Frequency Stability Requirements for Narrow Band Receivers

Very-narrow-band Global Positioning System (GPS) receivers have recently been proposed in order to help mitigate the eflects of scintillation. This paper will revisit the stability requirements placed on the local oscillator of such receivers. INTRODUCTION The need to have good local oscillator shortand medium-term stability for phase tracking receivers has been well documented, especially for space communications [l]. Recently there has been great interest in improving the tolerance of WAAS/GPS Reference Station receivers to the effects of ionospheric scintillation in order to improve their robustness. One means that has been suggested to accomplish this is to narrow the bandwidth of the receiver. Morrissey et al. [2] undertook a study to quantify scintillation effects, through simulation, on a WAAS reference receiver in order to determine optimum estimates for certain key parameters, such as loop bandwidth, discriminator type, and tracking loop order that would maximize performance of the receiver. However, one parameter that was not varied or simulated in their study was local oscillator stability. Obviously, this was because an atomic frequency standard, similar to the one used at the WAAS Reference Stations, was used as the local oscillator for the receiver being tested. But it must be kept in mind that the short-term performance of a local oscillator plays a more significant role when estimating the performance of a narrow bandwidth receiver than long-term performance. It should also be kept in mind that the short-term performance of an atomic frequency standard depends solely on the performance of the crystal oscillator selected by the manufacturer of the atomic frequency standard. The short-term performance of an