Testing of ILS Receivers in a Multipath Propagation

Environment

International Flight Inspection International Flight Inspection SymposiumSymposium

Oklahoma City, OK USA June 2008Oklahoma City, OK USA June 2008

Jochen Bredemeyer, FCSKlaus Theißen, R&S

Testing of ILS Receivers in a Multipath Propagation Environment

2 of 16IFIS 2008, OKC

Introduction

• FI receivers are not dedicated measurement equipment but mostly aviation type *)

• Current Calibration *)– generators providing static, non-interfered signals– Complex scenarios are excluded

• Improvement of receiver characteristics– achieved through use of reproducible interfered RF

environment– Example: ILS Localizer reception

*) ICAO references, remarks overleaf

Testing of ILS Receivers in a Multipath Propagation Environment

3 of 16IFIS 2008, OKC

Introduction (cont‘d)

*) ICAO DOC8071 states:

4. FLIGHT INSPECTION RECEIVERS AND RADIO COMMUNICATION EQUIPMENT

4.1 Flight inspection receivers are to be of the highest quality in order to obtain the accuracy required for flight inspection purposes and should provide additional measurement outputs specific to flight inspection [..]

Calibration of test equipment

1.12.10 [..] A test transmitter is connected to the radio frequency (RF) input of the receiver in order to input simulated signals. The receiver output is compared with the nominal signals; deviations are recorded either in a test protocol or in the memory of a computer [..]

Testing of ILS Receivers in a Multipath Propagation Environment

4 of 16IFIS 2008, OKC

Current receiver techniques

Mature Super-Heterodyne Principle in combination with IF A/D sampling

Example: R&S®EVS300

16 BIT ADCAntialiasMixer 2Mixer 1

Preamp / ATTPreselector

INPUT SW

Comb Gen

IF2 AMP /FILTER

IF1 AMP /FILTER

DigitalVGA75MHz

2

2108 .. 116MHz

320..340MHz

AD Signal

Processor

DSP 24 BIT

PLL1 PLL2

DDSTCXO 10MHz

To MMI

I/Q demodulation and data reduction performed by DSP or FPGA

Testing of ILS Receivers in a Multipath Propagation Environment

5 of 16IFIS 2008, OKC

Current receiver techniques

Digital Down Converter with fIF = fS/4

B: IF undersampling at twice the signal bandwidth

Testing of ILS Receivers in a Multipath Propagation Environment

6 of 16IFIS 2008, OKC

Derivation of Signal-in-Space

• Reference multipath scenario– taken from the Frankfurt A380 measurement campaign

at a specific position on taxiway S (close to threshold 25L, turn 23°)

– Derivation of maximum reflection coefficient

• Modelling constraints– simple A380 reflection simulation to obtain a roughly

realistic interfered signal model - not to replace complex modelling!

– Patterns of both the LLZ and the receiving antenna are not considered

– Process of reflection is approximated by Gaussian pulse

Testing of ILS Receivers in a Multipath Propagation Environment

7 of 16IFIS 2008, OKC

Geometry on airport surface

Simplified Reconstruction of A380 tailfin reflection

0

0.05

0.1

0.15

0.2

0.25

0.3

-700 -600 -500 -400 -300 -200 -100 0R

Distance to threshold / m

Gaussian Reflective Coefficient over distance

0m

50

100

150

200

250

300-4000 -3500 -3000 -2500 -2000 -1500 -1000 -500 0m

vtx ,)(

ry

xLLZ

A380

DDM = 0.328

Reflection coefficient approximated by Gaussian pulse

Frankfurt RWY 25L

Testing of ILS Receivers in a Multipath Propagation Environment

8 of 16IFIS 2008, OKC

0

0.05

0.1

0.15

0.2

0.25

0.3

0 20 40 60 80 100 120 140 160 180 200

Dep

th in

Mod

ulat

ion

Audio frequency / Hz

Spectrum of Audio signal THR dist: -420m

Simulated Signal-in-Space

IF (bandpass) vs. Audio (baseband) absolute spectrum

0

0.2

0.4

0.6

0.8

1

800 850 900 950 1000 1050 1100 1150 1200

rela

tive

Mag

nitu

de

Bandpass (intermediate) frequency / Hz

EDDF LLZ25L 'Course' THR dist: -420m

2nd order sidebands causing scallopings when within filter bandwidth

Transition is non-linear operation

Offset remains

Reflection of carrier and 90Hz sidebands becomes visible

Testing of ILS Receivers in a Multipath Propagation Environment

9 of 16IFIS 2008, OKC

RF signal synthesis

• Arbitrary ILS generator delivers multipath affected RF signal to be fed into real receivers

