ion gnss+ 2014 tampa, fl sept. 8-12, 2014 sbas satellite selection and performance monitoring at the...

ION GNSS+ 2014ION GNSS+ 2014Tampa, FLTampa, FL

Sept. 8-12, 2014Sept. 8-12, 2014

SBAS Satellite Selectionand Performance Monitoring at the Region

Where Multiple SBAS are Available

SBAS Satellite Selectionand Performance Monitoring at the Region

Where Multiple SBAS are Available

Takeyasu Sakai, Kazuaki Hoshinoo, and Ken ItoElectronic Navigation Research Institute

Takeyasu Sakai, Kazuaki Hoshinoo, and Ken ItoElectronic Navigation Research Institute

ION GNSS+ Sept. 2014ION GNSS+ Sept. 2014 - Slide - Slide 22

IntroductionIntroduction• SBAS Selection Problem:SBAS Selection Problem:

– We already have four operational SBAS served by the US, Japan, Europe, We already have four operational SBAS served by the US, Japan, Europe, and India;and India;

– A possible issue for users in Enroute to NPA flight modes: An SBAS does not A possible issue for users in Enroute to NPA flight modes: An SBAS does not broadcast any information on the service area of SBAS nor information on broadcast any information on the service area of SBAS nor information on which SBAS should be used; The current SBAS specifications do not define which SBAS should be used; The current SBAS specifications do not define any procedure, or satellite selection criteria, to be used by receivers to choose any procedure, or satellite selection criteria, to be used by receivers to choose SBAS satellites for use;SBAS satellites for use;

– In order to investigate the situation, the authors considered the expected NPA In order to investigate the situation, the authors considered the expected NPA (or RNP0.3) availability for some SBAS selection criteria based on the actual (or RNP0.3) availability for some SBAS selection criteria based on the actual broadcast message of MSAS and GAGAN.broadcast message of MSAS and GAGAN.

• Necessity of Monitoring:Necessity of Monitoring:– Monitoring the performance of the system operated by other StatesMonitoring the performance of the system operated by other States in a in a

specific airspace as a responsibility of ANSP for the airspace;specific airspace as a responsibility of ANSP for the airspace;

– As an example, a result of evaluation of the performance of GAGAN in As an example, a result of evaluation of the performance of GAGAN in Japanese territory is shown.Japanese territory is shown.


MotivationMotivation

• India began the operation of GAGAN in Feb. 2014 by removing MT0;India began the operation of GAGAN in Feb. 2014 by removing MT0;• In Japan, we can receive GAGAN signals in addition to MSAS signals;In Japan, we can receive GAGAN signals in addition to MSAS signals;• Which SBAS does the SBAS-receiver use for augmentation?Which SBAS does the SBAS-receiver use for augmentation?

Likely MSAS, but using GAGAN is allowed and possible;Likely MSAS, but using GAGAN is allowed and possible; There is some reports on observation of some receivers using GAGAN.There is some reports on observation of some receivers using GAGAN.

Geostationary Satellites:Geostationary Satellites:

MSASMSAS MTSAT-1R @140EMTSAT-1R @140EMTSAT-2 @145EMTSAT-2 @145E

GAGANGAGAN GSAT-8 @55EGSAT-8 @55EGSAT-10 @83EGSAT-10 @83E

GSAT-8GSAT-8

GSAT-10GSAT-10

MTSAT-1RMTSAT-1R

MTSAT-2MTSAT-2


MSAS NPA-Mode AvailabilityMSAS NPA-Mode Availability

• NPA mode (RNP0.3 for Enroute) availability for 3 days of May 25 to 27, 2014;NPA mode (RNP0.3 for Enroute) availability for 3 days of May 25 to 27, 2014;• MSAS has been serving NPA mode horizontal navigation with 100% availability over almost whole of MSAS has been serving NPA mode horizontal navigation with 100% availability over almost whole of

Fukuoka (Japanese) FIR;Fukuoka (Japanese) FIR;• In condition of 1 critical GPS out, the area with NPA service is largely reduced and roughly half of the FIR is In condition of 1 critical GPS out, the area with NPA service is largely reduced and roughly half of the FIR is

supported with 100% availability.supported with 100% availability.

