POINTING ACCURACY EVALUATION OF SOTM TERMINALS UNDER

REALISTIC CONDITIONS

Mostafa Alazab1, Wolfgang Felber2, Giovanni Del Galdo1, Albert Heuberger2, Mario Lorenz2,Markus Mehnert2, Florian Raschke2, Gregor Siegert2, and Markus Landmann∗2

1Ilmenau University of Technology, Digital Broadcasting Research Lab, Helmholtzplatz 2, 98693 Ilmenau,Germany, name.lastname@tu-ilmenau.de

2Fraunhofer Institute for Integrated Circuits IIS, Am Wolfsmantel 33, 91058 Erlangen, Germany,name.lastname@iis.fraunhofer.de

ABSTRACT

The Vehicle-Mounted Earth Stations (VMES) oper-ation requirements defined by the regulatory author-ities are bounding for terminal manufacturers. Test-ing the VMES for these requirements (e.g. pointingaccuracy and polarization alignment) is therefore anecessity. The disadvantages of involving operationalsatellites and having fixed separation between themin traditional test methods are overcome in the pro-posed test facility. The test facility comprises anantenna tower and a laboratory building. A sensorarray is mounted on the antenna tower with the cen-ter sensor emulating the satellite. In the laboratorybuilding, the VMES is mounted on a motion emu-lator which can replay realistic motion profiles. Inthis contribution, the main components of the testfacility are introduced and the performance of thede-pointing measurement system is verified for dif-ferent VMES motion profiles.

Key words: SOTM, VMES, mobile VSAT, de-pointing measurement, LMS channel, Ka band, Kuband, motion profiles, FCC 25.226, ETSI EN 302977.

1. INTRODUCTION

The need to access communication services such asthe Internet at all times and in all places has becomean integral part of our private and professional lives.Especially at places without any terrestrial commu-nication infrastructure, satellite based systems arethe only solution. In this context, stationary VerySmall Aperture Terminals (VSAT) are already com-monly used. During the past years, operators learnedthat the inaccurate operation of VSAT frequentlyleads to a degradation of the quality of the offeredcommunication services. A major reason for this

∗Contact e-mail: markus.landmann@iis.fraunhofer.de

degradation is a misalignment of VSAT, i.e., the socalled de-pointing of the antenna, which has to beavoided in any case to minimize interference to ad-jacent satellites.

The increasing demand for mobile applications cov-ering land, maritime, and aeronautical environmentspushes the development of Vehicle-Mounted EarthStations (VMES). For these applications, the mo-bility of the ground terminals represents a signifi-cant challenge in complying with the requirementsin terms of pointing accuracy. In this context, oper-ators and regulatory authorities are already aware ofthe need for testing VMES. For instance, in the USthe Federal Communications Commission (FCC) [1]and in Europe the European TelecommunicationsStandards Institute (ETSI) [2] have defined require-ments for VMES. These requirements are expressede.g. in terms of pointing accuracy, required polariza-tion alignment (if non-circular antennas are used),Equivalent Isotropic Radiated Power (EIRP) spec-tral density limits, and the behavior of the terminalif the satellite signal is lost.

The Fraunhofer Institute for Integrated Circuits IISin collaboration with Ilmenau University of Technol-ogy developed a test facility for Ku/Ka band termi-nals, which in the following is denoted as the IIS testfacility. It can realistically and cost efficiently repro-duce the operational environment of VMES regard-less of the current weather conditions. It allows real-istic testing and speeds up the development processof VMES, while lowering the risk of over engineeringnew systems. This contribution presents evaluationresults of a VMES under predefined motion profilesproduced at the facility. Based on the results, thecompliance with the regulatory requirements is dis-cussed.

In Section 2, the IIS test facility for Ku/Ka band ter-minals compliant to these requirements is presented.In Section 3, the performance of the de-pointing mea-surement system is discussed. Section 4 summarizesthe main features and outcomes introduced in this

2

contribution.

