Interference and Regulatory Aspects of GNSS Pseudolites

Abstract

Galileo, the European Satellite Navigation System, is currently under development. Even before first satellites of the constellation are launched, Galileo signals will be provided through ground based Navigation Signal Generators for the investigation of signal performance and characteristics. These low power devices, called Pseudolites (Pseudo-Satellites), will transmit signals equivalent to those which are transmitted by the in-orbit satellites. However, from the regulatory point of view they are not providing Radionavigation Satellite Service (RNSS) as defined in International Tele communication Union (ITU) Radio Regulations but "something else". This has to be investigated, because it is expected that Pseudolites (PLs) will, beyond their roles to evaluate signal performances in the early phase of the program, significantly extend the navigation service availability into areas where the critical RF propagation of direct line of sight to satellites is blocked. Sound experience over many years has already been gained worldwide through the research with GPS Pseudolites. Galileo will introduce sophisticated and ambitious new signal schemes initiating new designs for innovative Pseudolite solutions. Old and new signals will coexist for many years to come. Currently there are various projects ongoing to develop Pseudolites for Galileo. A practicable regulatory framework, taking specific operational conditions of Pseudolites into account, has to be developed by the regulatory authorities to encourage the implementation of Pseudolite-networks on one side. But, at the same time, it is important to set strict rules for the implementation to avoid harmful interferences created by Pseudolites to RNSS and other radio receivers operated in the vicinity of a Pseudolite-network. The creation of a clear regulatory framework has eventually to provide the planning security for Pseudolite-network operators and RNSS- provider considering service guarantees. Pseudolites, as well as other means to achieve a nearly seamless service availability have been an essential element of the Galileo system architecture from its early system studies. In the Galileo architecture, PLs are defined as a sub-group of the so-called Local Elements. Technically speaking, Pseudolites are low power transmitters that either transmit or repeat (Synchrolites) RNSS-equivalent signals on the same frequency bands allocated to RNSS as defined in the ITU-R Radio Regulations. The creation of a regulatory framework for the operation of Pseudolites, which is yet undefined, has recently received a growing attention in the spectrum engineering working groups and frequency management groups of the European Conference of Postal and Telecommunications Administrations (CEPT). So far, PLs are operated under experimental license only. In order to prepare inputs to this process, the performance requirements in typical application scenarios have been investigated. This paper presents initial considerations and preliminary results of investigations performed on the interference properties of general GNSS Pseudolites. It proposes a concept for typical scenarios that can serve as generic Pseudolite network architectures to be considered in the on-going process to determine a regulatory framework for future operational networks.