Navipedia - User contributions [en]

An intuitive approach to the GNSS positioning

2011-01-05T09:41:27Z

80.101.109.79:

{{Article Infobox2
|Category=FUNDAMENTALS
|Title={{PAGENAME}}
|Authors=Benoit Roturier, DGAC/STNA, France; Eric Chatre GSA, GNSS Supervisory Authority, Brussels, Belgium and Javier Ventura-Traveset, ESA, European Space Agency.
|Level=Medium
|YearOfPublication=2011
}}
The basic observable in a GNSS system is the time required for a signal to travel from the satellite (transmitter) to the receiver. This travelling time, multiplied by the speed of light, provides a measure of the apparent distance (pseudo range) between them.
The following example summarises, for a two-dimensional case, the basic ideas involved in the GNSS positioning:

Let's suppose that a lighthouse is emitting acoustic signals at regular intervals of 10 minutes and intense enough to be heard some kilometres away. Let's also assume a ship with a clock perfectly synchronised with the one in the lighthouse, receiving these signals at a time not being an exact multiple of 10 minutes, for
example, 20 seconds later <math>\left (t = n * 10^m + 20^s \right)</math>
These 20 seconds will correspond to the propagation time of sound from the lighthouse (transmitter) to the ship (receiver). The distance ρ between them can be obtained multiplying this value by the speed of sound <math>v \simeq 335 m/s</math>.
That is, ρ = 20 s * 335 m/s = 6.7 km.
Obviously, with a single lighthouse it is only possible to determine a single measure of distance. So, the ship could be at any point over a circle of radius ρ, see figure 1.1.

With a second lighthouse, the ship position will be given by the intersection of the two circumferences centred in the two lighthouses and radius determined by their distances to the ship (measured using the acoustic signals). In this case, the ship could be situated at any of the two points of intersection shown in figure 1.1. A third lighthouse will solve the previous ambiguity, nevertheless a rough knowledge of the ship position may allow us to proceed without the third lighthouse. For instance, in figure 1.1, one of the solutions falls on the ground (on an island).
[[File:Lighthouse 2D Positioning.png|none|thumb|400px|'''''Figure 1.1:''''' 2D positioning: With a single lighthouse, there is a circumference of possible ship locations. With two lighthouses, the possible solutions are reduced to two. In the figure one of them can be ruled out because it falls on an island.]]

==A deeper analysis of a 2-D pseudorange based positioning==
Right now, a perfect synchronism between lighthouses and ship clocks has been assumed, but in fact this is very difficult to assure. Notice that a synchronism error between these clocks will produce an erroneous measure of signal propagation time (because it is linked to such clocks) and, in consequence, an error in the range measurements.
Let's assume that the ship clock is biased by an offset <math>d \tau</math> regarding the lighthouses clocks (which are supposed to be fully synchronised). Thence, the measured ranges, <math>R_1\, and\, R_2</math>, will be shifted by an amount <math>dt\, =\, v.d \tau</math>:

<math>R_1 = \rho_1 + dt,\, R_2 = \rho_2 + dt \qquad \mbox{(1.1)}</math>

That is, the radius of the circles of figure 1.1 will vary by an unknown amount <math>d t</math>, see figure 1.2. From hereafter we will call <math>R_i</math> as pseudorange, because it contains an unknown error <math>d t</math>.
At first glance, it might seem that the intersection of these circles (with an undefined radius <math>R_i</math>) could reach any point on the plane (for an arbitrary <math>d t</math> value). However, they will only intersect on the branches of a hyperbola, whose foci are located at the two lighthouses, see figure 1.2. Indeed, as the clock offset <math>dt</math> cancels when differencing the pseudoranges, the possible ship locations must verify the following equation (which defines an hyperbola, see figure 1.2):