Upconversion to ILS carrier RF

to ILS receiverIF 200kHzArtificial g(IF)

signal-in-space

eneration ofdistorted ILS bandpass DAC

Testing of ILS Receivers in a Multipath Propagation Environment

10 of 16IFIS 2008, OKC

Receiver investigations

Optimizing AGC parameters: Wrong hysteresis

-40

-20

0

20

40

0 5 10 15 20 25 30 35 40

-64

-62

-60

-58

-56

-54

-52

-50

DD

M /

A

AG

C L

eve

l / d

Bm

Procedure time / s

EVS -- DDM over time, Reflective Coefficient r=0.30raw DDM

ICAO filteredAGC Level

Testing of ILS Receivers in a Multipath Propagation Environment

11 of 16IFIS 2008, OKC

Receiver investigations

Corrected hysteresis

-15

-10

-5

0

5

10

15

0 5 10 15 20 25 30 35

-64

-62

-60

-58

-56

-54

-52

-50

DD

M /

A

AG

C L

eve

l / d

Bm

Procedure time / s

EVS -- DDM over time, Reflective Coefficient r=0.30raw DDM

ICAO filteredAGC Level

Testing of ILS Receivers in a Multipath Propagation Environment

12 of 16IFIS 2008, OKC

Receiver investigations

Simulated DDM of scattering A380 tailfin at 60km/h

-100

-50

0

50

-700 -600 -500 -400 -300 -200 -100 0-6

-4

-2

0

2

4

6

DD

M /

A

Dop

ple

r S

hift

/H

z

Distance to threshold / m

DDM over distance, Reflective Coefficient r=0.30, Sample rate 100Hz

raw DDMICAO filtered

Doppler Shift to Reflector

Testing of ILS Receivers in a Multipath Propagation Environment

13 of 16IFIS 2008, OKC

Receiver investigations

Corresponding DDM of measurement receiver

-100

-50

0

50

-700 -600 -500 -400 -300 -200 -100 0-75

-70

-65

-60

-55

-50

DD

M /

A

AG

C L

eve

l / d

Bm

Distance to threshold / m

EVS -- DDM over distance, Reflective Coefficient r=0.30raw DDM

ICAO filteredAGC Level

Selectable increase of level during process

Testing of ILS Receivers in a Multipath Propagation Environment

14 of 16IFIS 2008, OKC

Receiver investigations

Simulated DDM of scattering A380 tailfin at 360km/h

-60

-40

-20

0

20

40

60

-700 -600 -500 -400 -300 -200 -100 0-20

-10

0

10

20

30

40

DD

M /

A

Do

pp

ler

Sh

ift /

Hz

Distance to threshold / m

DDM over distance, Reflective Coefficient r=0.30, Sample rate 100Hz

raw DDMICAO filtered

Doppler Shift to Reflector

Significant offset due to sideband filter bandwidth

Testing of ILS Receivers in a Multipath Propagation Environment

15 of 16IFIS 2008, OKC

-60

-40

-20

0

20

40

60

-700 -600 -500 -400 -300 -200 -100 0-75

-70

-65

-60

-55

-50

DD

M /

A

AG

C L

eve

l / d

Bm

Distance to threshold / m

EVS -- DDM over distance, Reflective Coefficient r=0.30raw DDM

ICAO filteredAGC Level

Receiver investigations

Corresponding DDM of measurement receiver

Significant offset due to sideband filter bandwidth

Testing of ILS Receivers in a Multipath Propagation Environment

16 of 16IFIS 2008, OKC

Conclusions

• Use of approx. real RF signals-in-space– is suitable to perform measurements under multipath

conditions– very useful during receiver design– source for bandpass (RF) signal can be converted

from complex hybrid computation results

• Accurately designed measurement receivers– produce fully comparable results in dynamic

conditions (ground or flight inspection)– improve safety level of ILS (and other NAVAIDs)