This article focuses on the design and optimization of the acquisition system for telemetry data, to better monitor the health of a satellite.
Precision analog signal chain solutions are required for data acquisition, processing, and response generation, including the monitoring and diagnostics of subsystems, and their optimization is a complex challenge.
In this article, produced in collaboration with global electronics manufacturer Texas Instruments (TI), a paying participant in the satsearch membership program, we take a deep dive into how to improve satellite health monitoring by optimizing data acquisition systems in space applications.
For more on this topic you can also take a listen to our podcast with TI on optimizing data acquisition systems in space applications, which covers similar information.
Monitoring the health of a satellite requires the use of accurate and timely telemetry data. Such data acquisition systems high channel count and versatility at high data rates. They must also operate reliably in a very harsh environment, but with low power consumption and a small physical footprint.
In addition, beyond the data acquisition system of a satellite’s payload — which requires high precision and high bandwidth — there is also a need for analog components capable of acquiring data for monitoring, diagnostics, and control of the satellite. In other words, the complexity of the data acquisition system goes far beyond fundamental functions such as the read-out of the Charge-Coupled Device (CCD) sensors in a camera system.
But analog components in data acquisition systems pose challenges to designers as they can often result in conflicting design goals. For example, the high radiation levels in space may lead to latch-up situations in a semiconductor component. The system must detect this and react fast enough to prevent permanent damage to the component which could, in extreme cases, lead to the loss of the entire payload.
This is just one example of the issues that electrical engineers must consider when developing the optimum monitoring architecture for a satellite. In the next section we take a look at a number of others.
Design challenges for telemetry applications
Whether in a Low Earth Orbit (LEO) mission, a manned space initiative, or deep space exploration, it is crucial for mission control center to know the health status of the spacecraft. Satellite health in particular is primarily monitored by analyzing onboard telemetry data.
But with more powerful subsystems on board the current generation of satellites, and more ambitious mission objectives being worked on, telemetry data acquisition systems are causing increasingly challenging design requirements. Two of the primary challenges are:
- The achievable technical performance of the system, and
- Adaptability to different orbit requirements and their radiation levels.
When it comes to assessing the first issue, typically designers are forced to fulfill the demanding signal-to-noise ratio (SNR) requirement of the data acquisition system, which is the heart of the telemetry application.
For example, one of the most crucial components in this system is the Analog-to-Digital Converter (ADC), and designers will need to take into account several factors on which the final resolution performance of an ADC is based, such as:
- Reference voltage accuracy,
- Clock jitter,
- Power Supply Rejection Ratio (PSRR), and
- Input stage configuration.
Another fundamental decision for determining the circuitry’s radiation protection requirements (which enable it to give accurate satellite health data) is the radiation hardness level that the satellite must exhibit in order to have a successful mission. This depends on operating orbits, and the importance of adaptability to different orbital radiation requirements is outlined in the following section.
The importance of radiation protection in space electronics
For high Earth orbits, such as Geostationary Orbit (GEO) or in deep space missions, there is typically no room to compromise on radiation requirements and designers will always try to use QMLV-RHA qualified products. It is important to note that such devices are often quite expensive and choices in the marketplace may be limited.
For Low Earth Orbit (LEO) missions there is an opportunity to go with a lower radiation hardness level. However, commercial-off-the-shelf (COTS devices) are often still not a good choice for such applications, unless the stay in orbit is planned to be only very short, and the total cost of the satellite, and risk appetite of the operator, allows for a potential loss.
For LEO missions, designers typically prefer ‘radiation tolerant’ products over the more expensive ‘radiation hardened’ products.
Satellites operating in low earth orbits fly a lot faster than the Earth’s rotation and can pass points on the surface multiple times per day, moving from full exposure to the sun and into the cold shadow side of the Earth and back.
In most cases, COTS devices are simply not robust enough for such extreme temperature cycling. In addition, while radiation levels are lower than for GEO missions they are still significantly higher than on the ground where the Earth’s magnetic field and the atmosphere provide a strong shield.
For further details about the different integrated circuit (IC) quality levels of space-based products, please take a look at our earlier article onSpace Enhanced Plastic products for NewSpace with Texas Instruments.
In more advanced systems memories and digital circuits have an increased risk of an error caused by radiation simply due to their larger size and higher complexity. Accordingly, semiconductor process technology must be improved for the best possible radiation hardness in such systems.
The potential to develop a high-performing system with adequate radiation protection eventually comes down to the circuit design itself. Next, let’s take a look at the circuit design of a data acquisition system and the different electronic components that comprise it.
Focusing on the data acquisition circuit
Data acquisition systems are designed to convert measured data, such as sensor-captured data, into electrical signals for transmission. These are conditioned by an Analog Front-End (AFE) circuit and then sampled by an ADC.
For the data acquisition circuit, designers typically start by defining the requirements of the ADC as this is usually the most expensive and power-hungry device in the data acquisition system.
For telemetry and health applications in particular a high number of signals need to be monitored. In order to avoid each signal requiring its own ADC, designers typically add multiplexers.
In many cases, such multiplexers are already integrated into the ADC. For example, TI offers a 12-bit ADC, the ADS128S102-SP with an integrated 8-channel multiplexer and is designed for such satellite applications.
Just upstream of the analog to digital conversion, the signal chain has to be conditioned too. For any signal conditioning needs, designers have to cover several different cases. Next let’s take a look at how to protect the signal chain.
