differential global navigation satellite system r-121

Recommandation pour le fonctionnement etla surveillance des services DGNSS dans labande de fréquence 283,5 – 325 kHz

Recommandation AISM R-121Juin 2001

Recommendation on the Performance andMonitoring of DGNSS Services in the

Frequency Band 283.5 – 325 kHz

IALA Recommendation R-121June 2001

20ter, rue Schnapper – 78100 Saint Germain en Laye – FranceTelephone : +33 1 34 51 70 01 Telefax : +33 1 34 51 82 05E-mail : [email protected] Internet : http://www.iala-aism.org

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IALA Recommendation R-121 on the Performance and Monitoring of DGNSS Servicesin the frequency band 283.5 – 325 kHz

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IALA Recommendation on the Performance and Monitoring of aDGNSS Service in the frequency band 283.5 - 325 kHz

THE COUNCIL

NOTING the function of the IALA with respect to Safety of Navigation, theefficiency of maritime transport and the protection of the environment,

NOTING ALSO IMO resolutions A.860(20) on Maritime Policy for the Future GlobalNavigation Satellite System (GNSS), and A.815(19) on World WideRadionavigation System,

NOTING FURTHER ITU-R Recommendation M.823-2 on the Technicalcharacteristics of differential transmissions for Global Navigation Satellite Systems(GNSS) from maritime radio beacons in the frequency band 285-325 kHz (283.5-315 kHz in Region 1),

RECOGNISING the need to ensure that Differential GNSS (DGNSS) services inthe frequency band 283.5 kHz – 325 kHz are operated in accordance with certainminimum standards that take into account relevant ITU-R Recommendations andIMO Resolutions,

RECOGNISING ALSO that the minimum standards should include the signalformat, reference datum, availability, continuity, integrity, accuracy, signalmonitoring, range and coverage, status reporting, validation, and the publication ofinformation about the system,

HAVING CONSIDERED the proposals made by the Radionavigation Committee:

ADOPTS the Minimum Standards for the Performance and Monitoring of DGNSSServices in the frequency band 283.5 – 325 kHz set out in the annex of thisrecommendation; and

RECOMMENDS National Members and other appropriate Authoritiesproviding, or intending to provide, DGNSS services in the frequency band 283.5 –325 kHz, to use the Minimum Standards set out in the annex to thisrecommendation.


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Annex

Minimum Standardsfor the

Performance and Monitoring of DGNSS Servicesin the frequency band 283.5 – 325 kHz

List of Contents

1. GENERAL 5

2. SIGNAL FORMAT 5

3. REFERENCE DATUM 6

4. AVAILABILITY AND CONTINUITY 6

4.1 AVAILABILITY 6

4.2 CONTINUITY 7

5. INTEGRITY 8

6. ACCURACY 8

7. RANGE/COVERAGE 9

8. TRANSMITTER PERFORMANCE 9

9. RECEIVER PERFORMANCE 9

10. MONITORING 10

11. VALIDATION 10

12. PUBLICATION OF INFORMATION 10

13. REFERENCES 12

APPENDIX 1 14

DEFINITIONS AND GLOSSARY OF TERMS USED IN GNSS 14


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1. GENERAL

A Global Navigation Satellite System (GNSS), currently GPS and GLONASS, is a space-basedpositioning, navigation and time distribution system designed for world wide use. Differential GNSS(DGNSS) is a means of improving the accuracy of GNSS and providing integrity monitoring to the civiluser. DGNSS involves having, at precisely known locations, reference stations that provide real-timecorrections to the GNSS signals and integrity monitoring.

This Recommendation describes the use of transmitters in the 285 – 325 kHz / 283.5 – 315 kHz LF/MFmarine frequency band to transmit DGNSS. It must be noted that DGNSS is an augmentation of GNSSand is not a stand alone radionavigation system.

The removal of Selective Availability (S.A.) in May 2000 did not remove the requirement for GNSSaugmentation. Without augmentation GNSS accuracy may contain significant errors for extendedperiods of time and no integrity information is provided. Although this may be acceptable for someusers, such as fishermen and pleasure boat owners, it is not acceptable for merchant vessels, normallyoperating with tighter safety margins and higher risks. On the other hand the removal of S.A. enablesthe operator of a DGNSS service to reduce the data link data rate, which brings about severaladvantages.

System performance is based on the assumptions that the system provider conforms with theseguidelines and that the user equipment meets the design and installation standards as specified inSection 9.

