Technology
GPS/GNSS Glossary

  • For Timing

GPS/GNSS receivers can calculate not only the position but also the accurate time by receiving satellite signals. Accurate time information is used in infrastructure such as cell phone base stations, digital terrestrial broadcasting stations, and wireless systems that require time synchronization and accurate frequency.
This glossary mainly focuses on time and explains various associated terms. In addition, related general GPS/GNSS terms will also be explained. The content relates to Furuno's GNSS timing and frequency receivers GF/GT-88 series.

Please note that a"*" denotes a Furuno's original terminology.

Terms related to satellite systems and satellite signals

GNSS
Global Navigation Satellite System (GNSS) is a general term describing any satellite constellation that provides Positioning, Navigation and Timing (PNT) information data on a global basis. GPS, Galileo, GLONASS and BDS/BeiDou are Global Navigation Satellite Systems.
Navigation Satellite System covering a more limited area, such as QZSS and NavIc, are classified as Regional Satellite Navigation Systems (RNSS).
ICD
An Interface Control Document (ICD) is a formal document that details the interface design information of a GNSS program. Each operator of a satellite constellation publishes its own ICDs. GNSS receivers are generally designed with reference to these documents. ICDs are subject to revisions, and some of the published content may be changed accordingly.
Satellite ICD
GPS IS-GPS-200
Revision H,IRN003
28 July 2016
GLONASS Navigation Radiosignal in Bands L1, L2
Version 5.1
2008
Galileo EUROPEAN GNSS(GALILEO) OPEN
SERVICE
SIGNAL-IN-SPACE
Issue 1 revision 3
December 2016
QZSS L1C/A IS-QZSS-PNT-002
29 January 2018
QZSS L1S IS-QZSS-L1S-002
13 April 2018
Satellite number
The satellite number is the number assigned to a GNSS satellite.
In order to distinguish the constellation of a satellite from those with duplicate satellite numbers, the GNSS System ID can be used. This information is indicated in the last field of the GSA-sentence and also as the Talker ID of the GSV-sentence.
Satellite Nb. MIN MAX Notes
GPS 1 32 Same as the PRN No
GLONASS 65 96 Same as the PRN No
Galileo 1 36 Same as the PRN No
QZSS L1C/A 93 99 PRN No minus 100
QZSS L1S 83 89 PRN No minus 100
SBAS 33 51 PRN No minus 87
SBAS
Satellite-Based Augmentation System (SBAS) is a regional geostationary satellite system that provides GNSS correction data to improve the precision of the position and time of GNSS receivers. Geostationary satellite systems like MSAS (Japan), WAAS (USA), EGNOS (EU), GAGAN (India) and some others are SBAS.
SLAS correction information
Sub-meter Level Augmentation Service (SLAS) correction information is transmitted by QZSS satellites on the L1S signal to reduce ionospheric delays, and orbit, clock and other errors. These corrections can only be used by GNSS receivers capable of receiving the QZSS L1S signal.
Ephemeris
An ephemeris is a type of short-lived information broadcasted by GNSS satellites. It mainly provides the satellite time and position which are needed by GNSS receivers to calculate the position and time. GPS satellites, for example, broadcast their ephemeris every 30 seconds, which are usually valid for 4 hours.
Starting a GNSS receiver with valid backup ephemeris and time is known as HOT START.
Almanac
An almanac is a type of information broadcasted by GNSS satellites. It mainly provides rough orbital and status information for each navigation satellite of the constellation, various corrections and parameters to correlate the constellation time to UTC time. GPS satellites, for example, broadcast the full almanac every 750 seconds.
Starting a GNSS receiver with backup almanac is known as WARM START.
Starting a GNSS receiver with neither ephemeris nor almanac information is known as COLD START.
Jamming signal
Jamming signals are electrical noises generated by other equipment or radios that are intentionally broadcasted by malicious actors with the intend to affect the performance of near-by GNSS receivers.
Jamming signals will interfere with the reception of genuine GNSS satellite signals, which may result in poor or failed positioning.
Anti-Jamming
Anti-jamming is the function that helps a GNSS receiver to sustain normal operation by minimizing the influence of jamming signals.
Spoofing signal
Spoofing signals are signals generated by malicious actors that mimic genuine GNSS satellite signals and carry specially crafted information with the purpose to steer GNSS receivers to calculate wrong position, velocity and time information.