<math>R_1\, - \,R_2 = \rho_1\, - \,\rho_2\, = \,ctt. \qquad \mbox{(1.2)}</math>

[[File:Unknown Offset.png|none|thumb|400px|'''''Figure 1.2:''''' An (unknown) offset <math>d t</math> in the ship's clock produces a shift in the measured ranges (<math>R_i\, = \,\rho_i\, +\, dt</math>) (or pseudoranges), varying the circumference radius. But, as both pseudoranges <math>R_1\, and\, R_2</math> have been measured with the same clock, this offset cancels on the difference of ranges <math>R_1\, - \,R_2 = \rho_1\, - \,\rho_2\, = \,ctt.</math> Thence, the ship is located at a branch of the hyperbola <math>R_1\, - \,R_2\, =\, ctt.</math>]]

A third lighthouse will reduce the uncertainty in the ship position to only two possible solutions. Such solutions are given by the intersection of two hyperbolas as it is illustrated in figure 1.3. Notice that, after estimating the ship coordinates, its clock offset <math>d t</math> can be found from equation (1.1).
[[File:Unknown Offset 2.png|none|thumb|400px|'''''Figure 1.3:''''' 2-D positioning with an unknown user clock offset.]]

To complete this analysis, the figure 1.4 shows another geometrical construction where the solution is in the centre of a circle, with radius equal to the clock offset <math>d t</math>, and which is tangent to the three circles of radii <math>\rho_i</math> and centred in the lighthouses.

[[File:Unknown Offset3.png|none|thumb|400px|'''''Figure 1.4:''''' Geometrical view 2-D positioning, complementing the figure 1.3.]]

Finally, and in order to simplify the explanation, let's go back to the situation of figure 1.1 where the ship and lighthouses clocks are assumed fully synchronised.
If the range measurements were perfect, the sailor could determine his position as the intersection point of the two circles centred at F1 and F2 lighthouses.
However, the measurements are not exact, having some measurement error <math>\varepsilon</math>. Figures 1.5 and 1.6 illustrate how this measurement error is translated to the coordinates estimate as an uncertainty region, which depends on the geometry defined by the ship and lighthouses relative positions.

[[File:Unknown Offset4.png|none|thumb|400px|'''''Figure 1.5:''''' The measurement noise <math>\varepsilon</math> is translated to the position estimate as an uncertainty region.]]

==Translation to the 3-D GNSS positioning==
Although the preceding example corresponds to a two-dimensional case, the basic principle is the same in GNSS:
* '''Satellites''' (as the lighthouses): In the case of the lighthouses, one assumes that their coordinates are known. In the case of the GNSS satellites, the coordinates are calculated from the navigation data (ephemeris) transmitted by the satellites, see [[GNSS systems description]] and [[GNSS Time Reference, Coordinate Frames and Orbits]].
* '''Pseudorange measurements''': In GNSS positioning, as well as in the example, distances between receiver and satellites are measured from the travelling time of a signal (in GNSS, an electromagnetic wave) from the satellite to the receiver, see [[GNSS systems description]] and [[GNSS measurements and datapreprocessing]].

Other comments:
* '''Clocks synchronisation''': The satellite clocks are one of the most critical components of a GNSS system. In order to assure the stability of such clocks, GNSS satellites are equipped with atomic oscillators with high daily stabilities <math>\Delta f\, / \, f\, \simeq\, 10^{-13}\, -\, 10^{-14}</math>. However, despite this high stability satellite clocks accumulate some offsets along time. The satellite clock offsets are continuously estimated by the Ground Segment and transmitted to the users to correct the measurements<ref group="footnotes">A perfect synchronism was assumed between lighthouses clocks in the previous example.</ref> (see [[GNSS measurements and datapreprocessing]]). The receivers, on the other hand, are equipped with quartz-based clocks, much more economical but with a poorer stability (about 10-9). This inconvenient is overcome by estimating its clock offset together with the receiver coordinates, as in the previous example.
* '''From 2-D to 3-D positioning''': It is not difficult to extend the previous 2-D geometrical construction to 3-D case of GNSS positioning, and to show that at least 4 satellites are needed to compute the three receiver coordinates plus clock. In this case, the previous circles and hyperbolas are generalised to spheres and hyperboloids, which intersect in two possible solutions. For a ground receiver, one of such solutions is on the earth's surface and the other far away in the space. An algebraic method to compute these two solutions (the [[Bancroft method|Bancroft's method]]). Nevertheless, the usual way to solve this non linear problem is to linearise the equations around an approximate user position and solve iteratively (see [[Solving navigation equations]]).
* '''Dilution of precision (DOP)''': The geometry of the satellites, i.e., how the user sees them, affects the positioning error. This is illustrated in figure 1.6, where the size and shape of the region change depending on their relative position. This effect is called Dilution Of Precision (DOP) and it is studied in [[Predicted Accuracy: Dilution of Precision]].
:[[File:DOP effect in positioning.png|none|thumb|400px|'''''Figure 1.6:''''' DOP effect in positioning: 2-D illustration of the variation of the uncertainty region with the geometry.]]

