Accuracy - In navigation, the accuracy of an estimated or measured position of a craft (vehicle, aircraft, or vessel) at a given time is the degree of conformance of the measured position with the true position of the craft at that time. Since accuracy is a statistical measure of performance, a statement of the accuracy of a navigation system is meaningless unless it includes a statement of the applicable uncertainty in position.

AFM - Means Federal Aviation Administration (FAA) approved airplane flight manual. This entire document is FAA approved and must be carried on all aircraft certificated under Federal Aviation Regulation (FAR) Part 25. The AFM contains operating procedures and limitations for the airplane and engine combination, as well as for all installed appliances, and must be readily accessible to the flightcrew during all operations.

Ambiguity - System ambiguity exists when the navigation system identifies two or more possible positions of the vehicle, with the same set of measurements, and no indication of which is the most accurate position. The potential for system ambiguities should be identified, along with a provision for users to identify and resolve them.

Anywhere fix - The ability of a receiver to start position calculations without being given an approximate location and approximate time.

Area navigation (RNAV) - Application of the navigation process providing the capability to establish and maintain a flight path on any chosen course that remains within the coverage area of the type of navigation sources being used. RNAV utilizing capabilities in the horizontal plane only is called 2D RNAV, while RNAV which also incorporates vertical guidance is called 3D VNAV. Time navigation (TNAV) may be added to either 2D or 3D systems. TNAV added to a 3D system is called 4D.

ARINC - An acronym for Aeronautical Radio Inc., a corporation largely owned by a group of airlines. ARINC is licensed by the Federal Communication Commission (FCC) as an aeronautical station, and contracted by the FAA to provide communication support for air traffic control (ATC) and meteorological services in portions of international airspace.

Availability - The availability of a navigation system is the percentage of time that the services of the system are usable by the pilot. Availability indicates the ability of the system to provide usable service within a specified coverage area. Signal availability is the percentage of time that navigation signals transmitted from external sources are available for use. Availability is a function of both the physical characteristics of the environment and the technical capabilities of the transmitter facilities.

Bandwidth - The range of frequencies in a signal.

C/A code - The standard (course/acquisition) global positioning system (GPS) code - a sequence of 1023 pseudo-random, binary, biphase modulation on the GPS carrier at a chip rate of 1.023 megahertz (MHz). Also known as the "civilian code."

Cabotage - The Standard Dictionary of the English language defines cabotage (for flight purposes) as "air transport of passengers and goods within the same national territory." The definition adopted by International Civil Aviation Organization [ICAO] at the Chicago Convention is, "Each state shall have the right to refuse permission to the aircraft of other contracting states to take on its territory passengers, mail, and cargo destined for another point within its territory."

Capacity - The number of system users that can be accommodated simultaneously.

Carrier - A signal that can be varied from a known reference by modulation

Carrier aided tracking - A signal processing strategy that uses the GPS carrier signal to achieve an exact lock on the pseudo random code. This is more accurate than the standard approach.

Carrier frequency - The frequency of the unmodulated fundamental output of a radio transmitter.

Channel - A channel of a GPS receiver consists of the circuitry necessary to tune the signal from a single GPS satellite.

Chip - The transition time for individual bits in the pseudo-random sequence. Also, an integrated circuit.

Circular error probable (CEP) - A U.S. Department of Defense (DOD) specification in terms of accuracy. CEP is defined as the radius of a circle containing 50 percent of all possible fixes. Specification of radio navigation system accuracy generally refer to one or more of the following definitions.

a. Predictable accuracy: the accuracy (in NM or feet) of a position with respect to geographic or geodetic coordinates of the Earth. Predictable accuracy is also known as geodetic or absolute accuracy.

b. Repeatable accuracy: the accuracy (in feet) with which a user can return to a position whose coordinates have been measured at a previous time with the same navigation system.

c. Relative accuracy: the accuracy (in feet) with which a user can measure position relative to that of another user of the same navigation system at the same time. This may be expressed also as a function of the distance between two users. Relative accuracy may also refer to the accuracy with which a user can measure position relative to his own position in the recent past. For example, the present position of a craft whose desired track forms a specific geometric pattern on search operations will be measured generally with respect to a previously determined datum.