Terms related to time

UTC time
UTC stands for Coordinated Universal Time and is the primary time standard in the world. It includes leap seconds and is the reference of the time we use on daily basis, that is without taking into account any time zone and daylight saving adjustments.
The UTC time is determined based on a network of atomic clocks located in many different countries all around the world. The time difference between all these reference atomic clocks is on the nanosecond scale.
For example, in the case of the United States, the UTC time is set by the United States Naval Observatory and is called the UTC(USNO). Similarly, in the case of Russia, it is called UTC(SU).
Leap second
A leap second refers to the occasional addition of a one second to the Coordinated Universal Time (UTC) in order to maintain its synchronization with the earth rotation.
In general, a decision on a leap second insertion is made at least one to two months before its implementation, and is announced by the IERS and broadcasted by the different national time keeping agencies; for example by the NICT in Japan. A leap second insertion is usually scheduled at the end of a quarter, but preferably on January 1st or July 1st.
Leap second insertions have taken place since 1972. Yet considering that the GPS and the QZSS satellites are operating with a starting date of January 6, 1980, the total leap second is sometimes simply referred to as leap seconds and accounts only the insertions since January 6, 1980.
Default leap second*
Some GNSS receivers allow users set the default leap second, which will be used by these receivers to compute UTC time before the leap second information is received from GPS satellites.
The default leap second information is usually backup in memory by these GNSS receivers allowing them to use this information at all consecutive starts.
UTC parameters
UTC parameters are information broadcasted by each GNSS satellite to allow converting a GNSS time system into UTC time. UTC parameters mainly include the leap second difference between a GNSS time and UTC time, the leap second insertion timing and the nanosecond scale correction information.
The GLONASS satellites do not broadcast a leap second difference as the GLONASS time system includes leap seconds and is therefore already aligned with UTC time.
The UTC parameters are included in a group of navigation messages, commonly referred to as an almanac, broadcasted at a regular time interval.
GPS Time
GPS Time is the time system of GPS and is broadcasted by the GPS satellites. It is a continuous time system that started on January 6, 1980 at midnight UTC. It does not take into account any leap seconds.
The GPS Time is broadcasted following the concept of the Time Of Week (TOW) expressed in seconds and the week number. The TOW is broadcasted every 6s with a resolution of 1.5s and ranging from 0 to 403,199. The GPS week is broadcasted every 30s and ranges from 0 to 1024, effectively rolling over every 19.8 year (approximatively). After receiving both information GNSS receivers can compute the current GPS time, modulo the GPS week epoch.
The GPS satellites broadcast UTC parameters, allowing GNSS receivers to also compute the current UTC(USNO) time.
In the last many years, the GPS Time is kept aligned to the UTC(USNO) time (modulo the leap seconds) within a few nanoseconds. There is no guarantee that this amount of error will be maintained in the future.
GLONASS Time
GLONASS Time is the time system of GLONASS and is broadcasted by the GLONASS satellites. It is synchronized with UTC(SU) minus 3 hours and includes leap seconds.
GLONASS Time is broadcasted in a format allowing GNSS receivers to uniquely compute the current date and time without having to account for any week number rollovers, unlike with the GPS Time.
Since the GLONASS Time includes leap seconds, the correct alignment to UTC(SU) including the leap seconds is obtained by GNSS receivers as soon as the GLONASS time is decoded without having to wait for the reception of the UTC parameters.
The GLONASS satellites also broadcast the time difference between the GLONASS Time and the GPS Time, simplifying the time alignment between two systems for the GNSS receivers.
Galileo time
Galileo Time is the time system of Galileo and is broadcasted by the Galileo satellites. It is a continuous time system that does not take into account any leap seconds and that started on August 22, 1999 at 00:00:00, 13s ahead of UTC time. Galileo Time, GPS Time and QZSS Time are consistent with one another.
Galileo Time is broadcasted in a format allowing GNSS receivers to uniquely compute the current date and time without having to account for any rollovers until February 19, 2078. Like for the GLONASS Time, this is a significant advantage compare to the GPS and QZSS Times.
The Galileo satellites broadcast UTC parameters, allowing GNSS receivers to compute the current UTC(EU) time. They also broadcast the time difference between the Galileo Time and the GPS Time, simplifying the time alignment between two systems for the GNSS receivers.
Week number rollover
GPS/QZSS satellites broadcast an incremental week number from 0 to 1023. The week number that follow week 1023 is again week 0. This event is called week number rollover.
Typical GNSS receivers when only relying on GPS and QZSS satellites are known to have a limited period of time (1024 weeks - about 19 years and 8 months) in which they can properly convert the time received from these satellites to the current date and time.
Such GNSS receivers, per their manufacturer design, have their own limited period of time in which they are able to establish the correct date and time if they operate without backup. Once the limited period of time has past, upon a restart without backup, such GNSS receivers may display a date and time of 1024 weeks ago or of a multiple of 1024 weeks ago. In this case it is possible to set the correct date and time by sending a command to the GNSS receivers or by enabling the use of GLONASS and/ or Galileo satellites if supported. In the case of a continuous operation, these GNSS receivers correctly maintain the current date and time upon a week number rollover.
Local Zone Time (LZT)
LZT stands for Local Zone Time.
It gives the time offset value between UTC time and the local time.

