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European Geostationary Navigation Overlay Service (EGNOS)

European Geostationary Navigation Overlay Service (EGNOS)


Hi, my name is Boris Vassilev I am former associate professor at Technical University of Sofia and I worked for 10 years in eurocontrol at European Space Agency in the field of validation of EGNOS system-
European geostationary navigation overlay service. Today we will talk about
EGNOS system and this system is part of so-called augmentation systems. For the
improvement of the performance of global navigation satellite systems are used at
first satellite-based augmentation systems
with satellite in orbit, which send correction for position determination. The next
group is ground-based argumentation system, when we use ground stations for
navigation corrections. And the third group is aircraft based augmentation system (ABAS),
RAIM, Inertials, Baro Altimeter Baro speed meters and so on,
and so on. A satellite-based augmentation system is
a system that supports wide area of regional augmentations who they use
additional satellites broadcast messages. Such systems are commonly composed of
multiple ground stations, located at the accurately-survey points. Today we use the
wide – area augmentation system, operated by the United States Federal Aviation
Administration, the next one is the European geostationary navigation
overlay service EGNOS, operated by ESSP, on behalf of the European Union GSA the third one is multifunctional satellite augmentation system, operated by Japan’s
Ministry of Land, infrastructure and transport Japan civil aviation Bureau and
the the fourth one is GPS aided Geo augmented navigation system being operated by India. The next
one Glonass system, Russian system for
differential corrections and monitoring(SDCM), operated by Russia, and the last one is
the satellite navigation augmentation system, proposed by China. On this slide are shown
the areas covered by this satellite-based augmentation systems.
Here are shown the typical characteristics and requirements (ICAO) for
navigation when we use satellite-based augmentation systems for
navigation. we have several phases, or several stages
of flight. The first one is en-route, the second is en-route continental,
when we don’t fly over the oceans and seas, the fourth is initial
approach, intermediate approach, and non-precision approach and departure. And the
last one when requirements are stringent is approach
operations with vertical guidance. And here are given horizontal accuracy,
vertical accuracy, integrity, time-to-alert, horizontal alert limit,
vertical alert limit, continuity and availability. All these parameters are determined by
ICAO for different stages of flight. Why we use satellite-based augmentation system?
Practically, global navigation satellite systems are very precise systems in
comparison with other navigation system like inertial, like VOR, like
DMN and so on. But we haven’t confidence in their measurement. We are
not sure if we don’t use some satellite, which doesn’t work properly. And in this
case, the error is very big, very big amounts. And satellite-based augmentation
system improve integrity of the system ,that means that the crew of the
airplane will be alarmed within six seconds, for instance, that there is
something wrong in the proper work of the system. Here is shown a diagram system European
geostationary navigation overlay service. We have ground segments, which includes, at first range integrity monitoring stations, and
nowadays we have 39 range integrity monitoring stations, which spread all over
Europe and Africa at first. At second, we have ground segment which includes four
master control centers, nowadays we use 2 control centers in Rome, Fiumicino
Airport and in Madrid Torrejón Airport and two additional
stations in Germany and England. Instead of these 4 stations, we have and navigational
land Earth station, which sends navigational messages for upgrading
navigation information and navigation corrections sent by geosatellites.
