The Inside Story on Unmanned Vehicle Signal Recorder Technology

Pentek, Inc. designs embedded computer boards and recording systems for DSP, software radio and data acquisition for both COTS and rugged environments. In an interview with Aerospace & Defense Technology, Vice President Rodger Hosking explained how signal recorder technology is being implemented in military unmanned vehicles.

Rodger Hosking, Pentek

A&DT: Why is real-time recording of wideband RF signals a critical part of radar, signal intelligence, and electronic warfare systems for unmanned vehicles?

Rodger Hosking: Many unmanned vehicles are deployed for gathering information about certain regions of interest. Real-time recorders can capture vital intelligence from radar and optical scans, and from communications receivers. This information is often acquired as raw, wideband signals that must be analyzed after the mission. These signals can yield information about which radars and radios are operational in the area, identify the type of enemy equipment, and determine which countermeasure systems are operating. They can also decrypt encoded signals for content and develop patterns of behavior that may guide battlefield tactics.

A&DT: What does an FPGA in the UV electronics systems offer?

Hosking: Unmanned vehicles abound with different types of sensors, signals, interfaces, and protocols. FPGAs excel at implementing specialized I/O interfaces with real-time processing engines to extract information. These include parallel and gigabit serial ports for data converters and digital down-converters for delivering baseband software radio signals. FPGAs also accommodate the complex timing required for data acquisition and waveform generation for radar systems. FPGAs can also perform specialized digital signal processing operations that include decoding, decryption, beamforming, and demodulation, as well as image processing tasks like pattern recognition, motion detection, target identification, and classification.

A&DT: What is the best way to provide phase coherency across all channels of recording systems?

Hosking: Phase coherency across channels means that analog signals across all channels are digitized and captured at precisely the same sample clock edge. Usually, this is in response to a hardware trigger that starts the acquisition and recording. Timing characteristics vary among the many different types of A/D converters, so care must be taken to ensure that the alignment between the trigger and sample clock provides consistently-aligned capture across all channels. Recorded files for each channel will thus have the first samples of each recording perfectly aligned in time. Of course, all A/D converters must operate using the same sample clock so that samples in the recorded files are also aligned in time across channels. Provisions for synchronization must be incorporated in the circuit design of the digitizer boards, not only for multiple A/Ds on each board, but also across multiple boards. Often this requires an additional sync and timing generator product that delivers the appropriate clocks and timing signals to each digitizer board. As sample clock rates get higher, these designs become more challenging.

A&DT: Why is SWaP important?

Hosking: One or more of these three critical factors – Size, Weight, and Power – often present harsh, non-negotiable limits for equipment in unmanned vehicles. Size is an obvious factor because of the fixed interior dimensions of any vehicle, but shape can be just as important since the various systems must be fitted into place like a puzzle. Weight is especially important for UAVs, because it drives the required launch power and duration of flight time. Power is another finite resource that often presents a tradeoff between both weight and size. It is usually the most important limitation to mission duration, but may also impact the effective range of sensors and communication systems.

A&DT: What is the best way to achieve precision time stamping?

Hosking: Recorders equipped with GPS receivers can time stamp each recorded file with the precise time of the first sample. If the sample rate is locked to the GPS frequency reference, each sample in the recording is precisely defined in time. Additionally, Pentek recorders can log latitude, longitude, and elevation at programmable intervals of time, so that each recording can be matched to the exact position of the vehicle as it moves throughout its mission.

A&DT: How are the best real-time recording rates achieved?

Hosking: Today, solid state drives (SSDs) provide the fastest read/write rates, now exceeding 500 MB/sec. RAID controllers aggregate both the speed and capacity of multiple SSDs to deliver rates to 8 GB/sec and higher. These systems use server class PCIe motherboards chip sets that connect the multi-core processor, SDRAM memory, and PCIe ports to provide various maximum speeds between each resource. By judiciously assigning the PCIe ports, and by carefully controlling memory buffer structures and DMA block transfer sizes, overall system recording rates can be maximized and guaranteed across all conditions.

A&DT: What are some thermal management techniques that allow for operation in harsh environments?

Hosking: In air-cooled environments, PCIe server-based recorders must be equipped with strategically-placed fans that direct airflow across the installed boards that often house FPGAs and other heat-generating devices. Air must also be forced through the chassis, usually entering from the front panel and exhausting through the rear. For very rugged environments, Pentek’s 1/2ATR Small Form Factor recorders feature a completely sealed system with an air channel and that pulls air through the center of the chassis to effectively remove heat from internal fins thermally connected to the heat-producing devices. In conduction-cooled systems, heat can be removed by forcing air across external fins, or by direct connection to a cold plate.

A&DT: What are some of the military specifications and what are the considerations for compliance?

Hosking: Since most military unmanned vehicles must remain operational across a wide range of adverse environments, they are required to pass many environmental tests defined in MIL-STD-810. These tests include upper and lower temperature limits, shock and vibration levels, air pressures at a wide range of altitudes, acceleration, and resistance to fungus, humidity, fog, rain, and dust. Other tests defined in MIL-STD-410 expose the units to high levels of electrical and magnetic energy to validate unimpaired operation. It also strictly limits the allowed levels of radiated and conducted electromagnetic emissions generated by the unit while operating.

A&DT: What are some best practice considerations for unmanned vehicle electronics?

Hosking: Apart from ruggedization, the equipment must be easily controllable from resources within the vehicle. Pentek products offer a high-level application programming interface (API) using intuitive command functions and parameters that simplify operation of the equipment and interrogation of status and system health. These commands and responses can flow over Ethernet between the equipment and the mission control computer, which often provides a radio link to a remote facility during a mission. For unmanned vehicles with onboard recorders, once the mission is completed, the storage drives holding the recorded data should be easily removable so they can be transferred to a facility for analysis. Pentek supports this with its QuickPac drive pack that holds eight SSDs for quick removal and replacement, minimizing the down time between missions.

A&DT: What are some defense applications for Small Form Factor RF Signal Recording?

Hosking: In addition to capturing the important radar and communications signals during the mission, small recorders in unmanned vehicles are also used for storing other sensor, transducer, navigational, and imaging information. Many of these sub-systems are migrating towards gigabit serial interfaces using lightweight optical cables to join them using SerialFPDP (sFPDP) as an increasingly popular protocol. Recorders equipped with sFPDP can thus store these digital streams to capture a comprehensive operational history of the vehicle and its environment during the mission.