Phased-array antennas and digital beamforming techniques are increasingly being used by satellites to maximise the radiation pattern for reception and transmission.
To deliver the next generation of satellite applications, spacecraft are increasingly using phased arrays to combine multiple individual antenna elements to improve overall performance, increase gain, cancel out interference, and steer the array so it’s most sensitive in a particular direction. This allows operators to change and optimise reception and transmission in response to changing link requirements in real-time.
Phased-array antennas and digital beamforming techniques are increasingly being used by satellites of all sizes, orbits and frequencies to maximise the radiation pattern for reception and transmission. Determining the direction of arrival of incoming signals improves the received signal strength, and reduces fading, interference, and side-lobe levels, increasing the capacity of high-throughput payloads. Higher spatial diversity, better frequency reuse, and more precise user positioning are also achieved.
When developing high-throughput satellites, phased-array antennas are tested throughout all stages of spacecraft development: from characterising the performance of individual elements and the complete array during the initial prototyping (EM) phase, and when integrated with the main payload. This is followed by entire spacecraft validation in a representative environment using thermal-vacuum chambers during the qualification (EQM) stage. Throughout operation, regular in-orbit checks of the transmission links are made to monitor and confirm quality of service (QoS) and by using beamforming techniques, to dynamically change and optimise the antenna's radiation pattern for receiving and transmitting in response to changing link requirements. Each of these development stages present unique test and measurement challenges to manufacturers of satellites.
A phased-array antenna receives and transmits a specified bandwidth of information over a specific range of frequencies with a particular gain, boresight (axis of maximum gain), efficiency, impedance, polarisation, and sidelobe levels. The antenna radiates a 3D field in a specific direction and all of these parameters must be tested and characterised. Each element contains a transmit/receive module as illustrated below and testing requires receive, transmission, and bi-directional measurements.
A key challenge for satellite manufacturers is how to accurately test a phased-array antenna in receive mode throughout all stages of spacecraft development. Prior to over-the-air (OTA) measurements, each low-noise amplifier (LNA) within the individual elements needs to be tested. Gain, noise figure, compression, and intermodulation distortion are characterised using either a vector network analyser (VNA) or a CW signal source and a spectrum analyser. A single VNA characterises the above metrics without re-connection.
For the OTA tests, the key measurements are the received power levels and phase differences between the individual elements to determine the direction of arrival. A multi-channel VNA such as the ZNBT or ZVA can be used to perform such tests as illustrated below. The receive antenna-under-test (AUT) is connected to a VNA and on the transmitting side, a horn antenna is broadcasting a known CW signal.
The second key challenge for satellite manufacturers is how to accurately test a phased-array antenna in transmit mode. Prior to OTA measurements, each power amplifier within the individual elements needs to be tested. Gain, noise figure, compression, and intermodulation distortion are characterised using a VNA or a CW signal source and a spectrum analyser. For OTA tests, the key measurements are effective radiated power (ERP) and pulse shape.
Multi-channel phase-coherent signals are used to drive the phased-array AUT and a spectrum analyser is connected to a reference horn antenna to measure the maximum received radiation pattern and the levels of side-lobe reduction and nulling as shown below.
When the CW test signal transmitted by the antenna-under-test is replaced by a modulated carrier, a signal analyser is used to measure and report metrics such as error vector magnitude (EVM) and bit-error rate (BER).
A concern for satellite manufacturers is how to reliably produce multiple phase-coherent outputs to test antennas. Coherency maintains a fixed, defined, relative-phase relationship between signal generators over time and a number of methods exist to stabilise the relative phase of carriers, e.g. coupling of the 10 MHz or 1 GHz references, or a common local oscillator (LO) connection. Neither of the first two techniques provide sufficient long-term stability due to component drifts, temperature differences and the fact that the phase noise of individual signal generators are uncorrelated in time as illustrated below.
LO phase-coherence options are available to assure stable relative phases between multiple instruments. This is not available for analogue signal generators as they lack the ability to set individual phases for each RF carrier with LO coupling.
OEMs struggle with measuring the transmission radiation patterns vs. frequency. To support this test, software is available which commands external signal generators to output up to four phase-coherent signals for electronic beamforming and steering. Using this software, it is possible to plot the resulting 2D azimuth of the antenna's radiation pattern. The free runtime Matlab executable can be downloaded here.
Until next month, the first person to tell me the difference between an omni-directional antenna and a Yagi will win a Courses for Rocket Scientists World Tour t-shirt. Congratulations to Gilles from Canada, the first to answer the riddle from my previous post.