DFMC
Satellite Selection
Discussions
Presented to: IWG # 26
Developed by: Jed Dennis and Mark Hemstad
Presented by: Jason Burns
Date: Feb 5-7, 2014
Federal Aviation
Administration
Background
• Legacy L1 SBAS: DO-229
– Section 1.3.3
“The operational concept for GNSS and Space-Based Augmentation
Systems is predicated on the combination of the different GNSS
elements without pilot intervention. As GNSS is a global system,
there should be no flight crew interaction based on airspace, so that
the flight crew should not be involved in the selection of different SBASs”
• SARPS Section 6.2.5 and 6.2.7
“while the State has responsibility to approve the use of one or more
SBAS signals in its airspace, due to the inability of current equipment to
deselect particular SBAS, States might effectively preclude use of SBAS
if the State does not approve use of all SBAS. ”
– Would presence of 3-4 SBAS deter State approval?
DFMC SBAS GEO Selection – Task 2
IWG # 26, Feb 5-7, 2014
Federal Aviation
Administration
2
SBAS in 2013
Minimal coverage overlap with little to no overlap in service areas
SBAS Selection
• En Route: No selection guidance
• Terminal: No selection guidance
• Approach: SBAS identified in FAS data block
DFMC SBAS GEO Selection – Task 2
IWG # 26, Feb 5-7, 2014
Federal Aviation
Administration
3
Future SBAS
•
-
Changes with DFMC
– Direct avionics mitigation of ionosphere
– Addition of GAGAN and SDCM
– Potential to use additional core constellations
Results in significant regions with 3 SBAS, some regions with 4 SBAS
Overlap of coverage in SBAS Service Areas
DFMC SBAS GEO Selection – Task 2
IWG # 26, Feb 5-7, 2014
Federal Aviation
Administration
4
Reference Problem
SBAS Service Areas
Supported by reference
network
SBAS # 3
SBAS # 2
Flight Path
• Coverage from three available SBAS
SBAS # 1
• Flight path through airspace of three
SBAS providers
• Which SBAS to use when?
DFMC SBAS GEO Selection – Task 2
IWG # 26, Feb 5-7, 2014
GEO coverage arcs
Federal Aviation
Administration
5
DFMC SBAS Selection
• Questions
– Is SBAS selection required? Desired?
• DFMC capable of meeting LPV requirements in most of use area
– Horizontal Alert Limit (HAL): 40 m, Vertical Alert Limit (VAL): 35 m
• Easily meets en route and terminal requirements
– En route HAL: 2 nm; Terminal HAL: 1 nm
• Suggests any SBAS service would be sufficient to meet
Performance Based Navigation (PBN) requirements
– Assumes SBASs provide similar performance
– Will future operations require tighter horizontal or vertical
performance?
• Automated Dependent Surveillance – Broadcast (ADS-B)
• Trajectory Based Operations (TBO)
– How well can automated selection means match SBAS Service Areas?
– Will standardized selection criteria help with State approval of SBAS?
DFMC SBAS GEO Selection – Task 2
IWG # 26, Feb 5-7, 2014
Federal Aviation
Administration
6
SBAS Selection Options
Cons
Protection Level
Assessment
Automated (auto), tied to
capability of SBAS
dDFRE (Clock/ephemeris
covariance)
Auto, measure of distance Requires track and decode of
to SBAS reference network second SBAS provider GEO
Maximum Common Space
Vehicle (NSV)
Auto, indicator of distance Undetermined regions
to SBAS reference network
GEO Elevation Angle
Automated and predictable Inconsistent with SBAS
reference network location
Broadcast Service Volume
Auto, SBAS provider/State
control
Message downlink capacity
Stored Service Volume
Auto, State Control
Database maintenance
Preplanned
Automated, simple
Pilot Error and preflight work
Defer to Manufacturer
Enable OEM options
Unknown behavior
Pilot Selected
Change any time
Pilot Error and workload
DFMC SBAS GEO Selection – Task 2
IWG # 26, Feb 5-7, 2014
Requires track and decode of
second SBAS provider GEO
Federal Aviation
Administration
Broadcast Integrity
Parameter Methods
Pros
Pre-defined
Methods
Method
7
Assumptions
• Use corrections from one SBAS at a time
• Select SBAS prior to GEO or ranging source selection
• Avionics may use any ranging source corrected by SBAS
– Not required to use all corrected ranging sources
• SBAS selection independent of ranging source selection
– Avionics able to track sufficient number of SVs corrected by SBAS
– Expect can be guaranteed if any of the following are true
• Each SBAS broadcasts corrections for common core constellation(s)
• All GNSS broadcast at same frequencies (ie L1/E1 and L5/E5a)
