Real World Receiver Testing

and the 1dB Criteria

Guy Buesnel, Mark Hunter, Charlotte Perry December 2018

2

Introduction Measurement of GNSS Receiver parameters

• Last year we examined measurement parameters when GNSS

Receivers were subjected to Adjacent band interference

• 1dB degradation in CNR used in European ETSI standard for ABC

– Established Interference Protection Criterion (USAF justification paper)

– Some agencies have proposed use of GNSS accuracy or Time to First Fix as a

preferable metric

• The 1dB IPC does not require the level of harmful RF interference to be

specified..

• Spirent approach – use automated test bench to carry out many

repeated measurements on sample receivers across increasing

interference levels

3

Test Set-up PNT Test Bench

RF Simulator

Scenario Generation

RF Navigation Data

Control

commands

Control

commands

Receiver

Data

GNSS

Device under test

DUT Receivers A,B,C,D,E,X

4

Test Method The GNSS RED Adjacent Band Interference test

Frequency band (MHz) Test point centre

frequency (MHz)

Adjacent frequency

signal power level (dBm)

Comments

1518 - 1 525 1 524 -65 MSS (space-to-Earth) band

1 525 - 1 549 1 548 -95 MSS (space-to-Earth) band

1 549 - 1 559 1 554 -105 MSS (space-to-Earth) band

1 559 - 1 610 GUE RNSS band under test

1 610 - 1 626 1 615 -105 MSS (Earth-to-space) band

1 626 - 1 640 1 627 -85 MSS (Earth-to-space) band

Frequency band (MHz) Test point centre

frequency (MHz)

Adjacent frequency signal

power level (dBm)

Comments

960 - 1 164 1 154 -75 AM(R)S, ARNS band

1 164 - 1 215 GUE RNSS band under test

1 215 - 1 260 GUE RNSS band under test

1 260 - 1 300 GUE RNSS band under test

1 300 - 1 350 1 310 -85 Radiolocation, ARNS, RNSS

(Earth-to-space) band

Parameter Value Comments

Frequency See tables 4-2 and 4-3

Power level See tables 4-2 and 4-3

Bandwidth 1 MHz See clause B.1 for details

Format AWGN

RED: “Radio Equipment Directive”

GNSS RED: ETSI EN 303 413

Detail of requirements and tests necessary for

GNSS receivers

Note: The GNSS RED also includes an emissions

test – not covered in this presentation…….

5

RFI effects on CNR and Position Manual Analysis 5 datasets, 60 samples

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

-120 -100 -80 -60 -40 -20 0

0

5

10

15

20

25

30

35

40

45

50

horizonta

l positio

n e

rror

(m)

RFI power (dBm)

CN

R (

dB

-Hz)

RFI Power and Horizontal position error at 1554MHz with measurement range (HPE) added - Receiver A

(dB-Hz) Pos Error (m)

RED test level CNR drop >1dB

• Monitoring CNR

and Horizontal

Position Error with

increasing RFI

• Full Range error

bars displayed

• RED Test level and

point of 1dB CNR

drop marked out

6

RFI effects on CNR and Position

• CNR and Horizontal

Position error plotted

for all Receivers with

increasing RFI

• Note that CNR drop

off (for all 5 TURs)

occurs before HPE

starts to fluctuate..

• Some of the TURs

clearly much more

susceptible to the

RFI than others….

Manual Analysis 5 datasets, 60 samples, All receivers

-120 -100 -80 -60 -40 -20 0

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0

5

10

15

20

25

30

35

40

45

50

-120 -100 -80 -60 -40 -20 0

Axis Title

HO

RIZ

ON

TA

L P

OS

ITIO

N E

RR

OR

(m

)

CN

R (

dB

-Hz)

RFI POWER

Horizontal Position Error and CNR changes with increasing RFI

A CNR B CNR C CNR D CNR E CNR A POS B POS C POS D POS E POS

7

RFI effects on CNR and Position

• Note Receiver A HPE

fluctuations once CNR has

degraded to minimum value of

around 22.5 dB-Hz

In-Band interference at GPS L1 Frequency

-5

0

5

10

15

20

-120 -100 -80 -60 -40 -20 0

0

5

10

15

20

25

30

35

40

45

50

HP

E (

m)

