This paper describes a technical approach for the development of RFI prediction models using carrier synchronization loop when calculating Bit or Carrier SNR degradation due to interferences for (i) detecting narrow-band and wideband RFI signals, and (ii) estimating and predicting the behavior of the RFI signals. The paper presents analytical and simulation models and provides both analytical and simulation results on the performance of USB (Unified S-Band) waveforms in the presence of narrow-band and wideband RFI signals. The models presented in this paper will allow the future USB command systems to detect the RFI presence, estimate the RFI characteristics and predict the RFI behavior in real-time for accurate assessment of the impacts of RFI on the command Bit Error Rate (BER) performance. The command BER degradation model presented in this paper also allows the ground system operator to estimate the optimum transmitted SNR to maintain a required command BER level in the presence of both friendly and un-friendly RFI sources.
Proc. SPIE. 9469, Sensors and Systems for Space Applications VIII
KEYWORDS: Electromagnetic coupling, Performance modeling, Satellites, Satellite communications, Telecommunications, Signal detection, Aerospace engineering, Systems modeling, Receivers, Signal to noise ratio
This paper describes innovative frameworks to develop RFI modeling and prediction models for (i) estimating the RFI characteristics, (ii) evaluating effectiveness of the existing Unified S-Band (USB) command waveforms employed by civil, commercial and military SATOPS ground stations, and (iii) predicting the impacts of RFI on USB command systems. The approach presented here will allow the communications designer to characterize both friendly and unfriendly RFI sources, and evaluate the impacts of RFI on civil, commercial and military USB SATOPS systems. In addition, the proposed frameworks allow the designer to estimate the optimum transmitted signal power to maintain a required USB SATOPS Quality-of-Service (QoS) in the presence of both friendly and unfriendly RFI sources.
We have developed an approach to multiple-access lasercom that adopts the commercial paradigm of sharing the most
expensive terminal resources among all users. Space-time division multiple access (STDMA), analogous to an optical
space-time switch, hops the transmit beam and receive direction among multiple users and exchanges data while the
beam dwells on a user. A key enabler of STDMA is electronic beam steering using liquid crystal optical phased arrays,
which provides fast, precise, and agile beam re-pointing. We have built the first optical STDMA terminal, combining
beam hopping between remote terminals with coherent combining of both transmit and receive apertures, which is an
effective means for increasing antenna gain in systems for which large aperture components are impractical. Coherent
beam combining provided the expected increase in antenna gain, and the terminal was found to re-point the beam among
users quickly and precisely enough to suffer only minor throughput degradation. Communications test were performed
using 10 Gb/s Ethernet for a single-aperture configuration. Performance is presented as a function of angle scan speed
and STDMA dwell time per remote terminal. The results suggest that STDMA is a viable technology for supporting
multiple-access space-based laser communication.
Liquid crystal optical phased arrays are an enabling technology for a variety of photonic and electronic beam manipulation functions, including steering, control of polarization, and amplitude and phase modulation. For applications in the emerging field of space laser communications, such devices would need to survive in the space environment for 10 to 15 years. To assess suitability and identify potential issues, a series of experiments were conducted in which nematic liquid crystal devices were subjected to three radiation environments: total dose (gamma), prompt dose (x rays), and fast neutrons. Tests were conducted using simple phase retarder devices, which served as surrogates for beamsteering devices. Impacts to optical and electrical characteristics of the devices at 1.55 µm were measured after incremental exposure trials. Modest effects were observed, but none were deemed significant enough to impact performance of the devices for space communication beamsteering applications.
For free-space optical (FSO) communications systems, sensitive optical receivers are the key to
closing the link over long distances in inter-satellite transmission scenarios, or to overcome large
atmospheric attenuation in terrestrial FSO systems. We present a 10.7 Gb/s optical transmitterreceiver
pair operating at 1550-nm, based on return-to-zero, differential phase-shift keying (RZDPSK).
The receiver is pre-amplified and uses an optical delay interferometer and a balanced
photo-receiver. The outer dimensions, the weight, and power consumption are 44×44×18 cm3,
14.1 kg, and 35 W, respectively. This optical receiver is single-mode fiber coupled. At 10.7
Gb/s, a receiver sensitivity of 27 photons/bit was measured, which yields a bit error rate of 1e-9.
