| GPS Receiver RF Front-End Enables Use of a Laptop PC for Soft Baseband Processing |
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| Aug 31 2007 | |
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advertisement: Before examining the design of the USB or PCIe hardware, let’s review a little of the basic theory for software GPS receivers. For the decoding of navigation messages and position calculations, the interested reader can refer to James Bao-Yen Tsui’s book, “Fundamentals of Global Positioning System Receivers: A Software Approach.”3
GPS Basics A GPS system consists of 24 space satellites or space vehicles (each identified by a unique PRN code), a ground-control station, and user equipment (receivers). For civilian GPS applications, the satellites communicate over the L1 band located at 1.57542 GHz. A GPS receiver requires line-of-sight “visibility” of at least four satellites to establish a reliable position. Acquisition and tracking of the signals is very complex, because each one varies with time as well as receiver location. Traditional GPS receivers implement acquisition, tracking, and bit-synchronization operations in an ASIC, but a software GPS receiver provides flexibility by implementing those blocks in software rather than hardware. By simplifying the hardware architecture, software makes the receiver smaller, cheaper, and more power-efficient. You can write the software in C/C++, MATLAB®, and other languages, and port it into all operating systems (embedded OS, PC, Linux, and DSP platforms). Thus, software GPS receivers offer the greatest flexibility for mobile handsets, PDAs, and similar applications.  Figure 2. Typical adapter configurations for theUSB dongle (a), and the PCIe minicard (b) showthe simple, low bill of materials design for eitheroption. The USB dongle solution can be simple, with just the MAX2741, a counter, and the USB interface controller needed to capture the signal convert it to digital and send it to the host PC. Software running on the host PC then performs all the baseband functions and displays the location on the PC screen. This article will only examine the civilian GPS signal on the well-known L1 band located at 1.57542 GHz. The GPS system is actually a simple spreadspectrum communication system4. First, the 50-bps navigation message is repeated 20 times to produce a 1,000- bps bit stream. The repeated signal is then spread by a unique C/A code with a length of 1,023 chips (a chip is the rate at which the pseudorandom noise code is applied). The result is a baseband signal of 1.023 megabits per second (Mbps). Hence, the 43-dB processing gain (G) of the GPS system permits it to resolve a signal well below the thermal noise level. |