Protecting the signal chain
There are many aspects to ensuring the satellite health monitoring signal chain is fully protected. In this section we illustrate a number of these with reference to the space-grade solutions that Texas Instruments offers.
Firstly, the circuit’s operational amplifier (OpAmp) must provide good performance with low drift and offset voltage. TI’s OPA4277-SP is designed to offer a wide supply voltage range and good signal conditioning performance, with ultra-low offset voltage and offset drift.
Another OpAmp, the LMP7704-SP, is suitable for interfacing with precision sensors that have high output impedances, as it offers an ultra-low input bias of less than ±500 fA. It is a true rail-to-rail amplifier, designed for versatility and that can be configured for the transducer, bridge, strain gauge, and also for transimpedance amplification. Ensuring such versatility simplifies the circuit design and reduces potential single points of failure for a higher level of protection.
In contrast to the LMP7704-SP, some amplifiers are specific to a single use case, such as shunt current sensing. For example, there is a dedicated current sense amplifier available from TI named the INA240-SEP. This device features enhanced PWM rejection and can sense drops across shunt resistors over a wide voltage range.
During satellite operation there can be situations where the supply voltage level or current level rises too fast, damaging or even destroying a system. It is important that the system reacts quickly to such events. A classic data acquisition system, with an ADC and a Microcontroller Unit (MCU), in the signal chain is often too slow in such cases.
Designers must typically implement a comparator in the analog domain to enable a faster response to such problems. As an example, in TI’s portfolio the TLV1704-SEP offers rail-to-rail inputs and a low propagation delay of 560ns.
In addition, with an open collector stage the output can be pulled to any voltage rail up to 36 V above the negative power supply, regardless of the TLV1704-SEP’s supply voltage. This device is therefore flexible enough to handle almost any space application, from simple voltage detection to driving a relay, with a low propagation delay that can account for rapid voltage and current changes.
Texas Instruments (TI) offers a broad range of precision analog products that enable designers to build applications to the required performance criteria and protect the signal chain at every level.
But these aren’t the only concerns when designing an electrical system for use in space. Satellite applications demand high levels of robustness in design along with sophisticated solutions for detecting faults.
Fault detection and fault tolerance
In the space industry there is obviously a consistent focus on making technologies highly robust against the harsh environment. But despite the highest standards and most rigorous testing requirements, there is always the chance of a major issue occurring, such as a fatal Single Event Effect (SEE) or a generic component failure.
To mitigate this, designers can add multiple monitoring and diagnostic capabilities into the system to ensure robustness and redundancy. Recovery strategies such as a controlled power cycle of the affected subsystem and a switch to a redundant component or module can also be used.
As mentioned, with the new generation of products in the electronic industry, the circuits are getting more and more complex. And this is bringing an exponential increase in potential failure mechanisms. Designers must make sure that any failure scenario is properly identified and understood, and that its impact is mitigated as much as possible.
In addition, with the cost-sensitivity that LEO constellations face, there is also a trend toward MCU-based implementations as radiation-hardened FPGAs are typically very expensive.
And whether such digital systems are FPGA-based or MCU-based, fault detection and fault mitigation becomes even more important in LEO. Components with integrated fault detection and fault mitigation technology will bring high value in any application, but particularly where margins are thin.
Now that we have covered the design requirements for data acquisition systems let’s have a look at future trends in space electronics in the next section.
Further trends in space electronics
Electronic manufacturers offering space products are expected to find a solution that is not only low cost, but is also smaller in size and has lower mass (to reduce launch costs), and meets the lower radiation requirements of the LEO orbit.
With these aims in mind, Texas Instruments is at the forefront of one new trend in the industry that is having a big impact on size, weight, power, and cost (SWAP-C) budgets – the availability and use of new devices in plastic packaging. Traditional QMLV devices are provided in ceramic packages which are physically larger and heavier than the more common commercial plastic devices used on Earth today.
TI has been working with its customers, the space community, and various government agencies to create new standards that will allow for greater use of plastic packages and substrates in space applications.
TI’s first space developments with plastic packages came with the introduction of the radiation-tolerant Space-enhanced plastic (Space-EP) portfolio. Customers were looking for lower-cost space devices as a solution for the higher volumes required for Low Earth Orbit satellites or in other NewSpace domains.
Plastic packages are attractive because the scale of the semiconductor supply chain can be leveraged to reduce costs, while maintaining performance, and provide a smaller overall solution. Today, Texas Instruments has 20 such products in production or sampling in their Space-EP portfolio with many more in development.
In addition, to take advantage of first-hand insights on current trends in the industry, TI’s experts provide support in assisting designers optimize their solutions. Such application materials enable designers to make faster evaluations of different architectures and rapid decision-making. It also provides a great head-start for actual development.
For example, you can view TI’s reference designs at this link and the company’s Spacecraft Circuit Design Handbook. Such references are invaluable for developing efficient, successful, fault-tolerant circuits that can take advantage of the high-performance payloads and subsystems on today’s market, while also accurately and consistently monitoring the health of a satellite during operations to ensure mission success.
You can find out more about the Texas Instruments space portfolio here, or you can use this link to access the TI E2E™ design support forums where you can engage further with the company.
In addition, you can also get more information from TI engineers on procuring electronic components and reducing power consumptions in space electronics on the satsearch blog.