Effect of Interference in the Calculation of theAmplitude Scintillation Index S4Rodrigo Romero

Ph.D. StudentDepartment of Electronics and Telecommunications

Politecnico di TorinoTurin, Italy +39-011-2276442

Email: rodrigo.romero@polito.it

Fabio DovisAssistant Professor

Department of Electronics and TelecommunicationsPolitecnico di Torino

Turin, Italy +39-011-0904175Email: fabio.dovis@polito.it

Abstract— Irregular electron content in the ionosphere affectsthe propagation of Global Navigation Satellite Systems (GNSS)signals with a phenomenon known as scintillation. Ionosphericscintillations are rapid fluctuations in the amplitude and phaseof the signal that can lead a receiver to lose lock. In recent yearsthe number of Ionospheric Scintillation Monitoring Receivers(ISMR) have increased dramatically. This paper presents aninitial analysis of the effect that interference may have in thecalculation of the widely used amplitude scintillation index S4.Interference is one of the growing threats to the performance ofGNSS. Our analysis shows that interference may have particulareffects on GNSS receivers that can lead them to output anerroneous measurement of the S4 index.

I. INTRODUCTION

Electron concentration along the propagation path ofsatellite signals in the ionosphere is known to causeGlobal Navigation Satellite Systems (GNSS) signals toexperience range delays when traveling through it, but undercertain conditions it can also induce another phenomenoncalled scintillation [1]. Ionospheric scintillations are rapidfluctuations in the received signal amplitude and phase,originating from a scattering effect in the ionosphere dueto zones with irregular electron concentration. Amplitudescintillations cause signals to fade. Phase scintillations mayinduce a frequency shift in the signal carrier that in somecases can go beyond the Phase Lock Loop (PLL) bandwidthof the GNSS receiver. Both effects are very challengingfor a receiver tracking block and may cause frequent cycleslips and losses of lock of the satellite signals during strongionospheric events.

Due to their global availability, GNSS signals providean excellent means to monitor and study ionosphericscintillations. The ever increasing reliance on GNSS systemscombined with the approach of the next solar maximum havedriven a marked research interest to improve the robustness ofthese systems to the threats posed by ionospheric disturbances[2]. In recent years, Ionospheric Scintillation MonitoringReceivers (ISMR) as in [3], [4] and [5] have been deployed indifferent regions of the world to measure scintillation indexeson amplitude and phase, namely the S4 and phase standard

deviation σφ, together with a collection of signal statistics inreal time.Given the crowded telecommunication environment where areceiver is likely to work, radio frequency interference fromother systems or devices is always a possibility. The presenceof unintentional interference is recognized to be one of thegrowing threats to GNSS performance [6]. Interference mayhave particular effects on the GNSS receiver that are similarto ionospheric scintillations, leading the receiver to output anerroneous measurement of the phenomenon.In this paper, we want to study the monitoring of amplitudescintillation from a user receiver perspective in the presenceof interference. Section II offers some background onionospheric scintillation and its effects on GNSS receivers.Section III describes the methodology used to generateour scintillation and interference scenarios together with adescription on how the amplitude scintillation index S4 iscalculated. Section IV presents the analysis of the results inour simulations. Section V presents the conclusion and futurework to be developed.

II. SCINTILLATION EFFECT ON GNSS RECEIVERS

As mentioned, scintillations are caused by a scatteringeffect in zones with irregular electron concentrations in theionosphere. How often scintillation affects GNSS signalsdepends on solar and geomagnetic activity, geographiclocation, season and local time. The geographic regionswhere the ionosphere is more active and less predictableare the equatorial and high latitude regions. Equatorialionospheric irregularities are produced after the local sunsetby the combined effects of the chemical recombination andthe electrodynamic lifting of the ionospheric F − region bythe Pre-Reversal Enhancement, in the band extending fromabout 20◦N to 20◦S geomagnetic latitudes. Auroral and polarcap disturbances are mainly the result of geomagnetic stormswhich are associated with solar flares, coronal mass ejectionsand coronal holes [7]. In equatorial regions the dominanteffect of scintillation is in the amplitude, with fades as bigas 20-30 dB during the most active ionospheric periods. Inpolar regions, as a consequence of lower ionization levelswith respect to equatorial areas, amplitude fades reach up

978-1-4799-0486-0/13/$31.00 ©2013 IEEE

Fig. 1. Simplified Receiver Architecture.

to 10dB only. However, due to increased velocity of theionospheric disturbances stronger phase scintillations mayappear as compared to what is observed at equatorial areas[8] [9].