2. SIGNAL FORMAT

The data format and transmission characteristics are detailed in ITU-R Recommendation M.823-2 (Ref.1)1.

Depending on requirements message types used should include: Type 1 (GPS Full Correction Set) orType 9 (GPS Partial Correction Set) or 31 (GLONASS Full Correction Set) or 34 (GLONASS PartialCorrection Set); Type 3 (GPS Reference Station Parameters) or Type 32 (GLONASS Reference StationParameters); Message Types 7 (DGPS Radiobeacon Almanac) or Type 35 (GLONASS RadiobeaconAlmanac); Type 6 (Null frame, used as transmission fill). Partial Correction Sets messages are preferredbecause of better performance in impulse noise conditions.

Message Types 3/32, and 7/35 should be broadcast at the minimum time intervals specified in ITU-RRecommendation M.823.

Message Type 5, which deals with GPS constellation health, is included in ITU-R RecommendationM.823, but is not normally used.

Message Type 16/36 may be used to broadcast special messages in text form, but should not normallybe used for integrity warnings, which are included in message headers. Type 16/36 messages may beused for status information, or for navigation related information such as hydrographic ormeteorological data. Type 16/36 messages should never be broadcast consecutively.

1 The ITU-R Recommendation incorporates the format developed by RTCM Special Committee 104.(Ref. 3)


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3. REFERENCE DATUM

The datum used should be stated in the Administration's publications. If a datum is used that is notconsistent with WGS84 in the case of GPS or PE90 in the case of GLONASS, Message Type 4 shouldbe broadcast to indicate the datum. It should be noted that use of the incorrect datum could result inerrors of up to several hundred metres.

4. AVAILABILITY AND CONTINUITY

IMO resolution A.xxx (21) (Formerly A.815(19)) provides for three standards of accuracy, availabilityand continuity from a satellite radionavigation system. They represent the situation when coverage isprovided by a system comprising the satellite transmissions alone, when a single additional transmittingstation augments the satellite transmissions and when two or more additional transmitting stationsaugment the satellite transmissions. The standards, in tabular form, are:

Area AbsoluteHorizontalAccuracy

Availability Continuity Augmentation

Ocean _100 m >99.8% over 30 days N/A None

Coastal/harbour withlow level of risk

_10m >99.5% over 2 years _99.85% over 3hours

Single station

Coastal/harbour withhigh level of risk

_10 m >99.8% over 2 years _99.97% over 3hours

2 or more stations

4.1 AVAILABILITY

The availability standard adopted for a DGNSS service in the band is related to the techniques used inplanning and implementing the service. The availability of the satellite constellation is not taken intoaccount in the availability of the differential service. It does not take into account the availability of theGNSS Constellation.

Availability is defined in IMO Resolution A.860(20) (Ref. 4) as:

“The percentage of time that an aid, or system of aids, is performing a required function under statedconditions. The non-availability can be caused by scheduled and/or unscheduled interruptions.”

Mathematically this can be written as:

Availability (A)* = _____MTBO____

MTBO + MTSR

Where: MTBO = Mean time between outages; based on a 2 year averaging period(30 days ocean phase) MTSR = Mean time to service restoration; based on a 2 year averaging period (30 days ocean phase)

This accounts for scheduled and unscheduled service interruptions, i.e. preventative and correctivemaintenance.___________________________________________________

* Alternatively expressed as UP TIME/TOTAL TIME where TOTAL TIME = 2 years


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Worked example (# 1):

Assuming a scheduled maintenance cycle of 6 months, i.e. mean time between scheduledmaintenance is 0.5 years – therefore there will be 4 scheduled maintenance breaks in 2 years.

Assuming a MTBF of 2 years. Therefore the average number of failures over 2 years isexpected to be approximately 1.

Summing, there will be 5 outages over the two year period. Therefore the mean time betweenoutages is 2/5 years or approximately 3500 hours.

If the average out of service time for scheduled maintenance is 6 hours. The total out of servicetime for scheduled maintenance over the two year period is 24 hours. Similarly, if theunscheduled maintenance period is 12 hours, the total time out of service over the two yearperiod is 36 hours. This covers 5 maintenance events and, therefore, the mean time to servicerestoration is 36/5 hours or approximately 7 hours.