Without GPS Outage 1 Critical GPS Out

AvailableAvailable


GAGAN NPA-Mode AvailabilityGAGAN NPA-Mode Availability

• For GAGAN, the area with NPA service is larger than MSAS;For GAGAN, the area with NPA service is larger than MSAS;• Without any GPS satellite outage, NPA mode availability is 100% over almost whole of Without any GPS satellite outage, NPA mode availability is 100% over almost whole of

India FIR, while the area with NPA service is largely reduced if 1 critical GPS is out;India FIR, while the area with NPA service is largely reduced if 1 critical GPS is out;• In Japan, GAGAN can provide NPA service with 95 to 100% availability.In Japan, GAGAN can provide NPA service with 95 to 100% availability.

Without GPS Outage 1 Critical GPS Out

AvailableAvailable


Other SystemsOther Systems• Russian SDCM (System for Differential Corrections and Monitoring):Russian SDCM (System for Differential Corrections and Monitoring):

– Already broadcasting test signal from their geostationary satellites.Already broadcasting test signal from their geostationary satellites.

– Consists of 3 geostationary satellites:Consists of 3 geostationary satellites: Luch-5A launched in 2011 at 167ELuch-5A launched in 2011 at 167E Luch-5B launched in 2012 at 16WLuch-5B launched in 2012 at 16W Luch-5V to be launched in 2014 and placed at 95ELuch-5V to be launched in 2014 and placed at 95E

– In Japan, signals from Luch-5A and Luch-5V are possibly received; We have In Japan, signals from Luch-5A and Luch-5V are possibly received; We have already received PRN140 signal from Luch-5A.already received PRN140 signal from Luch-5A.

• Chinese SNAS:Chinese SNAS:– Served as a part of BeiDou Satellite Navigation System;Served as a part of BeiDou Satellite Navigation System;

– Consists 3 Geostationary satellites at 80E, 110E, and 140E; All signals can be Consists 3 Geostationary satellites at 80E, 110E, and 140E; All signals can be received in Japan.received in Japan.

• Korean K-SBAS:Korean K-SBAS:– Potentially 1 or 2 more SBAS geostationary satellites by around 2020.Potentially 1 or 2 more SBAS geostationary satellites by around 2020.

SBAS in the WorldSBAS in the WorldION GNSS+ Sept. 2014ION GNSS+ Sept. 2014 - Slide - Slide 77

SDCMSDCM

SNASSNAS

SACCSASACCSA

OperationalOperational OperationalOperational

OperationalOperational

Since 2003Since 2003 Since 2011Since 2011 Since 2007Since 2007Since 2014Since 2014

OperationalOperational


SBAS Selection IssueSBAS Selection Issue• SBAS does not broadcast any information for selection:SBAS does not broadcast any information for selection:

– No information on the service area of SBAS and no information on which No information on the service area of SBAS and no information on which SBAS should be used by a user in NPA mode;SBAS should be used by a user in NPA mode;

– For PA mode, FAS (Final Approach Segment) data specifies SBAS to be For PA mode, FAS (Final Approach Segment) data specifies SBAS to be used.used.

• SBAS specifications:SBAS specifications:– The ICAO SBAS SARPS and RTCA WAAS MOPS do not define the specific The ICAO SBAS SARPS and RTCA WAAS MOPS do not define the specific

procedure, or satellite selection criteria, to be used by receivers.procedure, or satellite selection criteria, to be used by receivers.

– The RTCA MOPS states an option for SBAS selection as a note:The RTCA MOPS states an option for SBAS selection as a note:

““Selecting the SBAS satellite with the highest elevation angle that is broadcasting Selecting the SBAS satellite with the highest elevation angle that is broadcasting

applicable ionospheric corrections is normally sufficient.applicable ionospheric corrections is normally sufficient.””

– Receiver manufacturers have no restriction on the algorithm for the selection; Receiver manufacturers have no restriction on the algorithm for the selection; Each user receiver has its own satellite selection scheme if it is tracking Each user receiver has its own satellite selection scheme if it is tracking multiple SBAS signals; The situation depends on the implementation.multiple SBAS signals; The situation depends on the implementation.

• What are possible criteria for SBAS selection?What are possible criteria for SBAS selection?


