GNSS USER ASSESSMENT OF GPSGalileo Interoperability

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GNSS USER ASSESSMENT OF GPS/Galileo
Interoperability

• Dr. A.J. Van Dierendonck, AJ Systems
• The comments in this paper represent the
  opinions of the author and are not necessarily
  those of any other person or organization.

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Introduction What is addressed?

• An assessment of Signal Interoperability of
  proposed GPS and Galileo signals
• Civil Signals Only
• Signal interoperability issues with regard to
  design decisions already made, or to be made, on
  both GPS and Galileo

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GPS and Galileo Interoperability and Compatibility
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Interoperability and Compatibility

• What do these words mean? According to the New
  Space-Based PNT National Policy as follows
• Interoperable refers to the ability of civil
  U.S. and foreign space-based positioning,
  navigation, and timing services to be used
  together to provide better capabilities at the
  user level than would be achieved by relying
  solely on one service or signal
• Compatible refers to the ability of U.S. and
  foreign space-based positioning, navigation, and
  timing services to be used separately or together
  without interfering with each individual service
  or signal, and without adversely affecting
  navigation warfare

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Translation and Opinion

• Interoperable
• As stated, means that the different GNSS systems
  provide the same answer, within the advertised
  accuracy of each individual system Call this
  System Interoperability
• Should also mean that different GNSS systems
  provide signals, and required use of those
  signals, that are relatively similar so as to
  simplify GNSS receivers that use combined GNSS
  systems (or make them feasible) Call this
  Signal Interoperability or Optimized System
  Interoperability
• Compatible
• Means that GNSS systems do not interfere with
  each other, and that non-military signals can be
  jammed without adversely affecting the military
  signals

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Signal Interoperability at L1

• Galileo and GPS are essentially interoperable at
  L1 from a system point of view
• From a user point of view, there is still a lack
  of Signal Interoperability
• Symbol rates on L1 Galileo SOL/OS signal are 5
  times higher that GPS C/A code signal
• Perceived to cause problems for indoor users
• As a consequence, Galileo has defined E5a with
  lower symbol rates as their indoor user signal --
  Creates problems for both indoor and aviation
  users (indirectly)

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Signal Interoperability at L5/E5

• Should be a new topic of US/EU negotiations
• Certainly has been a topic at EUROCAE (Europes
  version of RTCA)
• E5a is signal interoperable with L5
• E5b is not interoperable with any GPS signal
• Unfortunately, required for European aviation to
  receive Galileo integrity information
• However, it is compatible in that it doesnt
  interfere with any GPS signal

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Interoperability/Compatibility at L2

• There is no Galileo L2 signal thus not signal
  interoperable with GPS, but, certainly, the GPS
  L2 signal does not interfere with any Galileo
  signal thus, compatible
• Of course, L2 is not in an ARNS band, so its
  relationship to Galileo does not affect aviation
• Given future availability of Galileo, L2C would
  probably not be ideal for indoor use

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GPS/Galileo Signal Interoperability Issues Details
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L2/L5/E5 Signal Interoperability Details

• Precise Land-Based Users
• For these users, not correct to discuss L5/E5
  signal interoperability without including L2
• Combination will provide ultimate performance for
  land-based users (coupled with L1)
• L2 is adjacent to E5 band (See Figure)
• 75 MHz receiver front-end covers all three
  bands, centered at about 1202 MHz
  technologically feasible for land-based receivers
• See ION-GPS/GNSS-2003 paper by Issler, et al
• Not feasible for aviation use
• Not all ARNS and too much visible interference

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L2/L5/E5 Spectral Relationships
1202 MHz
75 MHz
50 MHz
Not to Scale
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L5/E5 Signal Interoperability Details Aviation
Users - 1

• L5/E5 band covers about 50 MHz
• DME/JTIDS/MIDS/Radar pulse reception up to 100
  duty cycle at some defined saturation level
• Receiver processing limited to individual L5/E5a
  and E5b bands
• Antenna/LNA may still have to cover entire 50 MHz
  band because of inability to separate bands at RF
  frequencies
• Spectral separation would then be at IF
  frequencies
• Some pulse (radar and onboard) interference will
  still saturate LNA thus, interference in one
  band would affect the other

