ON-BOARD PROCESSING (OBP)

Classes of OBP

Prior studies cite two main clas ses of OBP, baseband and carrier. This author believes the subject can be more adequately discussed with three classes: baseband, carrier, and support (Gagliardi 1991, pp. 367-404).

The three main classes of OBP are:

  1. Bas eband processing and switching (routing) -- two subclasses: autonomous and ground-controlled,
  2. IF or RF switching (frequency or time domains), and
  3. Support processing.

OBP can provide greatly increased efficiency and performance in communications satellites with trade-offs in increased cost and complexity. The increased efficiency can be used for significant mass reduction or for increased capacity. With the current trends toward decreased launch costs/unit mass, the increased capacity appears to be the logical benefit of choice. However, with the current rapid deployment of mobile service satellites (MSS) with LEO and multiple satellites (tens of satellite s) in a network, the choice may tend toward lighter, low-cost designs.

The capability to develop, manufacture, and deploy a viable full OBS/OBP satellite depends on much more than the on-board controller and the associated electronic switch. A princi pal objective of OBP is to implement mesh networks. Mesh networks can best be implemented using digital baseband signals, electronically scanned (or agile) antennas, ISL, Ku- or Ka-band receivers and transmitters, digital modulation and coding, and multi ple access techniques. Many electrical or photonic device technologies represent significant capabilities toward implementation of full OBP.

Satellite-switched networking can be implemented via two primary approaches: (1) fully processed by the satel lite, and (2) support by terrestrial control. Existing commercial satellite systems such as ITALSAT and the soon-to-be-launched NASA ACTS rely on terrestrial control of satellite implemented on-board switching. This approach greatly decreases complexity and therefore increases reliability of the spacecraft. However, response time and throughput efficiency are compromised.

Class 1: Baseband Processing and Switching. Baseband processing and switching involves the demodulation and demultiplexing of the received signal, performing error detection and correction, removing routing and control information (if not transmitted in a common channel signalling mode), routing the data, pointing directed antennas, buffering the data, multiplexing the data, tra nsmitting the data. The data could be of three types: circuit switched, message switched, or packet switched. Required technologies include multiple beam antennas, signal processing, microprocessors, time and/or space switches, ISLs, protocol processor s, and stored- program switches. LEO systems require sophisticated position and pointing capabilities, satellite-to-satellite handover control, and beam-to-beam handover control.

Class 2: RF or IF On-Board Switching. On-board RF/IF switchi ng involves electronically controlled RF/IF switches which can be reconfigured on a near-real- time basis via ground control. OBP for carrier switching has become fairly common in recent years, the INTELSAT spacecraft being the common example. On-boards ignal regeneration (demod-remod) is also now being used fairly frequently to gain the signal to noise (thus low BERs). Baseband processing with message and packet switching is much less common and is generally used for special-purpose spacecraft only. H owever, with the rapidly increasing speed, power, and reliability of microprocessors, the more significant baseband processing and switching is expected to move forward rapidly.

Class 3: Support Function On-Board Switching. On-board support processi ng encompasses several functional areas. They include control of waveguide switching parameters, ephemeris calculations for small beamwidth, electronically scanned antennas, communications network protocol processing, special processing for such function s as handover for LEO spacecraft, error detection and correction, and elastic buffering and control.


Published: July 1993; WTEC Hyper- Librarian