Class I airspace - Short range navigation within the limits of the operational service volume of ground based navigational aids (navaids).

Class II airspace - Long range navigation beyond the limits of the operational service volume of ground based navaids.

Clock bias - The difference between the clock's indicated time and true universal time.

Coast-out fix - A navaid (or intersection "fix"), sometimes called a coastal fix or gateway fix, whereby an aircraft transitions between the domestic route structure and the oceanic route structure, such as an organized track system (OTS) or air traffic service (ATS) volume of ICAO standard navaids.

Control segment - A worldwide network of GPS monitoring and control stations that ensure the accuracy of satellite positions and their clocks.

Convergence - A term used by controllers relative to the lateral separation of aircraft. Aircraft are determined to be converging if their lateral separation is becoming narrower in width.

Coverage - The coverage provided by a radio-navigation system is that surface area or space volume in which the signals are adequate to permit the user to determine position to a specified level of accuracy. Coverage is influenced by system geometry, signal power levels, receiver sensitivity, atmospheric noise conditions, surface conductivity, and other factors affecting signal availability.

Crosstrack error - The perpendicular deviation that the airplane is to the left or right of the desired track.

Cycle slip - A discontinuity in the measured carrier beat phase resulting from a temporary loss-of-lock in the carrier tracking loop of a GPS receiver.

Data message - A message included in the GPS signal which reports the satellite's location, clock corrections, and health. Included is rough information on the other satellites in the constellation.

db - An abbreviation for decibels. It is a unit of relative power, voltage, or current, plotted on a logarithmic scale. An increase (or decrease) of 10 db means that something is either double (or half) of the original value. Db are used to compare one relative value to another.

Dead reckoning (DR) - Is a method of estimating the position of an aircraft without astronomical observations, based upon a previous known position and an estimate of the course and distance travelled within a given time increment. An estimation of the winds aloft is an integral part of the DR process.

Differential positioning - Precise measurement of the relative positions of two receivers tracking the same GPS signals.

Dilution of precision (DOP) - The multiplicative factor that modifies ranging error. It is caused solely by the geometry between the user and his/her set of satellites. Known as DOP or geometric dilution of precision (GDOP).

Domestic airspace - Airspace overlying the continental land mass of the United States, Alaska, Hawaii, and U.S. possessions. Domestic airspace extends to 12 nautical miles (NM) offshore.

Doppler aiding - A signal processing strategy that uses a measured Doppler shift to help the receiver smoothly track the GPS signal. Allows more precise velocity and position measurement.

Doppler shift - The apparent change in the frequency of a signal caused by the relative motion of the transmitter and receiver.

Drms - Refers to the "distance root mean square error." This fundamental parameter is the building block to the most common measure of navigation fix accuracy, "two (2) Drms."

Enroute - A phase of navigation covering operations between departure and arrival terminal phases.

Ephemeris - The predictions of current satellite position that are transmitted to the user in the data message.

Extended overwater - FAR Part 1 defines "extended overwater operation" for airplanes as an operation overwater at a horizontal distance of more than 50 NM from the nearest shoreline; and for helicopters, as an operation overwater at a horizontal distance of more than 50 NM from the nearest shoreline or more than 50 NM from an offshore heliport structure.

Fast multiplexing channel (FMC) - A single channel which rapidly samples a number of satellite ranges. "Fast" means that the switching time is sufficiently fast (2 to 5 milliseconds) to recover the data message.

Frequency band - A particular range of frequencies.

Frequency spectrum - The distribution of signal amplitudes as a function of frequency.

Fix dimensions - This characteristic defines whether the navigation system provides a linear, one-dimensional line of position, two-dimensional, or three-dimensional position fix. The ability of the system to derive a fourth dimension (for example, time) from the navigational signals is also included.

Fix rate - The fix rate is defined as the number of independent position fixes or data points available from the system per unit time.

Gateway fix - See " Coast-out fix."

GDOP - Refers to geometric dilution of precision. The degree of uncertainty of a position fix with respect to the crossing angles of the lines of position (LOP).

