Worldwide demand for low Earth orbit satellites is increasing at an unprecedented pace, driven by diverse needs such as faster and more affordable Internet access, and faster revisit rates with finer resolution for imaging data. The satellite payload instruments performing communications or imaging functions are becoming increasingly sophisticated and capable and require the collection of increasing amounts of telemetry data to ensure the safe and reliable operation of the satellite.
This is accomplished using multiple large, power-hungry, and heat-generating circuit cards that contain a variety of discrete components for monitoring payload health. The component count of these I/O cards and the area and power they consume can all be significantly reduced by leveraging the latest advances in radiation-hardened mixed-signal integrated circuits (ICs) combined with a new open instruction set architecture (ISA) for reduced instruction set computing called RISC-V.
Critical Telemetry Functions
Telemetry allows health monitoring and fault detection, isolation, and recovery from the satellite ground station, but also allows the satellite to autonomously control payload instrument loading to manage power consumption and thermal dissipation, which may be necessary to avoid overloading and to preserve the life of the satellite.
Today, telemetry data is captured by large circuit cards, commonly referred to as I/O cards, laden with discrete components such as analog multiplexers, analog to digital converters, current drivers, and voltage references. These components capture data regarding voltage levels and current consumption, temperature, mechanical strain, pressure, and magnetic field strength, all of which are necessary for monitoring the health of the payload. These I/O cards are typically very large, occupying 12 to 18 square inches of precious space inside each payload equipment. Complex payloads, such as digital channelizers for communications applications, or signal processing systems for imaging or radar applications, are often chassis-based and can require multiple I/O cards for telemetry purposes. Telemetry I/O cards burn power and generate heat as well as add considerably to the bill-of-materials cost of the payload equipment. Further, circuit cards designed around discrete devices are not flexible or configurable.
Cutting Size, Cost, and Power
Recent advances in the technology of radiation-hardened mixed signal ICs have resulted in a higher level of integration that can minimize component count and reduce the area consumed by these I/O cards. Functions such as multiplexers, amplifiers, filters, ADCs, and DACs that were previously accomplished with small-scale integration and discrete components can now reside in a single IC. This allows data to be read and processed from sensors that monitor critical satellite parameters with a dramatic reduction in board space. Such integration has the added benefit to satellite manufacturers of increased reliability due to fewer components and reduces time and cost required for screening, testing, and qualification of these numerous components since this is now accomplished in one effort with a single IC. Microsemi's LX7730 Telemetry Controller is an example of this advancement. It integrates these functions into a compact 132-pin quad package and is QML-qualified for both class Q and class V requirements for the most demanding space applications.
The benefits of this approach can be seen in applications such as the Ganymede Laser Altimeter (GALA), one of the scientific instruments being tested for use onboard the European Space Agency (ESA) Jupiter Icy Moon Explorer (JUICE) mission scheduled to launch in 2022. This altimeter system will measure the distance of the spacecraft to the surface of Jupiter's icy moons Ganymede, Europa, and Calisto by calculating the time it takes a laser beam to travel to the surface, be reflected, and return to the telescope within the instrument.
Laser altimeter system supplier Hensoldt Optronics chose the LX7730 telemetry controller to provide processing housekeeping for instrument data including temperatures, voltages, and supply currents. Within its small footprint, the LX7730 device takes an active part in several closed-loop controls necessary for an accurate laser operation with low electromagnetic interference (EMI) levels. Regular calibration procedures reduce temperature and lifetime dependent drifts and ensure the required accuracy of the acquired digitized values.
Additional benefits of this system architecture are available by leveraging the emergence of the RISC-V ISA, which has opened the door for inexpensive and extremely flexible processing that can be performed locally to the telemetry source, enabling datalogging, health monitoring, and load control to be performed autonomously at the payload, and relieving the satellite central computer system of the processing burden of managing telemetry in remote pay-load units. RISC-V is an open ISA whose instructions are also frozen, which enables several key benefits for space design. Because the instruction set is frozen, any software written for a RISC-V core will run forever on any RISC-V device. This is ideal for space applications where a code base may be reused multiple times on many different programs spanning decades.
In addition, the open ISA enables vendors to create soft CPUs tailored for customers’ specific requirements. These soft RISC-V cores can also have their RTL shared. This could be critical for designs where inspection is needed to enable trust for security-conscious applications. RISC-V processors have already been integrated into radiation-tolerant field programmable gate arrays (FPGAs) for spaceflight applications. An FPGA configured to implement a RISC-V processor can be used in each payload for telemetry processing purposes, firstly reading telemetry data from the mixed signal telemetry acquisition IC, secondly performing processing and decision-making using data from the mixed signal device, and thirdly reporting health status information to the satellite central computer using the prevailing command bus protocol. FPGAs are commonly used to implement standard spacecraft control buses such as MIL-STD 1553, SpaceWire, and CAN bus, as well as non-standard bus protocols that are proprietary to the satellite integrator.
A complete telemetry-gathering system can be implemented using a highly integrated IC and an FPGA integrating a RISC-V processor. This system can be demonstrated with Microsemi's Six Sensor Demo that utilizes the company's RTG4 FPGA in a RISC-V environment connected via SPI to the LX7730 Telemetry Controller. The controller IC is used to acquire data from a small network of sensors connected to it and displays the measured values on a laptop screen via a GUI. The FPGA sends the address, data and read/write bits of the SPI frame to the IC that returns the ADC data to the FPGA. Finally, the FPGA applies the necessary scaling to the ADC output and sends the scaled data to the GUI through a UART.
The latest solutions for telemetry-gathering greatly simplify datalogging tasks and free up the main processor for other tasks. At the same time, high levels of integration of the mixed signal functions dramatically reduce the overall size and weight of the telemetry logging subsystem while increasing reliability, thus addressing three critical requirements in today's satellite systems.
This article was contributed by Microsemi Corp., a wholly owned subsidiary of Microchip Technology, Chandler, AZ. For more information, visit here .