11 GPS L5 - Spectral Environment Dr. Chris Hegarty Mr. Mike Williams L2/L5 Industry Day May 2, 2001

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GPS L5 - Spectral Environment

Dr. Chris HegartyMr. Mike Williams

L2/L5 Industry DayMay 2, 2001

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Overview

• L5 will reside in the 960 - 1215 MHz band– Allocated internationally for Aeronautical

Radionavigation Services (ARNS)– Co-primary Radionavigation Satellite Services (RNSS)

allocation obtained in 2000 for 1164 - 1215 MHz

• Band is heavily used, e.g., by ground navaids and Link 16

• IGEB ad hoc Working Group 1 formed to validate compatibility

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L5 Electromagnetic Environment - Primary Contributors

DME/TACAN• Over 1700 U.S. ground beacons • 1 MHz channels across 960-1215 MHz• EIRP = 100 W - 10000 W• 3.5 s pulse width (1/2 voltage)• 2700 - 3600 pulse pairs/s

JTIDS/MIDS (Link 16)• Now 600 terminals (many airborne)• May be 4000 U.S. terminals by 2010• Hops over 51 3 MHz channels from 969-1206 MHz• 6.4 s pulse width • For uncoordinated exercises:

– Peak power = 200 W–396,288 pulses/12 s in 200 nmi radius

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Compatibility Analysis• Assumptions:

– Pulse blanking and robust selectivity incorporated into L5 user equipment

– L5 received power: -154 dBW

• Compatibility assessed using signal-to-noise ratio (SNR) approach– Goal is to maintain post-correlation SNR provided at L1

• On following charts, 5.8 dB is breakpoint between acceptable and unacceptable

SNR Degradation at 5,000 ft. Above Ground Level - DME/TACAN Only

Plots courtesy of Tae Kim and Swen Ericson, MITRE CAASD

Acceptable degradation Unacceptable degradation

SNR Degradation at 40,000 ft. Mean Sea Level - DME/TACAN Only

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SNR Degradation at 40,000 ft - All Known U.S. Emitters

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SNR Degradation at 40,000 ft - All Known U.S. Emitters with Reassignment of In-band

DME/TACANs

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Summary

• DME/TACAN is primary contributor to L5 electromagnetic environment, Link 16 secondary– Primarily a concern for high altitudes in only a few

regions of the world

• DME reassignments (within +/-9 MHz of L5), as necessary, will ensure excellent L5 reception at all altitudes in U.S.– Validated by simulation– Hardware tests planned in FY02

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Backup

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Receiver Requirements• Primary contributors to electromagnetic

environment near L5 are pulsed• More selective front-end (compared to L1

avionics) necessary to limit number of pulses desensitizing receiver

• “Pulse blanking” a low-cost, low-risk method to minimize effects of pulses on receiver performance– Performance standards should not specify design, but

will require operation in pulsed environment

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IGEB-assumed Receiver Selectivity

20 log|H(f) |

f (MHz)

0

5.5 dB/MHz

(10,0)

(22.7,-70)

RTCA consensus was that 5.5 dB/MHz is maximum RF/IF roll-off that can bereasonably achieved considering:• Component cost vs. performance• Package size of installed active antennas and avionics

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Receiver Noise Floor

• L1 avionics specifications assume noise floor of -201.5 dBW/MHz– Consistent with 4 dB Noise Figure

• Increased selectivity for L5 comes at a price:– Increased insertion loss raises noise floor

• WG1 developed values for L5:– 5 dB Noise Figure

– Noise floor of -200.0 dBW/MHz

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L5 Active Antenna Noise Figure Budget

• Allowance made for added filter, limiter losses for tough L5 RFI– increased antenna unit selectivity for less LNA off-channel overload– higher loss 2-stage limiter for high peak power on-board transmitters

• L5 Rx System Noise Temperature = 625 K (5.0 dB NF)

• Sky Temp. = 100 K Overall Ant. Input Temp. = 725 K (-200 dBW/Hz)

Z-mismatch -0.2 dB

3-4 pole-2.0 dB

BPF Lim LNA

cable Rx-Proc

-13 dB

-0.7 dB G=30 dBF=1.8 dB

L1

Diplexer

NF 8 dB

L5

2-3 pole-1.4 dB -0.3 dB

Courtesy of Bob Erlandson, Rockwell-Collins. A similar budget was independentlyderived by Dan Bobyn, a former Novatel RF design engineer.

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Example of Worst-Case DME/TACAN Environment

Victim aircraft at40,000 ft

over HarrisburgNote: Only TACAN/DMEs with frequency assignments from1157 - 1209 MHz are shown/analyzed.