L1/L5 WAAS Support
Todd Walter
Stanford University
http://waas.stanford.edu
Key DFMC MOPS
Components
 Operational goals (1.3)
 Equipment classes (1.4)
 Constellation assumptions (B)
 Equipment & tracking requirements (2.1, 2.2, 2.3)
 Environmental assumptions (2.4)
 Interference environment (C )
 Test requirements (2.5)
 Antenna requirements (separate MOPS)
 Signal specification (A)
 L5 physical layer (A.2 & A.3)
 Message structure and content (A.4)
2
 Protection Levels (J)
Recently Agreed to L5 MOPS
Changes
Expanded PRN mask
Removal of fast corrections
Single satellite correction message
Smaller quantization errors
Alert message with DFRECIs
When more than 51 SVs corrected
Full DFREs can be sent in 2 messages
Updatable DFRE table
Support for more SBAS orbit types
3
ICD Topics Still Under
Discussion
L5 reversionary mode
Optional ionospheric messages
Extra clock states in the G matrix
Separate state for each constellation
Correlation of tropospheric error
4
Clock parameters in MT-12 like
message as part of correction
Identification of service regions
Ephemeris content validation
Authentication of service
L5 Reversionary Mode
The FAA is not pursuing ionospheric
corrections on L5
L5/E5-only RAIM should provide lateral
guidance for terminal through en route
Other ANSPs may elect to define
messages to permit vertical guidance
in L1/E1-denied areas
These messages are not required to
support dual frequency vertical guidance
5
Separate Constellation Clock
States
SBAS corrections can align the time
offsets between constellations
May require separate clock terms
Differences in signal structures and
satellite filters can create receiver
dependent biases
Adding a clock state per constellation
addresses dynamic range and biases
Small increase in protection levels
6
Recommended by RTCA + EUROCAE
Inter-GNSS Clock
Parameters
Clock offset states can be broadcast
in MT-12 like message
MT-12 currently only used for timing
Not currently broadcast by WAAS
Could also be applied to corrections
to align GNSS timescales
7
Current offsets greater than dynamic
range of the corrections
Splits correction information across
different messages
Identification of Service
Regions
May want to assist user in finding
SBAS with highest availability
Interest expressed in specifying which
SBAS to use in certain areas
May want to broadcast information
about other SBAS satellites
Currently supported in L1, but not used
Additional messages could be defined
to meet these goals
8
Needs to be coordinated among SBASs
Ephemeris Content
Verification
Addresses issue of ephemeris content
change without IODE change
Instead of IODE, correction message
would contain a hash of full contents
Needs a large number of bits to fully assure
unique hash (>24)
Adjacent ephemerides must have different hashes
Requires more bits than easily available
Looking into alternate methods to achieve goal
Perhaps a low bit count hash with the IODE
9
Signal Authentication
FAA is interested in having ability to
authenticate SBAS signal
Level of authentication to be studied
Signal only
Data contents
Bandwidth requirements TBD
May only require a message per minute
May use Q-channel
10
Needs to be loss tolerant
Key management is an issue
Next Steps
Document agreed upon ICD elements
similar to appendices A & J of MOPS
Exchange documents to ensure
common understanding
Comment on descriptions
11
Identify missing details
Discuss options to address identified
issues
Update documentation with new
agreements
12