The frequency and intensity of rain attenuation affecting the communication between a satellite and an earth terminal is an important consideration in planning satellite links. The NASA Lewis Research Center Satellite Link Attenuation Model Program (LeRC-SLAM) provides a static and dynamic statistical assessment of the impact of rain attenuation on a communications link established between an earth terminal and a geosynchronous satellite. The program is designed for use in the specification, design and assessment of satellite links for any terminal location in the continental United States.
The basis for LeRC-SLAM is the ACTS Rain Attenuation Prediction Model, which uses a log-normal cumulative probability distribution to describe the random process of rain attenuation on satellite links. The derivation of the statistics for the rainrate process at the specified terminal location relies on long term rainfall records compiled by the U.S. Weather Service during time periods of up to 55 years in length. The theory of extreme value statistics is also utilized.
The user provides the following:
- the longitudinal position of the satellite in geosynchronous orbit
- the geographical position of the earth terminal in terms of latitude and longitude
- the height above sea level of the terminal site
- the yearly average rainfall at the terminal site
- the operating frequency of the communications link (within 1 to 1000 GHz, inclusive).
Based on the yearly average rainfall at the terminal location, LeRCSLAM calculates the relevant rain statistics for the site using an internal data base. The program then generates rain attenuation data for the satellite link. This data includes a description of the static (i.e., yearly) attenuation process, an evaluation of the cumulative probability distribution for attenuation effects, and an evaluation of the probability of fades below selected fade depths. In addition, LeRC-SLAM calculates the elevation and azimuth angles of the terminal antenna required to establish a link with the satellite, the statistical parameters that characterize the rainrate process at the terminal site, the length of the propagation path within the potential rain region, and its projected length onto the local horizontal.
LeRC-SLAM is available in two machine versions: a Macintosh version and an IBM PC version. The Macintosh version of LeRC-SLAM carries the NASA case number LEW-14977 and the IBM PC version of LeRC-SLAM carries the NASA case number LEW-14979. It was originally released as part of the COSMIC collection.