• Avionics can track all core constellations
DFMC SBAS GEO Selection – Task 2
IWG # 26, Feb 5-7, 2014
Federal Aviation
Administration
8
Simulation
• Used Stanford University
MAAST tool
– DF mode of operations
– Each SBAS run separately
– Post-processed for
selection assessment
– Uses all in-view satellites
• Scenario parameters
– 24 1-hour time epochs
– 2 deg by 2 deg grid
• Constellations
– GPS: 24 SV DO-229
MOPS constellation
– Galileo: 27+3 Walker
56°:27/3/1 constellation
• SBAS Use Areas
– Areas in which there are
or expect will have SBAS
Service Areas
Note: Simulation results presented at ION ITM 2014
DFMC SBAS GEO Selection – Task 2
IWG # 26, Feb 5-7, 2014
Federal Aviation
Administration
9
Assessment Metrics
• SBAS selection
– Percent of associated SBAS Use Area in which associated SBAS was
selected every epoch
– Transition Area: Percent of world in which more than one SBAS was
selected over the course of a day
– Predefined: Percent of associated SBAS Use Area in which
designated SBAS meets availability requirements
• Availability
– Percent of the world in which selected SBAS provided RNP 0.3
service at least 95% of the time
– For pre-defined methods, percent of world in which at least one SBAS
by itself provided RNP 0.3 service at least 95% of the time
DFMC SBAS GEO Selection – Task 2
IWG # 26, Feb 5-7, 2014
Federal Aviation
Administration
10
HPL Selection Criteria
• Choose SBAS based on minimum Horizontal Protection Level
GPS
GPS
Method
HPL
Percent of SBAS Service Area in Which the World
Transition
SBAS Was Selected
Availability Region
GPS
w/Galileo
WAAS
82.2
88.8
EGNOS
87.0
95.8
DFMC SBAS GEO Selection – Task 2
IWG # 26, Feb 5-7, 2014
MSAS
63.0
87.1
GAGAN SDCM
98.0
71.3
99.0
85.4
68.0
89.0
48.8
31.7
Federal Aviation
Administration
11
NSV Selection Criteria
• Choose SBAS that corrects the largest number of satellites
observed by the user (Number of Common SVs)
GPS
GPS
Method
NSV
GPS
w/Galileo
Percent of SBAS Service Area in Which
the SBAS Was Selected
WAAS
65.4
79.1
EGNOS
36.4
70.0
DFMC SBAS GEO Selection – Task 2
IWG # 26, Feb 5-7, 2014
MSAS
0.0
29.8
World
Transition
Availability Region
GAGAN SDCM
47.3
31.6
67.2
73.1
39.9
88.5
58.3
43.9
Federal Aviation
Administration
12
GEO Selection Criteria
• Choose SBAS based on highest GEO elevation angle
GPS
GPS
Method
GEO
Percent of SBAS Service Area in Which the World
Transition
SBAS Was Selected
Availability Region
GPS
w/Galileo
WAAS
86.8
86.8
EGNOS
58.2
58.2
DFMC SBAS GEO Selection – Task 2
IWG # 26, Feb 5-7, 2014
MSAS
81.8
81.8
GAGAN SDCM
82.4
26.2
57.4
82.4
26.2
80.7
0
0
Federal Aviation
Administration
13
dDFRE Selection Criteria
• Choose SBAS based on MT-28 covariance (dDFRE)
• Identify SBAS that has best dDFRE for each satellite
• Select SBAS that has largest number of selected satellites
GPS
GPS
Method
δDFRE
Percent of SBAS Service Area in Which the World
SBAS Was Selected
Availability
Transition
Region
GPS
w/Galileo
WAAS
75.7
87.9
65.8
37.2
EGNOS
73.2
92.4
DFMC SBAS GEO Selection – Task 2
IWG # 26, Feb 5-7, 2014
MSAS
89.0
94.6
GAGAN SDCM
91.5
76.5
44.6
100
89.1
84.4
Federal Aviation
Administration
14
dDFRE, GPS and Galileo
GPS
Smaller
transition
regions
GPS + Galileo
GPS
Expanded
Coverage
GPS + Galileo
Second constellation improves results
DFMC SBAS GEO Selection – Task 2
IWG # 26, Feb 5-7, 2014
Federal Aviation
Administration
15
Other methods
• Based on individual performance of SBAS in simulation
Method
Predefined, GPS
Predefined, GPS
w/Galileo
Defer to
Manufacturer
Percent of SBAS Service Area in Which the SBAS
Was Selected
WAAS
EGNOS MSAS
GAGAN SDCM
99.6
100
100
100
96.5
World
Availability
99.6
100
87.9
Unknown
Unknown Unknown
100
100
96.5
Unknown Unknown
65.3
Unknown
Availability lower than best automated selection method
DFMC SBAS GEO Selection – Task 2
IWG # 26, Feb 5-7, 2014
Federal Aviation
Administration
16
Comparison of assessed methods
– Protection Level
• Provides best results (SBAS selection, availability)
• Requires processing of multiple SBAS / GEOs
– Processing requirements
– Results in requirement for at least 4 GEO tracking channels
» 2 for Selected SBAS, 2 for alternate SBAS
– δDFRE
• Requires sufficient number of ranging sources for positive selection
• Potential to improve method if prefer over protection level method
– Number of common SVs
• Many regions lack clear selection
