exactView-9 Satellite Overview

exactView-9 is a part of the Automatic Identification System satellite constellation operated by exactEarth to monitor ships and their movements through busy shipping lanes and harbor areas around the globe to deliver information on global shipping movements to industry customers.

The Automatic Identification System is used by sea vessels that send and receive VHF messages containing identification, position, course and speed information to allow the monitoring of vessel movements and collision avoidance as well as alerting in the event of sudden speed changes.

These signals can be transmitted from ship-to-ship and ship-to-shore to allow the monitoring of a local area, but deploying space-based AIS terminals allows a broad coverage and data relay to ground stations for monitoring of large sea areas. However, due to the large footprint of satellites, overlapping and signal collisions become a problem, especially for frequented traffic routes, requiring a steady improvement in reception technology to separate the different signals.

EV9 hosts the next generation AIS receiver coupled with a high-speed data transmission system to be able to downlink a large amount of data during ground station passes.

The satellite is based on SFL’s Generic Satellite Bus that provides all the required subsystems for the operation of a variety of payloads leaving about 30% of its total volume open for use by payloads. Using the same platform for several previous missions led to a quick build-up of flight heritage and performance data which is of great value when conducting experimental missions.

The satellite bus features a cubical design with a 20-centimeter side length using aluminum exterior panels and two internal trays to host the various satellite subsystems and create a payload bay for simple integration of satellite payloads of different kinds.

Power is provided by four to ten triple-junction GaAs solar cells installed on each of the external panels delivering up to ten Watts of power using Peak Power Point Tracking provided by the Battery Charge/Discharge Units. Power is stored in two Li-Ion batteries with a capacity of 5.3 Ah. The power conditioning unit provides a 4-Volt unregulated power bus.

Attitude Determination is accomplished by a three-axis magnetometer, six sun sensors for fine and sun attitude determination and a star tracker for precise attitude determination. The Miniature Star Tracker provides three-axis attitude solutions at a control cycle at 0.5 Hz and an accuracy of 10arcsec. Attitude actuation is provided by three reaction wheels with a total mass of 185grams and a volume of 5 by 5 by 4 centimeters. The wheels have a momentum capacity of 30mNms and deliver a maximum torque of 2mNm. Momentum dumps are supported by three magnetotorquers.

Data handling and satellite control is provided by an ARM7 housekeeping computer that handles standard telemetry and communications while a second computer supports all attitude determination and control functions. A third computer board is in charge of the operation of the science payload and handles its data. Each processor board uses the ARM7/TDMI processor with a code memory of 256kB and 2MB of hardware SRAM memory used to store program variables and data. A 256MB flash memory is used for long-term data storage.

The communications system of the satellites includes a UHF receiver, an S-Band Transceiver and a VHF beacon. The UHF receiver will be used for the command uplink from the ground at a data rate of 4kbit/s in the amateur radio band using quad-canted monopole antennas for omni-directional coverage. The VHF beacon transmits the satellite’s identification and some basic telemetry values for satellite tracking and the initial commissioning of the spacecraft.

The S-Band system is used for data downlink to the ground . The system consists of two patch antennas installed on opposite side panels of the satellites connected to the main S-Band Transceiver. The downlink data rate can be selected between 32 and 256kbit/s.