Terms related to GNSS receiver PPS and frequency outputs

PPS
PPS stands for Pulse Per Second. The output of a GNSS receiver providing a one pulse per second is called 1PPS. 1PPS is equivalent to a 1 Hz clock.
In timing applications, the 1PPS edge is precisely synchronized with UTC time or GPS Time (or any other supported GNSS Times), providing a highly accurate time information to the outside world.
VCLK PPS*
The VCLK PPS is a PPS whose edge is synchronized with UTC time (or any other supported GNSS Times) and serves as the reference for the clock edge of the VCLK frequency.
VCLK frequency*
The VCLK frequency is the frequency outputted by the onboard voltage-controlled oscillator.
While receiving GNSS satellite signals, this frequency is adjusted based on the time acquired from GNSS satellites to provide a stable clock. Some GNSS receivers can continue to guarantee for a period of time the clock stability in free-running state, when GNSS satellite signals can longer be received (Holdover function).
GCLK frequency*
The GCLK frequency is an arbitrary frequency generated by a GNSS receiver from its system clock and built-in accumulator.
While receiving GNSS satellite signals, the arbitrary frequency is adjusted based on the time acquired from GNSS satellites to provide a stable and accurate clock.
As such arbitrary frequencies are generated using an accumulator, it is important to consider whether the jitter and spurious contained in the GCLK frequencies are acceptable for the intended application.
GNSS synchronization
GNSS synchronization is the state in which GNSS satellite signals are received and the oscillator is synchronized with UTC time or a given GNSS time. This is the counterpart to EPPS synchronization.
GPS synchronization
GPS synchronization is the state in which the time, the PPS and the VCLK frequency are all synchronized with GPS time. A GNSS receiver will transition into GPS synchronization when either set in this state or when the UTC parameters are not received.
UTC synchronization
UTC synchronization is the state in which the time, the PPS and the VCLK frequency are all synchronized with UTC time. It can be selected by command which particular UTC to synchronize with.
EPPS synchronization*
EPPS synchronization is the state in which the PPS and the VCLK frequency are not synchronized with UTC time or a GNSS time but with an external 1PPS provided on the EPPS input of the GNSS receiver.
RTC synchronization
RTC stands for Real Time Clock. The expression RTC synchronization is sometimes used to indicate that the PPS and/ or the VCLK frequency are in a free-running state either before the GNSS satellite signals are received and the time is fixed, or if the GNSS satellite signals are no longer received for a period of time or more.
Cable delay*
Cable delay refers to the time it takes for a signal to travel through a cable. It depends on the cable type and length.
The cable delay can be compensated using the PPS command of the GNSS receiver. The cable connected between the GNSS antenna and the GNSS receiver as well as the cable connected between the 1PPS output of GNSS receiver and the equipment should be considered.
Holdover
Holdover refers to the ability of a GNSS receiver to maintain as much as possible the accuracy and the stability of the 1PPS and the VCLK frequency during periods when GNSS satellite signals cannot be received.
Learning period (time)*
The learning period is a predetermined period of time during which the GNSS receiver, while continuously receiving GNSS satellite signals, learns the behavior of the onboard oscillator. Once the learning period is completed the GNSS receiver is capable of holdover.
Frequency control mode*
The frequency mode (frequency control mode) refers to the state of the VCLK frequency.
There are six different frequency modes: WARMUP, PULL-IN, COARSE LOCK, FINE LOCK, HOLDOVER, and OUT OF HOLDOVER.
WARMUP WARMUP is the state in which the internal VCLK frequency is waiting to reach stability immediately after power-on.
PULL-IN PULL-IN is the state in which the GNSS satellite signals are received and the PPS and the VCLK frequency are beginning to be synchronized with UTC time or a GNSS time.
COARSE LOCK COARSE LOCK is the state in which GNSS satellite signals are received and the PPS and the VCLK frequency are roughly synchronized with UTC time or a GNSS time.
FINELOCK FINE LOCK is the state in which GNSS satellite signals are received and the PPS and the VCLK frequency are precisely synchronized with UTC time or a GNSS time.
HOLDOVER HOLDOVER is the state in which GNSS satellite signals are no longer received after the learning period was completed and the PPS and the VCLK frequency are maintained to a certain accuracy and stability, by automatically taking into account the frequency aging characteristic and frequency-temperature characteristic of the oscillator, providing better performances than in free-running mode.
OUT OF HOLDOVER OUT OF HOLDOVER is the state in which GNSS satellite signals are no longer received either for period of time greater than the holdover period or before the learning period was completed. In this state the accuracy and stability of the PPS and the VCLK frequency are not maintained. The oscillator is in free-running mode.