This is less than 1 dB from the quantum limit (22 photons/bit). Coupled with a commercial
optical booster amplifier having an output power of about +37 dBm, a link loss of more than 80
dB can be bridged. In an inter-satellite communications scenario, this corresponds to several
tens of thousands of kilometers. Additionally, high link losses can also be experienced in
terrestrial systems as the result of atmospheric scintillation. To study this effect, the transmitter
and receiver combination were tested with simulated turbulence (scintillation). A turbulence box
was used to emulate different levels of scintillation under which the pre-amplified RZ-DPSK
system was investigated. Results of these tests are presented.
Extreme noise and vibration levels at lift-off and during ascent can damage sensitive payload components. Recently, the Air Force Research Laboratory, Space Vehicles Directorate has investigated a composite structure fabrication approach, called chamber-core, for building payload fairings. Chamber-core offers a strong, lightweight structure with inherent noise attenuation characteristics. It uses one-inch square axial tubes that are sandwiched between inner and outer face-sheets to form a cylindrical fairing structure. These hollow tubes can be used as acoustic dampers to attenuate the amplitude response of low frequency acoustic resonances within the fairing’s volume. A cylindrical, graphite-epoxy chamber-core structure was built to study noise transmission characteristics and to quantify the achievable performance improvement. The cylinder was tested in a semi-reverberant acoustics laboratory using bandlimited random noise at sound pressure levels up to 110 dB. The performance was measured using external and internal microphones. The noise reduction was computed as the ratio of the spatially averaged external response to the spatially averaged interior response. The noise reduction provided by the chamber-core cylinder was measured over three bandwidths, 20 Hz to 500 Hz, 20 Hz to 2000 Hz, and 20 Hz to 5000 Hz. For the bare cylinder with no acoustic resonators, the structure provided approximately 13 dB of attenuation over the 20 Hz to 500 Hz bandwidth. With the axial tubes acting as acoustic resonators at various frequencies over the bandwidth, the noise reduction provided by the cylinder increased to 18.2 dB, an overall increase of 4.8 dB over the bandwidth. Narrow-band reductions greater than 10 dB were observed at specific low frequency acoustic resonances. This was accomplished with virtually no added mass to the composite cylinder.
Experiments demonstrating several vibro-acoustic mitigation technologies will be tested on the Vibro-Acoustic Launch Protection Experiment 2 (VALPE-2) aboard a Terrier-Improved Orion sounding rocket slated for launch from Wallops Island Flight Facility in May 2003. Flight data collected in November 2002 from a nearly identical launch (VALPE-1) is being used to characterize the fairing environment and design the prototype hardware for the second flight. This paper discusses the various experiments that will be tested on the VALPE-2 flight, and presents some of the measured results and lessons learned from the first flight.
The authors are developing a cost- and weight-effective means for achieving an improved low- and mid-frequency acoustic environment in payload fairings for rockets at lift-off. The solution will be an active noise control system with an optimum selection of distributed active vibration absorbers (DAVAs) and acoustic actuators. High sound pressure inside a launch vehicle fairing during lift-off can damage delicate equipment in the payload. Space launch vehicle payload noise is a very important problem in the successful launch and deployment of space instruments and equipment. Measurements taken during the first few seconds of launch show very high sound pressure level (SPL) in the low frequency range of 60 to 250 Hz. High SPL is a severe problem because interior noise impinges on the instruments and equipment in the payload and can lead to their vibrational failure. Engineers have made moderate progress in addressing this problem by strengthening the instruments and by applying passive noise control treatment to the fairing. Both strategies incur significant penalties of added weight and financial cost and reduced allowable payload size. For further progress in suppressing low-mid frequency noise, another way is needed. The authors are developing a hybrid passive/active noise control system based on emerging technology of distributed active vibration absorbers (DAVAs). DAVAs are constructed from acoustic foam and area-distributed actuators. Passively it behaves as a tuned mass damper at low frequencies and a viscous damper at high frequencies. Actively a DAVA produces mechanical forces that are directed to reduce fairing vibrations.