Possible CriteriaPossible Criteria• SBAS Satellite Selection Criteria:SBAS Satellite Selection Criteria:

– In the literature by Jed Dennis at the ION ITM 2014, some possible criteria are In the literature by Jed Dennis at the ION ITM 2014, some possible criteria are listed and evaluated;listed and evaluated;

– Here we consider possible criteria which do not need human interaction Here we consider possible criteria which do not need human interaction and/or database information.and/or database information.

• Elevation Angle [EL]:Elevation Angle [EL]:– Selects SBAS signal coming from the highest elevation angle;Selects SBAS signal coming from the highest elevation angle;

– SBAS selection becomes a function of longitude of receiver location only; SBAS selection becomes a function of longitude of receiver location only;

– No direct relationship with distribution of SBAS ground network which No direct relationship with distribution of SBAS ground network which dominates the spatial extent to which the augmentation information can be dominates the spatial extent to which the augmentation information can be applied.applied.


Possible CriteriaPossible Criteria• Elevation Angle and Ionosphere [ELi]:Elevation Angle and Ionosphere [ELi]:

– Selects SBAS signal coming from the highest elevation angle and providing Selects SBAS signal coming from the highest elevation angle and providing the applicable ionospheric correction information;the applicable ionospheric correction information;

– Straightforward implementation along with the note of MOPS;Straightforward implementation along with the note of MOPS;

– Here we assume the determination if the applicable ionospheric information is Here we assume the determination if the applicable ionospheric information is or not is done by testing the availability of IGPs at the user location;or not is done by testing the availability of IGPs at the user location;

– This option does not directly reflect the distribution of ground network, but This option does not directly reflect the distribution of ground network, but ionosphere information cannot be obtained at the location far away from the ionosphere information cannot be obtained at the location far away from the service area.service area.

• Protection Levels [HPL] [VPL]:Protection Levels [HPL] [VPL]:– Selects SBAS signal giving lower protection level (HPL or VPL) which means Selects SBAS signal giving lower protection level (HPL or VPL) which means

better performance;better performance;

– In the NPA mode, it is natural to employ HPL for the selection because SBAS In the NPA mode, it is natural to employ HPL for the selection because SBAS gives just a horizontal navigation; Selection based on VPL is also possible;gives just a horizontal navigation; Selection based on VPL is also possible;

– Advantage: Reflects the performance of each SBAS in real time.Advantage: Reflects the performance of each SBAS in real time.

– Computational load inside the receiver is relatively high.Computational load inside the receiver is relatively high.


Possible CriteriaPossible Criteria• Number of Satellites [NSV]:Number of Satellites [NSV]:

– Selects SBAS signal which gives the largest number of GPS satellites Selects SBAS signal which gives the largest number of GPS satellites available;available;

– Reflects the spatial relationship between each GPS satellite and the ground Reflects the spatial relationship between each GPS satellite and the ground network in real time.network in real time.

• Degradation of Correction [dUDRE]:Degradation of Correction [dUDRE]:– Selects SBAS signal giving less dUDRE;Selects SBAS signal giving less dUDRE;– The dUDRE indicates spatial degradation of the accuracy of correction The dUDRE indicates spatial degradation of the accuracy of correction

information for each GPS satellite; Basically dUDRE grows larger if the GPS information for each GPS satellite; Basically dUDRE grows larger if the GPS satellite goes away from the ground monitoring network;satellite goes away from the ground monitoring network;

– This criterion is computed for each individual GPS satellite, not for the SBAS This criterion is computed for each individual GPS satellite, not for the SBAS itself; Here we assume that the receiver compares the number of GPS itself; Here we assume that the receiver compares the number of GPS satellites which gives lower dUDRE than the other SBAS, and selects the satellites which gives lower dUDRE than the other SBAS, and selects the SBAS which have the largest number of such GPS satellites.SBAS which have the largest number of such GPS satellites.

– Reflects the spatial relationship between each GPS satellite and the ground Reflects the spatial relationship between each GPS satellite and the ground network in real time.network in real time.

– The computational load would be medium.The computational load would be medium.


Possible CriteriaPossible Criteria

Criteria DesignatorReflecting

Performance

Reflecting Spatial

Relationship

Computational Load

Elevation Angle [EL] No No Low

Elevation Angle and Ionosphere

[ELi] No A Little Medium

Horizontal Protection Level

[HPL] Yes Yes High

Vertical Protection Level

[VPL] Yes Yes High

Number of Satellites

[NSV] No Yes Low

Degradation of Correction

[dUDRE] No Yes Medium

• Here we assume GAGAN-preferred receiver: It is assumed that GAGAN is selected Here we assume GAGAN-preferred receiver: It is assumed that GAGAN is selected in case of the equal evaluation between two SBAS for [NSV] and [dUDRE].in case of the equal evaluation between two SBAS for [NSV] and [dUDRE].


