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Manshadi impact

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Published on January 3, 2008

Author: Heather

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Impacts of Interference on the Receiving Systems of the Deep-Space Network (DSN) Earth Stations Operated by NASA due to Adjacent Band Emissions from Earth Exploration Satellites Operating in the 8025-8400 MHz Band :  Impacts of Interference on the Receiving Systems of the Deep-Space Network (DSN) Earth Stations Operated by NASA due to Adjacent Band Emissions from Earth Exploration Satellites Operating in the 8025-8400 MHz Band EESS X-band Workshop Toulouse, France June 22-24, 2005 Charles C. Wang, Miles K. Sue and Farzin Manshadi NASA/JPL Peter Kinman Case Western Reserve University Introduction:  Introduction The 8025-8400 MHz band used by EESS satellites for downlink data is adjacent to the 8400-8450 MHz band used by deep-space missions for downlinks The 8400-8450 MHz band is vital to deep-space missions and is extensively used By the end of 2005, 19 deep space missions will be using this band Adjacent band emissions from EESS satellites can interfere with the sensitive deep space earth stations A protection criterion of -221 dBW/Hz is established by the ITU-R (SA. 1157) to protect the deep-space Earth stations Characteristics of Deep Space X-Band Downlinks:  Characteristics of Deep Space X-Band Downlinks Deep space X-band downlink signal can be extremely weak (can be -240 dBW/m2 or lower) To ensure the integrity of deep space data, NASA’s Deep Space Network (DSN) employs Large aperture Very low noise receiver front-end Powerful error correction code ITU-R specified the 8400-8450 MHz protection criterion as -221 dBW/Hz or equivalently an increase of 1 dB in noise floor. 1dB loss is unacceptable for some missions. Within NASA, -231 dBW/Hz is used to protect deep space X-band downlinks. Location of deep space stations – strategically placed to provide 24 hr/day coverage. NASA’s Deep Space Network (DSN) have complexes in Goldstone, Calif, Madrid, Spain, and Canberra, Australia. There are many tracking stations at each complex. Other agencies have their own stations. In addition for relaying telemetry data, deep space X-band downlinks are used for radio science experiments Characteristics of received signal such as signal strength, phase, etc are used as observables. Difficult if not impossible to distinguish between an interference event or a natural phenomenon Characteristics of EESS Out-of-Band Interference to Deep Space X-Band:  Characteristics of EESS Out-of-Band Interference to Deep Space X-Band EESS satellites are typically in near polar low-Earth orbits EESS tracking stations typically have much higher flux density per bit than deep space downlinks Interference to deep space downlinks are typically characterized by brief interference events that occurs many times a year Example: A 150 Mbps (QPSK) EESS satellite at the 705 km sun-sync orbit with an isoflux antenna and transmitting at 20W EESS satellite exceeds deep space protection criterion at a DSN station ~700 sec/yr, but 700 sec/yr = 35 interference events with average event duration of 20 seconds while an interference event is short, the impact on the affected deep space downlink can last much longer! Overview of Deep Space Receiving System:  Overview of Deep Space Receiving System Mechanisms of EESS Adjacent Band Emission Affecting Deep Space X-Band Downlinks (I):  Mechanisms of EESS Adjacent Band Emission Affecting Deep Space X-Band Downlinks (I) EESS adjacent band interference to deep space downlink Degradation from gain suppression of RF front-end Degradation from discrete spectral lines Degradation from rise of noise floor (broadband interference) Mechanisms of EESS Adjacent Band Emission Affecting Deep Space X-Band Downlinks (II):  Mechanisms of EESS Adjacent Band Emission Affecting Deep Space X-Band Downlinks (II) Interference from RF front-end saturation The output of deep space X-band downlinks RF front-end can be suppressed if the EESS interference is sufficiently large Mechanisms of EESS Adjacent Band Emission Affecting Deep Space X-Band Downlinks (III):  Mechanisms of EESS Adjacent Band Emission Affecting Deep Space X-Band Downlinks (III) Interference from discrete spectral lines Discrete adjacent band emissions are typically caused by imperfection in the EESS transmitter design or ranging tones Very low power discrete spectral lines near the center frequency of a deep