==Notes==
<references group="footnotes"/>

[[Category:Fundamentals]]

Category:Fundamentals

2011-01-04T19:38:20Z

80.101.109.79: /* Description */

{| border=0 style="font-size: 90%"
|This category uses the form xxx.
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{| style="border:1px solid #aaa" cellpadding="5" cellspacing="2"
|-
| style="width:90%; vertical-align:top"|
===Description===
On this page you will find all GNSS fundamentals related articles on Navipedia.

A Global Navigation Satellite System (GNSS) involves a constellation of satellites orbiting at about twenty thousand kilometeres altitude over the earth surface, continuously transmitting signals that enable users to determine their three-dimensional position with global coverage.

| style="width:10%; vertical-align:top" |
[[File:Fundamentals.png|right|150px]]
|}

The SBAS Integrity Concept Standardised by ICAO: Application to EGNOS

2010-11-14T13:17:15Z

80.101.109.79: /* Example */

{{Article Infobox2
|Category=EGNOS
|Title={{PAGENAME}}
|Authors=Benoit Roturier, DGAC/STNA, France; Eric Chatre GSA, GNSS Supervisory Authority, Brussels, Belgium and Javier Ventura-Traveset, ESA, European Space Agency.
|Level=Medium
|YearOfPublication=2006
}}
==Abstract==
There have been a lot of debates, within the International Civil Aviation Organisation (ICAO) GNSS Panel (GNSSP) group of experts<ref>Currently (2006) known as Navigation System Panel (NSP).</ref>, on the proper way to ensure SBAS user safety while at the same time respecting the high availability requirement.
The group finally validated a method at the GNSSP Seattle meeting in June 2000 which is reproduced in the GNSS Standards And Recommended Practices (SARPs), published in November 2002<ref name="ICAO77">ICAO Amendment 77, Annex 10 to the Convention on International Civil Aviation, Aeronautical Telecommunications: International Standards and Recommended Practices, Volume 1, Radio Navigation Aids, November 2002.</ref> [1]. Although the technical relevant information for a SBAS system designer to implement the SBAS integrity concept is fully described in the SARPs, only the strict necessary information is reported there, and it is quite difficult to a non specialist to properly understand this important concept. Since the SBAS integrity concept is quite specific and new, some kind of complementary information to the SARPs was felt desirable. This is the main motivation of this paper, which will also illustrate how the integrity is being managed through the European EGNOS SBAS project.

==Introduction==
The integrity service of ICAO compliant GNSS systems may currently be provided by the three normalised augmentations known under the terms ABAS (Airborne Based Augmentation System), GBAS (Ground Based Augmentation System) and SBAS (Satellite Based Augmentation System)<ref name="ICAO77"/> [1]. ABAS integrity concept relies on the single observation through the airborne user receiver of redundant pseudo range information, while GBAS (resp. SBAS) integrity elaboration relies on the use of a single (resp. a network) of ground reference stations.

In addition to integrity service, GBAS and SBAS also provide to the user differential corrections to improve the precision in a restricted area around a single reference station for GBAS and over a wide area defined by a network of reference stations for SBAS.
Finally, the SBAS geo satellites also transmit a ranging navigation signal similar to a GPS satellite.

