Direct Broadcast

Direct Broadcast

The polar orbiting meteorological satellites, operated by the CGMS agencies, offer users worldwide the opportunity to receive in real time the observations collected by the satellite instruments. To acquire the direct broadcast data, the user needs a local reception system, capable of tracking the satellite when in visibility of the station and processing the acquired data to data products for use by weather forecasters and Numerical Weather Prediction (NWP) applications.

Manufacturers and operators of direct broadcast reception stations for the polar orbiting satellites critically depend upon support from the satellite operating CGMS agencies. This includes the coordination and the provision of technical specification of the direct broadcast, orbit information, software packages for product processing, auxiliary operational data for instrument processing as well as operational coordination.

CGMS is coordinating this support, primarily through the Direct Broadcast Services: LRPT/AHRPT Global Specification and CGMS Agency Best Practices in support to Local and Regional Processing of LEO Direct Broadcast data.

The latter document covers 10 Best Practices in support to Local and Regional Processing of LEO Direct Broadcast data, summarised as follows:

Planning and system implementation:

• BP.01 Globl specification for direct broadcast
• BP.02 Timely provision of space-to-ground interface control documents
• BP.09 Satellite direct broadcast and reception station performance requirements
• BP.04 Povision and maintenance of product processing software

Acquisition and product processing operations:

• BP.03 Provision of current orbit information
• BP.05 Provision of auxiliary data for instrument product processing
• BP.06 Recommendations of channel selection for hyperspectral instruments

Satellite instrument status:

• BP.07 Spacecraft and instrument operational status
• BP.08 Operational announcements
• BP.10 Operational routine mointoring of the direct broadcast

The following operators of LEO satellites have reported their implementation of the Best Practices in CGMS Working Papers: NOAA (CGMS-48-NOAA-WP-03), CMA (CGMS-48-CMA-WP-04) and EUMETSAT (CGMS-48-EUM-WP-03) .

The following summarises key information extracted from the above three CGMS Working Papers.

Space-to-ground interface control document

The Space-to-Ground ICD defines the radio frequency and encoding characteristics of the satellite downlink, which is essential for the design of the direct broadcast reception station antenna, feed, demodulator and bit synchroniser.

The Space-to-Ground ICD furthermore defines the layout and content of the DB data stream, which is essential for defining the interface to the processing S/W packages processing the DB data stream to level 0.

Product processing software providers

Product processing software packages for processing DB data to level-0 and level-1 are a prerequisite for any local processing and in particular for the DBNet regional services. Product processing packages for further processing DB data to level-2 are highly valuable.

Provision of current orbit information

A direct broadcast reception station requires satellite orbit information for the following three functions:

• Prediction and planning of future satellite passes;
• Antenna pointing during the acquisition of satellite data;
• Processing and geolocation of the sensor data.

For a direct broadcast reception station to have the best possible orbit information available at any time, without needing information about when manoeuvres are executed, the reception station will typically be configured to retrieve the orbit information from the satellite operator every few hours.

Auxiliary data for instrument product processing

Some product processing algorithms require the specifications of instruments and their performance, including instrument spectral response functions (SRFs), noise and FOV size. Some instruments (e.g. IASI) require auxiliary configuration data for the processing. Other instruments may require regular updates of instrument calibration data. See also WMO Guidelines on Best Practices for Achieving User Readiness for New Meteorological Satellites, WMO-No. 1187