A Low Cost, Open-architecture-based System for Portable Satellite Telemetry and Control
A comprehensive examination of the market demands for cost-reduced satellite telemetry and control stations is presented. These systems are implemented using flexible, open-architecture-based, high performance real-time systems. The trend for combining telemetry monitoring of satellite data with closed-loop satellite command and control functions is also discussed. This combined functionality opens up the possibilities for completely integrated, reduced-cost satellite control systems. In addition, the major requirements for the telemetry processing and command and control functionality of the integrated, reduced-cost satellite control system are described. The divergent requirements of performance, flexibility and price of these integrated, reduced-cost satellite control systems are made possible via the use of open-architecture building blocks that include standard versa module Eurocard (VME) boards combined with specialized real-time software drivers and user-oriented, flexible graphical user interface (GUI) software.
David R. Spielman
San Diego, CA
Historically, the concept of a low cost satellite telemetry and control station has been a contradiction in terms. By the very nature of its components (large autotracking dish antenna, wideband receivers, diversity combiners, microwave power amplifiers and telemetry processing subsystems), such a system probably will remain expensive for the foreseeable future. However, because of the proliferation of nonmilitary satellite applications in the US and the widening international deployment of commercial communications satellites, the requirement for lower cost telemetry and control equipment for satellite applications continues to grow. Traditional ground station equipment, with its custom components and proprietary architectures, is no longer cost effective in today’s highly competitive, cost- and schedule-sensitive international marketplace where satellite programs are undergoing ever-shortening development and deployment cycles. These facts mandate that the command, control and telemetry ground equipment used to support these missions be highly flexible, easily reconfigurable, supportable on an international level and, above all, based around a totally open system architecture with hardware and software components that adhere to published international standards. The use of equipment and components that are available from only one vendor or that are specific and limited in their focused application is becoming unacceptable.
Combining Telemetry Monitoring with Commanding
Ground systems that integrate telemetry processing functions with command and control are becoming more popular because the combination reduces program costs and development time for the end user. An example of a system that combines telemetry processing and monitoring with satellite command, control and ranging capabilities is the VMEstation telemetry system (VTS). Figure 1 shows a system block diagram. This system utilizes a real-time telemetry acquisition and analysis front end that is coupled tightly with a SPARC 20 UNIX workstation. The workstation acts as the display server for other X-Windows displays on the network. For support of a satellite that employs space ground link system (SGLS) communications, the real-time subsystem consists of a control central processing unit (CPU), IF receiver and subcarrier demodulator, telemetry bit synchronizer (with Viterbi decoder), pulse code modulation (PCM) decommutator and PCM simulator, Inter-range Instrumentation Group time code generator/reader and high performance small computer standard interface controller with real-time archival disk. Satellite uplink command and ranging functions are supported by an SGLS modulator, linear phase modulator and range/range rate card set.
The circuit cards in this system are housed in a 20-slot 6U VME chassis and have a totally open architecture from a hardware standpoint. For example, the decommutator board uses the standard VME and VMEbus subsystem bus (VSB) interfaces for the movement of information. Data are decommutated into a double-buffered, dual-ported memory that is accessible across the VSB rather than a private/proprietary bus. The approach used by this decommutator, where a large buffer of data is acquired before it is passed into the system, has several advantages over the conventional tag and data bus architecture historical to telemetry processing. First and foremost, a true open architecture is achieved that gives the end user much more flexibility in configuring CPU and input/output (I/O) capabilities. This open-architecture setup means that any board that has a VME/VSB interface, and there are many, may be used to interface with the telemetry data stream. The user of this system is not tied to any one vendor for system components and support. To upgrade capability, the addition of newer, faster CPUs and other interface and processing boards is straightforward. Standard VME boards from a multitude of vendors are available for most required I/O interfaces. By utilizing strictly VMEbus-compliant boards, the flexibility and growth potential for the system are increased significantly with minimal impact on cost.
Satellite Ranging Functions
Historically, with the possible exception of the telemetry processing subsystem, the functional elements that make up a typical satellite ground station (wideband receivers, diversity combiners, ranging equipment and antenna controllers) have all been box-level, rack-and-stack devices with their own respective power supplies, card cages and front-panel controls. However, this trend has begun to change over the last several years with the advent of VME-based receivers, diversity combiners and satellite ranging board sets that perform the same functions as their larger, bulkier, rack-and-stack counterparts. Of specific interest are the satellite range and range-rate board sets being developed to support SGLS and spaceflight tracking and data network satellite programs.