Therefore, the SBAS integrity service which is addressed here should protect the user from both:
* failures of GPS/GLONASS/GEO satellites (drifting or biased pseudo ranges) by detecting and excluding faulty satellites through the measurement of GPS signals with the network of reference ground stations;
* transmission of erroneous or inaccurate differential corrections. These erroneous corrections may in turn be induced from either:
** undetected failures in the ground segment;
** processing of reference data corrupted by the noise induced by the measurement and algorithmic process.
This last type of failure, which may occur when the system is in a nominal state (no GPS/GLONASS/GEO satellite failure, no ground segment/user equipment failure) is usually known as “fault free case”. Protection of the user against noise effects has been quite demanding during the process of definition and validation of the ICAO SBAS integrity concept. In fact, the potential for such non integrity events generated in fault free conditions is inherent to data measurement and processing, to provide users with basic and precise correction messages and is thus a permanent risk which has to be carefully managed. This has involved the definition of statistical error bounds called horizontal or vertical protection levels (HPL or VPL) which will be discussed in depth in section V.

Before dwelling in depth into the details of the elaboration of adequate parameters to protect users from non integrity events which might occur from system failure (section IV) or noise (section V), we will recall integrity requirements (section II) and integrity definitions (section 3).

==Integrity Requirements==
The elaboration of a high level fault tree for all phases of flight leading to a given objective in term of Target Level of Safety (TLS)<ref>The top TLS objective is that the probability of accident leading to hull loss should be inferior to 1.5 10-7 per flight.</ref> and further decomposition for a number of phases of flight into aircraft, airborne database and signal in space (SIS) contribution to this risk has been provided by the ICAO All Weather Operational Panel<ref name="ICAO AWOP15">ICAO AWOP/15 Report, 15th meeting, Montreal 26 September- 12 October 1994.</ref> (AWOP) [2]<ref name="ICAO AWOP16">ICAO AWOP/16 Report, 16th meeting, Montreal 23 June- 4 July 1997.</ref> [3].
[[File:Fault tree allocation for SBAS.jpg|none|thumb|400px|alt=Fault tree allocation for SBAS|'''''Figure 1:''''' Fault tree allocation for SBAS APV I, II and Cat I operations]]

The fault tree for approach with vertical guidance (APVI,II and Category 1 approach type) corresponding to the most demanding operations supported by SBAS derived from AWOP work is shown in Fig. 1<ref>The AWOP 2.10-7 figure for SIS integrity risk by approach (150 s) has been further decomposed by GNSSP into a 10-7/approach allocation for the ground system integrity risk and a 10-7/approach allocation for the fault free case.</ref>.
This paper will focus on Non Aircraft, signal in space (SIS) integrity risk corresponding to the bottom right part allocations of Fig. 1.
AWOP work has been used as input by GNSSP to define the high level integrity requirements summarised in Fig. 2.

{|{{Prettytable}}
|+ align="bottom"|'''''Figure 2:''''' ICAO SARPs high level integrity requirements on SIS.
!style="background-color:#D5D6D2"|Typical operation
!style="background-color:#D5D6D2"|Time to Alarm
!style="background-color:#D5D6D2"|Integrity
!style="background-color:#D5D6D2"|Hor. alert limit
!style="background-color:#D5D6D2"|Vert. alert limit
|-
|En-route
|5 mn
|1-10-7/h
|4 NM
|N/A
|-
|En-route
|15 s
|1-10-7/h
|2 NM
|N/A
|-
|En-route, Terminal
|15 s
|1-10-7/h
|1 NM
|N/A
|-
|NPA
|10 s
|1-10-7/h
|0.3 NM
|N/A
|-
|APV I
|10 s
|1-2x10-7/app
|40.0 m
|50 m
|-
|APV II
|6 s
|1-2x10-7/app
|40.0 m
|20 m
|-
|CAT I
|6 s
|1-2x10-7/app
|40.0 m
|15 - 10 m
|}

==Integrity Definitions==
The provisions for integrity in the SARPs are complex for a non expert, but also are the definitions of non integrity events and three levels of definitions may be identified which are further discussed in this section.
===High level definition of integrity===
The high level definition of integrity in the SARPs is ([1] §A.1):
A measure of the trust which can be placed in the correctness of the information supplied by the total system. Integrity includes the ability of a system to provide timely and valid warnings to the user (alerts).

