LOW-PROFILE AUTO-TRACKING ANTENNA SYSTEM FOR BROADBAND MOBILE SATELLITE COMMUNICATIONS

Tomihiko Yoshida, Naoki Honma, Hiroshi Tanaka, Tadahiro Sueyoshi, Toshihiro Tsuchida, and Hiroshi Kazama

NTT Access Network Service Systems Laboratories, NTT Corporation,

1-1 Hikarinooka, Yokosuka-shi, Kanagawa 239-0847, Japan {yoshida.tomihiko, honma.naoki, sueyoshi.tadahiro,

tsuchida.toshihiro, kazama.hiroshi}@lab.ntt.co.jp

Abstract One of the key components for broadband mobile satellite communications is an auto-tracking antenna. In this paper, we present a low-profile (90-mm height), cost-effective, auto-tracking, receiving antenna system. The antenna achieves high G/T, wideband characteristics, and high-accuracy tracking performance. We tested the antenna on an experimental microbus and test results demonstrate the antenna is applicable for mobile satellite communication. This paper describes configurations and performance of the antenna system.

1. Introduction The growth of high-speed Internet networks has created a demand for ubiquitous broadband access to the Internet. A satellite communication network is an effective system of ubiquitous broadband access for users who are passengers on automobiles, railways, sea-going vessels, and airplanes. We have been developing an auto-tracking antenna system [1,2] that is the key component to provide broadband mobile satellite communications. Most tracking antennas for commercial use are high-profile reflector antennas. However, airliners and high-speed railways require low-profile antennas to reduce aerodynamic effects. In particular, for Japanese high-speed railways, wind noise must be reduced for commercial use. In order to satisfy this requirement, a planar phased-array antenna, which can be mounted horizontally on the top of the airplanes or vehicles, must be used. However, there are several problems by using the phased-array antenna in the horizontal plane. One problem is that the effective area of the antenna aperture is reduced when the beam is directed to low-elevation range. For example, the antenna gain is reduced by 3 dB at a 30-deg. elevation angle because the projected aperture is reduced by half of the area. In order to satisfy the required antenna gain at a low-elevation angle, a large antenna aperture or low-loss feeder is required. Another problem is that the phased-array antenna has a frequency-dependent characteristic, especially at a low elevation angle. When the elevation angle becomes lower, the delay time difference of the received signal between distant antenna elements becomes larger. This delay time difference causes the frequency dependence of the phase of the received signal. Therefore, if the phases are adjusted at a specific frequency, the beam direction is fluctuated depending on the frequency and it affects the performance of the wideband reception. For instance, if the phase shifters adjust at 12.25 GHz, the antenna beam moves about 4 deg. and the gain decreases about 20 dB at 12.75 GHz at a 30-deg. elevation angle with a 1-m diameter antenna. Hence, the antenna can be used within a narrow band. Our developed receiving antenna presented in this paper addresses these problems and achieved low feeder loss and wideband characteristics. This paper describes antenna configurations and performance. Pursuing this further, the antenna was mounted on an experimental microbus to demonstrate tracking performance. We also show the experimental results of the tracking performance.

2. Requirements Antenna requirements are shown in Table 1. We supposed that the antenna system is mounted on a high-speed train that experiences angular displacement and temperature in Table 1, and we use the Ku band for practical broadband communications. The coverage of an elevation angle is decided from supposed service areas in Japan and angular displacement conditions. Antenna G/T is derived from a link budget. “Effective antenna G/T” means antenna G/T in operation, including pointing loss, feeder loss, polarization loss, and VSWR (Voltage Standing Wave Ratio) loss. However, loss caused by any protective cover (radome) is not included. From the link budget, the approved G/T is derived as 9 dB/K. We supposed that the radome loss is 1 dB, so the effective G/T is given as 10 dB/K. This value, 10 dB/K, must be satisfied at any frequency, any coverage, any angular displacement, and any temperature in Table 1.

Table 1 Requirements Effective antenna G/T 10 dB/K Frequency 12.25 – 12.75 GHz (Ku band) Coverage EL: 30 – 65 deg, AZ: -225 – +225 deg Angular displacement Roll: 10 sin(2πt/6) deg,

Pitch: 3 sin(2πt/7) deg, Yaw: 180 sin(2πt/130) deg

Temperature -10 – +55 degrees Celsius

3. Antenna plane configuration To achieve the low-profile antenna, we designed a planar phased-array antenna. Comparing a one-dimensional phased-array antenna to a full (two-dimensional) phased-array antenna, the full phased-array antenna requires a few hundred or more phase shifters. Therefore, we adopted the one-dimensional phased-array antenna that drives the main beam electronically to set the elevation angle, and mechanically to set the azimuth angle. In other words, several phase shifters and antenna arrays form the antenna pattern whose main beam direction becomes the desired elevation angle, and the antenna plane is mechanically rotated by a servomotor around the azimuth angle. A diagram of the antenna is shown in Figure 1. The antenna consists of about 3,000 elements.