2. THE IIS TEST FACILITY

For system validation and quantitative performanceevaluation of VMES, it is desirable to install and op-erate a test facility which allows for simple, repeat-able, and realistic real-time measurements withoutthe need for operational satellites. The Land MobileSatellite (LMS) channel, the motion of the vehicleand the earth coordinates at which the functional-ity of the VMES is tested are emulated to reproducethe real world conditions. One could propose to doso in an anechoic chamber, applying a motion emu-lator and a transmit antenna instead of a satellite.However, for high gain antennas (e.g. dishes with upto 90 cm diameter) this would be prohibitive, as tofulfill the far field condition the required chamberwould have to be enormous. In contrast to the lat-ter approach, a tower of 50m height located 100maway from the VMES antenna is used to emulatethe satellite. In this way, the far field conditioncan be guaranteed for apertures up to 90 cm diam-eter. The VMES is mounted on a motion emula-tor inside an anechoic chamber, having Line of Sight(LoS) through a RF transparent window towards thetower (See Figure 1). In this way, the IIS test facil-ity, as sketched in Figure 2, enables the testing ofthe overall functionality of the VMES; including an-tenna elements, Positioning, Acquisition, and Track-ing (PAT), and mechanical integration for a satelliteelevation of 16◦ and 24◦. In particular, the antennasub-system can be tested independently both undermovement and under the influence of the LMS chan-nel. At this point, the operational environment of a

window towards

antenna tower

window towards

operational satellites

~6,5 m

~1,7

m

~5,5

m

~2,3

5 m

~2,3

5 m

Figure 1. Anechoic chamber with RF transparentwindows towards the tower (satellite emulation) andtowards operational satellites (45◦ W to 45◦ E)

VMES can be reproduced realistically with the IIStest facility.

The IIS test facility comprises the following compo-nents (shown in Figure 2):

laboratory building

antenna tower

antennas

downlink-

converter

uplink-

converter

channel

emulator

device

under test

mo!on

emulator

channel-

emulator

op!onal

up-/downlink

converter

GPS

emulator

control unit / ground segment

over-the-air

connected

94 mdistance

Figure 2. SOTM test bed architecture

• Channel emulators: emulate the fading char-acteristics caused by the propagation envi-ronment; especially blocking/shadowing [3] atKu/Ka band and weather conditions indepen-dently for the uplink and downlink. Channelsmeasured as well as simulated can be employed.

• Motion emulator: emulates mechanical distur-bances that act upon a terminal mounted on dif-ferent types of vehicles (e.g. trucks, cars, ships,etc.) under various conditions (e.g. highways,gravel road, rough sea, etc.). Both generic andmeasured motion profiles can be applied. Forinstance, measurements for typical scenarios ofemergency aid organizations are available (seealso [4] and [5]).

• Navigation emulator: provides arbitrary GlobalPositioning System (GPS) RF signals for a givenset of real latitude and longitude coordinates,which may be necessary for antenna systems uti-lizing satellite navigation support.

• De-pointing measurement system: a crossshaped sensor array with five antennas ismounted on the antenna tower (see Figure 3).With this fundamental component of the IIS testfacility, the de-pointing angle in azimuth and el-evation can be accurately determined.

This last component of the IIS test facility is dis-cussed in detail in the following sections. In thiscontext, the performance of the antenna de-pointingmeasurement system is demonstrated.

3. ANTENNA DE-POINTING MEA-SUREMENT SYSTEM

To determine the VMES antenna de-pointing, a sen-sor array as shown in Figure 3 is used. Each box on

3

Figure 3. De-pointing measurement setup

the tower contains an antenna for the required fre-quency (Ku/Ka band) and a power detector. Theantenna de-pointing and the sensor positions are de-fined according to the coordinate system introducedin Figure 4.

f

q

A

P

O

Figure 4. Anetnna de-pointing coordinates. The sen-sor array is mounted such that the center sensor islocated at the origin (point O)

The center sensor is assumed to be at the origin of thecoordinate system (point O). The point A representsthe position of the antenna and the point P repre-sents the de-pointing direction of the antenna. Theangle φ is the antenna de-pointing in the horizontalaxis while the angle θ is the antenna de-pointing inthe vertical axis. The angle notations in Figure 4 areused comprehensively throughout this contribution.The separation between sensors 1 & 2 and betweensensors 3 & 4 can be varied in the range ∈ [1◦, 6◦], ac-cording to the beam-width of the terminal antenna,as explained in detail in Section 3.1.