It has to be noted that the integrity requirement in Fig. 2 includes both an alert limit in horizontal and vertical dimensions and an allocated time to warn the user. Moreover, the integrity is often specified by its inverse, integrity risk, as in Fig. 1. The integrity risk may be defined as the probability of providing a signal that is out of tolerance without warning the user in a given period of time.

The out of tolerance condition is defined in the SARPs in the user position domain. Although it might seem obvious from the high level definition of integrity given above that a non integrity event corresponds to the situation obtained when any user navigation system error (NSE) in horizontal or vertical dimensions is superior to Horizontal or Vertical Alert Limit (HAL or VAL), while not providing timely and valid warnings to the user, the definition which has been retained in the SARPs is a little bit more conservative (as shown in [4]), and is described in the next section.

'''The above situation (NSE > HAL or VAL) is often referenced as “Hazardously Misleading Information (HMI)” case.'''

===Non integrity event definition applicable to the ground system designer===
This definition (in the most demanding case of APVII or Cat I) may be found in [1] §B.3.5.7.5.1 :
“Given any valid combination of active data, the probability of an out-of-tolerance condition for longer than 5.2 consecutive seconds shall be less than 2 x 10-7 during any approach, assuming a user with zero latency. An out-of-tolerance condition is defined as a horizontal error exceeding the HPLSBAS or a vertical error exceeding the VPLSBAS (as defined in B.3.5.5.6).”
The Horizontal and Vertical Protection Level (HPL and VPL) are elaborated within the user receiver (cf [1] B.3.5.5.6) at each epoch by combining ground transmitted parameters, aircraft parameters and geometry of the user with respect to satellites used in the position calculation. They will be further discussed in section V.
This definition (NSE > HPL or VPL) is often referenced as “Misleading Information (MI)” case.
It has to be used by a SBAS system designer to prove by simulation and/or tests that the SBAS design is SARPs compliant with respect to integrity requirements. It is also a high level requirement for the calculation of ground parameters used in XPL elaboration by a SBAS system designer, as further discussed in section V.3.
However, since this definition implies the knowledge of the NSE, a standard user may obviously not apply this out of tolerance test to raise a flag in case of non-integrity event.

===Non integrity event definition applicable to a SBAS standard user===
The test to be done at user level to check the correctness of transmitted data is defined in SARPs ([1] §B.3.5.8.4.2):
“The receiver shall compute and apply horizontal and vertical protection levels defined in B.3.5.5.6”
This definition is not really explicit (!), but more may be found in the guidance material section ([1] §C.6.4.4):
“… If the computed HPL exceed the Horizontal Alert Limit (HAL) for a particular operation, SBAS integrity is not adequate to support that operation. The same is true for precision approach and APV operations, if the VPL exceeds the vertical alert limit (VAL).”
This test (HPL or VPL > HAL or VAL), which is implemented at each epoch, allows to declare the SBAS “system unavailable” for a given level of operation since in this case the probability of an MI (and HMI) event is high. Note that xPL and xAL (x stands either H or V) are now known by the user.
If a SBAS is SARPs compliant as defined in section II.2, then a user applying the above test will be protected to the required level.

===Example===
The three above discussed integrity tests (HMI, MI and system unavailable) appear more explicitly in figure 3:
[[File:Different non-integrity definitions and tests.jpg|none|thumb|400px|alt=different non-integrity definitions and tests|'''''Figure 3:''''' Example of the different non-integrity definitions and tests.]]

Another practical representation of these different cases is obtained through a 2D plot of the Vertical Position Error (VPE) against the VPL where each pixel corresponds to a measurement epoch as in Fig. 4. This is usually known in Europe as the Stanford diagram. Fig. 4 illustrates the trade off between integrity and availability (Stanford diagram) as obtained through EGNOS real measurements at the ESA EGNOS P.O. in Toulouse, France on March 2005. The diagonal traces the limit between the safe operation of the system (left side) and the unsafe conditions (right side). The EGNOS System is shown to be safe in the nominal test conditions of Fig. 4, with an availability of both APV-1 and APV-1 of 100% for this specific test period.

==References==
<references/>

[1] ICAO Amendment 77, Annex 10 to the Convention on International Civil Aviation, Aeronautical Telecommunications: International Standards and Recommended Practices, Volume 1, Radio Navigation Aids, November 2002.

