Continuous Data Storage and Replay Unit
By Dr. Suman Ganguly, Mr. Slavisa Zigic, Mr. Brijesh Sirpatil, Center for Remote Sensing, Inc.

Continuous Data Storage & Replay Unit
The novel, high-speed Continuous Data Storage and Replay Unit (CDSR) is used for a variety of advanced applications that include signal intercept, communication, signal analysis, and signal simulation. It is also used for test and evaluation of several systems such as communication, radar, sonar, navigation, and aeronautics. The CDSR units perform similar to a high-speed tape recorder, only they are magnitudes faster; the throughput per unit is 240 Mbytes/s and they have a storage capacity of several terabytes (1MB = 1 million bytes, 1 Tbyte = 1 trillion bytes). For faster throughput and larger storage capabilities, multiple units can be connected and synchronized. Although they use standard hard disks as storage media (similar to RAID systems), they operate in hard real time and are guaranteed to record and replay a continuous data stream.

The unit accepts both digital and analog signals, where the stored digital data can be replayed via multiple D/As and the analog signals are digitized using built-in state-of-the-art A/Ds.

Some of these units are equipped with a variety of RF front-ends (and back-ends), such as downconverters and upconverters covering HF to microwave and fiber optic bands. The large sampling speed attainable with each unit allows high fidelity sampling over large instantaneous bandwidths. Depending on the number of bits/channels, instantaneous bandwidths of 120 MHz can be attained with 8-bit sampling. A/D converters configured to sample at more than 240 MSPS at 8 bits are available (for larger dynamic range, 80 MSPS at 16 bits can be used).

The units can be used as self-contained, deployable systems for interception and storage of wide bandwidth RF spectrum when positioned anywhere within the RF spectrum. The stored signals can be replayed with high fidelity (at baseband or the original RF band) or transferred to suitable computers for analysis and processing. The transfer to and from other computers is provided through Ethernet.

While the signal acquisition, signal intercept, and storage are the primary uses for these units, they provide an extremely versatile platform for several other applications. Signal replicas generated by a computer can be stored in these units and played back at a high data rate. In this mode, one can use these units as the basic hardware for a variety of signal simulation work. The signal simulation and signal analysis software are separately available from CRS. The use of these software tools (or user supplied software) provides versatile signal simulation, as well as signal analysis capabilities that are not available in standard simulators. These units can be used for simulation of various communication signals including radio, radar, navigational, sonar, and audio signals. These simulations can be operated under all different situations. The effects or unusual propagation conditions, user motions, signal dynamics, obscuration, multipath, and fading can be modeled and stored, thus allowing the generation of live RF signals with high fidelity.

One of the major hallmarks of these units is its flexibility. It was realized that a versatile instrument would require diverse configurations. These units allow various configurations without sacrificing simplicity and operating convenience. An extremely friendly GUI (Graphical User Interface) allows the user to configure these systems. Users can select the sampling rate, number of channels, bit-depth, and other relevant parameters. This information is then logged and automatically used during the replay or analysis operations.

Configuration
The digital data recorder and replay unit was developed for upward scalability. An almost unlimited amount of data throughput can be achieved. Such units are necessary for high fidelity, high bandwidth, Space Time Adaptive Processing (STAP), and Space Frequency Adaptive Processing (SFAP) processing. For GPS, data from multiple antenna elements can be sampled and radiated, allowing Wavefront sampling and Wavefront generation.

The architecture is modular. Currently, each module has a throughput of 240 MBPS that can be increased if necessary. Each unit can store approximately two terabytes of data and can be played back in real time. Various frontends, A/Ds, D/As, and upconverters can be built into the unit. GPS units are supplied with a dual frequency GPS downconverter (up to 100 MSPS A/Ds and D/As) and a dual frequency upconverter. The user needs to provide antennas for input or output.

The architecture is simple. It has been built from scratch using proprietary hardware. After suitable multiplexing, buffering, and encryption, the sampled data is streamed directly into the storage medium. Continuous streaming during recording and playback is guaranteed. The data is not routed through a PCI bus and consequently, there is no bus related latency.

The data sampling rate, number of bits and channels are all user-selected by using a front-end keyboard. A 6.4" TFT monitor indicates the status.

Data can be transferred to and from a user PC via Ethernet. Currently, 100 MBPS is supported, with future upgrades to Gigabit Ethernet underway.

Currently, eight hard disks are used for storage. This can be increased to 16 or more per unit. A large number of disks are difficult to physically accommodate in a convenient rack mountable unit.

