Modular system simulates a space payload

Simulating the payload lets engineers test I/O interfaces.

Mathew Maher, Surrey Satellite Technology Ltd., Surrey, UK -- Test & Measurement World, 7/1/2011 12:00:00 AM

At SSTL, we recently designed satellite payloads for launch on partner-system satellite platforms. To minimize development time, we developed a payload simulator that lets us test the interfaces between the platform and its payload.

Figure 1 An instrumentation system simulates signals from a satellite.

The simulator simulates more than 500 I/O channels that cover several electrical standards and formats. It uses closed-loop control for its signals because it must reproduce dynamic payload signals based on inputs. Most of the hardware is standard off-the-shelf test equipment that resides in a 19-in. rack. Figure 1 shows the system diagram.

The host PC is a workstation-grade computer with a quad-core Intel Xeon processor that runs Scientific Linux. It provides a stable and close-to-real-time platform to run the bespoke command-and-control application software. A hardware-based remote-access device routes all user control over Ethernet to a remote console that has a keyboard, mouse, and monitor.

A GPIB link connects several instruments to the work­station. A 1200-W power supply from TTi provides 50 VDC to the integrated flight units. These flight units are used to provide functionality that is not cost-effective to simulate, such as RF communications. A TTi signal generator produces clock signals for the flight transmitters and receivers. Finally, two Agilent Technologies electronic loads add 15 channels of simulated current consumption. With those loads, the simulator can load the satellite power system in a way that represents the payload.

An MXI interface connects the workstation to a Geotest 20-slot PXI chassis that houses most of the electrical interfaces (Figure 2). The chassis provides a second line of defense in the event of overtemperature or overvoltage failure. Should either condition occur, the software will perform a controlled shutdown and advise the operator. Any equipment failure can easily cost more than $1.5 million in flight equipment and can push development programs beyond planned launch dates.

Figure 2 The system’s PXI chassis contains analog- output modules, an FPGA module, and switch modules.

The PXI chassis also houses two National Instruments 64-channel, 30-V input cards that detect 28-V pulses. These cards provide both rising-edge and falling-edge detection, which means we can calculate pulse width. This software pulse-width detection gives us a quick way to measure software performance. In one software test, we ran the system on both Linux and Windows and attempted to measure pulse width. Running Windows, the system couldn’t accurately measure pulses below 15 ms, whereas Linux consistently lets us measure pulses down to 1 ms.

We also use a National Instruments 96-channel digital-output card to provide basic TTL signaling, and we use a 64-channel analog-output card from Geotest to produce slow-update analog signals from 0 V to 5 V. The main satellite uses these analog signals to monitor slow-moving signals from the payload units.

Because space-standard interfaces often differ from commercial interfaces and protocols, we can’t always purchase off-the-shelf items for testing. In this instance, we use a Geotest FPGA card to provide a specialist six-wire serial-interface protocol. This reprogrammable card lets us reuse VHDL code developed for the satellite payload directly on the simulator with little modification. Attached to the FPGA card are custom-made differential signaling adapters that provide the space-standard electrical format. These external adapters also provide a level of fault isolation between test equipment and the DUT (device under test).

A Pickering Interfaces 32-way switch simulates RF-switch-feedback contacts, and a 10-A switch card connects satellite power sources to the electronic-load channels. Six precision-programmable resistor cards provide 36 channels of thermistor simulation from –55°C to +90°C. Both switch and resistor cards use mechanical relays, which isolate the simulator from the platform.

A Pickering Interfaces isolated power-supply card gives the system three +5-V power supplies to the external differential signaling adapters used on the serial-communications channels. This card employs onboard DC-DC converters that isolate the internal PXI chassis power rails from the DUT. T&MW

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