[2] ICAO AWOP/15 Report, 15th meeting, Montreal 26 September- 12 October 1994.

[3] ICAO AWOP/16 Report, 16th meeting, Montreal 23 June- 4 July 1997.

[4] Liu Fan, “Analysis of Integrity Monitoring for The Local Area Augmentation System Using The GNSS”, PhD. Report, Ohio University, August 1998.

[5] RTCA, “Minimum Operational Performance Standards for Global Positioning System/Wide Area Augmentation System Airborne Equipment”, RTCA-DO 229 C, November 2001.

[6] Bruce DeCleene, “Defining Pseudo Range Integrity – Overbounding” ION Conference, September 2000.

[7] M. Tossaint, J. Samson, F. Toran, J. Ventura-Traveset, A Tadjine, I. Delgado, “The Stanford – ESA Integrity Diagram: Focusing on SBAS Integrity,” Part 1 of this book.

[[Category:EGNOS]]
[[Category:EGNOS Fundamentals]]

The SBAS Integrity Concept Standardised by ICAO: Application to EGNOS

2010-11-14T13:10:07Z

80.101.109.79: /* Integrity Requirements */

{{Article Infobox2
|Category=EGNOS
|Title={{PAGENAME}}
|Authors=Benoit Roturier, DGAC/STNA, France; Eric Chatre GSA, GNSS Supervisory Authority, Brussels, Belgium and Javier Ventura-Traveset, ESA, European Space Agency.
|Level=Medium
|YearOfPublication=2006
}}
==Abstract==
There have been a lot of debates, within the International Civil Aviation Organisation (ICAO) GNSS Panel (GNSSP) group of experts<ref>Currently (2006) known as Navigation System Panel (NSP).</ref>, on the proper way to ensure SBAS user safety while at the same time respecting the high availability requirement.
The group finally validated a method at the GNSSP Seattle meeting in June 2000 which is reproduced in the GNSS Standards And Recommended Practices (SARPs), published in November 2002<ref name="ICAO77">ICAO Amendment 77, Annex 10 to the Convention on International Civil Aviation, Aeronautical Telecommunications: International Standards and Recommended Practices, Volume 1, Radio Navigation Aids, November 2002.</ref> [1]. Although the technical relevant information for a SBAS system designer to implement the SBAS integrity concept is fully described in the SARPs, only the strict necessary information is reported there, and it is quite difficult to a non specialist to properly understand this important concept. Since the SBAS integrity concept is quite specific and new, some kind of complementary information to the SARPs was felt desirable. This is the main motivation of this paper, which will also illustrate how the integrity is being managed through the European EGNOS SBAS project.

==Introduction==
The integrity service of ICAO compliant GNSS systems may currently be provided by the three normalised augmentations known under the terms ABAS (Airborne Based Augmentation System), GBAS (Ground Based Augmentation System) and SBAS (Satellite Based Augmentation System)<ref name="ICAO77"/> [1]. ABAS integrity concept relies on the single observation through the airborne user receiver of redundant pseudo range information, while GBAS (resp. SBAS) integrity elaboration relies on the use of a single (resp. a network) of ground reference stations.

In addition to integrity service, GBAS and SBAS also provide to the user differential corrections to improve the precision in a restricted area around a single reference station for GBAS and over a wide area defined by a network of reference stations for SBAS.
Finally, the SBAS geo satellites also transmit a ranging navigation signal similar to a GPS satellite.

Therefore, the SBAS integrity service which is addressed here should protect the user from both:
* failures of GPS/GLONASS/GEO satellites (drifting or biased pseudo ranges) by detecting and excluding faulty satellites through the measurement of GPS signals with the network of reference ground stations;
* transmission of erroneous or inaccurate differential corrections. These erroneous corrections may in turn be induced from either:
** undetected failures in the ground segment;
** processing of reference data corrupted by the noise induced by the measurement and algorithmic process.
This last type of failure, which may occur when the system is in a nominal state (no GPS/GLONASS/GEO satellite failure, no ground segment/user equipment failure) is usually known as “fault free case”. Protection of the user against noise effects has been quite demanding during the process of definition and validation of the ICAO SBAS integrity concept. In fact, the potential for such non integrity events generated in fault free conditions is inherent to data measurement and processing, to provide users with basic and precise correction messages and is thus a permanent risk which has to be carefully managed. This has involved the definition of statistical error bounds called horizontal or vertical protection levels (HPL or VPL) which will be discussed in depth in section V.

