39 glaser krag ispa 2008 praesentation

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Air Systems Division

Wandering “Performance Holes” in GNSS

Poster Session – ISPA 2008

Oswald Glaser, Thales ATM, Stuttgart

Holger Krag, Private Consultancy, Darmstadt

ISPA 200814 - 15 Oct 2008Bonn, Germany



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Wandering Performance Holes

Wandering “Performance Holes” in GNSS and RelatedTopics – ISPA 2008 – Poster Session

Authors

Oswald Glaser, Thales ATM, Stuttgart

Currently working as Systems Engineer at Thales ATM in Korntal Münchingen(near Stuttgart) on GNSS performance issues, including also future Galileo basedGBAS applications

Before: working on various projects applying GNSS in aeronautical applications

Holger Krag, private contribution to this publication, Darmstadt

from 2002 to 2006: responsible at Thales ATM for a Test Bed for the GalileoGMS; publications on impact of RAAN phase angle on performance of combined

GNSS Currently working at ESOC in Darmstadt on space debris




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Wandering Performance Holes – The Goals

General Goals of this Presentation

The Problem

GNSS performance simulations as well as performance measurements areaffected by variations (“performance beats”) over time that are difficult to explain

This makes it difficult to compare the results in performance determinationprovided by different parties

The presentation aims to indicate some causes and how to avoid them

The possible impact of inappropriate observation periods is discussed

Geographical spots of low performance (“Performance Holes”) are discussed, thatmay change shape and location over time

These considerations are extended from single to dual constellation

Based upon initial indications recommendations for future work are given

Observation periods shall be adjusted to real performance periods

The movements of “Performance Holes” shall be investigated on the basis of“House Keeping Strategies” of the GNSS service providers for single and dualconstellation in order to agree on common performance determination standards

Attention shall be given to the role of the initial RAAN phase angle for Galileo




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Overview

Overview on the Presentation

Issues Related to Observation Periods

The Single Constellation Case

GNSS Low Performance Areas are well known GPS-GBAS Low Performance Areas (Example)

Galileo-GBAS Low Performance Areas (Example)

Selection of a set of observation points

Do the “Performance Holes” move ? The Dual Constellation Case

Low Performance Areas in Dual Constellation

Performance Variations in Dual Constellation

Recommendations

References

Acknowledgements




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Adequate Observation Periods

Issues Related to Observation Periods

Actual general guidelines for observation periods

Observation Periods for Simulations as well as for Measurement Campaignsare taken as a multiple of a UTC day to ease the editing of the simulation

parameters However, the GNSS periods are a multiple of a sidereal day. The difference is

about 3 tenth of a percent. This is quite small. But the measurements in thattime gap can be average but also either very good or very bad. So this cancontribute to deviations between different measurements or simulations

The possible effects and pros and cons of the “correct” and simplifiedobservation periods should be investigated

Utilisation of Sub Periods

Already at the beginning of the Galileo investigations it was found [10] that theWalker Constellation used in Galileo shows sub-periods where a nearly identicalsituation appears like at the start, but satellites mutually change their role. Thiscan be used to limit the observation time to this sub-period, thus saving timeand effort.

In case of combined GPS and Galileo use, up to now a 10 Days observationperiod is used, as no matching sub periods were found so far.




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The Single Constellation Case

The Single Constellation Case - Overview

GNSS Low Performance Areas are well known GPS-GBAS Low Performance Areas (SF Example)

Galileo-GBAS Low Performance Areas (DF Example)

Selection of a set of observation points

Do the “Performance Holes” move ?




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GNSS Low Performance Areas are well known

Since the design studies of GPS it is well known, that a Global SatelliteNavigation System has areas of low performance of different shape anddifferent geographical distribution.

The below examples, showing the 98% DOP values over CONUS asperformance indicators, are taken from a lesson by AJ van Dierendonckin the year 2000 (Low performance represented by higher values).




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AMD-B

GPS-GBAS Low Performance Areas (Example)

Due to the in-homogeneous distribution of low performance spotsselection of a set of airport points or the selection of specificobservation areas may lead to different performance results

Impact of the selected coverage area:This example shows that also within the area “US-small”(60N-30N;120W-60W) the results will significantly depend

on the selection of sub-areas (see two indicated areas)

Single frequency GPS 24 AMD-B,

outage models applied,GSL F (2 CS), VAL=5m, coverage

“US-small” (=60N-30N;120W-60W),resolution 2°x2°x300s

Example: “Single Frequency GPS-24,

standard outage model applied, optimistic

Multipath (AMD-B class) assumption, twocritical satellites allowed” Does not meet CAT III Availability Requ.Simulation period: 24 Hours




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GPS-GBAS Low Performance Areas (Example)

Backup Slide:The full

simulation

result for the

previous page.