In a preliminary measurement, the received power atthe five sensors is measured for different known an-tenna de-pointing directions while the tracking sys-tem of the antenna is disabled. This data servesas reference for the de-pointing estimation, in whichthe motion emulator replays a certain motion profile

while the tracking system of the antenna is active.At this point, the estimation is carried out in threesteps:

1. measure the received signal at the 5 sensors ofthe VMES

2. calculate the correlation between the measuredsignal and the reference data

3. the antenna de-pointing estimate results fromthe maximum of the correlation

3.1. Optimum sensor positions

The de-pointing estimation accuracy is expressed asthe standard deviation calculated for a large numberof estimation realizations based on Monte Carlo sim-ulations. The optimum sensor positions that yieldthe best estimation accuracy will be derived in thefollowing. The estimation accuracy depends on threeparameters:

• the position of the 4 outer sensors

• the available Signal-to-Noise-Ratio (SNR) at thepower detectors

• the 3 dB beam-width of the antenna

The SNR and the 3 dB beam-width of the antennaare fixed parameters since they result from the trans-mit EIRP of the antenna and the fixed beam of theantenna. Therefore, the positions of the sensors arethe only variable parameters that can be adjusted toimprove the de-pointing estimation accuracy. In thefollowing, the optimum positions of the sensors arederived for the highest possible de-pointing estima-tion accuracy w.r.t. the SNR and the 3 dB beam-width of the antenna. Antenna patterns with dif-ferent 3 dB beam-widths are simulated and the de-pointing estimation accuracy is calculated w.r.t. thepositions of the sensors and the SNR. The simulationresults lead to an empirical equation for the optimumpositions of the sensors with:

∆ ≈ (a · ρ3 + b · ρ2 + c · ρ+ d) · w (1)

where

• ∆ is the distance of the outer sensor to the cen-tered sensor along horizontal as well as verticalaxes (see Figure 3)

• ρ is the SNR in dB

• w is the 3 dB beam-width of the antenna indegrees

• with the polynomial coefficients a = −1.3·10−06

4

• b = 1.8 · 10−04

• c = −7.2 · 10−03

• d = 0.709

Based on Equation (1), the optimum sensor posi-tions w.r.t. the beam-widths and SNR are depictedin Figure 5. The different lines correspond to the

ρ [dB]

w[deg]

4

6

6

7

8

9

10

10 20 30 40 50 60 70

6 [deg]5.5

5

5

54.5

4

4

43.5

3

3

32.5

22

21.5

111

0.5

Figure 5. Optimum sensor positions ∆ [deg] w.r.t.antenna beam-width and SNR

optimum sensor positions for different beam-widthsand SNR values. The maximum achievable estima-tion accuracy corresponding to the optimum sensorpositions are plotted in Figure 6. It can be shown

ρ [dB]

w[deg]

0.4

0.6

0.8

1

1

1.2 [deg]

2

3

4

5

6

7

8

9

10

10 20 30 40 50 60 70

0.84

0.44

0.52

0.36

0.28

0.2

0.2

0.12

0.04

0.02

0.014

0.012

0.008 0.

006

0.004

0.002

Figure 6. Estimation accuracy for the optimum sen-sor positions ∆ [deg] w.r.t. antenna-beam width andSNR

that at a fixed antenna beam-width, better estima-tion accuracy can be achieved by increasing the SNR.Assuming that the sensor positions can be adjustedfreely, the maximum accuracy as shown in Figure 6can be achieved. However, the adjustment of the sen-sors can be very time consuming in practice. If onewanted to test subsequently various terminals withdifferent antenna beam-widths, it would be prefer-able to keep the sensors at fixed positions for all

tests. By defining a minimum de-pointing estima-tion accuracy (e.g. 0.05◦) that has to be achievedin any case, a region w.r.t. sensor position and an-tenna beam-width can be defined achieving at leastthe minimum accuracy at a certain SNR. As an ex-ample of an estimation accuracy better than 0.05◦,these regions are shown in Figure 7 for different SNRvalues. According to Figure 7, the sensor positioncan now be chosen in a wider range. For example,

∆ [deg]

w[deg]

0.5

1

1 1.5

2

2 2.5

3

3 3.5

4

4 4.5

5

5 5.5

6

6

7

8

9

1070dB

65dB

60dB

55dB50dB

45dB40dB

35dB30dB

25dB

20dB

Figure 7. Regions with estimation accuracy betterthan 0.05◦

having an antenna with w = 5◦ and the sensors fixedat ∆ = 3◦, the following holds:

• With these sensor positions, the de-pointingestimation accuracy larger than 0.05◦ can beachieved if the available SNR is not below 35dB.