Figure 1. Antenna (outdoor unit)

There are mainly three configurations for combing and amplifying the received power of elements: 1) Combing the received power of elements is performed by using waveguides. After that, the combined power is amplified. 2) Combing the received power of elements is performed by using microstrip lines. After that, the combined power is amplified. 3) Amplifying the received power of

elements is performed. After that, the amplified power is combed. Configuration 1 uses waveguides for combing the received power. Thus, less feeder loss can be achieved. However, when using common waveguides, a high-profile antenna is achieved. Configuration 2 causes a larger feeder loss (a few dB), so a larger antenna aperture is required. Configuration 3 achieves minimum feeder loss. However, a few hundred or more low-noise amplifiers (LNAs), which significantly impact the cost, are necessary. Therefore, we developed low-profile waveguides and selected configuration 1. Two technologies are applied to the antenna to achieve low feeder loss and wideband characteristics; one is low-profile multistage waveguides and another is delay lines. The details are described in the following sections.

3.1. Low-profile multistage waveguides The antenna plane mainly consists of subarrays, waveguides, amplifiers, delay lines, phase shifters, combiners, and a 180 deg. hybrid, as shown in Figure 2. The antenna plane consists of about 40 subarrays that are arranged in parallel. A subarray consists of a number of subarray units that connect in series, and each unit consists of 6 x 2 or 4 x 2 elements that are formed as a microstrip antenna on a dielectric board. The received power of a subarray unit is combined on the dorsal side of the elements by using a low-profile waveguide. The output powers of the units are combined by a multistage waveguide as the output of the received power of a subarray. The paths of the multistage waveguide have a tournament-form as shown in Figure 2. Hence, the distance between each unit and output point of a multistage waveguide is almost the same. In other words, a subarray is divided into small subarray units, and unit powers are combined in phase. By using waveguides, the antenna achieves low feeder loss (less than 1 dB) compared with that achieved by using microstrip lines (3 dB or more). In addition, we developed low-profile multistage waveguides that achieve a low-profile antenna. By using the multistage waveguide, path lengths can be almost the same. Therefore, the phase shifts of the subarray unit signals through the multistage waveguide are the same and independent of frequency, so the subarray achieves wideband characteristics.

��������

��������

�� �

Σ Δ

���

��

��

��

��

�������

��������

�������� ����

������� �����

����� ����

����� ��!� "�#�!��$�

���

�������� ������������

����� ��!�

"�#�!��$�

���% �� ����� ��!�

"�#�!��$�

Figure 2. Antenna plane configuration

3.2. Delay lines Each subarray has LNAs, a delay line, and a phase shifter as shown in Figure 2. About 20 subarray powers that are half the power of the antenna are combined in a combiner. Sum and error signals of the output of two combiners are obtained by a 180-degree hybrid. About 40 phase shifters are adjusted to change the antenna beam direction for elevation angle. However, as described in section 1, for a low-elevation angle, the beam direction strongly depends on the frequency. In order to reduce the frequency dependency, we decided to use true-time delay lines. The length of a delay line is selected to obtain the same delay time from a satellite to a 180-degree hybrid through each element. The appropriate delay-line length is different depending on elevation angle. Thus, variable delay lines are desirable to obtain the best performance. However, the variable range is wide (about 200 mm for 30 – 65-deg. elevation angles with a 1-m diameter antenna) and low-

profile variable-delay lines are difficult to prepare. Therefore, we decided to use fixed-delay lines made of semi-rigid cables. Considering the beam-offset balance within the ranges of frequency and elevation angle, the cable length is adjusted for a 50-deg. elevation angle. These fixed-delay lines improve loss due to the beam offset, depending on frequency, from about a 7-dB loss without delay lines to a 1.7-dB loss. Using the above technologies, the antenna achieved a low-profile (90 mm height), low feeder loss, high G/T, and wideband (500 MHz) characteristics at a wide elevation angle. Moreover, we have to check sidelobe levels to avoid interferences from undesired satellites and acquisition errors due to sidelobe. Figure 3 shows antenna patterns measured through the elevation angle when beam directions are set to 30-deg., 45-deg. and 60-deg. elevation angles. In Figure 3, 0-deg. offset angle is identical to 90-deg. elevation angle. Those figures clarify that the antenna achieves low sidelobe levels and wideband characteristics for any elevation angle. Azimuth patterns also achieve low sidelobe levels. Therefore, it is shown that the antenna plane has the practicable electronic performance.

