- 19 April 2016
Any GNSS signal is a compromise between the needs of different users, and since GPS C/A code, other features have been added to the newer signals to meet these needs. However the advances mean energy split over two codes, over two lobes by BOC, faster modulation (i.e. shorter symbols), longer spreading codes... all of which make acquisition of the signal, i.e. finding initial lock in time and frequency domain, much more difficult, both intrinsically (more possibilities), and by defeating the high sensitivity algorithms developed for acquisition in the early 2000’s. The invention is to transmit two copies of the identical data symbols, one on each code, rather than data and a pilot signal with a secondary code symbol. The second copy of the data symbol is transmitted one symbol delayed. Thus in the receiver for acquisition purposes, the algorithm delays the first copy, so that again they are aligned for mathematical combination, the algorithm eliminating the data symbols during acquisition, allowing long-term integration of the energy on both codes for high sensitivity. In addition, the combination of the two samples separated by a delay allows the algorithm to measure the carrier frequency sensitively during acquisition. After acquisition, the mathematical algorithm is changed to preserve the databits, adding the energy and integrating only over one symbol, as required for demodulation of the data.
The main application areas are GNSS receivers required to operate with very high sensitivity, weak signals and compromised locations with good unassisted TTFF and high availability. While the idea also works for communications systems, its advantage is sensitive acquisition, rather than tracking only, so is particularly suitable for the consumer GNSS receiver.
Innovative aspects and main advantages
The concept of combining the two signal components has been proposed before, but what is new is allowing it to be done coherently because they both have the same data symbols (as proposed in an earlier patent) and now the same data symbols but with one symbol delay, which allows simultaneous frequency detection, removal, and long integration for sensitivity. As the data-symbol is known from the first component, it can be stripped from the second component without being a secondary code, leaving a pilot stream for phase locked loops when signal is clean and strong. When the signal is weak or distorted, the continuous, integrated frequency measure is still available.