The overall availability over the two year period is therefore (3500/(3500+7)) = 99.8%

Worked example (# 2):

Assuming a scheduled maintenance cycle of 6 months, i.e. mean time between scheduledmaintenance is 0.5 years – therefore there will be 4 scheduled maintenance breaks in 2 years.

Assuming a MTBF of 2000 hours. Therefore the average number of failures over 2 years (17520hours) is expected to be approximately 9. (actually 8.76 rounded up to 9)

Summing, there will be 13 outages over the two year period (4 scheduled + 9 unscheduled).Therefore the mean time between outages is 17520 hours/13 equals 1348 hours.

If the average out of service time for scheduled maintenance is 6 hours. The total out of servicetime for scheduled maintenance over the two year period is 24 hours. Similarly, if theunscheduled maintenance period is 67 hours (an average of approximately 5 hours per outage –some will be more and some will be less) the total time out of service over the two year periodis 91 hours. This covers 13 maintenance events and, therefore, the mean time to servicerestoration is 91/13 hours or approximately 7 hours.

The overall availability over the two year period is therefore 1348/(1348+7) = 99.5%

4.2 CONTINUITY

Continuity is the ability of a system to function within specified performance limits without interruptionduring a specified period (normally short term). Continuity was previously referred as “Reliability” andthe reliability of a service was defined as the probability that the service, when it is available, performs aspecified function without failure under given conditions for a specified period of time, and is thereforethe same as continuity over a longer term.

The equation below refers to an availability type calculation, i.e. the average over the continuity timeinterval (CTI) that the service is available or the probability that the service is available at any instantduring the CTI. It is more appropriate to calculate the probability that the service is available throughoutthe CTI. The suggested approach is given below:Assuming the service is available at the beginning of the operation, then the probability that it is stillavailable at a time t later is:


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exp (-t/MTBF)This is the standard expression for reliability and excludes scheduled outages (i.e. uses MTBF)assuming that planned outages will be notified and the operation will not take place. The probability thatthe service will be available after a time CTI, that is the continuity is then:

C = exp (-CTI/MTBF)

If MTBF is very much greater than CTI, this can be approximated to:

C = 1 – (CTI/MTBF)

Where:

MTBF = Mean time between failures; based on a 2 year averaging period

CTI = Continuity time interval; in the case of maritime continuity, equal to 3 hours

There is no need to include the availability at the beginning of the time period of the operation becauseif there is no service, then the operation will not commence.

Worked example (# 1):Using the figures in the previous example for a system with a 2 year MTBF, the continuity overa three hour period is 1-(3/17520) = 99.98%Worked example (# 2):Using the figures in the previous example for a system with a 2000 hour MTBF, the continuityover a three hour period is 1-(3/2000) = 99.85%

5. INTEGRITY

“Integrity is the ability to provide users with warnings within a specified time when the system shouldnot be used for navigation.” (Ref. 4)

The criteria used for identifying an integrity failure should be stated by the Administration in theirpublications. A position error of greater than 10m persisting for more than 20 seconds should be theleast stringent condition adopted. The delay in access to the transmitted signal caused by the format anddata rate will necessitate a smaller error and/or shorter time period for the detection level.

The time for broadcasting an integrity warning to the user, from the time the integrity problem isidentified, should not be greater than 10 seconds.

When it has been determined that the error condition no longer exists, there may be a delay, similar tothat used for initially identifying the error, before the error warning is withdrawn.

6. ACCURACY

The absolute horizontal accuracy should be better than 10m at the 95% probability level within thepublished coverage area.

The absolute horizontal accuracy of the position solution at the reference station should be about1metre. This can be achieved by using a high quality reference receiver with carrier smoothing and toachieve this level of accuracy, reference receiver antennae positions should be surveyed to a 3Daccuracy of better than 20cm.

Due to spatial decorrelation, achievable accuracy degrades at an approximate rate of 1 metre for each150 km distance from the broadcast site. Accuracy is further degraded by computational and otheruncertainties in user equipment and the ability of user equipment to compensate for other error sourcessuch as multipath and propagation distortions.


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7. RANGE/COVERAGE

Nominal ranges of stations over seawater paths should be published at stated field strength (for example50, 75 or 100 _ V/m) (Ref. 5). Published coverage diagrams are normally based on software modellingpredictions and should be verified by measurements. The modelling process can be quite complex anddifficult, especially over mixed land/sea paths. Advice regarding modelling can be sought throughIALA. In predicting coverage, each service provider should establish the required field strength bygiving consideration to the following factors:

_ Radiated power. Antenna system configuration, including horizontal and vertical polar diagrams.

