Measurements of atmospheric properties become progressively more difficult with altitude above the surface of the earth, and even surface measurements are difficult over the oceans. First balloons, then airplanes and rockets, were used to carry instruments aloft to make in-situ measurements. Now remote sensors, both ground-based and satellite-borne, are used to monitor the atmosphere. In this context, upper air means all of the troposphere above the first hundred meters or so and, in some cases, the stratosphere. There are many uncertainties associated with remote sensing, so there is a demand for in-situ sensors to verify remote measurements. In addition, the balloon- borne instrument package is relatively inexpensive. However, it should be noted that cost is a matter of perspective; a satellite with its instrumentation, ground station, etc. may be cost-effective when the mission is to make measurements all over the world with good space and time resolution, as synoptic meteorology demands. Upper air measurements of pressure, temperature, water vapor, and winds can be made using in-situ instrument packages (carried aloft by balloons, rockets, or airplanes) and by remote sensors. Remote sensors can be classified as active (energy emitters like radar or lidar) or passive (receiving only, like microwave radiometers), and by whether they “look” up from the ground or down from a satellite. Remote sensors are surveyed briefly before discussing in-situ instruments. Profiles of temperature, humidity, density, etc. can be estimated from satellites using multiple narrow-band radiometers. These are passive sensors that measure longwave radiation upwelling from the atmosphere. For example, temperature profiles can be estimated from satellites by measuring infrared radiation emitted by CO2 (bands around 5000 μm) and O2 (bands around 3.4μm and 15μm) in the atmosphere. Winds can be estimated from cloud movements or by using the Doppler frequency shift due to some component of the atmosphere being carried along with the wind. An active sensor (radar) is used to estimate precipitation and, if it is a Doppler radar, determine winds. The great advantage of satellite-borne instruments is that they can cover the whole earth with excellent spatial resolution.
A Heuristic Genetic Algorithm for Regional Targets’ Small Satellite Image Downlink Scheduling Problem