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JUICE. Medium Gain Antenna Subsystem (MGAMA)
- Client: ADS F/ ESA
- Country: Netherlands
- Start date: 2016
- Ending date: 2022
The Medium Gain Antenna Major Assembly (MGAMA) is a medium-gain dual-axis steerable antenna assembly that provides main uplink and downlink communications between the JUICE spacecraft and Earth.
The MGAMA will be used in the TT&C (Telemetry, Tracking and Command) Subsystem of the satellite. During flybys where the instruments require specific pointing of the spacecraft, the MGAMA is used for communication and radioscience.
The MGAMA will work on the following bands:
- A 2-way X-Band link
- A 2-way Ka-Band link
The X & Ka-band Medium Gain Antenna is composed of the following main components:
- ARA (antenna reflector assembly). The ARA includes the Main Reflector (MREF), the Sub-reflector (SREF) mounted on a thermally stable Tripod Struts Assembly and the structural frame (ARA Frame) which supports all these elements. The ARA frame supports in addition the Feed Assembly.
- The Boom and RF waveguides. It joins the ARA with the APM, composed of the waveguide sections connecting the Feeder with the APM elevation Rotary Joint. It also supports the thermal protection. An MLI protects the boom against solar radiation.
- The Hold Down and Release Mechanisms (HDRMs). They provide attachment points for the MGA in stowed/launching configuration to the S/C structure. The release system is a NEA and a bolt preloaded between two latches. Both latches of each HDRM are preloaded against the boom fittings and at the same time, the fittings of each boom are also preloaded between them due to the preload of the HDRM latch. There are two spherical surfaces in each contact between the latches and fittings and between adjacent fittings.
- Antenna Pointing Mechanism (APM), which is attached to the S/C structure and consists on a two axis electro-mechanism providing the required 2 degree-of-freedom (azimuth and elevation) and steerable capability to the MGA radiating element, in order to keep the MGA pointing towards Earth under the required S/C operational attitudes for scientific data download.
The APM provides as well angular position measurement capability in both azimuth and elevation rotational axes. The azimuth axis is driven by the GearHead Motor (GHM) geared to a rotating L bracket which supports the elevation actuator. The output of the elevation output bracket is linked to the MGAMA boom. Both azimuth and elevation GHMs are based on stepper motors with planetary reducers geared to the corresponding output brackets. An integrated X-band dual axis Rotary Joint Assembly (MGA-RJA) routes the RF energy through the APM in both TX and RX directions.
The APM includes:
- Antenna Pointing Mechanism Electronics (APME), which provides the control functionality of the APM, receiving the movement commands ordered by the on-board S/C computer. Includes the control algorithms to follow the requested movement profiles, and returns the required telemetry containing MGA health and status information.
- Harness connecting the APM to the APME.
- The medium gain antenna will transmit throughout the phase of the mission to the frozen moons of Jupiter, operating both in the environment of Mercury (temperatures above 250°C) and in Jupiter, at 5 AU astronomical units and temperatures of -210°C.
- The antenna will operate in the X band for the transmission of telemetry data with a gain of 39.80 dBi in uplink and 39.25 dBi in downlink. In the X band it will have a gain of 30.68 dBi in uplink and 28.68 dBi in downlink.
- The medium gain unit will perform the radio science experiment (RSE) with a phase stability of 4xe-15 (Allan Deviation) in 1000 seconds.
The maximum pointing error in 1000 seconds is 95 arc-seconds with a precision of greater than 0.02 degrees.
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