• Method requires refinement
– GEO elevation angle
• Not for SBAS selection, okay to chose GEO once SBAS selected
HPL Method seems best
DFMC SBAS GEO Selection – Task 2
IWG # 26, Feb 5-7, 2014
Federal Aviation
Administration
17
SBAS Selection Options
Method
Pros
Cons
Protection Level
Assessment
Automated (auto), tied to
capability of SBAS
dDFRE (Clock/ephemeris
covariance)
Auto, measure of distance Requires track and decode of
to SBAS reference network second SBAS provider GEO
Potential if
Improve
Maximum Common SV
(NSV)
Auto, indicator of distance Undetermined regions
to SBAS reference network
Acceptable
GEO Elevation Angle
Automated and predictable Inconsistent with SBAS
reference network location
No
Broadcast Service Volume
Auto, SBAS provider/State
control
Message downlink capacity
Potential
Stored Service Volume
Auto, State Control
Database maintenance
Potential
Preplanned
Automated, simple
Pilot Error and preflight work
No
Defer to Manufacturer
Enable OEM options
Unknown behavior
Current
Standard
Pilot Selected
Change any time
Pilot Error and workload
No
DFMC SBAS GEO Selection – Task 2
IWG # 26, Feb 5-7, 2014
Requires track and decode of
second SBAS provider GEO
Recommend
Federal Aviation
Administration
Yes
18
Conclusions
• Broadcast Integrity methods sufficient for SBAS selection
– Automated, no pilot intervention
– Slight improvement in availability
– Protection level assessment has best performance, but other methods
are close and acceptable
• Performance improved with more ranging satellites
• Trade-off between pilot responsibility, maintenance
responsibility and computational burden for SBAS avionics
• Need to reach concurrence on methods prior to working
specific requirements
DFMC SBAS GEO Selection – Task 2
IWG # 26, Feb 5-7, 2014
Federal Aviation
Administration
19
Backup
Presented to:
Developed by:
Presented by:
Date:
Federal Aviation
Administration
SBAS Systems
135
138 133
129 137
120
DFMC SBAS GEO Selection – Task 2
IWG # 26, Feb 5-7, 2014
126
140
Federal Aviation
Administration
21
VPL Selection Criteria
• Choose SBAS based on minimum Vertical Protection Level
GPS
Method
GPS
VPL
Percent of SBAS Service Area in Which the World
Transition
SBAS Was Selected
Availability Region
GPS
w/Galileo
WAAS
84.0
91.6
EGNOS MSAS
84.9
60.0
95.0
86.6
DFMC SBAS GEO Selection – Task 2
IWG # 26, Feb 5-7, 2014
GAGAN SDCM
92.0
77.8
95.9
85.4
67.9
89.0
51.1
37.1
Federal Aviation
Administration
22
SBAS Selection Dependency on Flight Mode
Navigation
Flight Mode
SBAS Selection
Oceanic
En-route
-
Horizontal
Terminal
Horizontal
And
Vertical
DFMC SBAS GEO Selection – Task 2
IWG # 26, Feb 5-7, 2014
Requires reassessment
during flight
Provide Guidance
or
Manufacturer
Implementation
LP
Transition to SBAS
identified by FAS
LPV / LPV-200
Per FAS data block
Federal Aviation
Administration
23
SBAS Assessment Triggers
• Event Based
– Mismatch in number of tracked versus augmented SVs
• Indicator of distance from SBAS Reference Stations (SRS)
– Good distinction in East-West direction
– Poor distinction for North-South when multiple SRS in same
hemisphere
– Change in longitude or latitude
• Set based on distance between SRS and flight speeds
– Distance flown
• Set based on distance between SRS and flight speeds
• Periodic
– Set time interval
• Set based on distance between SRS and flight speeds
DFMC SBAS GEO Selection – Task 2
IWG # 26, Feb 5-7, 2014
Federal Aviation
Administration
24
Constellation Selection
• Augmented by SBAS in use
• Augmented by another available SBAS
• Requires switching SBAS
• Permitted if not restricted by provider
identified in FAS data block
• As able to support RAIM
DFMC SBAS GEO Selection – Task 2
IWG # 26, Feb 5-7, 2014
Federal Aviation
Administration
25
Ranging Source Selection
• All in view for constellation(s) in use
• Simplicity, does not require complex algorithm for
satellite selection
• Best for RAIM integrity
• SBAS integrity uses subset with valid corrections
• Selection based on tracking limit of equipment
• Support for operation and integrity method
• If multi-constellation RAIM, will need an additional
SV(s) to resolve constellation biases
• Goal is to maximize availability en-route, ensure selection
of the matching service provider sufficiently before
commencing final approach
DFMC SBAS GEO Selection – Task 2
IWG # 26, Feb 5-7, 2014
Federal Aviation
Administration
26