Terms related to position calculation

PVT calculation
PVT calculation stands for position, velocity and time calculation. It refers to the calculation by a GNSS receiver of various information such as the 3D position (latitude, longitude and altitude), speed, time and orientation/ azimuth of the satellites and the receiver, based on the information received from the navigation satellites.
Pseudorange
A pseudorange refers to the calculation by a GNSS receiver of its distance from a GNSS satellite, based the travel time of the GNSS satellite signal to reach the GNSS receiver. This calculated distance inevitably includes some inaccuracies, so the term pseudorange is used.
Pseudoranges are some of the information used by GNSS receivers for the PVT calculation.
Doppler frequency
A Doppler frequency refers to the calculation by a GNSS receiver of the received frequency of a GNSS satellite signal. This frequency will depend mostly on the relative motion between the GNSS satellite and the GNSS receiver, so the term Doppler frequency is used.
Doppler frequencies are some of the information used by GNSS receivers for the PVT calculation.
LOS(LOS satellite)
LOS stands for Line Of Sight. It refers to the direct reception of a GNSS satellite signal at the GNSS receiver's antenna, without any blocking obstacles or reflections. It is synonymous with an unobstructed view between a GNSS satellite and the GNSS receiver's antenna. Such GNSS satellites are specifically called LOS satellites. The more LOS satellites are received, the more stable the observed signal levels are, and the more accurate the PVT calculation is.
Examples of Line Of Sight communication are FM radio, microwave and satellite transmission.
NLOS(NLOS satellite)
NLOS stands for Non Line Of Sight. A NLOS satellite is the opposite of a LOS satellite and indicates the presence of an obstruction between a GNSS satellite and the GNSS receiver's antenna. Strictly speaking, a GNSS satellite whose signal is not received at the GNSS receiver's antenna, is determined to be non-visible and is also included in NLOS satellites. However, we simply call such satellites as non-visible satellites and not as NLOS satellites.
A NLOS satellite defines a GNSS satellite whose signal cannot be received directly at the GNSS receiver's antenna, but only a faint signal of it that is reflected by surrounding buildings or attenuated by an obstacle. A signal that is received from a reflection and by travelling through an obstacle is referred to as multipath.
Using multipath GNSS satellite signals for the PVT calculation tends to result in poor position accuracy and time accuracy due to errors in the calculation of the pseudoranges and the Doppler frequencies. Determining which GNSS satellites are NLOS satellites, masking them appropriately, and using only LOS satellites for the PVT calculation improves the position accuracy and time the accuracy.
Position mode*
General GNSS receivers must receive four or more GNSS satellite signals for the PVT calculation, thus to calculate information such as latitude, longitude, altitude, speed, azimuth and time.
Yet, if a GNSS receiver operates at a known static position and the latitude, longitude and altitude of this position are provided to the GNSS receiver, it can calculate accurate time and output precisely synchronized PPS and VCLK frequency with UTC time or a GNSS time, while receiving only one GNSS satellite signal.
A GNSS receiver dedicated to time sensitive applications can operate in four different modes: NAV mode, which assumes the GNSS receiver does not operate at a static position and calculates the latitude, longitude, altitude, speed, azimuth, and time; TO mode, which assumes the GNSS receiver operates at a static position and precisely knows its position, and calculates only the time; SS mode and CSS mode which assume the GNSS receiver operates at a static position and precisely estimate this position while calculating the time.
Position estimation
(Estimated position)
Position estimation refers to the process of calculating a position fix with insufficient accuracy. The results of such calculation is referred to as the estimated position.
NAV mode NAV mode stands for Navigation Mode. In this mode the GNSS receiver calculates the latitude, longitude, altitude, speed, azimuth, and time every second.
A GNSS receiver dedicated to time sensitive applications must be set in NAV mode if it is used in non-static applications.
TO mode TO mode stands for Time Only mode. In this mode the GNSS receiver must operate in static position, must already know its precise position, and only calculates the time every second.
In TO mode, a GNSS receiver outputs a more accurate and stable time, PPS and VCLK frequency. In TO mode a GNSS receiver can continue to calculate the time even when it receives only one GNSS satellite signal.
SS mode SS mode stands for Self-Survey mode. In this mode the GNSS receiver must operate in static position and calculates the latitude, longitude, altitude and time every second. It calculates its static position with a high accuracy based on the positional information obtained over a long period of time. To do so it must receive four or more GNSS satellite signals, excluding SBAS satellite signals.
In SS mode, even if at a time less than four but more than one GNSS satellite signals are received, the GNSS receiver can continue to output accurate time using the fixed position information calculated up to that point of time and the same processing technic as in TO mode. This allows the GNSS receiver to keep outputting accurate time, PPS or VCLK frequency.
The SS mode is suitable for situations where the use of TO mode is desired but the exact static position of the GNSS receiver is unknown and must first be determinated. Once the GNSS receiver has calculated its static position for a long enough period of time and/ or with a certain accuracy (configurable), it automatically transitions into TO mode.
CSS mode CSS mode stands for Continuous Self-Survey mode.
While in SS mode, the GNSS receiver discards its surveyed position when powering off, in CSS mode, it backs it up in BBRAM (back-up memory) and continues calculating its position after powering on again, starting from the backed-up position.
Fixed position The fixed position is the 3D position (latitude, longitude, altitude) of the fixed point to be set in the GNSS receiver when using the TO mode.
Estimated accuracy of the time
The estimated accuracy of the time calculated by a GNSS receiver is the standard deviation (in nanoseconds) of the pseudorange (at 1sigma) of all the GNSS satellites used for the PVT calculation and reported in the GSA sentences.
This value is lower in open sky environments and higher in harsh environments such as indoor or in urban canyons. It is mostly due to the influence of multipath GNSS satellite signals in the PVT calculation. The estimated accuracy can be used to judge whether the reception environment is good or bad.
T-RAIM
T-RAIM stands for Time Receiver Autonomous Integrity Monitoring. It is a mechanism to identify and eliminate GNSS satellites that may adversely affect the PVT calculation. It is based on the principle of combination and majority rule and can be used when there are more received GNSS satellites than the minimum number required for the PVT calculation.

