NOAA KLM User's Guide
NOAA-P will contain an Advanced Data Collection System (A-DCS) instrument. Platforms(buoys, free-floating balloons and remote weather stations) collect relevant data and transmit them to the spacecraft via a PTT and Platform Messaging Transceiver (PMT) in UHF frequency. A-DCS uses Doppler information to enable the location of PTTs. The data are stored onboard the satellite for later transmission to the ground. A-DCS also includes a transmitter function to send stored messages to the PMTs, which have been uplinked via the Master Beacon.
The Argos A-DCS is a space-based, data telemetry system that provides a global means to locate and collect environmental data from fixed and moving, low-power transmitters; i.e., polar ice flows, ocean buoys, birds, mammals, etc. in near-real time (15 minutes to 3 hours). The Argos A-DCS transmits data for operational and research related environmental applications, e.g., meteorology, oceanography and protection of the environment, with the majority of users being government/non-profit agencies and researchers. Argos A-DCS customers are engaged in over 1,000 programs operating approximately 15,000 data collection platforms in 72 countries.
The environmental platforms transmit data to the A-DCS at a carrier frequency around 401.635 MHz, digital modulation format at 400 bps and a high data rate of 4800 bps. The A-DCS demodulates this signal and determines the carrier frequency and relative time of each transmission. This data is processed, formatted, and transferred directly to the ground and to the satellite for real-time or later transmission to the ground.
The receiver linearly converts the incoming signal to an intermediate frequency that is applied to the input of the search unit and to the equivalent of Data Recovery Units (DRUs). The search unit is basically a spectrum analyzer, which uses a Fast Fourier Transform to cover the 110 kHz operating frequency range. The receiver has a center frequency of 401.635 MHz and antenna polarization of RHCP.
The control unit sequentially scans the search unit channels. It makes a binary estimate of both the signal level and frequency. These two digital words are stored in the Control Unit and are used for the assignment of a DRU to a particular receiver output signal.
Processing software performs the following signal functions: acquisition of the carrier, signal demodulation, bit synchronization, frame synchronization, Doppler counting, decommutation; and a management software performs the formatting of the data.
The telemetry formatter interrogates the buffer in the equivalent DRUs. When the buffer is full, the encoder sends a command to shift the bits into memory. When the data transfer signal from the satellite is received by the encoder, it transfers the data out of the memory to the satellite.
The A-DCS instrument will be able to send messages to the user's mobile terminals through its UHF transmitter (465.9875 MHz bi-phase PM 200 or 400 bps). The user requests will be received at the Toulouse, France facilities of CLS, the CNES subsidiary in charge of the operations of the ARGOS system. These requests will indicate the identification of the destination terminal, the message to be transmitted at 400 bps (or 200 bps) and the time constraints (if any).
Taking into account the above and the status of the ARGOS system, the Downlink Message Management Center (DMMC) will prepare the uploading of the request to the instrument through one of the Master Beacons of the ARGOS system.
The best situated Master Beacon is selected by the DMMC and the message to be uplinked is sent to this Master Beacon through a terrestrial public network. This uplink message contains the information necessary to prepare the downlink message to be sent to the user terminal.
The uplink message is an ARGOS message, the content of which is analyzed upon reception by the instrument which in turn prepares the downlink message to be included in the downlink High-level Data Link Control bit stream transmitted by the UHF transmitter.
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