The data acquisition board has a built in programmable processor. If needed, this processor can be programmed for equalization. Currently, there is no processing and the raw data is stored.

Data Storage Unit Implementation Details
1. Analog Interface: Analog interface consists of DAC and ADC. Analog interface will be 32 bits wide. The bus may be shared between DAC and ADC. It shall use LVTTL signals.

2. DIO Interface: DIO interface uses differential signals. It has a 16 bit data bus and 10 bit control bus. The maximum clock frequency is 100 MHz, limiting the data throughput to 200 MB/s. The DIO interface has flow control. It does not have any error detection or correction features.

3.Sampling Clock Generation: a programmable synthesizer shall generate a sampling clock. The sampling frequencies will be user selectable in steps within a block of frequency ranges.

4. Bit packing: Bit packing will be implemented only on the analog interface. The options are 2, 4, and 8 bits.

5. FIFO: The FIFO shall have a continuous throughput of 250 MB/s. The interface to FIFO will be 128 bits wide and run at 66 MHz.

6. RAID: RAID shall have 8 hard disks.

7. IDE Controller: The IDE controller shall operate in PIO mode 2 and UDMA mode 4. In UDMA mode 4, the peak throughput is 66.66 MB/s. Each UDMA transfer will consist of 16 sectors. There is no CRC error recovery feature.

CDSR Overview
• Fastest recorder for continuous data stream (240 MBPS)
• Total storage (2 Terabytes)
• Built in A/D and D/As
• Built in front and back-ends (downconverters/upconverters)
• User selectable configuration – clock rate, number of channels, bit-depth
• Friendly, convenient operation
• Signal analysis and signal simulation through external computers (networkable)
• Recording and playback of wideband (100 MHz instantaneous bandwidth) signal –
  digital, analog (baseband) or RF to microwaves
• Signal analysis and signal simulation of complex signals – communication, radio,
  radar, navigation, audio, sonar, biomedical, avionics
• Signal intercept anywhere in the HF to microwave bands
• Signal simulation for difficult and complex situations – propagation effects, user
  dynamics, obscuration, fading, multiple antennas, multipath
• Multiple units can be used in synchronous fashion, allowing coherent processing of
  multiple antennas, larger data throughput and larger overall storage
• User accessible signal processing architecture for real time processing of filters,
  equalizers, calibrators, etc.
• Complete set of off-line signal analysis and simulation software available

The CDSR units are configured with both an analog and digital input stream from external sources. The CDSR unit can record and play back data using 16 bit raw digital interface. The input clock can vary from 1MHz to 100MHz. The output clock is programmable in steps from selected frequency ranges varying from 1MHz to 100MHz.

In parallel with the digital I/O, there are analog channels. The analog signals are immediately digitized and multiplexed to provide a digital data stream which is recorded in the conventional manner. The digitization can be configured with a variety of bit widths and sampling rates. All clocks in the system are synchronized so coherently and timing accuracies are maintained. Low sampling jitter is maintained through specialized electronics.

While the users can provide external analog and/or digital signals, VIA SCSI connectors on the near panel (from almost anywhere), some of the units are configured with a variety of analog front-ends and downconverters for recording signals anywhere in the RF band. The 240 MBytes of data transfer limit the total bandwidth of the system. However, the 240 MBps throughput can be configured with different combinations of sampling clock and A/D bit depth. The A/D bit depth effectively controls the dynamic range, whereas the sampling rate controls the bandwidth.

The combinations shown in Table 2 are available as COTS products where limitations are due to those of A/D converters. With improved A/Ds, we shall continue upgrading these sampling configurations.

Some users may require a larger sampling rate than what is needed by the Nyquist. This oversampling provides improved noise immunity and high fidelity.

Front-Ends
Users can provide any analog signal in the 1-volt range for the built-in A/D converter to digitize the signal. The A/D converter cards can thus provide digital downconversion and record the baseband signals. The users may select any of the standard downconverters that are built inside the CDSR housing.

For high frequency signals, RF front-ends with adequate gain and signal bandwidth are used. The HF band front-end does not require a downconverter and consists of a high performance amplifier/filter combination to limit the out-of-band signals beyond 30MHz. The user can connect a suitable HF antenna and record snapshot HF signals anywhere in the world. In the HF to microwave band front-end, a user can select bandwidth of 30 MHz or 60 MHz and can position the center frequency anywhere in the 0 to 2 GHz band. All of these selections are software controlled and are set up by the user using the simple and intuitive menu driven GUI.