• When exchanging the antenna with another an-tenna having 3 dB beam-width of 4◦, we do notneed to change the position of the sensors as wecan still ensure a minimum accuracy of 0.05◦

assuming that the SNR is not below 35 dB.

• Ensuring the same accuracy threshold of 0.05◦

with another antenna having a 3 dB beam-widthof 3◦ requires either:

– increasing the SNR to be around 10 dBlarger (45 dB) while keeping the same sen-sor positions or,

– changing the sensor positions to be in therange ∈ [1◦, 2.3◦] while keeping the SNR at35 dB.

3.2. De-pointing Measurement Results

To demonstrate the performance of the de-pointingmeasurement system, measurements with the follow-ing setup were conducted

5

• High gain antenna as the Device Under Test(DUT) with w = 1◦

• 5 sensors are included in the measurement asshown in Figure 3

• Sensor positions along vertical and horizontalaxes with ∆ = 1◦

• Ka band (27.5 GHz - 31 GHz)

• Maximum receive SNR between 25 and 35 dB

φ [deg]

θ[deg]

-60

-50

-40

-30 [dBm]

-55

-45

-35

-5-5

-4

-4

-3

-3

-2

-2

-1

-1

0

0

1

1

2

2

3

3

4

4

5

5

Figure 8. Received power (2D pattern) of the highgain antenna

The received power at the center sensor while ro-tating the DUT (using the motion emulator) in a2D (horizontal-vertical) grid is shown in Figure 8.Therefore, the values shown in Figure 8 when prop-erly normalized, may also be interpreted as the di-rectivity function of the high gain antenna.

The performance of the system is analyzed rotatingthe DUT around the horizontal (one-dimensional)and horizontal&vertical axes (2D) (see Figure 4).The motion profiles are sine functions with a fixedamplitude, frequency and phase. The estimationresults for the 1D and 2D case are discussed in thefollowing. During the movement, the DUT trackingmode has been switched off, which means that theestimated de-pointing should exactly correspond tothe excitation induced by the motion emulator.

1D de-pointing example: In this example, theantenna moves according to Equation (2)

φ(t) = αφ · sin(2πfφt+ ψφ) (2)

Where the antenna de-pointing follows a time vary-ing sine function along the horizontal axis only withamplitude (αφ), frequency (fφ) and phase (ψφ). Inthis example: αφ = 1◦, fφ = 0.1Hz and ψφ = 0◦.

The maximum SNR at the power detectors is 35 dB.The terminal antenna is balanced and it can be as-sumed that there is no play in any of the axes. InFigure 9, the excitation motion profile and the esti-mation results are depicted. The blue line represents

t [s]

φ[deg]

Excitation

Estimation

Est. Error

-1.2-1

-0.8-0.6-0.4-0.2

0

0

0.20.40.60.81

1.2

10 20

Figure 9. Estimation result for a sine excitation (f =1Hz) along the horizontal axis with αφ = 1◦

the motion excitation, the red dotted line representsthe estimation results and the black line representsthe estimation error. The Root Mean Square Error(RMSE) and the standard deviation of the estima-tion results by means of the confidence interval areshown in Figure 10. The RMSE is represented by theblue line and the confidence interval (i.e. standarddeviation) is represented using the red bars. TheRMSE and the confidence interval are calculated forat least 100 realizations at each de-pointing angle.

φ [deg]

Confidence

interval[deg] RMSE

Confidence interval

-0.2 -0.15 -0.1 -0.050

0

0.01

0.02

0.03

0.04

0.05

0.05 0.1 0.15 0.2

Figure 10. RMSE and confidence interval for a sineexcitation (f = 1Hz) along the horizontal axis withαφ = 1◦

From Figure 10 it can be seen that the estimationaccuracy is in the order of 0.005◦ on average.2D de-pointing Example 1: The induced motionalong the horizontal and the vertical axes is sinu-soidal:

φ(t) = αφ · sin(2πfφt+ ψφ) (3)

andθ(t) = αθ · sin(2πfθt+ ψθ) (4)

6

With αφ = αθ = 0.5◦, fφ = fθ = 0.1Hz, ψφ = 0◦ andψθ = 90◦. The SNR at the power detectors is around25 dB. The tracking mode of the terminal antenna isswitched off but a small play along the terminals axescan be observed. Furthermore, the antenna itself isnot fully weight balanced. The results are shown in a2D diagram w.r.t. the excitation along the horizontaland vertical axes (Figure 11). The blue circle repre-sents the excitation of the motion emulator while thered dots represents the estimation results.

