Investigation of Pulsing Schemes for Pseudolite Applications

Pseudo-satellites, or pseudolites, are ground-based transmitters of GNSS-like signals. Pseudolites have found applications in a variety of scenarios where GNSS signals are either unavailable for insufficient. Recently interest in pseudolites has been growing again due to interest in incorporation of pseudolite technology in the Galileo programme. While ground based transmitters have great potential for improving navigation capability in GNSS denied or degraded environments, they are not without their own problems and limitations. One of the most significant challenges to be overcome is that of the near/far problem. Simply put, satellites are far away, while ground based transmitters can be very near. A pseudolite-enabled receiver must, therefore, be capable of handling signals with an extremely large dynamic range. Most existing GNSS receivers take advantage of the limited dynamic range of open sky GNSS signals to greatly simplify the front-end and analog-to-digital conversion (ADC) components. Thus, so called non-participating receivers are likely to be completely jammed by pseudolites, once the receiver is within a certain minimum range of the transmitter. Similarly, even for pseudolite-only receivers a significant dynamic range may be required, greatly increasing receiver complexity. Pulsing of pseudolite signals has been the most effective method to overcome the near/far problem. By transmitting low-duty cycle pulses rather than continuous signals, pseudolites can limit their impact on both participating and non-participating receivers. Pulsing takes advantage of the fact that GNSS receivers average the received signal over at least one millisecond. Pulsing involves multiplying the pseudolite signal by a pulse train, which, without careful consideration, can result in significant modification of the spectrum of the transmitted signal. This, in turn, can affect navigation performance and can even, in extreme cases, result in an inability to track the pseudolite signals. In this paper a family of pulsing schemes are investigated based on the construction of sequences with low Hamming correlation. The Hamming correlation of two sequences of length L over an alphabet A is defined as the number of positions in which the elements of the two sequences are identical. For modulation schemes where all the elements of the alphabet A are modulated onto mutually orthogonal signals, the Hamming correlation is a suitable metric for the cross-correlation between two sequences. A low Hamming auto-correlation assures good spectral properties of the sequences, while low Hamming cross-correlation facilitates multiple access. This technique has been widely applied in frequency hopping systems. The focus of this work is on pulsing schemes for the Galileo E1-OS signals, with an emphasis on indoor applications, or indeed any applications where multi-access is an issue. The longer code duration, shorter data bit duration and CBOC sub-carrier of the Galileo signals introduce new opportunities and challenges for pulse sequence design. The impact of the pulsing scheme will be evaluated on a real Galileo E1-OS capable receiver using a simulator and a combined digital record and playback system. The primary emphasis of the analysis will be on the tracking of the pseudolite signals themselves. The impact on non-participating receivers, while clearly of great importance, is fundamentally a function of the AGC and ADC in the receiver front-end, which analysis is outside the scope of this work. Future work will evaluate the combined impact of the receiver front-end and the pseudolite pulsing scheme.

O'DRISCOLL Cillian;  BORIO Daniele;  FORTUNY GUASCH Joaquim; 

2012-06-05

Institute of Navigation

JRC65165

http://www.ion.org/search/view_abstract.cfm?jp=p&idno=9902,    https://publications.jrc.ec.europa.eu/repository/handle/JRC65165,