A noise insensitive solution to an ambiguity problem in spectralestimation

The instantaneous frequency measurement (IFM) receiver is capable of measuring the center frequency of single frequency pulses over a wide range (bandwidth) of center frequencies. Because of various constraints, the frequency resolution requirement results in long correlator delay times that reduce the single correlator bandwidth. A large bandwidth can be achieved only if two or more correlators are used. The problem of estimating frequency is then reduced to the simultaneous congruence problem of number theory. A design procedure is presented for solving the congruence problem for a given amount of noise protection, a stated frequency resolution, a minimum bandwidth, and a fixed level of precision (bits) in the IFM receiver     

Validation of aerosol measurements by the satellite sensors SAM II and SAGE

The satellite sensors SAM II and SAGE have been developing a global data base on stratospheric aerosols since they were launched in October 1978 and February 1979, respectively. The validity of this data base has been tested by numerous comparisons with other measurements made by lidars, balloon-borne particle counters, and aircraft-borne impactors and filters. Because the satellite sensors measure extinction and the correlative sensors measure other properties (e.g., backscatter, number, mass), special techniques are required to convert each measured property to other properties and to quantify conversion uncertainties and measurement uncertainties. Use of these techniques in two major comparative experiments shows that the SAM II and SAGE extinction measurements agree with each other and with values derived from dustsonde, lidar, and filter measurements. In addition, the comparative experiments have highlighted the uncertainties of each type of sensor and stimulated further efforts to reduce these uncertainties.     

Aerosol measurements from earth orbiting spacecraft

Since the fall of 1978, two Earth-orbiting spacecraft sensors, SAM II, for Stratospheric Aerosol Measurement II, and SAGE, for Stratospheric Aerosol and Gas Experiment have been monitoring the global stratospheric aerosol. These experiments use the Sun as a source to make Earth-limb extinction measurements during each spacecraft sunrise and sunset. This paper describes the global aerosol data base (climatology) that is evolving. Seasonal and hemispheric variations such as the springtime layer expansion with warming temperatures and the local wintertime polar stratospheric clouds (PSC's) will be described. The PSC's enhance extinction by up to two orders of magnitude and optical depths by as much as an order of magnitude over the background 1000 nm values of about 1.2 × 10^?4 km^?1 and 1.3 × 10^?3, respectively. The detection and tracking of a number of volcanoes whose effluents penetrated the tropopause are also described. The mass of new aerosol injected into the stratosphere from each volcano is estimated. The May 1980 eruption of Mount St. Helens, for example, produced about 0.32 × 10⁹ kg of new stratospheric aerosol enhancing the Northern Hemispheric aerosol by more than 100 percent.     

Resolution of a 2π ambiguity problem in multiple frequencyspectral estimation

An algorithm is proposed to resolve a fundamental 2π ambiguity problem occurring in multiple frequency spectral estimation. Given M frequencies f_m, and I separate frequency estimators with unambiguous bandwidths F_i, the ambiguity problem can be stated as solving for the fm, given the estimator outputs, α_mi, (1⩽m⩽M;1⩽i⩽I) where f_m=α_mi+K_miF_i and K_mi is some integer. The proposed algorithm exhaustively resolves all possible α_mi groupings into single frequency values using a noise insensitive technique that exchanges system bandwidth for noise protection. The correct multiple frequencies are then defined as the single frequencies that repeat a specified number of times. A complete analysis is included     

Aerosol observations for climate studies

The SAM II and SAGE satellite systems have provided to date more than 5 years and almost 3 years, respectively, of data on atmospheric aerosol profiles on a near-global scale. Studies with these unique data sets are developing a global aerosol climatology for the first time and have shown the existence and quantification of polar stratospheric clouds (PSC's) and tropical stratospheric cirrus. In addition, a tropospheric cirrus climatology is evolving. Since these two experiments were launched, a series of large volcanic eruptions have occurred which have greatly impacted the stratospheric aerosol loading. The aerosol layer produced by the eruption of El Chichon, for example, increased the 30 mb temperatures in the northern tropics by as much as 4°C for 6 months after the eruption. This paper will describe in detail, from a climate perspective, the evolving aerosol and cloud climatologies as a function of space and time, and show the stratospheric dynamics of volcanic injections and their enhancements on stratospheric optical depth and mass loading.     

Satellite stratospheric aerosol measurement validation

The validity of the stratospheric aerosol measurements made by the satellite sensors SAM II and SAGE has been tested by comparing their results with each other and with results obtained by other techniques (lidar, dustsonde, filter, impactor). The latter type of comparison has required the development of special techniques that (1) convert the quantity measured by the correlative sensor (e.g. particle backscatter, number, or mass) to that measured by the satellite sensor (extinction), and (2) quantitatively estimate the uncertainty in the conversion process. The results of both types of comparisons show agreement within the measurement and conversion uncertainties. Moreover, the satellite uncertainty is small compared to aerosol natural variability (caused by seasonal changes, volcanoes, sudden warmings, vortex structure, etc.). Hence, we conclude that the satellite measurements are valid.