Telemetry is a set of equipment and processes that supports the remote measurement of device operational parameters. It is used and practiced in aerospace environments, and in test, training, and verification ranges for the acquisition, distribution, and analysis of airborne instrumentation signals in a single data stream.
The telemetry workflow, as shown in the figure below, starts with the data acquisition unit that samples multiple data streams, combines them into a single digital stream, and then transmits them by an RF link to a receiving ground station. From the ground station the telemetry stream is forwarded to the operations center for real time display, analysis, and recording. In the past, multiple dedicated and unmanaged networks were deployed that required individual connection provisioning per telemetry stream, which was time consuming, expensive and impractical. However, as the proliferation of IP network technology continued from the desktop through the access network and to the network backbone, it has become preferrable to use COTS IP networking products that support reliable and affordable data distribution.
Figure 1: Telemetry Workflow
Raise the standard
Industry standards ensure muli-vendor interropability and stimulate the development of the best technical solutions while eliminating the need to rely on a single source for equipment. The Telemetry over Internet Protocol (TMoIP) Standards were established in 2007 by the Range Commanders Council (RCC) and Telecommunications and Timing Group (TTG). The standard’s goal is to get information from the test vehicle to the end users via ground network using packet-switched techniques. The TMoIP gateway in the ground network appends the TM stream with the appropriate IP formatting to transverse the IP network, enabling connectionless point-to-point and point-to-multi point paths, reducing bandwidth requirements and simplifying operational requirements.
The initial development of the TMoIP standard (RCC 218-07) enabled the test community to deploy IP networks to distribute telemetry streams across the test ranges. In the course of this deployment, additional functions and features were identified by the ranges that would further enhance the utility of TMoIP technology. The revised TMoIP standard (RCC 218-20) updates the initial standard to enhance multi-vendor interoperability, pricing competitiveness, and provide additional features that improve overall telemetry system operation.
What’s new
The updated standard provides support for Precision Time Protocol (PTP), enabling stream timestamping with nanosecond accuracy. The timestamp will support two operations, clock recovery and stream alignment. Using timestamps to support clock recovery will deliver faster recovery of the receive clock, and superior wander performance as compared to the legacy clock recovery method. In addition to the support of clock recovery, the higher accuracy timing source will enable the precision alignment of telemetry streams. The timestamping function will have the additional benefit of allowing telemetry streams to be aligned with ancillary streams, such as video source streams and mission timing sources.
Payload shaping with frame sync status and fragmentation was added to help receiving devices identify if the packet is expected to carry frame sync pattern or data quality metric bits. The updated IP header will allow devices to drop large packets rather than fragment them. Support for data quality metrics (DQM / DQE) enables range personnel to gain access to information that provides a quantitative indication of the quality of the downlink, so that they can make selection decisions of received streams In real time.
Always open
For many years we have championed open standards, COTS, and solutions that provide serial-to-IP capabilities to enable transparent low latency transfer of any digital and analog signals. Features that have gained attention for our products include “Flat Line Response System” and “Auto IP Payload length modes”, which enable each channel to maintain an IP processing latency of 10mS for PCM telemetry streams in the range between 10Kbit/sec & 50Mbit/sec. The benefits of these features include efficient use of IP bandwidth with no user configuration and no equipment setup changes needed. These features ensure that the user, without any adjustments to the equipment, will always maintain a fixed latency and time correlation between PCM telemetry streams with diverse rates. Additionaly, the Auto level input detection mode enables the reception of TTL, ECL, CML signal formats or any other type of bi-level input signal without any user intervention and no re-configuration required. This feature enables low-level TTL telemetry signals to be interfaced without the need for external amplifiers, as low signal levels are normally not accepted by a standard TTL inputs. Also, in installations where no PCM telemetry clock is available, a Bit-Sync function is provided that extracts and generates the telemetry stream as separate clock/data signals.
Products in practice
The Atlantic Test Range’s Eglin Air Force Base utilizes the multi-service capabilities of the IPtec TNP100 TMoIP gateways to enable service assurance for their timing, video, telecom, analog and telemetry services. At each ground station the IPtec gateway generates a test signal using the built-in test generator capability of the TNP100. The test signal is transmitted along with the telemetry and associated information streams to the Operations Center. At the Operations Center, the received test stream is evaluated for bit errors. This enables the test engineer to quickly evaluate the link quality and isolate errors, which is critical in a real time test environment. Additionally, range communications may be transmitted via the telecom circuit emulations service, and ultra-low latency 2ms video service is transmitted from each ground station to the operations center. The ability to combine these three services with an in band test signal greatly simplifies operations and maintenance.
Figure 2: Eglin AFB Multi-Service Use Case
Looking ahead
The telemetry range of the future will ultimately migrate to an all IP-based infrastructure, eliminating the need to packetize serial PCM streams. The TMoIP gateway function will move upstream from the ground station and into the data acquisition device on the test vehicle. The role of the TNP100 will evolve from a transport to a gateway function. The TNP100 is suited to this functional migration, as the core technology of the product is based on programmable hardware devices, which can be field upgraded. The TNP100 is truly a “future-proof” product that can be deployed by the telemetry community for current and future needs.
Figure 3: Telemetry Range of the Future
For more information visit VITEC at DSEI 2021, London Excel 14-17 September on Stand H6-162.
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