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Aerospace and Electronic Systems Magazine, IEEE

Issue 2 • Date Feb. 1991

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Displaying Results 1 - 5 of 5
  • Advanced concepts for launch vehicle control

    Publication Year: 1991 , Page(s): 23 - 29
    Cited by:  Papers (1)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (663 KB)  

    The objective of the advanced launch system (ALS) program is to develop a family of launch vehicles which provides a substantial improvement in reliability, operability, and economy over existing systems. This requires that autopilot design and verification procedures be of minimum sensitivity to recurring costs while providing adequate, but not necessarily optimal, vehicle performance. One approach to accomplishing this task, namely, the utilization of advanced control design and analysis techniques, is considered. It is shown that the techniques, which represent a cross-section of developments in control theory, attempt to solve the ALS control problem in one of two ways: given increased knowledge of the launch vehicle and its environment, through greater model fidelity and additional sensor data; and autopilot design in the presence of quantified model/disturbance uncertainties and less stringent sensing requirements. It is concluded that the methods are promising and cost effective.<> View full abstract»

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  • Transfer orbit stage gyrocompass alignment algorithm twist and sway environment for Mar's Observer mission on commercial Titan

    Publication Year: 1991 , Page(s): 3 - 7
    Cited by:  Papers (3)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (395 KB)  

    The problems presented by the launch of a booster vehicle with a strapdown system on an expandable launch vehicle are addressed. These include the Earth motion by the wind-induced twist and sway motion at the launch pad, vibration-induced motion, and various sensor errors, as well as the limited memory and processing power of the flight computer. A robust numeric gyrocompass alignment algorithm is presented, and its performance in this environment is examined.<> View full abstract»

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  • Curved path approaches and dynamic interpolation

    Publication Year: 1991 , Page(s): 8 - 13
    Cited by:  Papers (1)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (490 KB)  

    It is shown that to simplify the specification of a desired trajectory for some subset of the variables of a dynamic control system, it may be advantageous to designate a set of intercept points that the trajectory is required to pass through. The system controls can then be computed in terms of a spline function to meet these requirements for dynamic interpolation. Optimization of a cost function under continuity constraints can be imbedded in the determination of spline coefficients to obtain certain desirable geometric properties of the resulting trajectory. The optimal trajectories determined under the additional constraint of constant speed are useful in determining the acceptability of the results of dynamic interpolation.<> View full abstract»

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  • Target motion compensation in synthetic aperture radar

    Publication Year: 1991 , Page(s): 14 - 18
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (384 KB)  

    In a synthetic aperture radar (SAR) targets on the ground that are moving become smeared as a result of velocity components parallel to the motion of the radar and are moved to radically different angular positions if they have velocity components perpendicular to the motion of the radar. Methods for restoring moving targets to their correct size and position are described. The samples collected for SAR processing are frequency-modulated RF pulses. Mathematically this leads to spectra that are described by Fresnel integrals. For stationary targets, the spectrum is symmetrical around the origin. If there is a moving target in a range cell, its Doppler spectrum will be displaced from the origin and may undergo other changes as a result of its nonzero velocity. Proper compensation to locate the target at the correct position requires that the spectrum be translated to a position dependent on the along-track velocity rather than to the origin. From the central frequency, the along-range velocity component can be estimated and the length of the translation can then be found. After translation, the spectrum is converted back to the time domain and the customary SAR processing is performed.<> View full abstract»

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  • Coherent Doppler tomography-a technique for narrow band SAR

    Publication Year: 1991 , Page(s): 19 - 22
    Cited by:  Papers (1)  |  Patents (1)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (357 KB)  

    Some concerns regarding a technique of narrowband synthetic aperture radar (N-SAR) imaging called coherent Doppler tomography (CDT), which may be a good candidate for spaceborne applications, are addressed. Using a single-frequency signal, are addressed. Using a single-frequency signal, resolution of two tenths of a wavelength can be achieved in the point spread function if the radar platform circles the ground path to be imaged. However, the high sidelobe level of -8-dB in the point spread function results in an unacceptable dynamic range. To reduce the sidelobe level, two approaches are presented: coherent processing using multiple discrete frequencies and noncoherent subaperture processing. Simulation results demonstrate that the sidelobe level is substantially reduced by both methods. However, the resolution is degraded and the computational overhead is greatly increased for noncoherent subaperture processing. Also presented is a possible satellite geometry configuration that could utilize N-SAR processing to provide high-resolution global mapping capability.<> View full abstract»

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The IEEE Aerospace and Electronic Systems Magazine publishes articles and tutorials concerned with the various aspects of systems for space, air, ocean, or ground environments.

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