Analyzing the deviation from the induced excita-tion, it can be seen that the deviation is smallerthan 0.02◦ on average for both horizontal and verti-cal axes. This deviation most probably results fromthe weight imbalance of the terminal antenna andthe play along the axes of the terminal.2D de-pointing Example 2: Again the same set-tings apply as in the previous example but with-out phase shift between the two axes in motioni.e. ψφ = ψθ = 0◦ . The results are shown in Fig-ure 12. Again the deviation is smaller than 0.02◦ onaverage for both axes and most probably resultingfrom the weight imbalance and the play along thehorizontal and vertical axes.

The results discussed above primarily validate theperformance of the IIS test facility and approves itsusage for VMES performance testing and validation.

φ [deg]

θ[deg]

Excitation

Estimation

-0.5

-0.5

0

0

0.5

0.5

Figure 11. Sine motion excitation for horizontal andvertical axes with phase shift of 90◦

4. CONCLUSION

In this contribution the IIS test facility is described.The de-pointing estimation accuracy has been ana-lyzed and evaluated by measurements. The advan-tages of the IIS test facility and especially the demon-

φ [deg]

θ[deg]

Excitation

Estimation

-0.5

-0.5

0

0

0.5

0.5

Figure 12. Sine motion excitation for horizontal andvertical axes with phase shift of 0◦

strated de-pointing measurement system comparedto system verification with operational satellites canbe clearly identified as follows:

• With the proposed sensor array, the de-pointingangle can be determined without involving op-erational satellites in contrast to satellite to ter-minal and vice versa measurement.

• The distance between the sensors can be ad-justed w.r.t. beam-widths, which results in ahigher estimation accuracy of the de-pointingangle.

• De-pointing measurements in azimuth and ele-vation in contrast to azimuth only are available,which is relevant in case of asymmetric antennacharacteristics as in case of low profile antennas.

• Measuring the reference data includes far fieldradiation pattern measurement (far field condi-tion applies for aperture sizes of up to 90 cm).

• Full communication test of the terminal at Ra-dio Frequency (RF) or Intermediate Frequency(IF).

• Closed loop communication test while changingenvironment conditions.

• Real operational Geostationary Earth Orbit(GEO) satellites can also be used for testing.

• Comparable conditions for every test run.

• The satellite emulation is available for 16◦ and24◦ elevation.

• Cost-efficient and available at all times.

• Measurements can be carried out regardless ofthe weather conditions.

7

Future Extensions: The IIS test facility includessome components and features which are planned tobe implemented in the future. The possibility to em-ulate not only the target satellite but also interferingsatellites is planned. Further extensions for polar-ization de-pointing measurements are to be imple-mented.

REFERENCES

[1] FCC. FCC 25.226; Blanket licensing provisionsfor domestic, U.S. Vehicle-Mounted Earth Sta-tions (VMES), 2010.

[2] ETSI EN 302 977 V 1.1.2, Satellite Earth Sta-tions and Systems (SES); Harmonized EN forVehicle-Mounted Earth Stations (VMES) oper-ating in the 14/12 GHz frequency bands cover-ing essential requirements under article 3.2 of theR&TTE directive, 2010.

[3] Marie Rieche, Daniel Arndt, Alexander Ihlow,Markus Landmann, and Giovanni Del Galdo.Image-based state modeling of the land mo-bile satellite channel for multi-satellite recep-tion. In 34rd ESA Antenna Workshop on Chal-lenges for Space Antenna Systems, ESTEC, No-ordwijk, Niederlande, October 2012. ESA. zurVeroffentlichung angenommen.

[4] ESA Project ARTES-5.1 - 7-.022 - AO-6669: Mobile Tracking Needs Website under:http://telecom.esa.int/telecom/.

[5] Lorenz M., Mehnert M., and Heuberger A. Mea-surements of mechanical disturbances of vehiclemounted, mobile very small aperture terminals(vsat). In Proceedings of the 11th Workshop Dig-ital Broadcasting, pages 61–65, Erlangen, Ger-many, September 2010.