_ Land paths. Additional attenuation over land should be calculated from current ITU-R curves andpractice applicable at 300 kHz (Refs. 6 & 7).

_ Fading due to skywave propagation of the station’s signal. At night the field strength at every pointin the coverage should be not less than that specified at the nominal range for at least 95% of thetime. Night-time field strengths may be calculated in accordance with references 8, 9 & 10.

_ Atmospheric noise. Assumed levels of atmospheric noise should be in accordance with current ITU-R data and practice applicable at 300 kHz. It is recommended that the noise level be that which isnot exceeded more than 95% of the time on average throughout the year (Refs. 11 & 12).

_ Precipitation static. In those areas where precipitation static is known to be a significant problem, anappropriate factor should be added to the atmospheric noise (Ref. 13).

_ Man-made noise. In those situations, such as harbours, where man-made noise is significant incomparison with natural noise sources, the local man-made noise level should be taken into account(Ref. 14).

_ Bit rate. Transmission rates of 100 bits per second (bps) are generally used. However, in areaswhere greater accuracy is more important than maximum range, 200 bps may be used. 200 bpstransmissions achieve a higher message throughput in impulse noise conditions but are moresusceptible to Gaussian noise than 100 bps broadcasts, and will generally provide a lesser range. AsSelective Availability now has been permanently removed from GPS, 50 bps could be considered tomaximise range.

_ Interference. Interference from stations on the proposed channel and on channels up to 3 kHz eitherside should be taken into account. The protection ratios to be applied are those of the ITU-R (Ref.1), appropriate to the type of interferes and the frequency separation. Both groundwave andskywave propagation should be considered.

8. TRANSMITTER PERFORMANCE

The performance of the LF/MF transmitter and its antenna can be affected by weather conditions. Anautomatic Antenna Tuning Unit (ATU) should be used to minimise such effects.

Some administrations provide standby antennas to maintain service in the event of storm damage oroutages for maintenance. It is normal practice to duplicate the transmitter, reference station and powersupply.

There are well-established methods for the measurement of radiated power, field strength and antennaefficiency (Ref. 15).

9. RECEIVER PERFORMANCE


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In order to utilise the full DGNSS service capability, users should operate a GPS receiver conforming tothe technical specifications set out in IEC 61108-1, or a GLONASS receiver conforming to the technicalspecifications set out in IEC 61108-2, combined with a differential GNSS receiver conforming to thetechnical specifications set out in IEC 61108-4 (Refs. 16 & 17).

The performance of the LF/MF receiver and its antenna can also be affected by environmentalconditions, for example precipitation static in high latitudes and atmospheric noise from electricalstorms (Ref. 14). H-field antennas are usually better in this respect, although less sensitive than E-field(whip) antennas. Shipborne noise can also limit performance and it is important to provide goodgrounding for installations with E-field antennas. Mounting location on the vessel can also be importantand local interference and noise sources should be identified and dealt with (Ref. 17).

10. MONITORING

On-site integrity monitors should be provided at each transmitting station, to check the transmittedsignal and the data content. Monitoring may also be carried out using receivers placed at sites somedistance from the reference station but within the coverage area. Integrity monitors may be linked bycommunication lines to a central control and monitoring site. A second control and monitoring site maybe provided as a standby. Data may either be logged on station and downloaded periodically or passeddirectly to the central control site. It is recommended that this data be archived for a period sufficient tomeet local litigation requirements.

11. VALIDATION

The operation of a DGNSS service should be carefully validated concerning the following parameters,which are defined by IMO Recommendations:

- accuracy

- availability

- continuity

- integrity

This process, which is currently under development, should be repeated at intervals throughout the lifeof the service.

12. PUBLICATION OF INFORMATION

Individual Administrations are encouraged to publish service descriptions, including coveragepredictions; examples are given in references 18 to 23.

In addition to the information contained in the standard message types, described in Section 1, notice ofcurrent or planned signal unavailability should be provided to users through the appropriate service (e.g.coastal radio station, VTS, Navtex, Safetynet etc.)

Wherever practicable, information on scheduled and unscheduled off-air periods should be promulgatedto users as follows:

Scheduled maintenance Date and expected downtime At least 1 month in advance

Unscheduled outages Expected downtime As soon as practicable and not morethan 1 hour after the occurrence


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IALA Guidelines on Bilateral Agreements and Inter-Agency MOU’s on the Provision of DGNSSservices in the frequency band 283.5 kHz– 325 kHz contains examples of such documents. Theexamples include guidance on the information that should be exchanged between co-operating agenciesand, where appropriate, the circumstances and timing for it to be exchanged.