The GPS configuration has been widely used by the GPS communities in collecting snapshots of GPS signal structures anywhere in the world. For P-code signals, a bandwidth of 20MHz is necessary. For the M-code signal, the bandwidth requirements are larger. The built-in downconverter units provide exceptional capability and the CDSR units are similar in that users can record a true snapshot of the radio spectra anywhere in the 0-2 GHz range. This is done simply by connecting an antenna directly to the unit and pushing the record button. Several hours of signals are stored for future analysis and/or replay.

Signal Replay
The reverse of the recording process is performed in the replay process. Users can directly tap into the digital data stream during the replay process. The digital data is available over a 24 bit depth and can be de-multiplexed into various combinations of bit-depth, number of channels, and clock speed.

If the signals were recorded using the CDRS units, the combinations of clock rate, bit-depth, and a number of channels are stored and automatically used during the replay operation. Users may download the data stream into a computer or upload them from a computer, which they provide the configuration information during these processes.

Users can also obtain an analog replica of the baseband signal by passing the data directly through the D/A converter units built inside the CDSR units. They may also obtain a frequency translation (upconversion) into the desired frequency bands, similar to what is provided for downconversion. Signal coherency is maintained through both the downconverting and upconverting processes. By using the playback feature, we can obtain:

1. Digital data stream into suitable devices
2. Analog base band signal
3. Upconverted replica of the original signal

Thus, we can record a snapshot of the radio spectra and recreate the same signal conditions at a later time.

Upconverters with specification similar to the downconverters (see Table 3) are available and are built inside the CDSR enclosure. Users may employ multiple upconverters and downconverters to make use of the bandwidth and data throughput capabilities. A two-channel downconverter/upconverter configuration has been very popular in the GPS communities. This enables them to record, store, replay, and simulate both L1 and L2 signals with complete fidelity. In the future, the availability of L5 can be easily incorporated using another downconverter/upconverter combination.

For non-GPS users, some downconverter/upconverter combinations are available off-the-shelf. Specialized units can be easily built in and the users can easily attain bandwidths of 100 MHz or more (depending on the bit-depth) to meet their demands.

Computer Interfaces:
A digital data stream at a high speed (240 MBPS) can be transferred to and from the CDSR (and external hardware) through a standard DIO interface but is not accessible over standard computer interfaces. For the ease of communication between a standard PC and the CDSR units, Ethernet-based interfaces are provided. Thus, one can:

1. Generate a signal structure in the computer (see simulation section) and store the simulated signal in the CDSR. The stored signal structure in the CDSR then can be replayed back to provide live RF signals at baseband at any desired frequency band (depending on the upconverter) or to user specific instruments

2. When the signal is recorded off the air, the stored signal can be transferred to a PC and analyzed using the Signal Analysis toolset available from CRS

An Ethernet-based interface allows networking and remote operations using multiple CDSRs and/or multiple users.

Signal Analysis and Simulation
Recorded and stored signals in digital form can be transferred to a conventional PC using Ethernet. The data then can be analyzed by the user using his own (or any other) signal analysis toolsets or by using CRS’s proprietary signal simulation and analysis tool, “Impulse™.” The Impulse™ analysis tool is widely used.

The Impulse™ toolset (or any other signal generation software) also can be used to generate sample-by-sample replicas of any desired signal. The combination of software signal simulation and the ability to store and replay the signals provides extremely powerful capabilities. This allows the simulation of complex signals and systems, which is not possible with conventional real-time signal simulators.

The ability to synchronize multiple CDSRs with 10-pico second accuracy allows unprecedented precision in wave front simulation and other high precision applications. Large amounts of data from a variety of signal sources (GPS, communication, jammers, multipath) can be generated and stored – appropriate for multiple receiving antennas (wave front simulation) and necessary for anti-jamming systems. The antennas and receivers can be in a highly dynamic environment (for GPS/INS integration). Various signal deteriorations caused by multipath, ionospheric scintillation, and plasma effect can be modeled and RF wave fronts with high fidelity can be generated using this approach.

Software Tool Impulse™
CRS’s Impulse™ software is a Windows® development system allowing users to utilize a point-and-click interface for the design, simulation, and implementation of complex systems. It allows a one step process for design, simulation, and operation of a variety of systems. It offers high fidelity, rapid execution, scalability, interface to doctrinally correct modules, and built-in analysis.