SBAS Selection [EL] [ELi]SBAS Selection [EL] [ELi]

• GAGAN selection rate for [EL] and [ELi];GAGAN selection rate for [EL] and [ELi];• SBAS selection is a function of longitude of user location;SBAS selection is a function of longitude of user location;• The border is 111.5E = mean of 83E (GSAT-10) and 140E (MTSAT-1R);The border is 111.5E = mean of 83E (GSAT-10) and 140E (MTSAT-1R);• This option has the temporal invariance: The result is simply 0% or 100%;This option has the temporal invariance: The result is simply 0% or 100%;• With ionosphere option, the spatial relationship is reflected a little.With ionosphere option, the spatial relationship is reflected a little.

Selection by [EL] Selection by [ELi]


SBAS Selection [HPL] [VPL]SBAS Selection [HPL] [VPL]

• GAGAN selection rate for [HPL] and [VPL];GAGAN selection rate for [HPL] and [VPL];• Show which SBAS gives the lower HPL/VPL between MSAS and GAGAN as a function of user location.Show which SBAS gives the lower HPL/VPL between MSAS and GAGAN as a function of user location.• The performances of MSAS and GAGAN are different; At the transition region, GAGAN likely provides The performances of MSAS and GAGAN are different; At the transition region, GAGAN likely provides

the better performance than MSAS.the better performance than MSAS.

Selection by [HPL] Selection by [VPL]


SBAS Selection [NSV] [dUDRE]SBAS Selection [NSV] [dUDRE]

• GAGAN selection rate for [NSV] and [dUDRE];GAGAN selection rate for [NSV] and [dUDRE];• [NSV] Smooth and linear characteristics along with a line connecting the centroids of ground networks of both [NSV] Smooth and linear characteristics along with a line connecting the centroids of ground networks of both

SBAS.SBAS.• [dUDRE] Possibly remote monitor stations of MSAS in Hawaii and Australia contribute to the better orbit [dUDRE] Possibly remote monitor stations of MSAS in Hawaii and Australia contribute to the better orbit

determination and thus to provide the lower dUDRE.determination and thus to provide the lower dUDRE.

Selection by [NSV] Selection by [dUDRE]


NPA Availability: GAGAN OnlyNPA Availability: GAGAN Only

• Baseline GAGAN NPA availability with no SBAS selection: A case that MSAS Baseline GAGAN NPA availability with no SBAS selection: A case that MSAS is not available.is not available.

GAGAN Only GAGAN Only (1 GPS OUT)


NPA Availability [EL]NPA Availability [EL]

• NPA availability with SBAS selection of the option [EL];NPA availability with SBAS selection of the option [EL];• Switching two SBAS improves availability largely and expand the area where NPA mode is Switching two SBAS improves availability largely and expand the area where NPA mode is

available; Navigation is available if either two SBAS is available;available; Navigation is available if either two SBAS is available;• But this option is not the optimum solution.But this option is not the optimum solution.

Selection by [EL] Selection by [EL] (1 GPS OUT)


NPA Availability [ELi]NPA Availability [ELi]

• NPA availability with SBAS selection of the option [ELi];NPA availability with SBAS selection of the option [ELi];• Ionosphere restriction reduces availability at the edge of the service area of either Ionosphere restriction reduces availability at the edge of the service area of either

SBAS.SBAS.

Selection by [ELi] Selection by [ELi] (1 GPS OUT)Selection by [ELi] Selection by [ELi] (1 GPS OUT)


NPA Availability [HPL]NPA Availability [HPL]

• NPA availability with SBAS selection of the option [HPL];NPA availability with SBAS selection of the option [HPL];• This option gives the best availability characteristics among candidate criteria; This This option gives the best availability characteristics among candidate criteria; This

option is the optimum solution giving the largest area of 100% NPA availability.option is the optimum solution giving the largest area of 100% NPA availability.

