# Presentation: 803072

##  Brief Technical Analysis of Potential for Interference from GPS Re-Radiators submitted to National Telecommunications and Information Administrationin connection withSpecial Meeting of the IRAC Regarding GPS Re-Radiators

- By:  Robert Horton
- Oct. 30, 2005

## Introductions

- Robert Horton
- Chief Executive Officer, GPS Source, Inc.
- Education:
- BS Electrical Engineering, Univ. of Arkansas, 1993
- MS EE – Digital Comm. Theory & RF Circuit Design, NCSU, 1995
- Career Experience:
- USAF/ANG: 1985 – 1995
- Ericsson Mobile Communications:  1994 – 1998
- GPS Networking, Inc.: 1998 – 1999
- Sony Ericsson Mobile Communications: 1999-2005
- GPS Source, Inc.: 2005-present
- US Patents:  Eight patents awarded. Three patents pending.

- Robert Horton

## Outline

- NTIA Proposed Technical Criteria
- Resulting Signal Levels
- Proposed Repeater System
- Scenarios for Interference
- Analysis of Scenarios w/ Proposed System & NTIA Technical Criteria
- Conclusions

- Robert Horton

## NTIA Technical Criteria

- Maximum ERP must be such that emissions are no greater than -140dBm/24MHz at a distance of 100ft from the building.
- Compliant signals levels may be determined by calculations using free space propagation models
- Calculations showing compliance must be provided and should not have allowances for attenuation from the building.

- Robert Horton

## Resulting Signal Levels of NTIA Technical Criteria

**50m**

**150m**

-   The figure at the right illustrates the propagation losses of the repeated signal according to the NTIA technical criteria.
-   Note that the ERP is -140dBm at 100ft.
-   At 50m, the 1st point for the FCC E911 location criteria, the signal from the repeater is approximately 15dB below the nominal level from the satellites.
-   Beyond 150m, the 2nd point in the FCC E911 location criteria, the signal from the repeater falls below the most recently claimed minimum

- sensitivity levels of assisted GPS receivers, -155dBm (i.e. even an assisted GPS could not receive the GPS repeater signal beyond the range of the FCC location accuracy criteria.

- Robert Horton

## Application of NTIA Technical Criteria

- There are difficulties in the application of this criteria for current GPS Re-radiators which do not include Power Monitor & Control

- Analysis of this current GPS repeater system shows that the ERP level at 100 ft could vary from -129.1dBm to -142.1dBm!
- Cable lengths, and hence losses, can vary significantly from one installation to the next
- Such variation in system elements make it difficult, if not impossible, to confidently predict the system gain and hence the signal levels
- Systems without Power Monitor & Control are susceptible to alterations by the user

- Robert Horton

## Proposed GPS Repeater System

- GPS Source only supports authorization of GPS repeater   systems with the capability to monitor & control ERP
- GPS repeater systems with power monitor & control will be calibrated and ERP levels set to ensure that the system can never exceed NTIA ERP criteria

- Power monitor and control will automatically compensate for unknown variation in system elements (LNA gain, cable losses, Repeater amp gain, etc.) and will deter and prevent alteration by the user (e.g. insertion of additional gain stages to boost power)

- Robert Horton

## Scenarios for Interference

- Interference with a Typical GPS Tracking Receiver Separated by ***Less*** than One Chip Delay (***Short*** delay of  < Tc(1+d/2)
- Interference with a Typical GPS Tracking Receiver Separated by ***More*** than One Chip Delay (***Long*** delay of  > Tc(1+d/2)
- Interference with a Typical GPS Acquiring Receiver
- Interference with an Assisted-GPS Acquiring Receiver

- Robert Horton

## Less Than vs. More Than One Chip Delay

***Short*** Delay  < Tc(1+d/2)

***Long*** Delay  > Tc(1+d/2)

- Robert Horton

## Analysis of Interference with a Standard GPS Tracking Receiver Separated by Less than One Chip Delay

- Receiver in proximity of repeater treats signal in manner similar to multi-path
- Industry accepted models, scrutinized by research community, define theoretical bounds on the worst case Pseudo-Range (PR) error created by multi-path

- Example:
- Relative signal level () =0.25 (or -12dB),
- Correlator separations of         d = 0.1Tc, 0.5Tc, & 1.0Tc
- _Phase of repeated path assumes worst case constructive interference_
_**Does not account for propagation losses as the receiver moves away from the repeater antenna (see next slide)**_

- References:  Braasch (1996), Van Dierendonck *et. al.* (1992) and Enge (1999);
- Misra & Enge, Global Positioning System: Signals, Measurements and Performance, 2nd Ed., p321

- Robert Horton

## Analysis of Interference with a Standard GPS Tracking Receiver Separated by Less than One Chip Delay

_**Worst case**_ PR error (_i.e. not position error_) that is possible due to presence of a GPS repeater system configured according to NTIA technical criteria, and accounting for propagation losses:

- Assumptions:
- ERP s.t. -140dBm/100ft. (i.e. -74dBm)
- Nom. Level from Satellites = -130dBm
- Correlator separations of  d = 0.1Tc, 0.5Tc, & 1.0Tc
- _Phase for worst case constructive interference_
- _No allowances for attenuation due to the bldg. materials_
- _Does not account for delay through the GPS repeater system _
- _Receiver is located in boresight of repeater’s transmitting antenna_