a)

b)

c) Figure 3. Antenna pattern through elevation angle

Beam directions are set to a) 30-deg., b) 45-deg. and c) 60-deg. elevation angles.

4. Control system configuration The control system configuration is shown in Figure 4. To lighten an outdoor unit mounted on the rooftop of a vehicle, the control functions are divided into the outdoor unit and an indoor unit. The outdoor unit includes the antenna plane described in the above section, an azimuth motor that rotates the antenna plane, an azimuth encoder that measures the relative azimuth angle of the antenna plane, an elevation controller unit that adjusts the phase shifters and attitude and position sensors that consist of three gyros, an inclinometer, a magnetic compass, and a GPS device. On the other hand, the indoor unit includes a motor driver, a main controller, a beacon receiver that identifies the target satellite and sends monopulse information to the main controller, and a power supply (not shown in Figure 4).

MainController

PS

PS

PS

PS

Combiner

Combiner180-deg.HYB

BeaconReceiver

AZ Motor

Encoder

GyrosInclinometer

GPS

PS

PS

・・・

・・・

EL controllerunit

Sum

Error

MotorDriver

Indoor Unit

MagneticCompassAntenna Plane

Subarray

Subarray

Subarray

Subarray

Subarray

Subarray

・・・

・・・

Outdoor Unit

Figure 4. Control system configuration

The attitude of the outdoor unit is measured by combination of gyros, inclinometer, magnetic compass, and GPS device. Using the measured attitude and a target satellite direction that is given by satellite longitude and GPS information, the main controller estimates the relative antenna azimuth and elevation angles. We call this estimation an open-loop control. If the sensor accuracy is sufficiently high, and, therefore, the measured attitude of the vehicle is accurate, accurate tracking can be achieved by using only the open-loop control. However, high-accuracy sensors are extremely expensive. On the other hand, while monitoring the signals received from the satellite, the controller drives the beam direction and searches for the satellite direction. We call this search a closed-loop control. There are mainly two closed-loop control methods: 1) sequential lobing methods, e.g., step-track and conical scan and 2) simultaneous lobing methods such as monopulse. Generally, sequential lobing requires a long time for accurate tracking, and the simultaneous lobing range for accurate tracking is a narrow angle (within about half beam width). Considering the open- and closed-loop control characteristics, we applied both controls. Basically, relative antenna azimuth and elevation angles are estimated by the open-loop control, and estimation errors due to sensors errors are compensated by the closed-loop control. Using both controls, moderately accurate gyros can be used, and the total cost can be reduced. In the following sections, we describe the closed-loop techniques.

4.1. Scan-tracking method From the antenna plane configuration in section 3, the information about the pointing error around an azimuth angle is the received signal level. Therefore, the applied tracking method will be

one of the sequential lobing methods such as step-track or conical scan. We simulated a few methods, and we adopted a scan-tracking method that is like a one-dimensional conical scan. By scan tracking, the controller drives the antenna sinusoidally, estimates the pointing error by one cycle, and compensates for this error during the next cycle. This motion is given as, 0 sina a t btω= + , ( )/ 2b eω π= , (1) where a is the antenna azimuth angle in the closed loop control, 0a is the amplitude of the scan, ω is angular velocity, and e is the estimated pointing error. e is given by the following equation: ( ) ( )2 2 2 2 2

0 0 03 / 4 / 2.58h he I a T I a Tθ θ θ− − , (2)

where, I is the integral of the product of the relative received signal level and 0 sina tω during one

cycle, hθ is the half bandwidth, 0θ is the value that satisfies 20sinc 0.5θ = , and T is the cycle time,

that is 2 /T π ω= . Using the scan tracking given by equations 1 and 2, we achieved better tracking accuracy than that of the classic step-track. Moreover, the motion of the scan track is continuous, so the collision noise of the gear due to backlash can be reduced.

