal Time (UTC) from special servers called time servers and synchronising computer clocks on an IP network. It is used at the time coordinating standard on networked computers that share resources such as files and emails so they are synchronised. With inadequate time coordination an email could appear to arrive before being sent. Thus the time stamp is critical. GPS time is used to coordinate first level (stratum) servers but other stratums below this are subject to certain degradation in time accuracy.
O
One sigma (One RMS or one Standard Deviation)
This is an accuracy statistic which is part of the Standard Distribution model (the familiar Bell curve). The Standard Deviation model shows that multiple measurements (or in this case multiple GPS fixes) will group themselves in statistically predictable ways.63% of all fixes will fall within one Sigma (One SD) of the mean (the real value) and 95% within 2 Sigma,98 8% within 3 Sigma. The user has no way of determining whether a particular GPS fix for instance is within 1, 2 or 3 Sigma hence the frustrating fact that GPS accuracy is statistical in nature and at best is a probability. In the course of testing receivers manufacturers derive the distance from the mean that includes 63% of the fixes. They can then present these as an accuracy specification eg Receiver X will achieve a horizontal accuracy of 3m RMS (1Sigma)
OEM
Original equipment manufacturer. In addition to what they manufacture themselves OEMS buy in 3rd party chips, modules and components to provide functionality or complete the product.
Overdetermined Time Mode
Only one satellite signal is actually required to provide time but the best receivers continue to use data from other satellites as well. They run their own algorithms to refine the time to a greater accuracy than that provided by one satellite...
P
Patch antenna
An antenna based on a plated section of metal on a circuit board and it's associated electronics used for receiving GPS signals
P-code
The Precise or Protected code. A very long sequence of pseudo-random binary biphase modulations on the GPS carrier at a chip rate of 10.23 MHz, which repeats about every 267 days. Each 1-week segment of this code is unique to one GPS satellite and is reset each week.
Passive Antennas
Passive GPS antennas do not have a built-in amplifier and suit GPS applications requiring only a short distance (between a couple of centimeters and a meter) between the GPS receiver and antenna. Passive antennas also require an LNA on the GPS receiver.
Position
A latitude, longitude, and altitude which may be expressed in these terms or converted to a projection as a coordinate.
Positional Dilution of Precision (PDOP)
Measure of the horizontal geometrical strength of the GPS satellite configuration. The PDOP is calculated from how optimal the satellite positions are for calculating a robust position and other factors. It can also be said to be a reliability estimation. PDOP is usually expressed as a ratio. The receiver can be configured to exclude signals outside of the parameters set.
Post-processed differential GPS
In post-processed differential GPS the base and roving receivers have no active data link between them. Instead, each records the satellite observations that will allow differential correction at a later time. Differential correction software is used to combine and process the data collected from these receivers.
Pseudo-lite
A ground-based differential GPS receiver which transmits a signal like that of an actual GPS satellite and can be used by GPS receivers for obtaining a fix.
Pseudo-range
A distance measurement based on the correlation of a satellite transmitted code and the local receiver's reference code, that has not been corrected for errors in synchronization between the transmitter's clock and the receiver's clock.
Path loss
The attenuation of the line-of-sight signal as it moves from satellite to user
Pick and Place
Pick and place is a board assembly technique that suits larger production runs and factories with higher expertise.
PVT
Position, velocity and time are the three most important values that GPS generate. Some GPS are optimized for the most accurate timing output but all GPS do provide PVT. PVT is normally applied to GPS for which position is the main requirements.
Protocols
A protocol is a convention [standard] that controls or enables connections, data transfer, and communications between two computing endpoints. A protocol can be defined as the rules governing the syntax, semantics, and synchronization of communication. Protocols may be implemented by hardware, software, or a combination of the two. At the lowest level, a protocol defines the behavior of a hardware connection. Protocols are generally used to define real-time communications behavior.
Pulse per Second Output
A Pulse per second (PPS) is an electrical signal that very precisely indicates the start of a second. PPS signals are output by various types of precision clock, including GPS receivers designed for timing synchronisation. Depending on the source, properly operating PPS signals have an accuracy ranging from a few nanoseconds to a few milliseconds
Q
Quantisation
In digital signal processing quantization is the process of approximating a continuous range of values by a relatively-small set of discrete symbols or integer values. A common use of quantization is in the conversion of a sampled continuous signal into a digital signal by quantizing. Both of these steps (sampling and quantizing) are performed in analogue to digital converters with the quantization level specified in bits. A specific example would be CD audio which is sampled at 44,100 Hz and quantized with 16 bits.
