ANTENNA DESIGN AND TESTING FOR THE ACES MICROWAVE LINK

Jürgen Schmolke & Michael Zähringer

Kayser-Threde GmbH, Wolfratshauserstr. 48, D-81379 Munich, Germany, juergen.schmolke@kayser-threde.com

ABSTRACT The ACES Microwave Link (MWL) is a two-way crosslink microwave signal transfer system for comparison of the ACES space clock ensemble aboard the International Space Station (ISS) with high performance ground clocks. The system is currently developed for the European Space Agency and will be installed at the Columbus External Payload Facility (CEPF). The Microwave Link is consisting of a Flight Segment as part of the ACES payload and Ground Terminals installed world-wide at major time and frequency laboratories participating in the ACES experiment programme. The ACES MWL project successfully concluded Phase C0 (preliminary design review) and is presently in Phase C1/C2 (EM phase).

The MWL antenna system for the Flight Segment is consisting of fixed pattern antennas for Ku-Band uplink and downlink and for an additional S-Band downlink. For reasonable contact times a field-of-view of ± 70° is required with maximum amount of gain, beyond 70° the gain characteristic should decrease as much as possible in order to reduce multipath effects as a result of reflections on ISS structures. The antenna system for the Ground Terminal is consisting of a dual-band feed in combination with an offset reflector mounted on a steering unit providing full hemispherical coverage with high gain. For high-precision clock comparison between space and ground the knowledge of the antenna phase centers for the Ku-Band frequencies is required precisely for all aspect angles over the full temperature range.

The paper reports the design status of the Ku-Band and S-Band antenna systems for the Flight Segment and Ground Terminals. The challenging performance parameter, the test configurations for antenna stability and first measurement results are presented.

1. THE ACES MICROWAVE LINK

1.1 ACES Mission Overview

The main objective of the ACES (Atomic Clock Ensemble in Space) mission is to operate a new generation of clocks in space, which will exhibit a performance superior to that of today’s ground based clocks.

The ACES payload is currently developed for the European Space Agency (ESA) with EADS as the Prime Contractor. Kayser-Threde and Timetech are responsible for the Microwave Link (MWL). Major other components of ACES are PHARAO, a caesium fountain currently developed by CNES, and SHM, a Space Hydrogen Maser currently developed by the Observatory of Neuchatel.

The ACES payload will be mounted at the Columbus External Payload Facility (CEPF) of the International Space Station (ISS) as illustrated in Fig. 1.

Fig. 1 ACES Payload mounted at the ISS

During the mission the space-based clocks will be compared to ground based clocks to verify their performance, to perform scientific studies and to make available such high accuracy to world-wide users.

1.2 Microwave Link Principle

For the space to ground clock comparison a Microwave Link was selected. The MWL system is consisting of a Flight Segment (FS) which is integrated into the ACES payload and a number of Ground Terminals (GT) installed word-wide at major time and frequency laboratories.

The system concept of MWL is based on the two-way frequency and time transfer principle as used by metrological institutes since many years. The major challenges for MWL are due to the operational environment and constraints on-board the ISS. In particular the orbital characteristics of the ISS, the variable thermal environment and the variable

propagation path are introducing effects not found in traditional clock-comparison systems.Clock comparisons during the ACES mission are limited to the infrequent and variable propagation passes of the ISS. Fig. 2 shows the key orbital parameters for one pass.

Max Distance: 1800 km

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Fig. 2 Key Orbital Parameters for ACES MWL

For an orbit height of 400 km a pass through zenith has a duration of about 480 s. Typical passes are at lower elevation with contact times of about 300s.

1.3 Fundamental Performance requirements

The performance requirements for MWL are mainly driven by the stability of the ACES clocks. The link shall introduce only a negligible additional uncertainty with respect to the uncertainties of the clocks to be compared. For one pass this leads to the following short-term specification for the Time Deviation (TDEV) of MWL:

σx (10 s ≤ τ ≤ 300 s) ≤ 4.1 x 10-12 x τ-1/2 (1)

The long-term performance shall be kept over time periods starting at one orbit and extending to over 10 days:

σx (1000 s ≤ τ ≤ 1000000 s) ≤ 1.7 x 10-14 x τ+1/2 (2)

In Table 1 the performance requirements are summarized for some specific time intervals.

