Some considerations

o c to b e r 20 0 6 www.insidegnss.com the term “Correlator” is often used in discussions of GPS and GNSS receiver design. It has been used to describe devices as simple as a single exclusive OR gate through to complete “baseband” chips that include a microprocessor. Most usually, and in this article, the term describes the hardware or software that produces all of the required correlation data for a single signal from a specific GNSS satellite signal. This is also sometimes termed a “channel.” With the open GPS civil signal at the L2 frequency (L2C) now becoming available on Block IIR-M satellites, receiver designers have the opportunity to work with a markedly different GNSS signal resource. The first IIR-M spacecraft (designated SVN53/PRN17) was launched September 25, 2005, and the second is scheduled to go into orbit on September 14, 2006. (IIR-Ms also transmit the new GPS L1/L2 military (M-code) signal, but we will not treat this issue here.) Against that historic backdrop, then, this article examines some of the novel elements of the L2C signal and its implications for GNSS receiver correlators. Our focus will be on a technically challenging aspect of receiver operation: initial acquisition of the signal and its processing by the correlator. But first we will review some of the key functional aspects of GNSS correlators and some of the signal parameters that affect their operation. correlators’ essential role The GPS L1 signal is a code-divisionmultiple-access (CDMA) signal. Each of the signals transmitted by different satellites in the L1 band has a distinct pseudorandom noise (PRN) code, which is a 1,023-chip sequence of 1s and 0s. (It’s called “chip” rather than “bit” because the code sequence carries no information) These chips are BPSK-modulated (where BPSK is binary phase-shift key in which only 0° and 180° phases are used); so, rather than 1s and 0s, the code could be considered to be 1s and –1s “multiplied” by the 50-bps data signal. The code rate is 1.023 Mchip/sec, which means that the very narrowband With the launch of the first modernized GPS block IIr satellite in September 2006, GNSS product designers have an additional, fully open signal to use in their work: L2c. but the new signal also has different parameters than the L1 c/Acode signal available for the last 30 years. What does that mean to the people who are designing the correlators for new generations of GNSS receivers? Lockheed Martin correlators for L2c Some considerations