Architecture of the Air Force Satellite Control Network
- Sunday, 01 April 2012
An AFSCN LP was created that integrates several physics-based models of antennae patterns, thermal noise, and signal gains. SNR is largely dependent on the signal power from the transmitter.
SNR normally refers to the carrier power over the noise power spectral density. This value is important because it is needed to determine the Bit Error rate (BER) of the subcarriers. The subcarriers are what contain the data needed by the users. Certain types of data require that the BER not be above a certain threshold. Therefore, the SNR is important because it is directly linked to BER. By knowing the predicted BER or SNR of their respective links, the users then know, within a margin of error, what the performance of that link will be and when/how long they should schedule their AFSCN support and/or how much power to expend.
Each user requires telemetry, tracking, and commanding (TT&C) support from the AFSCN. The users are composed primarily of SOCs and external users supporting communication services, navigation, surveillance, reconnaissance, environmental/wea ther, research and development, and launch. Users must interface with the Network operations center (NOC) to request support from the AFSCN. The NOC is responsible for de-conflicting requests and disseminating the Network Tasking Order (NTO) to all of the users and Remote Tracking Stations (RTS), or ground stations. The NTO tells the network when each spacecraft will be supported at each RTS.
It is a common occurrence in the AFSCN that the users request more time than needed and the support is cut short. This results in wasted time on the network that could be used for another support.
There are link prediction products currently available. These products utilize the same functionality required by the AFSCN, but are not tailored specifically to it. The Dynamic Link Analysis (DLA) tool is a MATLAB-based tool that was designed to predict link performance during launches on the Eastern and Western Launch ranges operated by the Air Force. DLA GUI allows a user to select the space vehicle and earth station desired. This tool then predicts the performance of the link based on known parameters. These selections then translate into a predicted SNR. Most link analysis is static, which means it assumes constant performance throughout the contact based on worstcase performance.
The noise temperature is all of the power added to the carrier from environmental and manmade sources. This added noise makes it difficult for the receiver to distinguish between the noise and the desired signal. Noise comes from natural sources like the Earth and Sun. It is also radiated from the receiving equipment, which imparts additional gain but also additional noise.
The system noise temperature for downlink will vary over time for all spacecraft contacts. This will yield different system performance at each interval of the contact. This fluctuation in noise temperature will be less pronounced for geostationary or geosynchronous orbits because they remain more or less stationary with respect to the spacecraft. However, the noise temperature will vary greatly for low earth orbiting (LEO) orbits because of the system noise temperature’s dependence on elevation.
To determine the link geometry, Analytical Graphics’ STK space systems modeling application generates geometric arrays for each link. The three parameters used to predict the performance of each link are: elevation angle, degrees off boresight, and range. The ground stations are selected from the online database provided by STK, and generic spacecraft orbits were defined using STK’s orbit modeler. STK automatically generates time-based arrays of any orbital location parameter given a ground station location, spacecraft location and orbit, and support start and end times.