_**Considerations:**_
- This is Worst case PR error possible in 1 satellite signal.  2DRMS error in final location result will be based on multiple pseudo-ranges – most or all of which will not encounter this worst case scenario
- Assumes worst case phase of repeated signal for constructive interference
- Even slightest attenuation from building material will result in significant reduction in potential error
- MP from repeater has very narrow BW time correlation, therefore easy for Multipath filters to track

- Robert Horton

## Analysis of Interference with a Standard GPS Tracking Receiver Separated by Less than One Chip Delay

**Consider urban Fire Department Scenario where repeater must only cover a range of 10ft. and area is accessible to the public, but direct signal is attenuated 10dB.**

- Assumptions:
- ERP for Fire Dept. App is = -90dBm
- Nom. Level from Satellites = -140dBm
- Correlator separations of  d = 0.1Tc, 0.5Tc, & 1.0Tc
- _Phase for worst case constructive interference_
- _No allowances for attenuation due to the bldg. materials_
- _Does not account for delay through the GPS repeater system_
- _Receiver is located in boresight of repeater’s transmitting antenna_

_**Considerations:**_
- This is Worst case PR error possible in 1 satellite signal.  2DRMS error in final location result will be based on multiple pseudo-ranges – most or all of which will not encounter this worst case scenario
- Assumes worst case phase of repeated signal for constructive interference
- Even slightest attenuation from building material will result in significant reduction in potential error
- MP from repeater has very narrow BW time correlation, therefore easy for Multipath filters to track

- Robert Horton

## Analysis of Interference with a Standard GPS Tracking Receiver Separated by More than One Chip Delay

- Beyond delay of  > Tc(1+d/2) (~one chip), the repeated signal appears as additive white noise due to mathematical properties of the GPS Gold codes
- Highest possible signal level at the range where this scenario applies is approx. -188.8dBW in a 25 to 30MHz BW (effectively -220dBW/Hz, well below KTF)
- Assume a typical receiver noise floor, of about -106dBm (KTBF), the noise floor of the receiver will be increased from -106dBm to -105.99998dBm
- Hence, there is no possibility for interference from the repeater in this scenario

- Example:

- Robert Horton

## Analysis of the Interference with a Typical GPS Acquiring Receiver

- Acquiring GPS receiver will be presented two signals:
- the direct overhead signals
- the GPS repeater signal.
- Search in Doppler space (*f**D** *) will not be effected so long as GPS operator velocity is low relative to satellite velocity
- Search in code phase space () will be successful, but will input to the receiver’s DLL initial estimates of the code phase that contain some error, according to previous multi-path analysis

- Consider also Reduced Sensitivity in Acquisition Phase:
- Due to uncertainty in Navigation data, maximum acquisition coding gain for an unassisted receiver is approximately 43dB (10log 20*1023)
- Assuming 7dB Eb/No is required for reliable demodulation of navigation data, and a receiver KTBF of -106dBm, the minimum unassisted acquisition sensitivity is approximately -142dBm
- Even if no direct signal is available at all, the GPS receiver could not acquire the repeated signal beyond 130ft. Even if the satellite signals were 100% blocked (assuming repeater system set to NTIA technical criteria: -140dBm/100ft.)

- Robert Horton

## Analysis of the Interference with an E911Assisted-GPS Acquiring Receiver

- FCC Criteria is specific, thus GPS repeater kit would be considered to be causing interference if resulting reported location has greater than 50m error, more 33% of the time and greater than 150m, more 5% of the time.
- Scenario 1:  Repeater inside, A-GPS outside
- At nominal level outside of -130dBm, desired is greater than repeater signal by at least 10dB beyond 100ft (30m) (assuming NTIA technical criteria of -140dBm/100ft & no atten. from bldg.)
- This is the most common scenario and will easily produce a result well within the FCC 50m criteria.
- Scenario 2:  Repeater inside building w/ dimensions <150m, A-GPS inside
- A-GPS has high probability of generating fix based on repeated signal.  Resulting position fix will be compliant due to definition of scenario (i.e. can’t be more than 150m away from true position due to dimensions of the building.
- Scenario 3:  Repeater inside building w/ dimensions >150m, A-GPS inside
- Signal level from repeater is at or near A-GPS min. sensitivity at 150m, thus the receiver could not acquire the repeater system beyond the FCC criteria, even if no detectable signal from over head were available (see slide 5).

- Robert Horton

## Conclusions

- Potential for harmful interference is negligible _**IF**_:
- A reasonable regulatory regime is implemented
- Foundation of that regime is based on the NTIA technical criteria
- Specific product enhancements, such as frequency selectivity and power monitor/control are implemented to ensure continued compliance with adopted criteria, regardless of installation and system variables.

- Robert Horton

## Thank You!

- Thank you for your valuable time and consideration.
- If you would like for Robert to review these slides with you, or if you have questions or concerns, please feel free to contact Robert Horton:
- _[rhorton@gpssource.com](mailto:rhorton@gpssource.com)_
- (719) 242-3800

- Robert Horton