GPS L5 Spectral Environment

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

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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
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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

12
IGEB-assumed Receiver Selectivity

• 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

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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
G30 dB F1.8 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.
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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.