Callisto | Research & Development: Spread Spectrum Techniques

Callisto has an established history of projects involving spread spectrum techniques and CDMA. The following provides a summary of Callisto’s experience in this area of technology.

Feasibility study into combining CDMA data communications over an FDMA service. The FDMA service is an existing communication diag2 channel on the European METEOSAT satellite. The channel allows earth based unmanned Data Collection Platforms which collect environmental data to transmit the data via a transponder on the geostationary satellite to the central processing facility. The study involved simulation of the communications channel with FDMA and CDMA transmissions using MatLab Simulink™. A particularity of the METEOSAT series satellites is that they are spin-stabilised, and the communications antennas are electrically de-spun, which give rise to amplitude and phase ripple on the communications links. The simulation model included elements to simulate the satellite antenna effects.

GALILEO TTC Modem.The Galileo System is considered a strategic asset for Europe and the space-to-ground interfaces need to be protected against intentional and unintentional interference. diag3 For that reason the Galileo Programme decided on the need to develop on-board and ground equipment for the TTC services, which would use spread spectrum techniques to assure communication security. In addition to the normal TTC services (telemetry, telecommand, two-way ranging), there was an additional requirement for Integrity up-links and two-way time transfer, which had to use spread spectrum communications also. ESA launched a pre-development of the Galileo secure TT&C communication system in 2000. Under a consultancy contract to ESA, Callisto supported the definition and specification of the Ground Modem equipment for the Galileo Secure TTC system. The Callisto consultant played a lead role in the preparation of the equipment specifications, which were released under an ITT to European Industry in 2002.

Noise Temperature Measuring System. During this period Callisto undertook a study for ESA into a method for continuously measuring the ground station receive noise temperature by means of a test loop. The test loop used a spread spectrum signal to ensure low interference between the test signal and operational signals. The study included full theoretical analysis of spread spectrum communications as applied to the method of the measurement signal. This was followed by the specification, design and implementation of a prototype system. The prototype design has been presented in papers at AP2000 – Millennium Conference on Antennas & Propagation, AMTA 2001 and at the 2nd ESA Workshop on Tracking and Telemetry (TTC 2001). In 2000 Callisto received an order from ESA for a pre-production Noise Temperature Measurements System for deployment in the ESA Perth TTC station.

Technical study into the possible use of CDMA techniques for the METEOSAT Data Collection Platform system. Data Collection Platforms are Earth based, unmanned platforms which collect environmental data and periodically transmit the data via a transponder on the Meteosat satellite to the central processing facility. The DCP transmission scheme uses FDMA as means to allow multiple Users to access the transponder. Callisto proposed, for study, a direct sequence spread spectrum code division multiplexing scheme (CDMA), with improved traffic capacity and improved interference immunity over the current method. A significant problem with the DCP service, which operates in uplink at 402 MHz, is the large level of interference from terrestrial communications services. Spread spectrum communications have a built-in advantage in interference environments. The study results were presented at the EUMETSAT Data Collection System User's Conference in Athens in 1993.