Table 1 MWL Performance Specification (TDEV)

Short Term Long Term

10 s 1.30 ps 1 d 5.10 ps

100 s 0.41 ps 10 d 16.20 ps

300 s 0.24 ps

1.4 MWL System Design

The ACES MWL is designed as a coherent, dual frequency, two-way crosslink in Ku-Band. In Fig. 3 the signal links between Flight Segment and a Ground Terminal are depicted. A second downlink exists in S-Band for measurements of the ionospheric refraction.

The Flight Segment consists of two transmitters (S-band and Ku-band) and four Ku-band receiver channels, allowing simultaneous contacts with up to four Ground Terminals. Each Ground Terminal has two receivers (S-band and Ku-band) and one Ku-band transmitter.

All signals are PN-coded, which spreads the signal energy over a bandwidth roughly equal to the chip rate. Correlation receivers are used to reconstruct the signal at the receive side, both for the PN-code and for the carrier.

Ku-Band, Up-link Power Tx: 2 W Carrier: 13.475 GHz PN-Code: 100 MChip/s 1pps: 1 time marker /s S/C: 4 Receiver Channels

S-Band, Down-link Power Tx: 0.4 W Carrier: 2248 MHz PN-Code: 1 MChip/s 1pps: 1 time marker /s

Ku-Band, Down-link Power Tx: 0.4 W Carrier: 14.70333 GHz PN-Code: 100 MChip/s 1pps: 1 time marker /s

Fig. 3 ACES MWL Signal Links

Measurements are performed by phase comparison of the reconstructed signals with respect to the local clock. Internal phase delays resulting from changing thermal conditions, varying signal levels and frequency shifts due to Doppler will be thoroughly calibrated out during data post-processing.

The antenna system for the Flight Segment comprises a S-band antenna and a Ku-band antenna, both with wide-angle radiation patterns to operate several Ground Terminals simultaneously. For the Ground Terminal a steering antenna with a combined S-band/Ku-band feed will be used.

2. MWL ANTENNA DESIGN

2.1 Antenna Performance Requirements

2.1.1 Phase Center Uncertainty

During one pass, the direction of the microwave signal path changes up to ± 70° with respect to the FS antenna. Also the rise and set point of the ISS are varying from pass to pass. In addition, the attitude profile of the ISS in roll and pitch has to be taken into account. Therefore the phase center for the uplink and the downlink frequency will move continuously with respect to a mechanical reference point. Phase center uncertainties between both Ku-band frequencies are resulting in a significant error contribution to the MWL short term performance (see table 1). For that reason exact measurement and calibration of the FS antenna under varying aspect angles and environmental conditions are mandatory.

For the FS Ku-band antenna the remaining differential phase center uncertainty shall be below 0.18 ps (corresponding geometrically with 55 µm). The absolute position of the phase center (downlink) is needed with an accuracy of 0.6 mm.

The GT antenna is less critical since it is equipped with a steering unit.

2.1.2 Gain Characteristic

For a typical pass of the ISS through zenith the distance between FS and GT changes between 400 km and 1800 km (see Fig. 2) resulting in a signal dynamic of about 13 dB. Due to phase shift which will be introduced in particular by AGC (automatic gain control) components within the receiver, signal dynamic at the receiver input should be limited as far as possible.

For that reason the gain characteristic of the FS Ku-band antenna shall ensure a receiver input signal dynamic during one pass of ≤ 15 dB.

To ensure fast signal acquisition and stable tracking, at the receiver input a minimum C/No of 40 dBHz is needed. For the GT antennas this leads to the following gain requirements: S-band antenna: ≥ 16 dBi Ku-band antenna: ≥ 32 dBi

2.1.3 Multipath Suppression

Due to the MWL signal design and processing, multipath signals which are time shifted by less than 1.5 PN chips will introduce measurement errors. This corresponds to a path difference of about 5 m. A detailed Field of View (FOV) and Multipath Analysis [3] was performed for MWL taking into account the complex three-dimensional structure of the ISS. The solar generators and radiators, which moreover are changing their alignment towards the sun throughout the orbital motion, will lead to regular disturbance of the FOV of the nadir-looking antennas.

