Two airborne spectrum analyzers were developed for acquiring dynamic data, characterized by frequencies up to 25 kHz, from a hypersonic-crossflow-transition experiment aboard the air-launched Pegasus® space booster rocket (see Figure 1). Real-time transmission of time histories of the dynamic data via a pulse-code modulation (PCM) telemetry encoder would have required sample rates of at least 50,000 s -1. The telemetry bandwidth necessary to support rates as high as this was not available; however, conversion of the data to the frequency domain aboard the rocket would make it possible to trade frequency resolution for a reduction in telemetry bandwidth. The two airborne spectrum analyzers implement this type of conversion. One spectrum analyzer is based on a swept-tuned receiver; the other is based on digital signal processing technology. The remainder of this article describes the swept-tuned spectrum analyzer.
Spectrum analyzers are typically used for radio-frequency measurements. Because the measurement bandwidth in the hypersonic-crossflow-transition experiment extended only slightly beyond the audio range, commercially available analog function modules (rather than radio-frequency components) were used in constructing the swept-tuned spectrum analyzer. They included an analog multiplier instead of a mixer, a root-mean-square-to-direct-current (rms-to-dc) converter as an envelope detector, and a digital sine-wave generator combined with a voltage-to-frequency converter for a local oscillator (LO). In lieu of a conventional display, the amplitude and frequency outputs of the analyzer were sampled by a PCM encoder. Then the display was obtained on the ground (where the calibrations were applied) by plotting the amplitude readings against the frequency readings.
The lower part of Figure 2 shows two vibration spectra obtained during one flight. The dashed line represents the vibration level while the rocket was held by the launching aircraft prior to launch; the solid line represents the vibration level at a high angle of attack after ignition of the rocket motor.
Calibration of the spectrum analyzer was done by Philip J. Hamory of Dryden Flight Research Center. Design was done by John K. Diamond of Langley Research Center. Analysis of data was done by Arild Bertelrud of Analytical Services and Materials, Inc. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Electronics & Computers category. DRC-00-12
This Brief includes a Technical Support Package (TSP).
Airborne Multichannel Swept-Tuned Spectrum Analyzer
(reference DRC-00-12) is currently available for download from the TSP library.
Don't have an account?
Overview
The document outlines the Technical Support Package for the Airborne Multichannel Swept-Tuned Spectrum Analyzer, developed under NASA's Commercial Technology Program. It serves as a comprehensive resource detailing the design, functionality, and applications of a sophisticated spectrum analyzer used in hypersonic flight experiments, specifically within the PHYSX (Pegasus Hypersonic Experiments) initiative.
The PHYSX program aims to enhance the understanding of aerodynamic and aerothermodynamic phenomena at hypersonic speeds, particularly the transition from laminar to turbulent flow, which is crucial for the design of future air-breathing hypersonic vehicles. The experiments are conducted aboard the Pegasus launch vehicle, targeting Mach numbers between 5 and 8 at altitudes exceeding 100,000 feet.
Key technical aspects of the spectrum analyzer include its ability to process high-frequency data, with a requirement for power spectral densities (PSD) and real-time signal processing. The document highlights the challenges faced in data acquisition, such as the need to transmit data at a maximum telemetry rate of 0.8 Megabits per second, while the required data rate for comprehensive analyses could reach 10 Megabits per second. To address this, onboard signal processing equipment was developed to either downlink meaningful data or perform real-time analyses.
The document also discusses the operational requirements for the FX-1 experiment, which was integrated as a secondary payload on the Pegasus vehicle. This integration necessitated minimal schedule impact and the ability to operate independently from the primary payload. The onboard instrumentation included a four-channel swept-tuned spectrum analyzer, which provided critical data for two hot-films and high-frequency pressure transducers, and a wideband RMS system for monitoring signal levels.
In addition to the technical specifications, the document emphasizes the importance of thorough data analysis, including the comparison of downlinked time-series records to ensure they accurately represent the entire data set collected during the flight. The findings from these experiments are expected to contribute significantly to the validation of transition codes and the overall understanding of hypersonic flight dynamics.
Overall, this Technical Support Package encapsulates the innovative approaches and technologies developed by NASA to advance hypersonic research, with implications for both scientific understanding and practical applications in aerospace engineering.