IALA will continue to maintain the master list of radiobeacon DGNSS stations and should be notified ofany changes. IALA will provide a formatted DGNSS almanac file on the Internet for the purpose ofdownloading. The format of the almanac file is a comma delimited ASCII file. The format has a shortheader that provides for general information about the beacon almanac followed by the site(s)information terminated with an asterisk.

The file contains the following information:

Header information:

File Name, Last Time Updated, Organisation (Country) and Total Number of Sites in the file.

Site Information:

Each site will have the following fields of information with commas to separate each field.

Field Attribute

Site name Max. 20 ASCII characters (including white space)

Transmitter ID Integer, 0-999

RS1 ID Integer, 0-999

RS2 ID Integer, 0-999

Latitude DD MM

North/South N/S

Longitude DDD MM

East/West E/W

Radio frequency Float, 283.5-325.0

Datum Integer, 0 = WGS-84 for GPS or PE90 for GLONASS,

1 = Local DatumOfficial operational Status 0 = fully operational

1 = test/preliminary

2 = not in operation


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13. REFERENCES

1. International Telecommunications Union - Radiocommunication Sector (ITU-R), “TechnicalCharacteristics of Differential Transmissions For Global Navigation Satellite Systems (GNSS)from Maritime Radio Beacons in the Frequency Band 283.5-315 kHz in Region 1 and 285-325kHz in Regions 2 and 3” Recommendation M.823-2, Geneva 1997.

2. IMO Resolution 815(19), “World-Wide Radionavigation System”, Publications Section,International Maritime Organization, 4 Albert Embankment, London SE1 7SR, Tel: +441725873241, Fax: +44 1715873210

3. Radio Technical Commission for Maritime Services, “RTCM Recommended Standards forDifferential GNSS (Global Navigation Satellite Systems) Service Version 2.2”, RTCM SpecialCommittee NO.104, Radio Technical Commission for Maritime Services, 1800 Diagonal Drive,Suite 600, Alexandria, Virginia 22314-2840 U.S.A.

4. IMO Resolution 860(20), “Maritime Policy for a Future Global Navigation Satellite System(GNSS)”, Publications Section, International Maritime Organization, 4 Albert Embankment,London SE1 7SR, Tel: +44 1725873241, Fax: +44 1715873210

5. ITU Radio Regulations Volume 1, Geneva 1998.

6. ITU, “Groundwave Propagation Curves for the frequencies between 10kHz and 30MHz”.ITU-R P.368-7, Geneva 1992.

7. ITU, “World Atlas of Ground Conductivities,” ITU-RP.832-2, Geneva 1999.

8. ITU, “Prediction of Skywave field strength at frequencies between about 150 and 1 700kHz.”ITU-R P.1147-1, Geneva 1999.

9. Last, J.D., Poppe, D.C., “Effect of skywave interference on coverage of radiobeacon DGPSstations.” IEE Proceedings on Radar, Sonar and Navigation, Vol.144, No.3, June 1997, pp.163-168.

10. Poppe, D.C., “Coverage and Performance Prediction of DGPS Systems EmployingRadiobeacon Transmissions.” Ph.D. Thesis, University of Wales, Bangor, October 1995.

11. CCR, “Characteristics and Applications of Atmospheric Noise Data,” ITU Report 322-3,Geneva 1990.

12. ITU. “Radio Noise”, ITU-R P.372-6, Geneva 1996.

13. Enge et al, “The Impact of Precipitation Static on the Range and Availability of a RadiobeaconBroadcast of Differential GPS Data”. DSNS ‘95, 1995.

