Telemetry ground stations distribute a variety of real-time and post-flight products to a diverse group of customers. Typical ground station output for visualization is an 8-channel continuous pen strip chart recorder. The advent of powerful color graphic workstations and PCs has not replaced recorders for a number of reasons.

For many workstations, chart recorders continue to be important, despite the fact that technology has advanced from ink pens or heat styli to thermal arrays with laser printer resolution. The most common recorder interface continues to be analog, even though both the recorder and ground station are digital. Ground systems must therefore first convert a measurand’s digital representation to analog; then the recorder digitizes the signal for presentation as an analog wave. Hardware architectures designed for telemetry incorporate schemes to ensure continuous deterministic data output. Such architectures permit selective output of any processed or derived parameter, even archived ones.

As far as PCs go, lack of determinism by the PC’s operating system prevents sending real-time prime, let alone processed parameters to digital-to-analog converters (DACs) over the ISA or PCI bus. Therefore, in PC-based telemetry ground systems like L-3’s Visual Test System (VTS), selected raw digital data measurands are taken directly from a decom (over the top as opposed to the system bus) to a DAC or analog port for output to the strip chart’s analog input. The DAC module may incorporate a DSP chip to perform polynomial conversions to scale and produce output in engineering units. (Processed parameters require a more exotic, auxiliary processor solution.)

Local and wide area networks (LANs and WANs) permit distribution of real-time data to multiple client workstations, where separately data can be viewed, analyzed, and archived. The same applications software running on the ground system PC will function on the clients, producing an identical look-and-feel. The workstation’s non-determinism issue, seen with video strip charts, is not encountered as the display is discrete pixel based as opposed to the pure time base of recorders. That is, paper moves whether data arrives or not, while the display’s trace only moves when new data arrives.

Another real-time output is parallel digital words (discretes). This output will drive annunciator panel lamps, showing the status of important measurands, control switches, etc. Many of the annunciator panel functions can be replaced by workstation displays where the entire screen or smaller window is a completely reconfigurable annunciator.

Large ground systems often have a requirement to distribute real-time data to heritage mainframes and proprietary memory mapping networks. When interfacing to mainframes and nodes, care must be taken to ensure that they are not overwhelmed by high-rate telemetry data. Large buffers allow a non-deterministic system to cope like a rubber band. But unless the heritage system has adequate performance, the buffer will overrun. One way to alleviate this problem is through data compression, i.e., reducing the amount of data transferred by taking only every nth sample, an average, or changes in values, or by using a current value table. In the last example, you can periodically interrogate a memory table for current measurand and/or processed parameter values. Both systems must support anomalies such as stale data (value(s) previously collected) since only a portion of the samples may be acquired and the application on the external system must be aware of data loss.

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