Global Positioning System (GPS) - A long range navigation system based on a constellation of 24 satellites orbiting the earth at a very high altitude that provide signals which through triangulation can identify a precise location.

Gross navigational error (GNE) - Pilots are expected to fly their aircraft along the centerline of their assigned route and to stay within the accuracy limits of their installed navigation systems. (For example, the accuracy tolerance limits for Loran C equipment used in instrument flight rules (IFR) oceanic navigation is 5.8 NM). If an aircraft becomes off- course, it is usually for one of the following reasons; a "loop" (or communications) error between the pilot and the controller, a pilot intentionally entering a waypoint which is not along the assigned route of flight (for example, a pilot deviation due to weather without prior ATC approval or without declaring an emergency), an unintentionally entered waypoint not along the assigned route, and a navigational equipment error or failure. Navigational errors that are greater than 20 NM are investigated by the various countries that provide ATC service.

Handlers - Individuals within specific countries who may be hired to accompany a flight and take care of the unique regulatory and cultural requirements associated with a flight into a foreign country. These individuals are locals who know the procedures and can assist in clearing customs, immigration, and airport security. These individuals also have varying degrees of expertise on other matters such as lodging, rental cars, flight restrictions, passenger ground travel, local prohibitions, health problems, etc.

High seas - Any body of water outside the 12 NM limit.

Handover word - The word in the GPS message that contains synchronization information for the transfer of tracking from the C/A to P-code.

IFR navigation - Navigation by electronic means or by use of a flight navigator. Navigation techniques may include use of ICAO standard navaids supplemented by accurate DR, pilot operated electronic long range navigation equipment, or use of a flight navigator. IFR oceanic (enroute) navigation requires that the aircraft adhere to a particular level of navigational accuracy.

Independent fix - An independent fix means that a position does not depend on a previous or following measurement.

Independent receiver function - "Independent" means that the function of any part of a receiver does not depend upon the functioning of any part of another unit. Today's receivers can be single-sensor, multisensor, or "embedded" as part of a multifunction flight management (and navigation) system (FMS). A combined communication-navigation (com/nav) system meets the requirements for an independent navigation receiver.

Inertial navigation system (INS) - An RNAV system which is a form of self-contained navigation. See "Area navigation (RNAV)."

Integrity - Integrity is the ability of a system to provide timely warnings to users when the system should not be used for navigation.

Ionosphere - The band of charged particles 80 to 120 miles above the earth's surface.

Ionospheric refraction - The change in the propagation speed of a signal as it passes through the ionosphere.

L band - The group of radio frequencies extending from 390 MHz to 1550 MHz. The GPS carrier frequencies (1227.6 MHz and 1575.42 MHz) are in the L band.

Minimum navigation performance specifications (MNPS) - A set of standards which require aircraft to have a minimum navigation performance capability in order to operate in MNPS designated airspace. In addition, aircraft must be certified by their state of registry for MNPS operation.

MNPS airspace - Designated airspace where MNPS procedures are applied between MNPS certified and equipped aircraft. MNPS airspace is located over certain areas of the North Atlantic (NAT) and over Northern Canada. An example is NAT MNPS airspace. This airspace is defined as the volume of airspace between flight level (FL) 275 and FL 400 bounded by certain geographical coordinates. To obtain MNPS approval, each operator must show compliance with the following conditions:

a. each aircraft is suitably equipped and capable of meeting MMS standards;

b. operating procedures are established which ensure that MNPS standards are met; and

c. flightcrews are capable of operating with sufficient precision to consistently meet MNPS requirements and are aware of the emergency procedure specific to MNPS airspace.

Multichannel receiver - A GPS receiver that can simultaneously track more than one satellite signal.

Multipath error - Errors caused by the interference of a signal that has reached the receiver antenna by two or more different paths. Usually caused by one path being bounced or reflected.

Multiplexing channel - A channel of a GPS receiver that can be sequenced through a number of satellite signals.

Navigation - The means by which an aircraft is given guidance to travel from one known position to another known position.

Navigation guidance - The calculation of steering commands to maintain the desired track from the present aircraft position to a new position.

