Exploring GPS L5 and Electromagnetic Environmental Challenges: Insights from Dr. Chris Hegarty & Mr. Mike Williams

1. GPS L5 - Spectral Environment Dr. Chris Hegarty Mr. Mike Williams L2/L5 Industry Day May 2, 2001 1

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 ms 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 ms 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

5. SNR Degradation at 5,000 ft. Above Ground Level - DME/TACAN Only Acceptable degradation Unacceptable degradation Plots courtesy of Tae Kim and Swen Ericson, MITRE CAASD

6. 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) | (10,0) 0 5.5 dB/MHz (22.7,-70) f (MHz) • RTCA consensus was that 5.5 dB/MHz is maximum RF/IF roll-off that can be • reasonably 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 L5 BPF Lim LNA Diplexer cable Rx-Proc 3-4 pole -2.0 dB -0.7 dB Z-mismatch -0.2 dB G=30 dB F=1.8 dB L1 2-3 pole -1.4 dB -13 dB -0.3 dB NF  8 dB • 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) Courtesy of Bob Erlandson, Rockwell-Collins. A similar budget was independently derived by Dan Bobyn, a former Novatel RF design engineer.

15. Example of Worst-Case DME/TACAN Environment Victim aircraft at 40,000 ft over Harrisburg Note: Only TACAN/DMEs with frequency assignments from 1157 - 1209 MHz are shown/analyzed.