14. Speckter, H.E, Hoppe, M, and Walterfang, M, “Performance of DGPS Beacon Receivers in aNoisy Environment”, XVth IALA CONFERENCE, HAMBURG 1998, “Radio Aids toNavigation”, AISM/IALA Headquarters, Tel: +33134517001 Fax: +33134518205 E-mail:[email protected]

15. Goodman, C.W.S. and Ward, N, “Procedure For Measuring Radiobeacon Antennae”, GeneralLighthouse Authorities of the United Kingdom and Ireland, Development DepartmentInformation Sheet INF.47 Issue1. Trinity House Lighthouse Service. East Cowes. Isle of Wight.Tel +44 1983 292651 Fax: +44 1983 294 317. E-mail: [email protected]


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16. International Electrotechnical Commission (IEC), 61108-1(under review) “Global navigationsatellite systems (GNSS), Part 1: Global positioning system (GPS) ‘ Receiver equipment ‘Performance standards, methods of testing and required results.” 1996-06; and,61108-2 (under review)“Maritime navigation and radiocommunication equipment and systems‘ Global navigation satellite systems, Part 2: Global navigation satellite system (GLONASS) ‘Receiver equipment ‘ Performance standards, methods of testing and required results.” 1998-06.

17. International Electrotechnical Commission (IEC), Future Publication 61108-4 “ShipborneDGPS and DGLONASS Maritime Radio Beacon Receiver Equipment”

18. Lysejko, A and Dziewicki, M, “Implementation of Satellite Navigation - DGPS System inPoland during1995/97”, XVth IALA CONFERENCE, HAMBURG 1998, “Radio Aids toNavigation” AISM/IALA Headquarters, Tel: +33134517001 Fax: +33134518205 E-mail:[email protected]

19. Royal Danish Administration for Navigation and Hydrography, “Description of the DanishSystem including Monitoring”, Tel: +45 32689500, Fax: +45 31574341

20. Swedish Maritime Administration, “Technical Characteristics for the Swedish Maritime DGPSService”, Tel: +46 11191512 Fax: +46 11126791 E-mail: [email protected]

21. Swedish Maritime Administration “Study of Skywave Effects on Coverage for the MaritimeGPS Service”, Tel: +46 11191512 Fax: +46 11126791 E-mail:[email protected]

22. Finnish Maritime Administration, “Finnish DGPS Transmission Specification, 1998”,Downloadable from: www.fma.fi/radionavigation. Tel: +358204484262 Fax: +358204484470E-mail: [email protected]

23. United States Coast Guard, “Broadcast Standard for the USCG DGPS Navigation Service”,Tel: +17033135900 Fax: +17033135920 E-mail: [email protected]

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APPENDIX 1

DEFINITIONS AND GLOSSARY OF TERMS USED IN GNSS

Accuracy. The degree of conformance between the estimated or measured parameter of a craftat a given time and its true parameter at that time. (Parameters in this context may be positioncoordinates, velocity, time, angle, etc.)

- Absolute accuracy (Geodetic or Geographic accuracy). The accuracy of a position estimatewith respect to the geographic or geodetic coordinates of the Earth.

- Geodetic or Geographic accuracy. See Absolute accuracy.

- Predictable accuracy. The accuracy of the estimated position solution with respect to thecharted solution.

- Relative accuracy . The accuracy with which a user can determine position relative to that ofanother user of the same navigation system at the same time.

- Repeatable accuracy. The accuracy with which a user can return to a position whose co-ordinates have been measured at a previous time using uncorrelated measurements from thesame navigation system.

Augmentation. Any technique of providing enhancement to the GNSS in order to provide improvednavigation performance to the user.

- Satellite-based augmentation system (SBAS). A system providing additional satellite signalsin order to enhance the performance of the GNSS service.

- Ground-based augmentation system (GBAS). A system providing additional signals from aground-based station in order to enhance the performance of the GNSS service.

Availability. The percentage of time that an aid, or system of aids, is performing a required functionunder stated conditions. The non-availability can be caused by scheduled and/or unscheduledinterruptions

- Signal availability. The availability of a radio signal in a specified coverage area.

- System availability. The availability of a system to a user, including signal availability and theperformance of the user's receiver.

- Service (orSignal) Availability (based on signal strength, noise and number of broadcasts)

Continuity. The probability that, assuming a fault free receiver, a user will be able to determineposition with specified accuracy and is able to monitor the integrity of the determined position over the(short) time interval applicable for a particular operation within a limited part of the coverage area.

Continuity Time Interval. The period of time that it normally takes to complete a maneuver or seek safeanchorage. When determining Continuity, this time period is 3 hours.

Correction. The numerical value of a correction is the best estimate that can be made of the differencebetween the true and the measured value of a parameter. The sign is such that a correction that is to beadded to an observed reading is taken as positive.


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Coverage. The coverage provided by a radionavigation system is that surface area or space volume inwhich the signals are adequate to permit the user to determine position to a specified level ofperformance.