4.2. Offset monopulse We applied the monopulse by using sum and error signals as the closed-loop control of elevation angle. However, there is a big difference between the typical monopulse and a monopulse applied in our antenna. Our antenna has the characteristic that the beam direction depends on the frequency, and in order to minimize this frequency dependency, the phase shifters are adjusted at the center frequency of the required frequency band, that is 12.5 GHz for our antenna. However, the receiving frequency of the beacon receiver that is used for the monopulse is the beacon frequency that is at the edge of the required frequency band, which is about 12.25 or 12.75 GHz. Therefore, using the monopulse signals at the edge frequency, the pointing error at the center frequency must be derived. We call this monopulse an offset monopulse. The relationship between the monopulse signal (error/sum signals) at 12.25 GHz and the pointing error at 12.5 GHz is shown in Figure 5. Using this relationship, accurate tracking can be achieved.

���� �� ���� �� ���� � ��� � ��� � �����

��

��

��

��

�

�

�

�

�

�

�� ��� ���� � ���� ��� �����

Δ �

Σ ���������

������� ����� ��

������� ����� ��

������� ����� ��

Figure 5. Monopulse signal vs. pointing error

5. Experimental Results We mounted the antenna on the rooftop of an experimental microbus shown in Figure 6 and tested it. The microbus was driven on city roads and the highway, and we measured the approximate tracking accuracy of the antenna. The maximum motion of the microbus was as follows: 10 deg. (roll and pitch angle), 10 deg./s (roll), 8 deg./s (pitch) and 20 deg./s (yaw). The approximate pointing errors of the relative antenna azimuth and elevation angles are shown in Figure 7. The errors that occurred during the periods of driving through shadowing areas such as behind buildings and inside tunnels are omitted. The exact attitude of the microbus cannot be measured, so deriving accurate pointing errors is difficult. However, using accurate gyros mounted in the microbus, approximate pointing errors were obtained, such as those shown in Figure 7. The experimental results indicate that the pointing error was about 1 deg. RMS. While the bus was in motion, the antenna always tracked the satellite and received the satellite signals very well, except in shadowing environments. Those results demonstrated that the antenna system is applicable to mobile satellite communications. Finally, the antenna performance is summarized in Table 2.

Figure 6. Experimental microbus

The developed antenna was mounted on the center of the bus and covered by a low-profile flat radome. The antenna at the rear of the bus is our previous antenna[1] (350-mm height) for comparison.

� ��� ��� ��� ��� ���� ������

��

��

��

�

�

�

�

�

�� ���

����������������

����

Figure 7. Pointing error in city

Table 2 Antenna performance

Antenna type One-dimensional phased-array antenna (about 3,000 elements)Effective antenna G/T Over 10 dB/K Frequency 12.25 – 12.75 GHz (Ku band) Coverage EL: 30 – 65 deg, AZ: -225 – +225 deg Tracking accuracy Less than 1.0 deg RMS Size D 1,350, W 1,290, H 90 mm (outdoor Unit)

D 500, W 500, H 300 mm (indoor unit) Weight 120 kg (outdoor unit), 18 kg (indoor unit) Temperature -10 – +55 degrees Celsius

6. Conclusion We presented the low-cost, low-profile, auto-tracking antenna system for mobile satellite communication. The antenna configuration is a one-dimensional phased-array antenna that drives the beam direction electronically for the elevation angle and mechanically for the azimuth angle. In order to achieve a low profile, high G/T, and wideband characteristics, the antenna plane uses the low-feeder-loss multistage waveguides and fixed-delay lines. In addition, in order to obtain cost-effective high-accuracy tracking, we adopted the combination of the open loop and the closed-loop controls. In the closed-loop control, we applied the scan tracking and the offset monopulse to obtain high accuracy. In order to demonstrate the tracking performance, we tested the antenna on an experimental microbus. While the bus was in motion, the antenna always tracked the satellite within about 1 deg. RMS. The experimental results demonstrated that the antenna system is applicable to mobile satellite communications.

References [1] F. Nagase, H. Tanaka, M. Nakayama, T. Seki, H. Kazama, and H. Mizuno, “Mobile Multimedia

Satellite Communication System at Ku Band,” IEICE Trans. Commun., vol. E84-B, no. 4, pp. 903-909, Apr. 2001.

[2] T. Yoshida, K. Ohata, S. Ozawa, and M. Ueba, “High Accuracy Auto-Tracking Antenna System Using H-infinity Control for Earth Stations on board Vessels,” AIAA Guidance, Navigation, and Control Conference, AIAA 2003-5731, Aug., 2003.