space downlink from the EESS adjacent band emission can cause deep space tracking loops to lose lock Mechanisms of EESS Adjacent Band Emission Affecting Deep Space X-Band Downlinks (IV):  Mechanisms of EESS Adjacent Band Emission Affecting Deep Space X-Band Downlinks (IV) Interference from rise in deep space noise floor Sidelobes from the modulation of EESS satellites can raise the noise floor of deep space X-band downlinks Interference affects a deep-space receiver’s tracking loop performances and data integrity, as well as antenna pointing Example Effects of EESS Adjacent Band Interference on a Deep Space X-Band Downlink (I):  Effects of EESS Adjacent Band Interference on a Deep Space X-Band Downlink (I) Interference can cause a deep space receiver to lose lock during the interference event. The receiver will need to reacquire the weak deep space downlink which can take many minutes. When receiver is out-of-lock in a DSN tracking station, an operator has to recognize the anomaly and reinitiate the acquisition sequence DSN receiver acquisition may require Loading predict to estimate the relative location and velocity of the spacecraft Antenna pointing acquisition Perform FFT to get accurate estimate of downlink carrier frequency Carrier tracking loop acquisition Subcarrier tracking loop acquisition Symbol tracking loop acquisition Frame synchronization Effects of EESS Adjacent Band Interference on a Deep Space X-Band Downlink (II):  Effects of EESS Adjacent Band Interference on a Deep Space X-Band Downlink (II) Acquisition time for suppressed-carrier deep space downlinks Acquisition time for residual-carrier deep space downlinks Effects of EESS Adjacent Band Interference on a Deep Space X-Band Downlink (III):  Effects of EESS Adjacent Band Interference on a Deep Space X-Band Downlink (III) It can take hundreds of seconds for various carrier and symbol tracking loops to acquire lock For frame synchronization, a frame is typically around 10,000 bits. Depending on the error correction code used, it can take around 400 seconds for frame synchronization for a downlink rate of 100 bps. About 15 minutes is needed for a deep space receiver to re-acquire for a 100 bps downlink after an interference event and much longer for weaker links. Effects of EESS Adjacent Band Interference on a Deep Space X-Band Downlink (IV):  Effects of EESS Adjacent Band Interference on a Deep Space X-Band Downlink (IV) Antenna Pointing DSN station’s antenna pointing is aided by the knowledge of relative motion of a deep space spacecraft and the antenna Conscan (conical scan) is a close-loop tracking algorithm that uses carrier power to noise spectral density to finely adjust the pointing of the antenna to correct small pointing errors caused by wind and/or atmospheric effects A rise in noise floor due to EESS adjacent band emission can cause Conscan to deviate from the intended target and cause the antenna to lose track of the spacecraft. Impacts of EESS Interference to Deep Space X-Band Downlinks (I):  Impacts of EESS Interference to Deep Space X-Band Downlinks (I) EESS adjacent band emission affects a deep space downlink mainly in the following four areas Command Telemetry Radio science Ranging Impacts of EESS Interference to Deep Space X-Band Downlinks (II):  Impacts of EESS Interference to Deep Space X-Band Downlinks (II) Command When antenna is off-pointed, both uplink and downlink are lost Telemetry Significant amount of data may be lost. Some data may be lost permanently Radio Science It is difficult if not impossible to distinguish an interference event versus an observed natural phenomenon Effects of EESS Interference to Deep Space X-Band Downlinks (III):  Effects of EESS Interference to Deep Space X-Band Downlinks (III) Ranging Ranging is used to determine the location of a spacecraft and is performed by measuring round-trip or one-way delay of range signal Each range sequence takes several minutes to perform and a gap in a ranging sequence renders the whole sequence useless Conclusion:  Conclusion ITU-R has established a protection criterion for 8400-8450 MHz band of -221 dBW/Hz Average fractional interference time of EESS adjacent band emission to a deep space X-band downlink is short, but it is consist of many short interference events each can cause the deep space downlink to lose tracking time of fifteen minutes or more. Interference impacts deep space downlinks in the following areas Command Telemetry Radio science Ranging

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