Instead of that we have GPS constellation, Glonass constellation, and
future we will have Galileo constellation and Compass constellation as
navigational satellites. These satellites are augmented by three Geo satellites,
which use information from ground and send navigation corrections
to user segments, which include users of ground, sea and air. A geostationary
satellite lie on geostationary orbit, which is a circular geosynchronous orbit in the
plane of the Earth’s equator with a radius of approximately 42,164
kilometers (measured from the center of the Earth). A satellite in such an
orbit is at an altitude of approximately 36,000 kilometers above sea
level. That means that the satellite rotates with the same angular velocity
of the Earth, and all the time the satellites are over given point, and with constant
coordinates with WGS, geodetic coordinate system 84. This picture shows
the used nowadays geosatellites. Just to see how many satellites are used for
different purposes – for navigation, for meteorology, for communication, TV broadcasting and so on and so on. Practically, for navigation EGNOS uses up to 3
geosatellites. There are 4 basic system parameters, which describe any satellite-based augmentation system, accuracy, integrity, availability and
continuity. They must guarantee that the user is informed on
his position with sufficient accuracy and is alerted when the system exceeds
tolerance limits. For this aim, horizontal and vertical protection levels(PL) are
computed, determined by the receiver to protect users from potential degradation
of the system, expressed in terms of horizontal and vertical position errors.
Above a certain user level, called “Alert limit”. This alert limit is determined
for aviation purposes by the International Civil Aviation
Organization, for different stages of flight. A situation, in which the position
error exceeds the alert limit without annunciation is called non-integrity event.
If protection level exceeds the AL, the system is declared unavailable
since in this case the probability of system non-integrity is high. Decision heightis the specified height above
the ground in an instrument approach procedures in which the pilot or crew
must decide whether to initiate an immediate missed approach if the pilot does not see the required
visual reference, or to continue the approach. This decision, I think, is
different for different situations. I have in mind APV1 approach, APV2
approach, CAT1, CAT2 and so on. Example of the different non-integrity
definition is shown at this diagram. Practically, we have vertical navigation
system air and all the time satellite-based augmentation system,
included our receiver determines vertical position error. Requirements these all
the time vertical position has to be protection level, has to be higher, bigger than position error. Meanwhile, we have vertical alert limit, as I
said, determined by the International Civil Aviation Organization,
and when protection level is lower than AL, we have available
navigation system. In the case, if the protection level is higher then AL,
the system is unavailable. The case when navigation system error is
higher than PL, but lower than AL, is a case when we have misleading
information. And situation when PL is lower than SE, meanwhile SE is higher than AL, we have hazardous misleading information. Very
dangerous case. And the last situation when SE is higher than PL,
but both are higher than AL, that means that the crew will be alerted.
For this situation we have misleading information and system is
unavailable. Practically, in aviation, are used the so-called
Stanford-ESA integrity diagram: focusing on satellite-based augmentation system
integrity. A long x-axis we have, for instance, verticall protection position error,
along the y-axis we have protection level, in our case vertival protection and the
most acceptable case will be well all errors, all measurements are above the
line, determined by the both axis – X and Y, and in this case the system is
available, and we have no misleading information or integrity
events, as I explained, situation when position error is bigger than protection
level, is called misleading information. In the case when this misleading information
is bigger than protection alert limit, we will have hazardous misleading
information. Our talk will continue with explanation of EGNOS system service
provider. With their monthly performances report, concerning September,
part of September characteristics for 2015. Here we will see EGNOS characteristic availability
performances, the open services availability performance is defined as
the percentege of time when the instaneous HNSE is lower than 3 meters and the instanteneous VNSE is lower than 4 meters
over the total number of samples with valid PA navigation solution. So, here
we are shown for different stations and we can see that for Sofia availability is 99,73% and for different stations is
different and we saw that for Central Europe this availability is
close to 100%. So, the next diagram shows EGNOS approach with VNSE
availability and approach and this is is defined as the percentage of epochs in a month, in which the protection levels are
below alert limits for these services. Protection level
lower than 40 meters and vertical protection level lower then 50 metres
over the total period. And we saw that for Sofia protection level is