Selection by [HPL] Selection by [HPL] (1 GPS OUT)


NPA Availability [VPL]NPA Availability [VPL]

• NPA availability with SBAS selection of the option [VPL];NPA availability with SBAS selection of the option [VPL];• The characteristics is similar to [HPL] but sub-optimum because availability of The characteristics is similar to [HPL] but sub-optimum because availability of

NPA mode is determined based on HPL.NPA mode is determined based on HPL.

Selection by [VPL] Selection by [VPL] (1 GPS OUT)


NPA Availability [NSV]NPA Availability [NSV]

• NPA availability with SBAS selection of the option [NSV];NPA availability with SBAS selection of the option [NSV];• Availability is similar to [HPL] and [VPL] in case of no GPS outage;Availability is similar to [HPL] and [VPL] in case of no GPS outage;• In case of 1 critical GPS out, availability is better than [EL] and [ELi], but lower than [HPL] In case of 1 critical GPS out, availability is better than [EL] and [ELi], but lower than [HPL]

and [VPL], somewhere for example Philippine Sea.and [VPL], somewhere for example Philippine Sea.

Selection by [NSV] Selection by [NSV] (1 GPS OUT)


NPA Availability [dUDRE]NPA Availability [dUDRE]

• NPA availability with SBAS selection of the option [dUDRE];NPA availability with SBAS selection of the option [dUDRE];• Availability degrades from [NSV] although this option requires more computational Availability degrades from [NSV] although this option requires more computational

load.load.

Selection by [dUDRE] Selection by [dUDRE] (1 GPS OUT)


Combined Use of SBASCombined Use of SBAS• Simultaneous usage of multiple SBAS:Simultaneous usage of multiple SBAS:

– The MOPS allows receivers in the NPA mode to use multiple SBAS The MOPS allows receivers in the NPA mode to use multiple SBAS simultaneously;simultaneously;

The receivers have an option to use a combination of GPS satellites augmented by an The receivers have an option to use a combination of GPS satellites augmented by an SBAS and other GPS satellites augmented by another SBAS which may be operated SBAS and other GPS satellites augmented by another SBAS which may be operated by a different service provider;by a different service provider;

– Ionosphere correction is made by GPS navigation message other than SBAS; Ionosphere correction is made by GPS navigation message other than SBAS;

– In the current situation, receivers in South East Asia receiving MSAS and In the current situation, receivers in South East Asia receiving MSAS and GAGAN may operate in such a way.GAGAN may operate in such a way.

• Computation of availability achieved by multiple SBAS:Computation of availability achieved by multiple SBAS:– Assumed procedure in the receiver:Assumed procedure in the receiver:

(i) Determine the primary SBAS based on one of the selection criteria;(i) Determine the primary SBAS based on one of the selection criteria;

(ii) Apply augmentation information provided by the primary SBAS to GPS satellites (ii) Apply augmentation information provided by the primary SBAS to GPS satellites augmented by the SBAS;augmented by the SBAS;

(iii) For remaining GPS satellites, apply augmentation information provided by the other (iii) For remaining GPS satellites, apply augmentation information provided by the other SBAS with consideration of time offset by adding 100ns to uncertainties of range SBAS with consideration of time offset by adding 100ns to uncertainties of range measurements of these GPS satellites as described in the MOPS.measurements of these GPS satellites as described in the MOPS.


Baseline: SBAS Selection [HPL]Baseline: SBAS Selection [HPL]

• NPA availability with SBAS selection of the option [HPL];NPA availability with SBAS selection of the option [HPL];• Investigates if combined use of SBAS improves availability from this baseline or not; In Investigates if combined use of SBAS improves availability from this baseline or not; In

other words, identifies advantage of receivers which have the algorithm of combined use other words, identifies advantage of receivers which have the algorithm of combined use of multiple SBAS.of multiple SBAS.

Selection by [HPL] Selection by [HPL] (1 GPS OUT)


Combined UseCombined Use

Combined Use: GAGAN+MSAS Combined use: GAGAN+MSAS (1 GPS OUT)

• Combined use of SBAS based on [EL] achieves availability better than the result Combined use of SBAS based on [EL] achieves availability better than the result of single SBAS selection by option [HPL];of single SBAS selection by option [HPL];

• Results of other options are very similar.Results of other options are very similar.