As a result, to minimize multipath effects the gain characteristics of the MWL FS antennas should decrease as much as possible beyond aspect angles of 70°.

2.1.4 Polarization

The MWL antennas shall have circular polarization.

2.2 Flight Segment Antenna Design

2.2.1 S-band Antenna

The baseline design for the S-band FS antenna is a turnstile antenna equipped with a protective radom to withstand kick-loads aboard the ISS. The design of the antenna is illustrated in Fig. 4.

Fig. 4 S-band Turnstile Antenna with Radom

A prototype of this antenna was manufactured and tested in combination with different chokering configurations in order to investigate their influence on the radiation pattern. The envelope of the prototype device (including radom) is about 100 mm x 90 mm (W x H).

2.2.2 Ku-band Antenna

The baseline design for the Ku-band FS antenna is a turnstile antenna with a protective radom scaled down from the S-band model. A prototype of this antenna was manufactured and tested in combination with different chokering and ground plane configurations. Fig. 5 shows a version with four chokerings (outer diameter 80 mm).

Fig. 5 Ku-band Antenna with four chokerings

2.2.3 Antenna Configuration at ACES Payload

During ACES system integration both MWL Flight Segment antennas will be mounted at the outer surface of the ACES payload as illustrated in Fig. 6.

Fig. 6 ACES Payload with MWL Antennas

The installation of the ACES payload at the ISS ensures that both antennas are regularly looking to the nadir during mission. For aspect angles beyond about ±70° the FOV will be limited as mentioned in 2.1.3.2.3 Ground Terminal Antenna Design

For the Ground Terminal a commercial offset reflector (0.6 m) with a dual-band feed system will be used. The feed system is consisting of a cylindrical helix for the S-band with an integrated horn antenna for the Ku-band. Fig. 7 is showing the mechanical outlines of the feed.

Fig. 7 Dual-band Feed System for the GT Antenna

A steering unit will allow pointing the antenna into any direction around the local horizon and at any elevation with a pointing accuracy of ± 0.5° in order to allow tracking the ISS throughout the entire hemisphere. Fig. 8 shows a draft design of the MWL Ground Terminal with reflector antenna and steering unit.

Fig. 8 MWL Ground Terminal with Steering Antenna

3. ANTENNA TESTING

3.1 Measurement Configuration

Most of the antenna measurements were performed at the ‘Compensated Compact Range’ (CCR) test facility of the Laboratory of Satellite Communication at Munich University [4].

The CCR is based on the principle that a spherical wave coming from a source antenna is converted into a

plane wave by means of two focusing, precision reflectors, arranged in optimized geometry in a closed anechoic chamber. Fig. 9 shows the two reflectors within the chamber.

Fig. 9 Compensated Compact Range Test Facility

Due to the concept of two compensated doubly curved reflectors, no test system introduced cross-polarization occurs in the test zone for both linear and circular polarization. Excellent amplitude and phase uniformity in the test zone is obtained by the large equivalent focal length of 37 m of the two-reflector system.

Fig. 10 shows the ground plan of the test facility. The complete test chamber is covered with pyramidal absorbers with a 10 to 30 cm height. The absorber layout is determined by the operation frequency range of 2.0 to 200 GHz and the main course of the rays.

During the measurements the DUT (Device under Test) is mounted on a positioner, which allows a three axes movement (azimuth, polarization and one linear axis). Especially for the determination of the phase centre of the DUT, the linear axis movement is applied.

For further improvement of the measurement accuracy, a ‘hardgating’ system is available [5]. This device suppresses interfering signals while maintaining the real-time test capability.

Feed

DUT

7,05 m

Main reflector

Measurementarea

Control area

Controlsyste m

Feed system

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Fig. 10 Top View of Antenna Test Facility

The CCR test facility was used during the current design activities for the measurement of the gain and phase pattern of the Ku-band and S-band prototype antennas for the Flight Segment.

3.2 Measurement Results

3.2.1 Flight Segment Ku-band Antenna

A series of measurements was performed using various configurations of the Ku-band antenna to investigate in particular the influence of chokerings and of different mounting position on top of the ACES payload. For each configuration gain and phase diagrams were recorded within an azimuth range of -120° to +120° and for different polarization angles.

