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Mission Data Downlinks
DMSP downlinks mission data stored on-board Stored Mission Data (SMD) once per orbit and has two real time data transmissions, the Real time Data Smooth (RDS) and Real Time Data fine (RTD). Onboard pre-processing of data by the OL-S sensor system provides for the various modes of data output. OL-S data consists of both visual or Light data (L data) and infrared or Thermal (T data) modes. Infrared Fine resolution data (TF data) can be collected continuously, day and night. Visible fine (LF data) is collected during daytime only. Fine resolution data has nominal linear resolution of 0.3 nm. Storage capacity and transmission constraints limit the quantity of fine resolution data (LF or TF) which can be provided in the SDF (Stored Data, Fine) mode. Data smoothing permits global coverage in both the infrared (TS) and visible (LS) spectrum to be stored on the primary tape recorders in the SDS (Stored Data Smoothed) mode. Smoothing is accomplished by electrically reducing the sensor resolution to 1.5 nm in the along scan direction, then digitally averaging five such 0.3 x 1.5 nm samples in the along track direction. A nominal linear resolution of 1.5 nm results. Additionally, a photomultiplier tube will allow collection of visible (LS) data under night-time conditions at 1.5 nm nominal linear resolution. The data rate is 1.3312 megabits per second for SDF (TF or LF) or 2.6624 megabits per second if SDS is interleaved bit-by-bit (TF/LF or TS/LS). The DMSP RTS Mux accepts either data rate and formats Equipment Status Telemetry data with the incoming stored data stream. This 3.072 megabits per second data stream is then transmitted via a DOMSAT communications satellite link to AFWA and FNMOC for processing. DMSP transmits two S-Band downlinks in real time: Real Time Data (RTD) link containing OL-S fine (TF/LS or LF/TS) data; and Real Time Smooth (RDS) data stream containing OL-S smooth data (not currently in use). The smooth data in the RTD mode has not been digitally smoothed, so that a smooth sample is 0.3 nm in the along track direction times 1.5 nm in the along scan direction. Both the RDS and RTD data streams include OL-S data as well as the special sensor data, though the RDS data rate is lower than RTD due to smoothing of the OL-S data. For the current F15 and below spacecraft, the RTD is transmitted at 2252.5MHZ with a data rate of 1.02 Mbps unencoded. If operational, RDS is transmitted at 2267.5 MHz with a data rate of 66 kbps unencoded (133 kbps encoded). There are also two back up frequencies for either RTD or RDS: 2207.5 and 2235.5MHZ. Beginning with F16, there is a dedicated S-band for RTD at 2222.5Mhz. The date rate of the RDS will increase to 88.75kbps unencoded (177.5 kbps encoded). 
All DMSP downlinks are encrypted. The MarkIV-B and SMQ-11 require KG44 decryption equipment with satellite specific chips for the RTD signal. The RSSs use s KG 144s for the RTD. The KG144 require a tape reader device and satellite specific cartridge tapes.DMSP downlinks mission and state-of-health telemetry data stored on-board once per orbit. Real-time telemetry is also being sent while the stored data is being down linked in a station circle. Additionally all spacecraft transmit real-time mission data continuously.The dedicated DOMSATS are being phased out and stored data will flow through the AFSCN cloud. Hawaii and the Enhanced AFSCN site now use the cloud. All enhanced AFSCN site are capable of receiving stored DMSP mission sensor data and telemetry. However, AFWA is not ready to use partial orbits so this capability is normally only used during early orbit. |