Ionospheric scintillations are also a frequency dependentphenomenon: drops on signal power or large phase errorseffects are greater on the L2 band when compared to L1. Thismakes signals transmitted on L2 frequency more susceptibleto suffer outages during strong scintillation conditions. Theimpacts of scintillations are not mitigated by the samedual-frequency technique that is effective at mitigating theionospheric delay. On user receivers, ionospheric scintillationcan lead to loss of the GNSS signals or increased noise onthe remaining ones. Most often it will only affect a number ofsatellites at a time, but if the user has poor satellite coverageeven modest scintillation levels can cause an interruption touser operations. Other effects on GNSS receivers are theloss or corruption of the data bits [10]. For these reasons,scintillation have become one of the most significant threatsfor GNSS operating in the near equatorial and polar latitudes.

III. METHODOLOGY

We made use of a software receiver to study the effectsof interference on the calculation of the amplitude scintil-lation index S4 [11]. A simplified model of the standardconfiguration of the receiver can be seen in Figure 1. Forour scintillated scenario, signals were generated followingthe Cornell Scintillation Model presented in [12] and [13].Interference was injected afterwards to observe its effect onthe calculation of the amplitude scintillation index.

A. Scintillation Generation

The Cornell model is a statistical model that synthesizesequatorial ionospheric scintillation perturbations for testingcarrier tracking loops of squaring type PLLs [13]. The inputparameters to the model are the scintillation index S4 and thedecorrelation time τ0. The simulator is driven by a stationaryzero-mean complex White Gaussian Noise process and outputsa complex scintillation time history Z(t). The complex timehistory is used as in (1) to simulate scintillation fluctuationsin amplitude and phase for the GNSS signals.

δA(t) = |Z(t)| δϕ(t) = ∠Z(t) (1)

Where || represents the magnitude and ∠ the phase of thecomplex time history. The mathematical expression for GPSL1 civil signal with scintillation is:

S(t) = A∗δA(t)∗D(t)∗C(t)∗sin(2πfL1t+ϕ+δϕ(t)) (2)

Where A is the signal amplitude, δA(t) is the amplitudefluctuation due to scintillation, D(t) is the navigation data,C(t) is the PRN spreading code, fL1 is the radiofrequencycarrier, ϕ is the initial carrier phase and δϕ(t) is the phasefluctuation due to scintillation.

B. Interference Addition

Two types of interference, WideBand (WB) and Contin-uous Wave (CW ), were fed into the system along with thescintillated signal. Similar to (2), at the input of the receiverprocessing block the complete expression for the scintillatedsignal at intermediate frequency affected by WB interferenceis:

S(t) =A ∗ δA(t) ∗D(t) ∗ C(t) ∗ sin(2πfIF t+ ϕ+ δϕ(t))

+N(t) +NWB(t)(3)

Where fIF is the intermediate frequency value, N(t) is thenoise term, and NWB(t) is a wideband noise with 250 KHzbandwidth centered at a frequency f = fIF + 1MHz. Simi-larly, in the case of CW interference the expression becomes:

S(t) =A ∗ δA(t) ∗D(t) ∗ C(t) ∗ sin(2πfIF t+ ϕ+ δϕ(t))

+N(t) +ACW ∗ sin(2πfCW t+ θ)(4)

Where ACW , fCW , and θ are respectively, the amplitude,frequency (also with a 1MHz frequency separation from theintermediate frequency value) and phase of the CW . Figure 2shows the spectrum of GNSS signals affected by the generatedinterferences.

C. Calculation of the Amplitude Scintillation Index S4

S4 index measures the amount of amplitude scintillationin GNSS signals. It is the root mean square deviation ofthe detrended Signal Intensity (SI), normalized by its meanpower. As described in [4] and [14], S4 is calculated fromthe in-phase and quadrature-phase accumulation samples ofthe prompt correlator, normally over 60 second intervals. Thecomputation is as follows: Raw SI samples can be calculatedas the difference between Narrow Band Power (NBP ) andWide Band Power (WBP ) by following (5) through (7):

NBP = (

20∑i=1

Ii)2 + (

20∑i=1

Qi)2 (5)

WBP =20∑i=1

(I2i +Q2

i ) (6)

SIraw = NBP −WBP (7)

0 2 4 6 8−240

−230

−220

−210

−200

−190

−180

Frequency [MHz]

Pow

er S

pect

ral D

ensi

ty [d

B/H

z]Signal Spectrum

(a)

0 2 4 6 8−260

−240

−220

−200

−180

−160

Frequency [MHz]

Pow

er S

pect

ral D

ensi

ty [d

B/H

z]

Signal Spectrum

(b)

Fig. 2. Spectrum of GNSS signal affect by WideBand Interference (a) andContinuous Wave Interference (b).