Combined UseCombined Use

Select GAGAN or MSAS Combined use: GAGAN+MSAS

• Comparison of NPA availability between single SBAS and combined use of SBAS in case Comparison of NPA availability between single SBAS and combined use of SBAS in case of 1 critical GPS out;of 1 critical GPS out;

• Combined use of SBAS even based on selection option [EL] achieves availability better Combined use of SBAS even based on selection option [EL] achieves availability better than the result of single SBAS selection by option [HPL].than the result of single SBAS selection by option [HPL].


Monitoring SBASMonitoring SBAS• Monitoring SBAS:Monitoring SBAS:

– Another issue might be the necessity of monitoring the performance of the Another issue might be the necessity of monitoring the performance of the system operated by other States;system operated by other States;

– SBAS is used as a primary means of navigation; Is it allowable to fly in the SBAS is used as a primary means of navigation; Is it allowable to fly in the airspace of a State dependently upon a primary means of navigation provided airspace of a State dependently upon a primary means of navigation provided by another State?by another State?

– A possible way to relax legal issues caused by using SBAS operated by A possible way to relax legal issues caused by using SBAS operated by another State is to monitor the performance of the SBAS; If necessary, warn another State is to monitor the performance of the SBAS; If necessary, warn to users in the airspace in case that the SBAS should not be used;to users in the airspace in case that the SBAS should not be used;

– Actually, in Spring, MSAS has broadcast messages with illegal preambles.Actually, in Spring, MSAS has broadcast messages with illegal preambles.

• GNSS Monitor System:GNSS Monitor System:– In order to monitor an SBAS, one also needs to monitor GPS satellites In order to monitor an SBAS, one also needs to monitor GPS satellites

augmented by the SBAS to check correctness of augmentation information;augmented by the SBAS to check correctness of augmentation information;

– Japan intends to implement monitor systems for GNSS including GPS and Japan intends to implement monitor systems for GNSS including GPS and SBAS whose signal can be received in Japanese territory.SBAS whose signal can be received in Japanese territory.


GAGAN Integrity at TokyoGAGAN Integrity at Tokyo

All-in-View ModeAll-in-View Mode All Possible CombinationsAll Possible Combinations

• Triangle chart comparing HPL and the associated position error provided by GAGAN GSAT-10 in Tokyo, Japan;Triangle chart comparing HPL and the associated position error provided by GAGAN GSAT-10 in Tokyo, Japan;• The availability of NPA mode is 97.8% for all-in-view receivers;The availability of NPA mode is 97.8% for all-in-view receivers;• In terms of safety assurance, all possible combinations should be tested like the chart at the right; Unsafe condition that In terms of safety assurance, all possible combinations should be tested like the chart at the right; Unsafe condition that

the error exceeds HPL did not occur for this period;the error exceeds HPL did not occur for this period;• Furthermore, all possible combinations of active SBAS messages should be tested.Furthermore, all possible combinations of active SBAS messages should be tested.


ConclusionConclusion• SBAS Selection Problem:SBAS Selection Problem:

– The current SBAS specifications do not define any procedure, or satellite The current SBAS specifications do not define any procedure, or satellite selection criteria, to be used by receivers to choose SBAS satellites for use;selection criteria, to be used by receivers to choose SBAS satellites for use;

– The expected NPA (RNP0.3 for Enroute) availability for some SBAS selection The expected NPA (RNP0.3 for Enroute) availability for some SBAS selection criteria is computed based on the actual broadcast message of MSAS and criteria is computed based on the actual broadcast message of MSAS and GAGAN;GAGAN;

– Criteria based on HPL or NSV have advantage to other options like a Criteria based on HPL or NSV have advantage to other options like a selection by elevation angle;selection by elevation angle;

– Combined use of SBAS also improves availability.Combined use of SBAS also improves availability.

• Necessity of Monitoring:Necessity of Monitoring:– Monitoring the performance of the system operated by other States;Monitoring the performance of the system operated by other States;

– As an example, a result of evaluation of the performance of GAGAN in As an example, a result of evaluation of the performance of GAGAN in Japanese territory was shown;Japanese territory was shown;

– It is necessary to test all possible combinations of ranging sources and It is necessary to test all possible combinations of ranging sources and augmentation information for integrity assurance.augmentation information for integrity assurance.

ion gnss+ 2014 tampa, fl sept. 8-12, 2014 sbas satellite selection and performance monitoring at the...

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