Differential system. An augmentation system whereby radionavigation signals are monitored at aknown position and the corrections so determined are transmitted to users in the coverage area.

Dilution of precision. The factor by which the accuracy of the GNSS position and time co-ordinates aredegraded by geometrical considerations of the constellation of GNSS satellites used by the receiver.

- Geometric dilution of precision (GDOP). The factor for the combined 3D-position and timeaccuracy.

- Position dilution of precision (PDOP). The factor for the 3D-position accuracy.

- Horizontal dilution of precision (HDOP). The factor for the horizontal position accuracy.

- Vertical dilution of precision (VDOP). The factor for the vertical accuracy.

- Time dilution of precision (TDOP). The factor for the time accuracy.

Distance root mean square (dRMS). The root mean square of the radial distances from the true positionto the observed positions obtained from a number of trials.

Failure. The unintended termination of the ability of a system, or part of a system, to perform itsrequired function.

Fix. A position determined by processing information from a number of navigation observations.

Fix rate. The number of fixes per unit time.

GLONASS. (Globalnaya Navigatsionnaya Sputnikovaya Sistema) - This is a space-based, radiopositioning, navigation and time-transfer system operated by the Government of the Russian Federation.

GNSS - Global Navigation Satellite System. A world-wide position, time and velocity radiodetermination system comprising space, ground and user segments. The system relates to the propertiesof the GNSS service plus the receiver.

GPS - Global Positioning System. This is a space-based, radio positioning, navigation and time-transfersystem operated by the United States Government.

GNSS service. The service relates to the properties of the signal in space provided by the space andground segments of the GNSS.

Integrated navigation system. A system in which the information from two or more navigation aids iscombined in a symbiotic manner to provide an output that is superior to any one of the component aids.

Integrity. The ability to provide users with warnings within a specified time when the system shouldnot be used for navigation.

Integrity monitoring. The process of the determination whether the system performance (or individualobservations) allow use for navigation purposes. Overall GNSS system integrity is described by threeparameters: the threshold value or alert limit, the time to alarm and the integrity risk. The output ofintegrity monitoring is, that individual (erroneous) observations or the overall GNSS system can not beused for navigation.

- Internal integrity monitoring is performed aboard a craft

- External integrity monitoring is provided by external stations


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IMO - International Maritime Organization

ITU - International Telecommunication Union

ITU-R - ITU Radiocommunication Sector

IEC - International Electrotechnical Commission

Latency. The time lag between the navigation observations and the presented navigation solution.

MTBF - Mean time between failures. The average time between two successive failures of a system orpart of a system.

MTBO - Mean time between outages.

MTRS - Mean time to restore system to full operation.

Navigation. The process of planning, recording and controlling the movement of a craft from one placeto another.

PZ-90 geodetic system. A consistent set of parameters used in GLONASS describing the size and shapeof the Earth, positions of a network of points with respect to the centre of mass of the Earth,transformations from major geodetic datums and the potential of the Earth, developed in 1990.

Radionavigation. The use of radio signals to support navigation for the determination of position ordirection, or for obstruction warning.

Receiver autonomous integrity monitoring (RAIM). A technique whereby the redundant informationavailable at a GNSS receiver is autonomously processed to monitor the integrity of the navigationsignals. (See also craft autonomous integrity monitoring.)

Redundancy. The existence of multiple equipment or means for accomplishing a given function inorder to increase the reliability of the total system.

Repeatability. The accuracy of a positioning system, taking into account only the random errors. Therepeatability is normally expressed in a 95% probability circle.

Root mean square error (RMS). RMS error refers to the variability of a measurement in one dimension.In this one dimensional case, the RMS error is also an estimate of the standard deviation of the errors.

RTCM - Radio Technical Commission for Maritime services

Threshold value (or alert limit) is the maximum allowable error in the measured position –duringintegrity monitoring - before an alarm is triggered.

Time to alarm . The time elapsed between the occurrence of a failure in the system and its presentationon the bridge.

True position (2D). The error-free latitude and longitude co-ordinates in a specified geodetic datum.

True position (3D). The error-free latitude, longitude and height co-ordinates in a specified geodeticdatum.

VTS - Vessel Traffic Services


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World geodetic system (WGS). A consistent set of parameters describing the size and shape of theEarth, positions of a network of points with respect to the centre of mass of the Earth, transformationsfrom major geodetic datums and the potential of the Earth.

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differential global navigation satellite system r-121

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