Terms related to communication with GNSS receivers

(communication) protocol
A communication protocol is a communication procedure, which includes rules and formats, for sending and receiving data from a device (here a GNSS receiver) using a communication port (here a serial COM port).
Command*
A command refers to a set of data to be sent to a GNSS receiver to configure or query it.
Sentence*
A Sentence refers to a set of data received from a GNSS receiver.
Serial data
Serial data is a generic term referring to a set of data sent or received sequentially on a communication port.
The term "serial data output" is sometime use instead of "sentences".
NMEA
NMEA stands for the NATIONAL MARINE ELECTRONICS ASSOCIATION.
However the term NMEA often used to refers to the NMEA 0183, a serial ASCII (American Standard Code for Information Interchange) based communication protocol, defined by the NMEA.
ACK
ACK stands for Acknowledgement. ACK is a unique signal to indicate that a command has been received successfully. When a command is sent to a GNSS receiver and is accepted as appropriate, ACK is returned by the GNSS receiver as a response sentence.
NACK
NACK stands for Negative-Acknowledgement. NACK is a unique signal to indicate that a command has been received unsuccessfully. When a command is sent to a GNSS receiver and is deemed as improper, NACK is returned by the GNSS receiver as a response sentence.
If NACK is returned, it is often because the command is either not properly formatted or checksummed.

Terms related to GNSS receiver storage area

BBRAM
BBRAM stands for Battery Backup Random Access Memory. This storage area can be used as a backup area only when a backup voltage is applied to the GNSS receiver while it is turn-off. In this case, the GNSS receiver's RTC also keeps the current time and date.
Ephemeris data, almanac data, last position and configuration values are sequentially stored in this area. This storage area is read out at the time of start-up or restart. The stored information can be deleted by removing the backup voltage or invalidated by sending specific commands to the GNSS receiver.
FLASH ROM(FLASH)
FLASH ROM (FLASH) refers to a storage area contained in a FLASH ROM.
By sending the FLASHBACKUP command to the receiver, it will save ephemeris data, almanac data, last position and configuration values to FLASH. This storage area is read out at the time of startup or restart.
Once baked up to FLASH, the settings can only be erased by either using the FLASHBACKUP command again or updating the GNSS receiver software. However these settings can be invalidated by sending specific commands to the GNSS receiver.
If a setting for the same item is stored in both BBRAM and FLASH, the BBRAM setting takes precedence. Yet, if the data in BBRAM are invalid for reasons such as a loss of backup voltage, the FLASH data will be applied at the next startup or restart.

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