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L5/E5 Signal Interoperability Details Aviation
Users - 2

• Signal Reception simultaneously at L5/E5a and
  E5b, even if possible in interference
  environment, complicates aviation receiver design
• Wideband antenna likely will not meet ARINC size
  and profile requirements
• This problem has not been solved being
  investigated by Chelton (UK parent of COMANT in
  US) for Galileo
• E5b close to radar bands
• LNA saturation would cause signal loss in both
  bands using wideband LNA approach
• Separate IF path required
• Increases aviation receiver cost

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A Few Words About Galileo Integrity

• Of course, very expensive to implement
• Aviation community in Europe being forced to use
  Galileo integrity as opposed to receiving
  integrity information from SBAS systems
• Not all problems have been solved still
  marginal with respect to aviation requirements
• Still have to receive GPS integrity from
  SBAS/GBAS
• Increases signal data rate to 125 BPS
• Perceived to be not interoperable with indoor
  operations
• May be encrypted to separate from OS signal????
• Not feasible for GPS must maintain legacy

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Indoor Receiver Considerations 1

• Galileo signal designs seem to give Indoor GNSS
  Reception priority over aviation
• Because of high volume sales
• Dominates frequency/bandwidth considerations
• Galileo integrity broadcast presents a conflict
• High rate integrity data (250 sps) broadcast on
  L1 SOL signal also appears on OS signal
• Prevents indoor data recovery at low SNR
• 10 dB degradation relative to 50 sps on E5a
• Does not affect tracking threshold because of
  data-less (pilot) carrier on Galileo signals
  only affects data recovery
• If data is received via cell phone link, not
  required for indoor use

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Indoor Receiver Considerations 2

• Because of high data rate at L1, Galileo proposes
  moving indoor operation to L5/E5a
• Taking advantage of low data rate on L5
• Assign integrity data broadcast to E5b
• Aviation users in Europe required to use Galileo
  integrity data
• Requires reception of E5b
• At the same time, reception at L5 required for
  GPS use and for reception of GPS integrity from
  EGNOS and WAAS (and other SBASs)
• Galileo by itself considered a degraded mode in
  the future
• At present, broadcast of Galileo integrity not
  planned for EGNOS, but most likely will be on
  WAAS and other SBASs

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Indoor Receiver Considerations 3

• How does Galileo expect that use of GNSS in cell
  phones will migrate from L1 to L5/E5a?
• Especially since dual constellation will be
  available at L1 long before it will be available
  at L5/E5a
• Dual frequency receivers not likely
• Required antenna and receiver technology not
  commensurate with low-cost/low-power requirements
• Migration to L5/E5a would have to be a clean
  break
• In the interim, what is the tradeoff between more
  satellites and a the lower data rate on Galileo
  E5a?
• I would think that manufacturers and users would
  pick the advantages of more satellites

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Signal Interoperability Problems at E5/L5

• Obvious that priority for Indoor Users has causes
  GPS/Galileo signal interoperability problems at
  E5/L5 for Aviation Users
• Is it correct to compromise Safety-of-Life
  applications for new technology that is dubious
  at best?
• Just to be able to improve data recovery
  performance for data that should be available via
  the cell link anyway?

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Indoor Use Data Recovery Issues

• Why is data recovery from GPS/Galileo signals
  required?
• Receiver integration with cell phone (or other
  communications device) is required to connect to
  E911
• So why not collect data from E911 network?
• Real reason not to may be related to intellectual
  property in the near term, most manufacturers
  may have to pay royalties
• Any patents will probably expire in another 10
  years
• Certainly not an integrity issue integrity data
  would require the higher data rate anyway

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Summary Conclusions

• Definitions of GPS/Galileo Interoperability and
  Compatibility were reviewed
• Official definition of Interoperability does not
  account for user signal processing requirements
• Consequently, GPS and Galileo are not optimally
  interoperable
• Optimal Interoperability inconsistencies shown
  for Aviation and Indoor Users
• Galileo (and GPS) Indoor Use seems to have taken
  precedence
• Major E5/L5 interoperability problem for Aviation
  Users