Before dwelling in depth into the details of the elaboration of adequate parameters to protect users from non integrity events which might occur from system failure (section IV) or noise (section V), we will recall integrity requirements (section II) and integrity definitions (section 3).

==Integrity Requirements==
The elaboration of a high level fault tree for all phases of flight leading to a given objective in term of Target Level of Safety (TLS)<ref>The top TLS objective is that the probability of accident leading to hull loss should be inferior to 1.5 10-7 per flight.</ref> and further decomposition for a number of phases of flight into aircraft, airborne database and signal in space (SIS) contribution to this risk has been provided by the ICAO All Weather Operational Panel<ref name="ICAO AWOP15">ICAO AWOP/15 Report, 15th meeting, Montreal 26 September- 12 October 1994.</ref> (AWOP) [2]<ref name="ICAO AWOP16">ICAO AWOP/16 Report, 16th meeting, Montreal 23 June- 4 July 1997.</ref> [3].
[[File:Fault tree allocation for SBAS.jpg|none|thumb|400px|alt=Fault tree allocation for SBAS|'''''Figure 1:''''' Fault tree allocation for SBAS APV I, II and Cat I operations]]

The fault tree for approach with vertical guidance (APVI,II and Category 1 approach type) corresponding to the most demanding operations supported by SBAS derived from AWOP work is shown in Fig. 1<ref>The AWOP 2.10-7 figure for SIS integrity risk by approach (150 s) has been further decomposed by GNSSP into a 10-7/approach allocation for the ground system integrity risk and a 10-7/approach allocation for the fault free case.</ref>.
This paper will focus on Non Aircraft, signal in space (SIS) integrity risk corresponding to the bottom right part allocations of Fig. 1.
AWOP work has been used as input by GNSSP to define the high level integrity requirements summarised in Fig. 2.

{|{{Prettytable}}
|+ align="bottom"|'''''Figure 2:''''' ICAO SARPs high level integrity requirements on SIS.
!style="background-color:#D5D6D2"|Typical operation
!style="background-color:#D5D6D2"|Time to Alarm
!style="background-color:#D5D6D2"|Integrity
!style="background-color:#D5D6D2"|Hor. alert limit
!style="background-color:#D5D6D2"|Vert. alert limit
|-
|En-route
|5 mn
|1-10-7/h
|4 NM
|N/A
|-
|En-route
|15 s
|1-10-7/h
|2 NM
|N/A
|-
|En-route, Terminal
|15 s
|1-10-7/h
|1 NM
|N/A
|-
|NPA
|10 s
|1-10-7/h
|0.3 NM
|N/A
|-
|APV I
|10 s
|1-2x10-7/app
|40.0 m
|50 m
|-
|APV II
|6 s
|1-2x10-7/app
|40.0 m
|20 m
|-
|CAT I
|6 s
|1-2x10-7/app
|40.0 m
|15 - 10 m
|}

==Integrity Definitions==
The provisions for integrity in the SARPs are complex for a non expert, but also are the definitions of non integrity events and three levels of definitions may be identified which are further discussed in this section.
===High level definition of integrity===
The high level definition of integrity in the SARPs is ([1] §A.1):
A measure of the trust which can be placed in the correctness of the information supplied by the total system. Integrity includes the ability of a system to provide timely and valid warnings to the user (alerts).

It has to be noted that the integrity requirement in Fig. 2 includes both an alert limit in horizontal and vertical dimensions and an allocated time to warn the user. Moreover, the integrity is often specified by its inverse, integrity risk, as in Fig. 1. The integrity risk may be defined as the probability of providing a signal that is out of tolerance without warning the user in a given period of time.