QoS
In the fields of packet-switched networks and computer networking, the traffic engineering term Quality of Service (QoS) refers to the probability of the telecommunication network meeting a given traffic contract, or in many cases is used informally to refer to the probability of a packet succeeding in passing between two points in the network within its desired latency period.
R
Real-time differential GPS
A base station which computes, formats, and transmits corrections usually through some sort of data link (e.g. VHF radio or cellular telephone) with each new GPS observation. The roving unit requires some sort of data link receiving equipment to receive the transmitted GPS corrections and get them into the GPS receiver so they can be applied to its current observations.
Reaquisition time
Re-acquisition times are often given to specify the time it will take a receiver to lock back on to the satellite signals after an interruption. This is likely to be less than 2 seconds. A-GPS is a technique designed to provide ephemeris information to the receiver via other networks so that the receiver has advance information that tells it where to look for satellites.
RTCM SC-104.
The special committee of the Radio Technical Commission for Maritime Services that developed recommended standards for DGPS.
RINEX
Receiver INdependent EXchange format. A set of standard definitions and formats to promote the free exchange of GPS data and facilities the use of data from any GPS receiver with any software package. The format includes definitions for three fundamental GOS observables: time, phase, and range.
RF Jamming
The broadcast of a frequency (or near that frequency) that is being used by another service eg GPS will stop reception of the service. Manufacturers have to build filters in antennas or circuitry to filter out the unwanted frequencies.
RG 59 Cable
RG-59/U is a specific type of coaxial cable, often used for low-power RF signal connections like GPS antennas. The cable has a characteristic impedance of 75 ohms. It can be used at base band video frequencies or, for short distances, at broadcast frequencies. Its high-frequency losses are too great to allow its use over long distances at broadcast frequencies; in these applications, RG-6 is used instead.
RG 58
As for RG 59 - coaxial cable with an impedance of 50 or 52ohms Ω
Reference signal - GPS
The GPS clock reference signal is a stable and highly accurate time reference that doesn't drift over time.
RAIM (Receiver Autonomous Integrity Monitor)
This is essentially a fault detection scheme that is applied to the GPS measurements to ensure signal integrity.
RoHS and RoHS compliant
The RoHS (Restriction of Hazardous Substances) Directive, came from the European Parliament in January 2003. It specified the restriction of the use of certain hazardous substances in electrical and electronic equipment. The RoHS Directive requires manufacturers to reduce or eliminate Lead (Pb), Mercury (Hg), Hexavalent Chromium (Cr6+), Cadmium (Cd), Poly Brominated Biphenyls (PBB) and Poly Brominated Diphenyl Ethers (PBDE) from their products sold in the EU starting July 1st, 2006.
S
Signal sampling by the receiver
SNR can account for signal attenuation of 5-6dBm. Receiver bandwidth if large enough can avoid SNR loss. However the tradeoff requires higher sampling rates with a corresponding increase in power consumption and processing loads, a factor which is detrimental to low-cost, low power consumer applications.
Signal to Noise ratio (SNR)
The ratio of a given transmitted signal to the background noise of the transmission medium.
Starter Kits
Starter Kits are a way of helping integrators get a running start when developing an application with GPS. They include RF signal traces, RF connectors, I/O connections, power converter, power adaptor, GPS antenna and communication software – in fact everything that is required for the test and development phase.
SAW (Surface Accoustic Wave) Filters
A circuitry filter used by some manufacturers that is designed to remove interference around the main GPS frequency (1575Mhz) in the form of splatter and secondary harmonics. It is in effect a selective jamming technique similar to bandpass.
Satellite constellation
The arrangement in space of a set of satellites whose signals are being received by a GPS receiver. The relationship of the satellites to each other and the receiver has great significance for signal reception, reliability and hence the accuracy of the positional fix that the receiver eventually makes.
Selective availability (S/A)
Intentional degradation of the performance capabilities of the NAVSTAR satellite system for civilian use by the U.S. military, accomplished by artificially creating a significant clock error in the satellites. Turned off in 2000 but can be re activated in time of conflict.
Standalone GPS Modules
A standalone module provides the entire GPS function from tracking the GPS satellites to computing the finished position, velocity and time (PVT solutions) which are then communicated to the host computer for use in the overall solution.
Static positioning
Location determination when the receiver's antenna is presumed to be stationary in the earth. This allows the use of various averaging techniques that improve the accuracy by factors of over 1000.
Schedule Track
This is an operation mode that enables developers and integrators to schedule when a receiver will power up from a sleep mode and output a position. It provides hot start performance but very low power consumption.
SCADA
SCADA is the acronym for Supervisory Control And Data Acquisition. It usually refers to a central system that monitors and controls a complete site.