The SGLS provides full duplex communications for commanding, tracking, telemetry and ranging between a spacecraft and ground stations around the world. Command uplink services are provided via an L-band (1750 to 1850 MHz) microwave link, and telemetry, tracking and ranging services are provided on an S-band (2200 to 2300 MHz) downlink. The SGLS downlink system provides two downlink carriers, carrier 1 and carrier 2, that can be received simultaneously. The function of carrier 1 is to provide for antenna autotracking, range and range-rate tracking, and low speed PCM or analog telemetry. The carrier 1 downlink may also contain multiple subcarriers. When command uplink and ranging functions are being performed, one or two subcarriers at frequencies of 1.024 and 1.7 MHz, respectively, are often used. Carrier 2 is always located at a fixed-frequency offset that is 5 MHz below the frequency of carrier 1 and is used for PCM bit streams employing phase-shift keying modulation. The sample system presented in this article uses only the carrier 1 downlink.
In the SGLS, the commanding and ranging functions are combined in the uplink, as shown in Figure 2 . Commands sent to the satellite are first frequency-shift keying modulated and then combined with a 1 Mbps pseudorandom noise (PRN) ranging code and a 500 kHz square wave to form the composite uplink. This composite signal then is linear phase modulated to provide the uplink. The receiver on the satellite detects the ranging code, multiplexes it with vehicle data (telemetry) and retransmits it on the carrier 1 downlink. The carrier 1 signal is received by the ground station where the ranging and telemetry information is extracted from the carrier. Satellite range then is determined by a range processor (RP). The RP determines the slant range of the satellite by processing the range-tone echo recovered from the carrier 1 signal. The RP accepts this echo and reference signals, and makes the necessary measurements of the slant range. Coarse range is determined by a correlation reception of the 1 Mbps PRN sequence. Fine range is determined by measuring the coherent phase angle difference between a 500 kHz square-wave reference signal and the same square-wave signal recovered from the carrier 1 echo.
Integrating Radio Receivers with Telemetry and Commanding Systems
The combination of telemetry processing and monitoring with command and control capabilities within the same VME chassis opens up possibilities for completely integrated, reduced-cost satellite control systems. A further step toward this goal is migrating the radio reception of the satellite downlink channels into the VME chassis where the digital baseband processing occurs. This step allows truly portable, small-footprint systems capable of receiving and processing telemetry data to be fielded. Principally, these systems are used in the ground testing of satellites during assembly and later during prelaunch testing where the radio range between satellite and test system is limited. This limited capability is a factor because the nature and size of high power microwave transmitters used in the command uplink do not allow easy integration with other components in a small-footprint VMEbus package.
The reception of the microwave downlink channel from a satellite (both in orbit or on the ground) has been implemented successfully within the VMEbus environment by several telemetry equipment vendors. For example, the IF receiver used in this system is a single, 6U VME card that provides onboard FM, linear phase modulation (PM) and binary phase-shift keying (BPSK) demodulation for data rates up to 5 Mbps. The receiver’s superheterodyne-based design with programmable (tunable) input preselector and multiple IF bandwidths can support a wide range of telemetry and satellite applications. All receiver functions are set up from the VMEbus and include input frequency, IF bandwidth, filter bandwidths with readbacks of the actual bit rate, synthesizer and demodulator lock, and signal and loop stress. To process the subcarrier present in SGLS carrier 1, a separate subcarrier demodulator is used. Like the IF receiver, the subcarrier demodulator used in this system is a single, 6U VME card that provides onboard BPSK or quadrature PSK (QPSK) demodulation with programmable (tunable) input carrier frequency, bit rate, loop bandwidth and filtering.
Further refinements in receiver design and packaging have resulted in receivers that employ not only traditional analog designs, but digital signal processing (DSP) implementations as well. These new receivers offer true wideband reception at data rates up to 40 Mbps with PM, BPSK and QPSK demodulation implemented via high speed DSP. In addition to these new receivers, a series of companion diversity combiners has been developed that provides true AM/automatic gain control weighted optimal ratio combining for both postdetection only and predetection/postdetection scenarios. These efforts have resulted in VMEbus-based telemetry receiver/diversity combiner products that are being used currently to control autotracking antennas, and receive and process telemetry data from satellites that have been placed into orbit.
System Requirement and Implementation
The major requirement involving a system that combines telemetry processing with satellite command and control is that the system hardware and software must support in real time the generation of commands derived in whole or in part from conditions that are revealed in the satellite’s telemetry data. This condition implies that the satellite’s telemetry downlink processing and the transmission of the command uplink are coupled tightly in a closed-loop system where real-time performance is critical. (The example system described in this article is such a system.)
