Integrated store-and-forward payload for nanosatellites

with amateur VHF transmitter

A.Addaim, A.Kherras

Centre of Space Research and Studies

Mohammadia School of Engineers, B.P 765, Rabat, Morocco

E.B.Zantou

Royal Centre of Space Research and Studies

Riad, Rabat, Morocco

The satellite, as small as the Cubesat concept, requires employment of compact and small boards. In the modular architecture, each single subsystem has a dedicated hardware and software. The approach that has been taken consists of the integration of the maximum subsystems within the same unit taking into account that single subsystems can be setup without modifying the operation of the remaining subsystems. This paper describes in details the design of integrated of store-and-forward APRS (Automatic Packet Reporting System) payload and OBDH (On Board Data Handling) subsystems using one chip based on the fixed point DSP (Digital Signal Processor) to design a low-cost Nanosatellite. In this paper, we consider the problem of designing a low-complexity AFSK and GMSK modems for a satellite communication link where the constraints on satellite power budget, antenna size, and data rate are such that the available transmit power, aboard the Nanosatellite, is 1 Watt. To cut down the cost of the Nanosatellite, we give a particular attention to software optimization due to use of small amount of 8K word PROM memory containing the application program. We have defined a methodology approach based on two design steps: algorithmic optimization, and optimal programming using assembler language. By an optimal software design that takes the maximum advantages from DSP architecture facilities, good real time experimental performances have been obtained for the APRS payload and OBDH implemented modules. Due to the environment in space and the constraints of the Nanosatellite, different measures had to be taken when the system was designed.

The mission of the Nanosatellite is to provide various APRS services, such as mobile localization of ships and data collection from autonomous weather stations in inaccessible sites using a store-and-forward payload. The APRS was developed by Bob Bruninga. The Cubesat project aims to send a satellite, with dimensions 10´10´10(cm) and mass one kilogram into Low Earth Orbit (LEO). The orbit, with altitude of 650 Km, will be achieved by means of low cost dedicated launch using Dnepr vehicle Launch. The Nanosatellite uses the AX.25 packet radio protocol, at 1200 bit/s for APRS payload, and at 4800 bit/s for Telemetry/Telecommand (TM/TC) operations.

As the satellite community transitions towards inexpensive distributed Nanosatellites, new methodologies need to be employed to replace traditional design techniques.љ This paper will contribute to the development of these cost saving methodologies. The goal of the integrated store-and-forward APRS Payload and OBDH subsystems design is to minimize component expenditures while still providing the reliability necessary for mission success.

Associating low cost ground terminals with a low cost Nanosatellite allows developing countries to access space communications with a very economical system. The present work, dealing with the design of the integrated store-and-forward APRS Payload and OBDH subsystems, shows hardware and software solutions adopted to cut down the system cost. The hardware utilizes commercial low cost components and the software is optimized using assembler language. The OBDH APRS unit is small device that can be mounted on any small satellite platform to serve applications such as mobile localization and data collection. By using a single Nanosatellite and low-cost communications equipments, store-and-forward systems can be kept at the extreme low end of the satellite communications cost spectrum.