Navigation information - Aircraft parameters such as position, velocity vector and related data such as track angle, ground speed, and drift angle used for navigation guidance.

Navaids - Are visual or electrical devices which may be used while airborne or on the surface, which provides point-to-point guidance information or position data to an aircraft in flight. Examples of standard ICAO navaids include very high frequency (VHF) omnidirectional range (VOR), with or without distance measuring equipment (DME), and nondirectional ground-based beacons (NDB).

NM or nm - means distance measured in nautical miles. One nautical mile is equivalent to 6,080.27 feet and is the fundamental measurement unit used in both sea and air navigation. It is based on the length of a minute of arc along an arc of a great circle around the Earth.

Oceanic airspace - Airspace over the oceans of the world, considered international airspace, where ICAO oceanic separation and procedures are applied. Responsibility for the provisions of ATC service in this airspace is delegated to various countries based generally upon geographic proximity and the availability of the required resources.

Oceanic airspace - Airspace over the oceans of the world is considered international airspace where aircraft separation and air traffic procedures are standardized by ICAO. The responsibility for ATS in oceanic airspace is delegated to the various ICAO member States according to geographic proximity and availability of the required resources. Specific procedures are defined by ICAO Document 7030.

Oceanic navigational error report (ONER) - A report filed when an aircraft exiting oceanic airspace has been observed by radar to be off course. ONER reporting parameters and procedures are contained in FAA Order 7110.82, "Monitoring of Navigational Performance in Oceanic Areas."

Offshore airspace - The airspace between the United States' 12 NM limit and the oceanic flight information region (FIR) boundary. An alternate definition is "within the limits of conventional land based navaids."

Omega - An RNAV system designed for long range navigation based upon ground based electronic navaid signals.

Operational service volume - Defines the reception limits of VOR/DME and NDB navaids which are usable for random/unpublished route navigation and which are flight checked periodically to reconfirm these limits of coverage. The operational service volume of NDBs used in oceanic navigation; that is, beyond the 75 NM standard service volume, must be individually flight checked and identified as such on the appropriate charts before they can be used for navigation.

Overwater - Section 91.511 of the FAR defines "overwater" as more than 30 minutes flying time or 100 NM from the nearest shore. This definition differs from the "extended overwater" definition found in FAR Part 1.

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 approximately every 267 days. Each I week segment of this code is unique to one GPS satellite and is reset each week.

Parallel offset path - A desired track parallel to, and left or right of, the "parent" track specified in nautical miles of offset distance.

Parent track - The planned track between two waypoints.

Parent waypoint - A waypoint used for route definition or progress reporting. The geographical position of a parent waypoint is not altered when RNAV equipment is operating in a parallel offset mode.

Pilotage - Aerial navigation by means of visual identification of landmarks.

POH - Pilot's Operating Handbook. The POH is the result of a 1976 industry developed specification for the operation of FAR Part 23 certificated aircraft. Only Sections I and II of the POH are FAA approved. These sections contain operating limitations for the airframe and engine combination. Section IX of the POH also contains additional operating instructions and FAA approved limitations for all supplemental installed equipment, including Loran C. Some type certificates issued for airplanes manufactured after 1977 - 1978 require that the POH be carried on the airplane (and therefore accessible to the pilot during flight) as a condition to meeting its type design.

Precise positioning service (PPS) - The most accurate dynamic positioning possible with GPS, based on the dual frequency P-code.

Primary navigation - See "Sole means of navigation." A "primary means" system is not required to reference a magnetic compass as it is totally independent of all other reference systems.

Pseudolite - A ground based differential GPS receiver which transmits a signal like that of an actual GPS satellite and can be used for ranging. The data portion of the signal contains the differential corrections that can be used by other receivers to correct for GPS errors.

Pseudorandom code - A signal with random-noise like properties. It is a very complicated but repeated pattern of Is and Os.

Pseudorange - 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.

P static - Is precipitation static, a form of background noise caused by rain, hail, snow, or dust storms in the vicinity of a receiving antenna, and measured at frequencies less than 10 MHz.

Reliability - The reliability of a navigation system is a function of the frequency that failures occur within the system. It is the probability that a system will perform its function within defined performance limits for a specified period of time under given operating conditions. Formally, reliability is one minus the probability of system failure.

