P. Vijay Kumar
Global Navigation Satellite Systems (GNSS) employ periodic sequences having low correlation, termed as spreading codes, to carry out the satellite acquisition and ranging tasks necessary to provide accurate positioning, time, and velocity information to the user. The spreading codes employed have period related to the frequency, 10.23 MHz, of the onboard atomic clock. In particular, operations in the L1 frequency band of two major GNSS systems, the Global Positioning System (GPS) and BeiDou Navigation Satellite System (BDS) employ spreading code having length 10230. In these two systems, the length 10230 is achieved by padding and truncating, respectively, a family of Weil sequences having period that is a prime number, either 10223 or 10243. As is well known, either truncation or padding leads, in general, to a degradation in correlation performance.
At IISc, in collaboration with ISRO scientists Dileep Dharmappa of ISTRAC, ISRO, and Sugandh Mishra, SAC, ISRO, we adopted a different approach and interleaved 5, Z4-linear sequences of period 2046 to achieve period 10230. The resultant design is termed as the Interleaved Z4-linear (IZ4) family of spreading codes. In terms of performance evaluation, the balance and even-correlation performance of the IZ4 family equals or improves upon the corresponding performance of the GPS and BDS signal sets. In particular, the new IZ4 family has maximum even cross-correlation value that is better by 4.4 dB than that of the truncated or padded Weil sequences employed in these GPS and BDS systems. The sequence family also turns in comparable odd-correlation performance. The sequences can be generated using a simple, shift-register-based implementation. A joint IISc-ISRO Indian patent as well as an international PCT application on the novel IZ4 design have been filed.
ISRO will incorporate the IZ4 family of spreading codes in the new civilian L1 signal of NavIC (Navigation with Indian Constellation).
Acknowledgement: This work could not have been accomplished without the strong encouragement and support contributed by senior administration and faculty within IISc and senior administration and scientists within ISRO.