11

GPS L5 - Spectral Environment

Dr. Chris HegartyMr. Mike Williams

L2/L5 Industry DayMay 2, 2001

2

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

3

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

4

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

7

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

8

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

DME/TACANs

9

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

10

Backup

11

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

12

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

13

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

14

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.

15

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.