Otherwise, the relative phase of the locally generated PRN sequence and the oscillator frequency are adjusted to update the decision variable, and the above process is repeated.It then details a low-cost method for adding GPS capability to laptop PCs through a USB dongle with the MAX2769 universal GPS receiver.
Usb Gps Receiver For Laptops Software Techniques ForCommunication and navigation engineers are increasingly interested in using software techniques for global positioning systems (GPS).Usb Gps Receiver For Laptops Code GPS SignalsThanks to very-large-scale integration (VLSI) development, powerful CPUs and DSPs can now detect and decode GPS signals in real time using software.
Usb Gps Receiver For Laptops Update The DecisionThe resulting software-based GPS receivers offer considerable flexibility in modifying settings to accommodate new applications without redesigning hardware, choosing an IF frequency, or implementing future upgrades. With the MAX2769 GPS receiver RF front-end, a simple USB dongle or PCI Express (PCIe) Mini Card format can be used to add low-cost GPS capability to laptop PCs. The MAX2769 transfers raw recovered data to the PC host, which executes the baseband decoding in software, thus eliminating the cost of the baseband ASIC typically required in stand-alone GPS systems. In short, the MAX2769 enables designers to implement an adapter capable of handling GPS and Galileo signals using a single chip. This article provides an operational overview of GPS systems and, then, details Maxims software-based GPS receiver solution. A GPS system consists of 24 space satellites or space vehicles (each identified by a unique pseudorandom-noise code), a ground-control station, and user equipment (receivers). For civilian GPS and Galileo applications, the satellites communicate over the L1 band located at 1.57542GHz. A GPS receiver requires line-of-sight visibility of at least four satellites to establish a reliable position. The acquisition and tracking of signals is very complex because each one varies with time and receiver location. Figure 1. The signal generation block for the civilian GPS signal uses a spread-spectrum approach for sending the signal. The GPS system is actually a simple spread-spectrum communication system. Figure 1 shows the signal generation block for civilian applications. First, the 50bps navigation message is repeated 20 times to produce a 1000bps bit stream. The repeated signal is then spread by a unique coarseacquisition (CA) code with a length of 1023 chipsa chip is the rate at which the pseudorandom-noise (PRN) code is applied. The result is a baseband signal of 1.023Mcps. Hence, the 43dB processing gain (G) of the GPS system permits it to resolve a signal well below the thermal-noise level. Because the Gold code exhibits excellent auto- and cross-correlation properties, it is widely used in CDMA communication systems, such as WCDMA and cdma2000. The baseband signal is modulated with binary phase-shift keying (BPSK), and upconverted to the L1 band for transmission. Because GPS is a CDMA communications system, the receiver must synchronize the PRN code as a prerequisite to demodulating the data. Code synchronization is usually achieved in two steps: code acquisition for coarse-code alignment, and code-phase tracking for fine alignment. More explicitly, a GPS receiver must first determine whether it has line-of-sight visibility to certain satellites or not. As we know, each satellite is distinguished by a unique CA code. When the satellite is visible, acquisition determines the signals frequency and code phase, which in turn establishes the corresponding demodulation parameters. The received-signal frequency varies due to the Doppler effect, 7 which causes the frequency to deviate from its nominal value by 5kHz to 10kHz, depending on the speed of the satellite with respect to the receiver. In the receiver, the GPS signal is first downconverted to in-phase and quadrature (I and Q) components. A pair of IQ correlators then correlate the I and Q baseband signals with the locally generated PRN sequence. After integrating over the duration of one bit, the I-Q correlator outputs are summed to provide an output-decision variable. Whenever the decision variable exceeds a certain threshold value, the system assumes that the corresponding acquisition was successful and proceeds to tracking mode.
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