Availability of

Integrity

over a24 Hours

observation

period.




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Galileo GBAS Low Performance Areas (Example)

This example of a GBAS performance simulation shows that the Galileo 27-Walker-Constellation is less sensitive to changes in East-West.

Impact of the selected coverage area:This example shows significantly differentAvailability results whether selecting the area “US-large”

(=80N-20N;170W-40W) or the area “US-small” (60N-30N;120W-60W) which is indicated by a red rectangular

Note: When goingtowards latitudesnear the equator,also the Galileo

GBAS performanceshows sensitivityin East-Westdirection.

Example: “Dual Frequency

Galileo-27, standard outage,optimistic Multipath (AMD-B class),only 1 critical sat allowed” Does not meet 99.998 % for the

large area but for the small area(important note: no ephemeris

bounding applied in this example!)

Simulation period: 10 days




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Selection of a set of observation points (1)

Example (1) for a selection of observation points This figure (FAA) shows a set of airport locations for which GBAS was planned at

a certain time. In face of the inhomogeneous distribution of spots of lowperformance in this area, it could be reviewed, whether it is more significant totake into account the performance on exactly these locations, the performance

over the whole area or to think of the performance at a Worst User Location.




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Selection of a set of observation points (2)

Example (2) for a selection of observation points The following figure (EUROCAE WP) shows a set of 35 airport locations in

Europe and US that were used in some cases in the past for GPS and GalileoGBAS performance simulations. In case that several observation points are closeto be “touched” by a moving low performance spot, small changes may affect the

results. Also here the performance over the whole area or a consideration of theperformance at a Worst User Location could lead to more stable results.

-150 -100 -50 0 50 100 150

-80

-60

-40

-20

0

20

40

60

80




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Movement of “Performance Holes” in a free running GNSS

“Performance Holes”are changing shapeand location over time.Details see R. Piriz [9].

At the ION GNSS in 2005Ricardo Píriz et al [9] showed in“The Galileo Constellation Design:A Systematic Approach”, a Galileoconstellation optimised towardsminimal corrective satellite manoeuvres

The figures taken there from showhow the performance of a Galileoconstellation would change over

years without “House KeepingActions”. In that case “PerformanceHoles” are changing shapeand location over time.

However, these assumptions do notcomply with Galileo’s House Keeping.

The question, how and to what extent“Performance Holes” will move canonly be answered by long termSimulations, based upon Galileo’sHouse Keeping Strategy.




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General Issue: Appropriate area for observation Due to the inhomogeneous distribution of spots of low performance, the selection

of the observation points or observation area significantly affects the results of theperformance determination.

Selection of a set of airport points may not be sufficient, but also the selection of a

dedicated area, perhaps a WUL concept should be applied Do the low performance areas move, e.g. in East – West direction ?

Question 1: Can the low performance areas be affected by constant drifts, forexample due to the difference of UTC day (earth rotation period) and sidereal day(satellite revolution period), or, due to the RAAN drift (≈14°per year for GPS and≈

9°per year for Galileo) Answer 1: The orbital revolution of the constellation is not a multiple of a sidereal

day but adjusted to account for the RAAN drift so that each satellite periodicallymatches the same footprint. (This has for example advantages for elimination ofstationary and deterministic multipath)

Question 2: Despite of such drifts, what movements are possible ?

Answer 2: The requirements for the “House Keeping Strategy” of the GNSSservice provider usually say that the RAAN values shall not deviate more than +/-2°. From this we can assume that in worst case the “Performance Holes” canwander +/- 222 km However, these are heuristic findings, investigations areneeded to substantiate this.




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“Performance Holes” in Dual Constellation

Single Constellation Summary

Preliminary we can assume that for the minimally guaranteedconstellation the “Performance Holes” may only move in East Westdirection within a tolerance of +/- 222 km.

Dual Constellation Issues:

The performance of a dual constellation GNSS (e.g. Galileo and GPSor Galileo and Glonass) may be affected by variations over time due to

the relative attitude the two constellations. In addition to the +/- 222 km tolerances of each single GNSS, the

effects of the relative RAAN angle between these systems have to beconsidered

Due to the selection of the orbit radius the effect of the RAAN drift onthe ground-track is compensated.

These effects should be investigated in detail by simulations




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Low Performance Areas in Dual Constellation

Several publications in the past years [2], [3], [7], [8], pointed out that

The performance of a combined constellation depends on the initial

relative phase angle in the RAAN parameter of the two GNSSconstellations (Right Ascension of the Ascending Node)

The Phase angle may change over time, but the ground-tracks ofthe joint constellation will remain constant (only depending on the

initial value)

The below figure shows the 6 GPS orbital planes and the 3 Galileo orbital planes

in a simplified presentation (does not give account to the different inclinations)




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Performance Variations in Dual Constellation

In previous publications [2], [3], [7], [8], TheGNSS performance as a function ofdifferent initial RAAN phase angleshave been investigated

It was shown that “In-phase” constellations

have low, “out-of-phase” constellationshave higher performanceHence, the initial RAAN phase angle

between Galileo and GPS willdetermine the performanceachievable in combined use.

