# Presentation: 718780

##  GPS L5 - Spectral Environment

- 1

-  GPS L5 - Spectral Environment

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

## 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

## 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

## 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

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

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

## 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

## Backup

## 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

## 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

## 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

## 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 dB
- F=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 independently
- derived by Dan Bobyn, a former Novatel RF design engineer.

**Notes:**
The L5 limiter loss allowance is higher to accommodate a 2-stage limiter for better receiver protection from closely spaced high power, on-board pulse xmtrs., DME, TCAS/Mode-S.

Since noise figure depends on source Z, the Z-mismatch allowance is for production noise figure impedance match tolerance between passive antenna and active circuitry 

## 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.