Impulse™ combines the speed and ease of graphical programming with the efficiency of C++ and utilizes the processing power of various hardware devices. It is intuitive and easy to use. It allows scientists and engineers to design, develop, simulate, optimize, and operate functional systems without the help of programmers. The time required for implementing advanced tasks can be reduced by an order of magnitude.

The open-architecture concept of Impulse™ can be used for a variety of applications ranging from radio, radar, sonar, medical, and various other systems involving signals and signal processing. The basic version comes with the General Modules and Display Module. Specialized modules include Signal Processing, EW Analysis, Communication, Radar, and GPS. More modules are being developed and will be available soon.

Various software-based components, or building blocks, are provided in each module. These components or building blocks are like functional Integrated Circuits (ICs) in a hardware design. These components are simple and are defined by Input/Output structure and State Engines. Simple menu-based parameters for these components allow flexibility and easy operation. State Engines allow the same elements to be configured differently under various conditions. Users can develop their own components and a “Wizard” helps the user interface his component (written in C++) to the Impulse™ DLL.

Operation of the complete system consists of a few easy steps:

• Select the components; join them as desired
• Select input and output
• Set the parameter for each component
• Set the states (optional)
• RUN

Intuitive buttons (RUN, STOP, single-step, SAVE, etc.) allow the user to get operational in a matter of minutes.

Software Features
• Impulse™ provides software building blocks like hardware components (chips)
• Once the system is assembled using software building blocks, the system can be simulated and tested under different user defined conditions
• The simulated system is ready for real-time operations
• Various hardware options for real-time input/output are available
• Graphical User Interface for design and simulation
• Built-in object oriented components allow easy adjustments
• Menu-based parameter selection for most of the components
• User-defined modules and components can be easily integrated
• User-defined components can be in C, C++, Fortran, assembly
• Wizard to support the development of user-defined components
• Fully extensible, with new component packs being developed
• Built-in analysis tools
• Built-in visualization allows real-time feedback from any component
• Proprietary development of components is available
• Can be used for one step process for design, simulation, and operation
• Accepts real-time signal as well as stored (simulated) data
• Simulations and runs can be saved at any point

Available Modules
General Module: Included with the basic Impulse™ package, this module includes such basics as Clock, Adder, and File Handlers.

Display Module: Included with the basic Impulse™ package, this module contains 2-D and 3-D plotting such as oscilloscope and phase plots. In addition, this module contains probes for measuring specific values.

GPS Module: This add-on module contains all the required components to create a fully working GPS receiver. Capabilities include C/A and P code, as well as dual frequency. Various code-based, codeless, and semi-codeless architectures are built in. Additional advanced elements such as those with multiple antennas (beamforming, STAP, SFAP, anti-jam), and M-code are available. The GPS signal simulation module is available separately. The signal simulation module has all the components necessary for building a software simulator.

Signal Processing Module: this module contains various processing toolsets, including a variety of fully customized filters, such as IIR, FIR, FFT, wavelets, adaptive filtering, correlation, coherence, multi-channel processing (including STAP), adaptive beamforming, nulling, and DOA estimator.

There are other modules under development.

The signal capture signal analysis capabilities provide the users an entirely different dimension in understanding the signal and systems behaviors under difficult conditions. These can be used for various test and measurement applications, evaluation of receiver functions, and improved receiver design and development. Thus, it can be used to record the signal behaviors under high dynamics flight conditions, for complex propagation conditions, as well as other applications.

This approach has been used in the design and development of GPS receivers that are relatively immune from ionospheric scintillation effects. An example of the signal recorded during ionospheric scintillation is shown in Figure 6. The scintillating signal is used to design the GPS receiver, which remained stable during the scintillation period. Figure 6 shows the output of the GPS receiver correlator outputs, showing the stability of the receiver.

Applications
Availability of a continuous throughput recording and replay unit with an extremely high data rate opens up a plethora of opportunities and applications. Some of these applications are based on immediate and known requirements and needs. The high-speed tape recorder-like device also opens up new vistas in signal analysis and related applications. The applications are limited only by the user’s imagination.

There are numerous uses for recording of signals in different radio bands. The recorded signals can be analyzed using the IMPULSE™ software and/or user generated software. Various R&D activities could use such systems. Several such units can be used for Very Long Base Line Interferrometry (VLBLI) or other similar research.