good, higher than 99.9%, but there are regions and these regions are in the border of EGNOS
coverage, where the availability is lower than 99, 97%. Here, we see the station’s range
integrity monitoring station, used for determination of EGNOS
parameters. Here are shown EGNOS APV availability for certified airports for
utilization of EGNOS approach. This diagram, this figure is shown only to see
how many airports in Europe are certified for utilization of EGNOS
approaches. And now, I will continue with EGNOS analysis for PRN 120 geosatellites,
which are used by EGNOS, for Europe for Burgas phase, where we have receiver Novatel(OEM
receiver) for 9 of October 2015. This report constitute a brief overview of the
performance of EGNOS signal in space as observed in Burgas, as I said, for the
period of 24 hours from 9 October. So, here we are shown the characteristics of this receiver – the lattitude, the longitude, the altitude, the GHS 84 coordinate system, the requirements for APV-1, APV-2, CAT-1 approaches Horizontal AL, vertical AL
image and determine the availability for this airport. As we saw, we have 97.791% for these days availability for APV-1 approach, for
APV-2, we have about 82% and for CAT-1 11%. That means that we don’t
satisfy the requirements of ICAL for instrumental approach using
EGNOS. Here is shown the type of navigational messages, sent by
geosatellites and content of any message. Meanwhile I want to say that any geosatellite,
sends any second navigation message and this navigation message
is different type. We use 64 navigation messages, an here are shown
navigation messages 1, up to 28 navigation messages, and we see that
any navigation message contained different information. For PRN mask and we
have to know this navigation message which satellites concerns. We have fast
correction navigation message from 2 up to 5 integrity information, fast
correction the degradation factor, GEO navigation message(time X Y and Z),
geosatellites almanac health and so on, mixed fast corrections, long term satellite error corrections.
Long term satellite error correction contains navigation message 25. Clock coverage metrics and so on and so on. Here we are shown message type over time for
Burgas, for this date. As we showed, there is an interuption in navigation messages caused by different reason. And this reasons are send
to users, which use navigation message, and we saw that we
have two interruptions in work of PRN120 and 1 interruption in PRN136 for this time
period. Next diagram is very interesting diagram. With blue line, we see horizontal
position error, this position error is within two metres, even lower than one metre.
Meanwhile, protection level, shown with green line, is within 20 metres from
6 metre up to 20 metres. What is the conclusion? The conclusion is that
protection level is too conservative measurement, and this is information which
crew knows. Practically, the crew in the air doesn’t know information for position error, but it is
convinced that the probability that position error is bigger tham PL
equals to 10, about 10 in power -7 (10E-7). That means that we know
our protection level. With red line is shown the number of used satellites, number of satellite
vehicles, which are used. As we showed, practically, we use satellites from 5
up to 13 satellites. And when we use 4 satellite the protection level is too
high. That means that geometricallt the level of
precision is very bleak and protection level is too conservative, because we are not
sure which satellites could be used for
position determination. We have three regions, when the protection level is
higher, within 20 metres, and we can see that three cases are characterized with
long number of used satellite, despite of these that we see at least 8,9,10 satellites.
This is caused by the reason that Burgas is on the border of EGNOS region.
Practically, information for all visible satellites, this receiver hasn’t.
The receiver has information only for satellites, which are over Europe and close to the
border. But satellites which are all over Russia or Asian and so on and are
visible for receiver, are not used by this this receiver when this receiver
uses corrections, received by geosatellites. This picture shows once again
information for position, vertical position, vertical protection level and once
again number of used satellites. And this number
is the same like when we determine horizontal protection level. And we see
once again the position error is very low despite of the vertical position level is little bit higher, bigger than horizontal
position error. Here is shown measured horizontal deviation from reference. We
have to have in mind that the coordinates of the antenna used in Burgas is determined
with very high accuracy. This accuracy is in WGS. This accuracy is within 2+-2 centimeters. This accuracy in
position determination of the antenna give us possibility to determine
our error and this diagram shows our error in horizontal plane deviation from
the reference, from the exact antenna position, and we see this deviation is within
2 meters in east direction, this deviation is lower than 1 meter, in Western deviation this
error is lower than 2meters, and in northern and South deviation, we have, once
again error, which is in lower than 2 metres. Thank you for your attention!


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