Achievable Loop Bandwidth Research Articles

Sensitivity and Performance Analysis of Doppler-Aided GPS Carrier-Tracking Loops

The performance of a Doppler-aided GPS receiver is analyzed. It is shown that Doppler aiding increases the receiver's robustness to interference and jamming. This is achieved by narrowing the noise bandwidth of the carrier-tracking loop using an external estimate of the Doppler shift of the GPS signal. The Doppler estimate is computed using an inertial navigation system and is incorporated into the carrier-tracking loop in a feed-forward fashion. The minimum achievable loop bandwidth is a function of phase noise on the receiver oscillator, satellite clock phase noise, and the quality of the external Doppler estimate. Using the criterion for loss of lock of 15 deg phase jitter, it is shown analytically that for an unaided receiver, loss of lock occurs at a signal power equal to approximately 26 dB-Hz. With Doppler aiding and a nominal-quality receiver clock, an additional 4 dB-Hz margin can be achieved, allowing signals with power as low as 22 dB-Hz to be tracked.

Robust design of sampled-data systems with application to an electro-hydraulic position controller

Attention is drawn to the importance of the sampling frequency as a design parameter of sampled-data feedback control systems and consideration is given to its implication to the ability of the systems to withstand gross variations of the plant without unacceptable deviation from the desired performance. It is seen that the robustness of a system is intimately related to the sampling frequency as a consequence of the limitations which it imposes on the achievable loop bandwidth. Conversely, for any specified degree of robustness, the minimum frequency of sampling is well defined. However, for plants with high-frequency dynamics such as the electro-hydraulic servo discussed within the paper, time constraints imposed by a dedicated microprocessor add to the restrictions of the sampling process in such a way that it may not be possible to guarantee the existence of a discrete compensator to satisfy the given specifications.