In Fig. 11 some typical results from relative gain measurements at 14.7 GHz for the antenna configuration with four chokerings (Fig. 5) are shown. The corresponding phase diagrams are depicted in Fig. 12.

Munich University of Applied Sciences - Laboratory for Satellite Communications, 14. Apr. 2004ACES Ku-Band Antenna, with Coke Ring, without Metal Plate, f = 14.7 GHz, with Radom, RHC

DUT-Pol. = 90° DUT-Pol. = 120° DUT-Pol. = 150°

DUT-Azimuth / °1201101009080706050403020100-10-20-30-40-50-60-70-80-90-100-110

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Fig. 11 Measured Gain Pattern in Ku-band

Munich University of Applied Sciences - Laboratory for Satellite Communications, 14. Apr. 2004ACES Ku-Band Antenna, with Coke Ring, without Metal Plate, f = 14.7 GHz, with Radom, RHC

DUT-Pol. = 90° DUT-Pol. = 120° DUT-Pol. = 150°

DUT-Azimuth / °1201101009080706050403020100-10-20-30-40-50-60-70-80-90-100-110

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Fig. 12 Measured Phase Pattern in Ku-band

From the measured gain pattern the expected variations of the signal power at the Ku-band receiver input were calculated taking into account the orbital behaviour of the ISS. For a pass through zenith, typical results are plotted in Fig. 13 and Fig. 14 for the full elevation range and over time respectively. The resulting signal dynamic range of about 14 dB is fully compliant with the requirements.

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with chokerings, without metal base plate, 14.7 GHz, with radom, RHCFile: RHC_Phi_120.dat

Fig. 13 Receiver Input Signal Power wrt Elevation

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File: RHC_Phi_120.dat with chokerings, without metal base plate, 14.7 GHz, with radom, RHC

Fig. 14 Receiver Input Signal Power wrt Time

Similar measurements were also performed with an antenna configuration with a flat ground plane instead of the chokerings. In direct comparison the chokering design shows a better decrease of gain beyond 70° with positive impact on multipath suppression.

For investigation of the influence of the ACES base plate, the chokering antenna was mounted on top of a metal plate (510mm x 510 mm). The distance between antenna and metal plate was changed between 0 mm and 75 mm. Fig. 15 and Fig. 16 are showing exemplary measurement results at 13.5 GHz with and without metal plate. With metal plate the diagram shows characteristic ripples which are resulting from interferences of the direct wave with signals reflected from the metal plate. By varying the distance between antenna and plate the ripple pattern changes accordingly. By reducing the distance the number of ripples decreases and their amplitudes increases.

Munich University of Applied Sciences - Laboratory for Satellite Communications, 14. Apr. 2004ACES Ku-Band Antenna, with Coke Ring, without Metal Plate, f = 13.5 GHz, without Radom, RHC

DUT-Pol. = 0° DUT-Pol. = 30° DUT-Pol. = 60°

DUT-Azimuth / °1201101009080706050403020100-10-20-30-40-50-60-70-80-90-100-110

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Fig. 15 Gain Pattern without Metal Plate

Munich University of Applied Sciences - Laboratory for Satellite Communications, 14. Apr. 2004ACES Ku-Band Antenna, with Coke Ring, with Metal Plate, f = 13.5 GHz, without Radom, Distance = 75 mm, RHC

DUT-Pol. = 0° DUT-Pol. = 30° DUT-Pol. = 60°

DUT-Azimuth / °1201101009080706050403020100-10-20-30-40-50-60-70-80-90-100-110

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Fig. 16 Gain Pattern with Metal Plate

An additional test session was performed to investigate the influence of MLI (Multi Layer Insulation) on top of the ACES surface. For the test the MLI was bulged partly for some millimeter. The effect on the phase diagram is depicted in Fig. 17. The recorded phase changes which are up to 4° are not acceptable with respect to the required phase center stabiliy. As a consequence another kind of thermal insulation will be used for the ACES payload.