RFI Power (dBm)

CN

R (

dB

-Hz)

RFI Power and Horizontal position error at 1575.42MHz, In Band - Receiver A

(dB-Hz) Pos Error (m)

8

RFI effects on CNR and Position

• Interesting comparison

of receiver behaviour –

for both out of band and

in-band interference

CNR drop-off seen to

precede degradation in

HPE values…

In-band and out of band interference comparison

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

20.0

-120 -110 -100 -90 -80 -70 -60 -50 -40 -30 -20

0

5

10

15

20

25

30

35

40

45

50

HP

E (

m)

RFI Power (dBm)

CN

R (

dB

-Hz)

1533MHz and GPS L1 RFI – Receiver A

InBand SNR 1554 SNR InBand POS 1554 POS

9

HPE and TTFF under RFI TTFF and HPE Comparison – Test Receiver A

-10.00

-5.00

0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

40.00

0.00

500.00

1000.00

1500.00

2000.00

2500.00

3000.00

-70 -60 -50 -40 -30 -20 -10 0

TTFF

(s)

RFI Power (dBm)

TTFF and HPE under LTE RFI (1533 MHz)

TTFF Initial Error Av Error

HP

E (m

)

• We see that (for this

receiver –and all

others we’ve tested

so far) the HPE

increases before any

degradation in TTFF

10

HPE and TTFF under RFI TTFF and HPE Comparison – Test Receiver A

-10.00

-5.00

0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

40.00

0.00

200.00

400.00

600.00

800.00

1000.00

1200.00

1400.00

1600.00

-65.0 -60.0 -55.0 -50.0 -45.0 -40.0 -35.0 -30.0 -25.0 -20.0

Po

siti

on

Err

or

(m)

TTFF

(s)

RFI

TTFF and HPE under 1554MHz RFI - Receiver Y

TTFF Initial Error Av Error

• Whilst CNR not plotted

for these two tests, we

saw that CNR was a

precursor to increasing

HPE on this receiver

11

Variability in GNSS measurements Example – 3D positioning accuracy

• Receiver “X” – this time 800

measurements taken using

automated test bench

• Probability distribution can now

be modelled and monitored

across interference levels…

• Too few measurements could

be problematic….

12

Variability in GNSS measurements Example – Time To First Fix

-144 dBm

-30dBm

Receiver “X” with 1533MHz

interferer (simulating LTE base

station)

30 Measurements taken using

automated Test Bench

• Any evidence of anomalous

behaviour?

13

Variability in GNSS measurements Example – Time To First Fix data

-144 dBm

-30dBm

Receiver “X” with 1533MHz

interferer (simulating LTE base

station) – Tester makes 3

measurements at each power

level…

3 measurements

3 measurements

“Unlucky” – this small set of

measurements might result in incorrect

conclusions/decisions being made…..

14

Spirent Insights …and further work

• Our results serve to confirm that under test conditions a 1dB degradation in CNR whilst subject

to interference, is a precursor to reduced or erratic performance (HPE etc…) in GPS receivers

• Rather than just testing to a mandated level of RFI, the work we’ve done highlights testing past

the 1dB degradation in CNR provides much deeper understanding of receiver behaviour

• Results also demonstrate stochastic nature of GNSS measurements – Too few measurements

in a Live Sky scenario (with even more variables) can give misleading results

• So far concentrated on single interference source – however LTE deployment plans means

there is a need for multiple interferer scenarios to test against…

15

Spirent Insights …and further work

• We are building up our “Big data” characterization/certification of receiver behaviour

including more parameters including:-

– RFI vs • TTFF (Cold, Warm, Hot)

• Reacquisition

• 1PPS and NTP/PTP accuracy

– Control Segment Errors

– Signal and Navigation data health status

– RAIM

– Multipath (Sim3D)

• RTK and MCMF

Join the GNSS Vulnerabilities group on Linked In to find out more

about GNSS jamming and spoofing the

Trust but Verify

guy.buesnel@spirent.com

mark.hunter@spirent.com

https://www.spirent.com/PNT/Measuring-Resilience

Spirent® Communications, Inc. and its related company names, branding, product names and logos referenced herein,

and more specifically “Spirent” are either registered trademarks or pending registration within relevant national laws.