The “natural” relative RAAN drift (5°as adifference of GPS-14°minus Galileo-9°) has no impact on the ground-tracks

However, the differences in the RAANdrifts in GPS and Galileo will lead to

a time shift of the two constellationson their ground-tracks – this is onlymeasurable in scales of months oryears (requires long termsimulations considering house-keeping strategy)




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Recommendations

Recommendations on further actions and research This presentation provides only first indications of effects

Further investigations on these issues is recommended, possibly resulting thereafter incommon and agreed guidelines for measurement and simulation campaigns

Selection of appropriate Measurement and Simulation Period

In some cases it may be of advantage to adjust the measurement and simulation periods tothe real GNSS periods (approximately the sidereal day instead of UTC day) to avoid evensmall deviations

This may ease mutual comparison of simulations as well as with measurements

Investigation of the real movement of “Performance Holes” The change of location and shape of low performance areas was considered here on the

basis of heuristic findings only

The movement of the “performance holes” shall be observed by simulation and measurement,taking into account the “house keeping strategies” of the GNSS service provider

Investigation of long them effects in combined constellation The differences in the RAAN drift has no effect on the ground-tracks of the combined

constellations (however, the selection of the initial angle has)

The initial RAAN phase angle between GPS and Galileo should be selected such that thecombined constellation provides high performance (similar to the “out-of-phase” constellationin [7])

Changes over time in the ground-track timing of the Galileo and GPS constellations should beassessed by long term simulations based upon the “House Keeping Strategy”




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References

References [1] Bruce P. Ayati, Paul D. Massatt, „Feasibility of Managing a Dynamic Constellation to a

Fixed Constellation Definition“, ION Annual Meeting June 2001, Albuquerque, New Mexico

[2] Arian Leonard, Holger Krag, Gerard Lachapelle, Kyle O’Keefe, Carsten Huth, CedricSeynat, „Impact of GPS and GALILEO Orbital Plane Drifts on Interoperability Performance

Parameters“, European Navigation Conference ENC, GNSS 2003 Graz [3] A. Leonard, H. Krag, E. Blomenhofer, „Impact Of Orbital Precession On The Combined

Galileo-Gps Performance“, 54th International Astronautical Congress of the InternationalAstronautical Federation, the International Academy of Astronautics, and the InternationalInstitute of Space Law 29 September - 3 October 2003, Bremen, Germany

[4] ESA / Galileo Industries, „GALILEO Common Integrity Algorithm Assumptions for Service

Volume Simulations“, DOC. No.: GAL-TNO-GLI-SYST-I/0636, ISSUE: 1, 13 October 2004

[5] RTCA DO-245A

[6] RTCA DO-229B

[7] H. Krag, A. Leonard, P. Ranaudo, E. Blomenhofer, Global Implications of GPS and GalileoRAAN Drifts on GNSS Performance, ION GPS 2003, Portland, Oregon

[8] Clifford W. Kelley, Kenneth F. Davis, Daniel M. Nguyen, „GNSS – Coordinating the GPS-Galileo-GLONASS Constellations”, ION GNSS 2004,

[9] Ricardo Píriz, Belén Martín-Peiró, Miguel Romay-Merino, “The Galileo ConstellationDesign: A Systematic Approach”, ION GNSS 2005,

[10] H. Krag, “Sub-Periods in Galileo Constellation”, Internal Technical Note, Thales 2004




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Acknowledgements

Acknowledgements

The work presented here is a “spin off” of two recent projects where Thales ATMprovided contributions. These projects are GIANT and GalTeC.

A very fruitful e-mail discussion with Curtis Shively (MITRE) started who providedparticular explanation on his simulations done for RTCA DO-245/245A and who inaddition to that provided further data to support the comparison of results.

Following considerations in Steven Rowson’s (former Thales) publications on GPS-30 constellation for GBAS, a helpful e-mail exchange with Michael C. Moreau (NASA)started on possible future constellation changes in GPS that might affect theperformance of GBAS in “GPS only mode” as well as in case of the use of combinedconstellations.

With respect to RAAN drifts and changes in the relative alignment of Galileo withGPS an e-mail conversation with ESA’s Daniel Navarro-Reyes was of great help,pointing out that ESA’s “house keeping strategy for Galileo” is different from theapproach presented by Ricardo Piriz 2005 in [9]

39 glaser krag ispa 2008 praesentation

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