Munich University of Applied Sciences - Laboratory for Satellite Communications, 29. Apr. 2004ACES Ku-Band Antenna, with Choke-Ring and Metal Plate, Distance = 75 mm, f = 13.5 GHz, with MLI on right side

MLI without Paper Nr. 1 MLI without Paper Nr. 2 MLI with Paper Nr. 1 MLI with Paper Nr. 2

DUT-Azimuth / °7065605550454035302520151050-5-10-15-20-25-30-35-40-45-50-55-60-65

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Fig. 17 Influence of MLI on Phase Pattern

3.2.2 Flight Segment S-Band Antenna

The S-band antenna was tested with several ground plane configurations (outer diameters 262 mm) with and without chokerings in order to investigate their influence on the radiation pattern.

No significant improvement of the gain characteristic with chokerings could be found. On the other hand the chokering design implies a significant increase in mass. For that reason the baseline design for the S-band antenna will be a turnstile feed on a flat metallic ground plane. Fig. 18 and Fig. 19 are showing typical measurement results for gain and phase pattern.

Fachhochschule München - Labor für NachrichtensatellitentechnikS-Band Turnstile Antenne, f=2,248 GHz, Amplitude

RHC, DUT:0° RHC, DUT:15° RHC, DUT:30° RHC, DUT:45°

DUT Azimut [°]9080706050403020100-10-20-30-40-50-60-70-80

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Fig. 18 Measured Gain Pattern for S-band

Fachhochschule München - Labor für NachrichtensatellitentechnikS-Band Turnstile Antenne, f=2,248 GHz, Phase

RHC, DUT:0° RHC, DUT:15° RHC, DUT:30° RHC, DUT:45°

DUT Azimut [°]9080706050403020100-10-20-30-40-50-60-70-80

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Fig. 19 Measured Phase Pattern for S-band

3.2.3 Ground Terminal Antenna

Up to now measurements were performed for the dual-band feed system alone. Fig. 20 and Fig. 21 are showing the radiation pattern for the S-band frequency and for the Ku-band transmit frequency respectively.

ACES, Dualband FeedMeasured S-Band Pattern, f = 2250 MHz

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Fig. 20 Measured Radiation Pattern of S-band Feed

ACES, Dualband Offset AntennaCalculated Ku-Band Pattern, f = 13475 MHz

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Fig. 21 Measured Radiation Pattern of Ku-band Feed

From these measurement results the radiation pattern for the GT antenna including the reflector were calculated. The results are depicted in Fig. 22 and Fig. 23. The calculated data are fully compliant with the gain requirements with some additional margin.

ACES, Dualband Offset AntennaCalculated S-Band Pattern, f = 2250 MHz, 0.5mm RMS

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Fig. 22 Calculated Radiation Pattern for S-band

ACES, Dualband Offset AntennaCalculated Ku-Band Pattern, f = 13475 MHz

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Fig. 23 Calculated Radiation Pattern for Ku-band4. FUTURE WORK

In this paper the current design status and first test results for the ACES MWL FS and GT antennas are presented. In particular the measurements of the FS antennas were based on prototype devices.

In a next step the final optimization of the FS antennas will be performed to reach Engineering Model status. Special emphasis will be put on the mechanical precision of the feeds and the optimization of the phase shifter and matching networks.

During the next test campaign also precise phase center measurements are planned with the FS antennas. As the prototype measurements have shown, it is of greatest importance for this task to use a realistic mock-up for the ACES mounting plate. For the GT antenna, measurements with the completely integrated device will be performed.

The above activities are planned for summer 2005.

5. REFERENCES

1. Microwave Link for ACES, Phase A Study, Final Report, ESTEC Contract 13671/99/NL/JS, 1999.

2. Föckersperger S., Bedrich S., Schäfer W., Design Status of the ACES Microwave Link, Proceedings of the EFTF Congress, Poster Session, 2004

3. ACES MWL Field-of-View and Multipath Analysis, Project Technical Note, 2003

4. Hartmann J., Fasold D., Analysis and Performance Verification of Advanced Compact Ranges, Proceedings of 29th European Microwave Conference, 1999.

5. Hartmann J., Fasold D., Identification and Suppression of Measurement Errors in Compact Ranges by Application of an Improved Hardgating System, Proceedings of 22nd ESTEC Antenna Workshop, 1999.

ACKNOWLEDGEMENTS

The work is funded by the European Space Agency.

Prof. D. Fasold from Munich University is acknowledged for his support during the measurement campaigns.

Special thanks to Mr. W. Schäfer from Timetech GmbH, as the MWL system engineer, for his support and valuable contributions to this paper.