The out of tolerance condition is defined in the SARPs in the user position domain. Although it might seem obvious from the high level definition of integrity given above that a non integrity event corresponds to the situation obtained when any user navigation system error (NSE) in horizontal or vertical dimensions is superior to Horizontal or Vertical Alert Limit (HAL or VAL), while not providing timely and valid warnings to the user, the definition which has been retained in the SARPs is a little bit more conservative (as shown in [4]), and is described in the next section.

'''The above situation (NSE > HAL or VAL) is often referenced as “Hazardously Misleading Information (HMI)” case.'''

===Non integrity event definition applicable to the ground system designer===
This definition (in the most demanding case of APVII or Cat I) may be found in [1] §B.3.5.7.5.1 :
“Given any valid combination of active data, the probability of an out-of-tolerance condition for longer than 5.2 consecutive seconds shall be less than 2 x 10-7 during any approach, assuming a user with zero latency. An out-of-tolerance condition is defined as a horizontal error exceeding the HPLSBAS or a vertical error exceeding the VPLSBAS (as defined in B.3.5.5.6).”
The Horizontal and Vertical Protection Level (HPL and VPL) are elaborated within the user receiver (cf [1] B.3.5.5.6) at each epoch by combining ground transmitted parameters, aircraft parameters and geometry of the user with respect to satellites used in the position calculation. They will be further discussed in section V.
This definition (NSE > HPL or VPL) is often referenced as “Misleading Information (MI)” case.
It has to be used by a SBAS system designer to prove by simulation and/or tests that the SBAS design is SARPs compliant with respect to integrity requirements. It is also a high level requirement for the calculation of ground parameters used in XPL elaboration by a SBAS system designer, as further discussed in section V.3.
However, since this definition implies the knowledge of the NSE, a standard user may obviously not apply this out of tolerance test to raise a flag in case of non-integrity event.

===Non integrity event definition applicable to a SBAS standard user===
The test to be done at user level to check the correctness of transmitted data is defined in SARPs ([1] §B.3.5.8.4.2):
“The receiver shall compute and apply horizontal and vertical protection levels defined in B.3.5.5.6”
This definition is not really explicit (!), but more may be found in the guidance material section ([1] §C.6.4.4):
“… If the computed HPL exceed the Horizontal Alert Limit (HAL) for a particular operation, SBAS integrity is not adequate to support that operation. The same is true for precision approach and APV operations, if the VPL exceeds the vertical alert limit (VAL).”
This test (HPL or VPL > HAL or VAL), which is implemented at each epoch, allows to declare the SBAS “system unavailable” for a given level of operation since in this case the probability of an MI (and HMI) event is high. Note that xPL and xAL (x stands either H or V) are now known by the user.
If a SBAS is SARPs compliant as defined in section II.2, then a user applying the above test will be protected to the required level.

===Example===
The three above discussed integrity tests (HMI, MI and system unavailable) appear more explicitly in figure 3:

==References==
<references/>

[1] ICAO Amendment 77, Annex 10 to the Convention on International Civil Aviation, Aeronautical Telecommunications: International Standards and Recommended Practices, Volume 1, Radio Navigation Aids, November 2002.

[2] ICAO AWOP/15 Report, 15th meeting, Montreal 26 September- 12 October 1994.

[3] ICAO AWOP/16 Report, 16th meeting, Montreal 23 June- 4 July 1997.

[4] Liu Fan, “Analysis of Integrity Monitoring for The Local Area Augmentation System Using The GNSS”, PhD. Report, Ohio University, August 1998.

[5] RTCA, “Minimum Operational Performance Standards for Global Positioning System/Wide Area Augmentation System Airborne Equipment”, RTCA-DO 229 C, November 2001.

[6] Bruce DeCleene, “Defining Pseudo Range Integrity – Overbounding” ION Conference, September 2000.

[7] M. Tossaint, J. Samson, F. Toran, J. Ventura-Traveset, A Tadjine, I. Delgado, “The Stanford – ESA Integrity Diagram: Focusing on SBAS Integrity,” Part 1 of this book.

[[Category:EGNOS]]
[[Category:EGNOS Fundamentals]]

The SBAS Integrity Concept Standardised by ICAO: Application to EGNOS

2010-11-14T13:04:39Z

80.101.109.79: /* Integrity Requirements */