Where Ii and Qi are the 1KHz in-phase and quadrature-phase prompt correlator samples. Before calculating the S4,the raw SI samples must be detrended to remove fluctuationsdue to satellite motion and possibly multipath. The signal trendis typically obtained by filtering the raw samples with a 6thorder Butterworth filter with cutoff frequency fc = 0.1Hz, butcan also be obtained as the mean value of the SI samples overthe 60 seconds span. The detrending is performed by dividingthe raw samples by the calculated trend, as in (8). DetrendedSI samples fluctuate around a value of 1.

SIdetrended =SIrawSItrend

(8)

Total S4 is then calculated as:

S4T =

√〈SI2〉 − 〈SI〉2

〈SI〉2 (9)

1 2 3 4 5 6 7 8 9 10 11 1238

40

42

44CN0

Time (minutes)

C/N

o[dB

Hz]

0 2 4 6 8 10 120

0.5

1

Time (minutes)

S4

Amplitude Scintillation Index

Butterworth filter detrendedMean value detrended

Fig. 3. (top) Estimated C/N0 of the Interfered Signal. (bottom) CalculatedS4 for every minute.

Where 〈〉 represents the average value over the interval ofinterest (60 seconds). If the carrier to noise density C/N0

can be estimated during the interval, it is possible to have anestimate of the S4 due to noise:

S4n =

√100

C/N0

(1 +500

19 ∗ C/N0

) (10)

Subtracting the square of this value from the square of (9)gives the revised S4 without the noise contribution:

S4 =√S42T − S42n (11)

By definition, the S4 index satisfies the following condition[13]:

S4 ≤√2 (12)

IV. RESULTS

A. Interference Scenario

To observe the effects of Interference in the calculationof the S4 we first processed scenarios in which only theinterference is affecting the L1 signal, absent scintillation.Figure 3 shows the estimated C/N0 of a signal as in (3)that last for 12 minutes, together with the corresponding S4calculated every minute. From minute 4 until 9 the signalis affected by a WB interference of −130dBW of powerthat causes a drop in the estimated C/N0. In the absence ofscintillation, the S4 value would be close to zero, but twopeaks are noticeable in the S4 which match the time instantswhen the interference starts and ends.

To understand the presence of such peaks, we can takea look at the signal intensity samples and their detrendedversions in Figure 4 for both the filter and the mean valuedetrending methods. It is noticeable from the top plots ofFigures 4.a and 4.b that at the time instant when the inter-ference begins, the estimated value of the signal intensity isgreatly reduced.. Likewise, when the interference ends the

signal intensity recovers and a sudden raise is observed. Sucheffect points to the Automatic Gain Control (AGC) of theuser receivers which adjust its gain depending on the signalamplitude and, in the presence of interference and its increasedsignal amplitude dynamics, means that the gain of the usefulpart of the signal is not constant anymore and it is in factreduced. When extracting the trends within such signals thedifferent detrending methods have different behaviors on howfast they can react to these sudden changes introduced by theAGC. As presented in (8), the detrending operation consistson the division of the signal intensity samples by its trend,which means that if the trend does not quite follow the signalthe division will not be around 1. As can be seen in the bottomplots of Figures 4.a and 4.b, the intervals where the AGCchanged its gain introduced errors in the detrended signalintensity increasing its variance, and thus, inflates the S4 valuecalculated.

1 2 3 4 5 6 7 8 9 10 11 120

2

4

6x 10

11 Signal Intensity vs Signal Intensity trend

Time (minutes)

Am

plitu

de

Signal IntensityTrend Butter filter

1 2 3 4 5 6 7 8 9 10 11 120

1

2

3

Time (minutes)

Detrended Signal Intensity

Am

plitu

de

(a)

1 2 3 4 5 6 7 8 9 10 11 120

2

4

6x 10

11 Signal Intensity vs Signal Intensity trend

Time (minutes)

Am

plitu

de

Signal IntensityTrend Mean Value

1 2 3 4 5 6 7 8 9 10 11 120

2

4

6

Time (minutes)

Detrended Signal Intensity

Am

plitu

de

(b)

Fig. 4. Raw and Detrended Signal Intensitiy for Filter Detrending Method(a) and Mean Value Detrending Method (b).