The described system is implemented using a totally open hardware and software architecture, ensuring a flexible system that can incorporate a wide range of processor and I/O devices that are available from a large number of sources. By employing an open architecture, the user can grow and adapt the system to a variety of mission requirements. By adhering to industry standards for software and network technology, the user can change, add to or enhance the system’s functionality by creating additional software programs to support new satellite launches.
In the example system described previously, the VTS software consists of low level drivers that control the boards in the system and higher level application software that allows setup and control of the telemetry, commanding and I/O boards. Asynchronous I/O utilities provide low overhead access to the telemetry and I/O hardware.
The VTS application software consists of real-time VME-based software running under the VxWorks operating system. This software controls the acquisition and processing of the PCM telemetry stream, generation of uplink commands, and the UNIX workstation-based setup and control graphical displays. The setup and control software running on the UNIX host communicates with the real-time processor over an industry-standard Ethernet link.
VTS software provides the high level control required to operate the system as a fully integrated product. The graphical setup and display portion of the software allows each real-time task to be configured and controlled graphically and intuitively from the host workstation using a series of icons, setup menus and display devices (referred to as widgets). The software architecture for the VTS system is shown in Figure 3 .
The industry-standard X-Windows/Motif-based GUI software provides system setup, telemetry and command parameter definition, display definition and real-time display capabilities. It performs error checking on user-input parameters and signals the user when an invalid parameter is entered. Hard copy output of the system’s configuration is supported. The system configuration allows the user to graphically set up each of the telemetry I/O interfaces, the other I/O interfaces and the network. The parameter definition capability allows the user to enter and edit parameter-processing definitions. The parameter ID and input information, as well as the various algorithms to be applied to it, are defined. The display definition capability allows the user to select from a number of widgets (numeric display, gauge, bar graph, strip chart, x-y plot or scrolling tabular display) and define the colors, ranges, positioning, size and format of each output window. Parameters (with their respective out-of-limit values) are associated with the widget to define a display. Displays are grouped to windows and windows are grouped to a configuration. Save, restore and edit capabilities are available for each user-configuration item.
The goal of the VTS GUI in the example system is to show the current state of the real-time side of the system using a block diagram format and to allow the user to modify the operation of the real-time system easily by manipulating this diagram. The top-level VTS screen contains the data flow diagram, which is a graphical representation of the flow of data between various processes in the real-time system. This diagram consists of a group of icons that are interconnected to show the desired routing of data. The top-level VTS screen also displays the main menu, which contains all of the commands necessary to configure and run the system. Figure 4 shows the top-level VTS screen.
The PCM bit synchronizer in the example system is configured via the BIT SYNC icon, and the PCM decommutator (frame synchronizer) is configured via the DECOM icon. All PCM stream definition and processing are defined and configured using the telemetry-processing (TM PROC) icon. Double clicking the mouse on any of these icons will bring up the appropriate setup menu(s) for the function selected. Figure 5 shows the bit synchronizer setup screen. When selected, the PCM stream definition setup menu provides an easy-to-use interface for defining major/minor frames, raw PCM measurands, derived parameters and parameter processing. Figure 6 shows a widget screen.
As the design and deployment time lines for military and nonmilitary satellite programs continue to shrink, the need for open-architecture, lower cost telemetry and control systems will continue to grow. The development and procurement of proprietary telemetry equipment and components that are available from only one vendor no longer can be justified in today’s marketplace.
The trend for the 21st century and beyond is for less expensive, simpler satellites that can be mass produced and deployed quickly into low earth orbits. This capability will require low cost, flexible, easily reconfigurable telemetry and commanding equipment that can be used and reused to accommodate the fast pace and rapidly changing satellite development environment.
The author wishes to thank Chuck Stephens of Premmco and John Reeser of Berg Systems International for their assistance in preparing this article.
1. Advanced Processing Laboratories, VME-station Telemetry System User’s Guide , San Diego, CA, October 1993.
2. The Aerospace Corp., Air Force Satellite Control Network Space/Ground Interface , (TOR-0059-6110-01)-3, El Segundo, CA, March 1992.
David R. Spielman is a systems development engineer at AP Labs’ Real-time Systems Group, San Diego, CA, which supplies real-time product building blocks and systems expertise. He has more than 16 years of experience in the integration and deployment of complex telemetry and satellite communications systems using commercially available, off-the-shelf components. During his career with AP Labs, Spielman has been involved in systems development, and sales and marketing activities.