Space Communications

This paper describes a research programme carried out by the Institute of Satellite Navigation (ISN) and Racal Survey Ltd. to examine the performance of differential positioning using both the GPS and GLONASS satellite systems for vehicle positioning. The work involved developing a differential GNSS positioning system for a vehicle using GNSS receivers developed by the ISN and Ashtech and using Racal's LandStar regional differential system. The programme has involved extensive real time vehicle positioning trials in urban areas. Two aspects of vehicle positioning performance have been concentrated on, (i) fix density, and (ii) positioning accuracy.

The results show that combining GPS and GLONASS significantly improves both stand alone and differential positioning accuracy compared to using just GPS alone. The results also show that combined GNSS supports a greater differential correction age without the same accuracy degradation that GPS exhibits. This, in turn, increases the GNSS fix density. Even with the same correction age, GNSS has a better fix density by about 10%. This increase in fix density will allow improved vehicle positioning in an in‐car navigation system with a lower level of augmentation.

In summary, these results have shown that GNSS has significant advantages over GPS as the satellite positioning component of a vehicle positioning system.

JSAT is presently operating four satellites. JCSAT‐2, JCSAT‐3, JCSAT‐4 and JCSAT‐5 at orbital slots 154, 128, 124 and 150 deg East, respectively. JCSAT‐2 and 5 provide domestic and international service with Ku‐band transponders. JCSAT‐3 and 4 also provide similar service using Ku and C‐band transponders with various interconnect capabilities.

In addition to the existing four satellites, JCSAT‐6 is under construction at Hughes which will be launched in July this year. With this fleet of satellites, JSAT plans to frame the information infrastructure in the digital communication era for domestic and international usage in the Asia Pacific Region.

For microsatellites, a versatile power supply sub‐system is of paramount important. To maximize the power generated by the solar generators (SG), the Maximum Power Point Tracking (MPPT) technique is considered as a strong competitor to the conventional fixed shunt Pulse Width Modulation (PWM) technique. The paper presents an implementation based on a single power probing sensor associated with a micro‐controller. The laboratory test shows that this MPPT controller functions correctly with few components. It is capable of controlling both SG Maximum Power Operation during battery charging period and Reduced Power Operation between battery charges. It is also possible to use the MPPT controller as a local telemetry controller.