Route - A defined path, consisting of a course in the horizontal plane, which aircraft transverse over the surface of the earth.

Satellite constellation - The arrangement in space of a set of satellites.

Selcal - Selective calling, a term used by the chart services on high/low altitude charts and others to indicate specific frequencies available for aircraft with selcal installed to be contacted on demand.

Self-contained navigation - Systems which are not dependent on external navigation sources on a continuous basis to determine position or navigation track. Self-contained navigation systems must be updated periodically with station-referenced or earth-referenced navigation systems to maintain their accuracy.

SNR - Signal to noise ratio. SNR is the ratio of the radio field intensity of a received radio wave to the radio noise field intensity received along with that signal.

Sole means air navigation systems - An approved navigation system that can be used for specific phases of air navigation without the need for any other navigation source.

Space segment - The part of the whole GPS system that includes the satellites and the launch vehicles.

Spread spectrum - A system in which the transmitted signal is spread over a frequency band much wider than the minimum band-width needed to transmit the information being sent. For GPS, this is done by modulating the carrier with a pseudo-random code.

Standard positioning service (SPS) - The normal civilian positioning accuracy obtained by using the single frequency C/A code.

State aircraft - Aircraft used exclusively in the service of any government or of any political subdivision thereof, including the government of any state, territory, or possession of the United States or the District of Columbia, but not including any government owned aircraft engaged in carrying persons or property for commercial purposes.

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 accuracy by factors of over 100.

Station referenced navigation - Position determination which is referenced to a stationary source.

Strap down navigation equipment - Navigation equipment that is temporarily installed in an aircraft, usually for the purpose of ferry flights. The installation is FAA approved for "form, function and fit" and placed on FAA Form 337.

Supplemental air navigation system - An approved navigation system that can be used in conjunction with a sole-means navigation system.

Standard service volume - Defines the reception limits of VOR/DME and NDB ground based navaids which are usable for random/unpublished route navigation. Standard service volume is a calculated value that has not been flight checked. Coverage limits for VOR/DME systems are published in the "Federal Radio-navigation Plan," published biennially by the FAA and available to the pilot community.

Statistical measure of accuracy - Navigation system errors generally follow a known error distribution. Therefore, the uncertainty in position can be expressed as the probability that the error will not exceed a certain amount. A thorough treatment of errors is complicated by the fact that the total error is comprised of errors caused by instability of the transmitted signal, effects of weather and other physical changes in the propagation medium, errors in the receiving equipment, and errors introduced by the human navigator. In specifying or describing the accuracy of a system, human errors usually are excluded. Further complications arise because some navigation systems are linear (one dimensional) while others provide two or three dimensions of position. When specifying linear accuracy, or when it is necessary to specify requirements in terms of orthogonal axes (for example, along track or crosstrack), the 95 percent (or two fl) confidence level is used. Vertical or bearing accuracies is specified in one-dimensional terms at the two fl, or 95 percent, confidence level. When two-dimensional accuracies are used, the 2 Drms (distance root mean square) uncertainty estimate is employed. Two Drms is twice the radial error or Drms. The radial error is defined as the root-mean-square value of the distances from the true location point of the position fixes in a collection of measurements. It is often found by first defining an arbitrarily oriented set of perpendicular axes, with the origin at the true location point. The variances around each axis are then found, summed, and the square root computed. When the distribution of errors is elliptical, as it often is for stationary ground-based systems, these axes can be taken for convenience as the major and minor ellipse. Then the confidence level depends on the elongation of the error ellipse. The range of confidence levels is from 95 to 99 percent. As the error ellipse collapses to a line, the confidence level of the 2 Drms measurement approaches 95 percent.

System capacity - System capacity is the number of users that a system can accommodate simultaneously.

User interface - The way a receiver conveys information to the person using it. The controls and displays.

User segment - The part of the whole GPS system that includes the receivers of GPS signals.

VFR navigation - Navigation by pilotage (that is, DR) or electronic means. There are no published accuracy standards for visual flight rules (VFR) oceanic (enroute) navigation.


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