# Presentation: 827547

## 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.

- CGSIC

- 12 September 2005

## 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

- CGSIC

- 12 September 2005

## GPS and Galileo Interoperability and Compatibility

- CGSIC

- 12 September 2005

## 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

- CGSIC

- 12 September 2005

## 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

- CGSIC

- 12 September 2005

## 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)

- CGSIC

- 12 September 2005

## Signal Interoperability at L5/E5

- Should be a new topic of US/EU negotiations
- Certainly has been a topic at EUROCAE (Europe’s 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 doesn’t interfere with any GPS signal

- CGSIC

- 12 September 2005

## 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

- CGSIC

- 12 September 2005

## GPS/Galileo Signal Interoperability Issues Details

- CGSIC

- 12 September 2005

## 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

- CGSIC

- 12 September 2005

## L2/L5/E5 Spectral Relationships

**~****75 MHz**

**~****1202 MHz**

- Not to Scale

**~****50 MHz**

- CGSIC

- 12 September 2005

## 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

- CGSIC

- 12 September 2005

## 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

- CGSIC

- 12 September 2005

## 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

- CGSIC

- 12 September 2005

## 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

- CGSIC

- 12 September 2005

## 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

- CGSIC

- 12 September 2005

## 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

- CGSIC

- 12 September 2005

## 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?

- CGSIC

- 12 September 2005

## 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

- CGSIC

- 12 September 2005

## 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

- CGSIC

- 12 September 2005