TABLE I

SUMMARIZING RESULTS OF S4 PEAKS DUE TO INTERFERENCE

Type Power (dBW)S4 variations

Filter Detrending Mean Detrending

WB

-120 Not Valid 0.4 - Not Valid

-125 Not Valid 0.4 - Not Valid

-130 0.2 0.4 - 1.1

-135 0.1 0.3 - 0.6

CW

-120 0.4 0.4 - 1.1

-125 0.15 0.3 - 1

-130 0.08 0.25 - 0.5

-135 0.06 0.25

1 2 3 4 5 6 7 8 9 10 11 1230

40

50CN0

Time (minutes)

C/N

o[dB

Hz]

0 2 4 6 8 10 120

0.2

0.4

Time (minutes)

S4

Amplitude Scintillation Index

Butterworth filter detrendedMean value detrended

Fig. 5. (top) Estimated C/N0 of the Scintillated Signal. (bottom) CalculatedS4 for every minute.

The same test was repeated changing the power of theinterference signal, both for CW and WB. The results can beseen in Table 1, where the observed variation of the S4 withrespect to the power and type of interference are summarized.For cases when the interference power is high with respectto the signal and in particular for WB, S4 values that wentbeyond the theoretical maximum value were found in thecalculations and were readily discarded. The term ”Not Valid”identifies such values in the table. It can also be observed thatthe mean value detrending method is more prone to producelarger S4 errors than the Butterworth filter due to its slowerreaction to the sudden gain variations introduced by the AGCin the signal intensity.

B. Scintillation plus Interference scenario

We chose a reference scintillation scenario by generatingan L1 civil signal with C/N0 = 45dB and driving thescintillation model with S4 = 0.4 and τ0 = 1. The estimatedC/N0 and S4 after the tracking stage are shown in Figure 5.

As explained before, S4 values that go beyond the theoret-ical maximum are readily discarded, thus, strong interferencethat causes this can never mask as scintillation. However, a

different case may present itself with lower levels of inter-ference. As shown in Table 1, for interference power rangingfrom −130dBW to −135dBW , the errors introduced in theS4 by the AGC could perfectly mask as scintillation. Thiscan be better observed on Figure 6 where, building upon thereference scintillation scenario, we added WB interferenceand recalculated the S4 index.From Figures 6.a and 6.b we can see how in some cases theinterference can be completely masked as scintillation, particu-larly more in the case of the Butterworth filter detrending. Thedifferences with respect to the reference scenario are mainlyduring minutes 4 and 9 which are when we have the changesof gain. During minutes 5 through 8 when interference isstill in full effect, we barely see any changes in the S4 withrespect to the reference scenario. This is because the S4 is anormalized index and, even if interference is present, as longas the gain was relatively constant during the whole minutethe index should not be greatly affected.

0 2 4 6 8 10 120

0.1

0.2

0.3

0.4

0.5

0.6

Time (minutes)

S4

S4 Index from Filter Detrending

S4 Interfered SignalS4 reference scenario

(a)

0 2 4 6 8 10 120

0.2

0.4

0.6

0.8

1

1.2

1.4

Time (minutes)

S4

S4 Index from Mean Value Detrending

S4 Interfered SignalS4 reference scenario

(b)

Fig. 6. Calculated S4 for Scintillated Signal with added WB Interference(-130dBW) with Filter Detrending (a) and Mean Value Detrending (b)

V. CONCLUSION

We demonstrated the effect of interference in the calculationof the amplitude scintillation index S4 on GNSS receivers.Two possible forms of interference were analyzed, Widebandand Continuous Wave. The results showed the decreased per-formance they can bring to ionospheric monitoring receivers inparticular for cases where the AGC is present in the frontend.An effect that may be difficult to spot when the interferencepower is low due to the fact that it can mask as scintillation,enhancing the measurement of the scintillation index. Themost common methods for signal detrending also formedpart of our analysis and we showed how their capability tofollow the sudden changes of gain introduced by the AGCcan also influence the amount of error in the calculation ofthe index. For future work we intend to study with simulatedand real cases whether available mitigation techniques, onceinterference is detected, can successfully suppress the harmfuleffects presented here while still retaining the scintillationcharacteristics of the signal.

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