An extremely useful application is the simulation of signals in different bands.
The Impulse™ software can be used to simulate various situations and scenarios covering an entire radio spectrum. Propagation conditions under different situations that include multipath, ground clutter, f oilage, and obscuration can be simulated. Extremely unusual signal conditions, such as those produced by plasma effects surrounding the space vehicles, can be easily simulated.

Extensive developments have been made in the simulation of various GPS signals and the upcoming Galileo signals. These signals have been validated by the GPS/JPO and the simulator tools have been used to provide a realistic replica of a GPS signal condition under jammed conditions (multiple-unlimited number of jammers), unusual propagation conditions, and shipborne conditions (JPALS), among others. Once stored in the CDSR unit, the simulated signals can then be played back to provide live RF signals anywhere in the RF spectrum.

The use of the CDSR units for simulation applications is well proven. Simulations of advanced communication and radar signals have been demonstrated. Numerous presentations and papers describing these simulations have been presented (see references). An example of the GPS signal simulation with C/A-, P-, and M-codes is shown in Figure 7. Some proposed Galileo signal simulations are shown in Figure 8. Figure 9 shows the simulated radar signals with two moving targets.

The use of similar simulation techniques in various communications, radar, and sonar technologies are highly desirable and would significantly reduce the simulator-developed efforts and provide a cost-effective platform capable of vertical upgrade and flexibility.

Signal Recording
Video Signals:
commerical video

Audio Intercepts:
(telephone records)

RF Intercepts:
radio, radar, communications, GPS, surveillance, and jamming

Test Signals:
instrumentation receiver testing flight-testing

R&D:
new Signal Structure design repeatable signal source recording unusual signal structures
Radio astronomical signals medical & biological signals

Signal Analysis:
EW & surveillance post analysis of radio signals in any band de-coding and de-encrypting
radio surveillance radar surveillance Communications intercept medical, biological, radio astronomy

Simulation:
communication
radar
sonar
navigation

• unusual propagation
• condition
• multipath
• foilage, clutter
• jamming
• live signal in any RF band

Advanced Features:
Each of the current units is limited to a maximum throughput of 240 MBPS. This throughput can be divided among multiple channels and different bit-depth and sampling rates. Although this is adequate for most of the applications, some specialized applications may demand larger data rates. Some examples include recording and simulation of wave fronts for phase array implementation where several parallel streams of signals are needed. In order to facilitate these higher data streams, means for synchronization between multiple units are provided.

The synchronization accuracy is typically better than 10-pico seconds and is limited by external connection/cables to the DSU inputs/outputs. Users can calibrate these timing offsets and, to some extent mitigate their effects through built-in real-time equalizers and filters.

For high-speed recording/replay, the timing synchronization between different channels is critical. A dedicated multiplexer/de-multiplexer unit is available for high precision applications. The MUX/De-MUX units allow timing synchronization of the order of pico seconds. The data rate can be increased almost indefinitely. Any limitations will arise through physical interconnection and switching through MUX/De-MUX units.

The current available CDSR unit offers a guaranteed recording and playback at 240MBPS for more than two hours. There is no other such unit in the market that has similar capabilities and competing products fall short in guarantying the throughput or they can only record and playback only for short periods of time. Also, competing units do not offer the various other features such as selectable number of channels, selectable sampling rate, network connectivity, and raw digital interface.

References
[1] Open Architecture Dual Frequency Software GPS Receiver and Applications, A. Jovancevic, S. Ganguly, A. Brown, M. Kirchner, S. Zigic, L. Scott, and P. Ward, ION 57th Annual Meeting, Albuquerque, N.M. June 11-13, 2001.
[2] Ionospheric Scintillation Monitoring Using Dual Frequency Software GPS Receiver; S. Ganguly, A. Jovancevic, A. Brown, M. Kirchner, S. Zigic; presented at the ION GPS conference, Portland, OR 2003.
[3] Advanced GPS Simulator/Receiver Prototyping System, S. Ganguly, A. Brown, M. Kirchner, M. Nguyen, P. Schnick, S. Green, and E. Weston; presented at the ION GPS/GNSS, Portland, OR, 2003.
[4] Open Architecture development System for GPS and Galileo; S. Ganguly, A. Jovancevic and A. Brown; presented at the ION GPS/GNSS Portland, OR, 2003.
[5] Real Time Dual Frequency Software Receiver; A. Jovancevic, A. Brown, S. Ganguly, J. Goda, M. Kirchner, and S. Zigic; presented at the ION GPS/GNSS, Portland, OR 2003..

Center for Remote Sensing, Inc.
www.cfrsi.com
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