Surface Mounted GPS Chips
Surface mount technology (SMT) is a method for constructing electronic circuits in which the components are mounted directly onto the surface of printed circuit boards (PCBs). Electronic devices so made are called surface-mount devices or SMDs. It is not suitable for small runs because set up costs and it is not suitable for plants without adequate expertise. In particular the of connections have to be of a high quality. The Trimble embedded module product suitable for surface mounting is Copernicus. The previous construction method of fitting components with wire leads into holes in the circuit board (also called through-hole technology) is not used so much. An SMT component is usually smaller than its leaded counterpart because it has no leads or smaller leads. It may have short pins or leads of various styles, flat contacts, a matrix of balls (BGAs), or terminations on the body of the component (passives).
Smart Antenna
Smart antennas have both receiver and antenna in the same housing. The proximity of the receiver and antenna overcomes difficulties of keeping signal purity up and overcoming interference. The position calculations do not lose their integrity when transmitted.
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TTFF
Time To First Fix - The time it takes for a GPS receiver to lock onto the satellite signals and determine the initial position.
Time synchronization and its problems
In assisted GNSS systems designed for rapid positioning (a few seconds) where weak signals are used, the receiver does not have enough time to read unambiguous time from the received signal itself.
Timing accuracy
Timing accuracy is simply a measure of the amount that the timing signals vary from the mean. For GPS and GPS synchronization the accuracy is reported in nanoseconds. The best accuracies are UTC +/-20 nanoseconds (one sigma or 66% of the signals will fall inside this accuracy probability.)
TSIP (Trimble Standard Interface Protocol)
This protocol is based on the transmission of packets of information between the user equipment and the unit. Each packet contains an identification code that identifies the meaning and form of the data that follows.
Transition cables
Where a chip or module is not directly placed onto a PCB of an integrators application, such that all of its RF, power, antenna etc connections are made directly, the GPS module will be connected to an antenna using a cable known as a 'transition cable' or 'tail'.
T-RAIM (Time-Receiver Autonomous Integrity Monitor)
This is a fault detection scheme that is applied to the GPS measurements for PPS integrity.
TrimCore™ Software Technology
One of the Patented technology innovations from Trimble that is an evolutionary software approach to segmenting GPS into modular, reusable building blocks that more closely meet customer resource requirements to implement GPS whether it is on Digital Signal Processors (DSPs), microprocessors or GPS chipsets.
TrimCore™ GPS Navigation Engine (NE)
A software license product in the TrimCore™ family of Trimble core Global Positioning System (GPS) software technology. An innovative licensed software positioning product. TrimCore is an evolutionary software approach to segmenting GPS into modular, reusable building blocks on Digital Signal Processors (DSPs), microprocessors or GPS chipsets.
Trimble
One of the original pioneering GPS companies given its name by Charles Trimble in the early 80s.Trimble has more GPS patents than any other manufacturer and is a world leading GPS company with large product ranges and high market shares in specific sectors and industries including component technology, construction, surveying, mapping, tracking and agriculture.
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UTC
Universal Time Coordinated is the international time standard and was formerly called Greenwich Mean Time [or GMT] which it replaced in 1972. It differs from local time by the number of hours of your time zone diverges from Greenwich and is approximately 13 seconds different to GPS time. UTC is occasionally corrected by the insertions of leap seconds so it is approximately synchronized with the earth's rotation.
Universal Transverse Mercator (UTM)
A commonly used and specific grid coordinate system onto which spherical global sections can be projected to enable calculations specific zones.
Urban canyon
An urban environment with numerous multi-story buildings that reduce the number available to a receiver and increase multipath - hence making computation of a position both more difficult and less accurate.
Update Rate
The GPS receiver update rate is the number of times it sends the position information per second. An update rate of 1 Hz will send out the position information once every second, a 5 Hz receiver will send out the position information five times per second.
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Voltage options for GPS receivers - 3.3Vdc or 5Vdc?
5Vdc was the original standard for GPS. However subsequent technology advancements have reduced the overall impedance in the GPS modules (and chips) and enabled the voltage to be reduced. The standard value chosen was to 3.3Vdc. Some receivers are made to accept only specific voltages while other have a design that can tolerate a range. Wi-Sys Communications make all of their antennas capable of using 2.7 -5.0 Vdc. Other receivers (the Trimble Thunderbolt) can use 19-34 Vdc
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WGS84
World Geodetic Survey 1984. The geodetic map datum reference onto which GPS receivers compute their position in the first instance. WGS 84 is the datum that provides the best general fit for the earth's surface.
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