Fundamentals of GPS Receivers: A Hardware Approach
Format: PDF / Kindle (mobi) / ePub
Fundamentals of GPS receivers covers GPS receivers' theory and practice. The book begins with the basics of GPS receivers and moves onward to more advanced material. The book examines three types of GPS receiver implementations: first is the custom design by the author; second is an industry standard design, now part of the open source network; the third relates to the receiver designed by JPL /NASA. Each receiver is unique allowing the reader to see how each design solves the same problems. Chapters discuss carrier phase measurements and GPS time and frequency measurements. The overall text is measurement oriented as opposed to processing the measurements. With a focus on the fundamentals of measurements the reader will be building their intuition for the physical phenomenon at work.
complication. The value computed for tk is “biased” to a mid week value. This results in a simple compare and addition/subtraction operation. 62 4.4 4 Solving for SV Position GPS SV Orbit Description Figure 4.1 shows a single GPS SV in orbit around the earth. The angles, rates, and times indicated are the primary information needed to solve for the SV position. In addition to this information are second- and third-order terms. These terms are not shown in Fig. 4.1. These other terms are
conversion process. 9.1.2 Digital Sampling Creates IF@ 1.405 MHz The 4.039 MHz third IF is digitally sampled to create a fourth IF frequency at 1.405 MHz. This IF is a digital IF and does not have an analog output. In particular, it is a rather messy digital waveform at the output of the GP2015. The Digital output is available as a SIGN and MAGNITUDE bit. This approximates a sinewave as shown in Fig. 9.2. The low number of bits used to capture the GPS signal may seem like a crude
MHz, the spectrum is repeated at various spots in the frequency domain. Since no filtering is provided by the GP2015, almost all of these possible spectral locations are available to process by subsequent digital methods. For the GP2015/2021 chip set, the difference spectrum at 1.405 MHz is chosen for processing. Centered here is the GPS signal in its Uncorrelated, wideband form with a bandwidth set by the filter at 4.309 MHz (approx. 2 MHz). This is essentially the lower side band product from
implementation. It needs a computer to complete the control loops associated with C/A code Scan and track as well as Doppler Scan/Track, Data demodulation, etc. What the GP2021 provides the host computer is all the control points and data gathering registers to do Code Lock, Doppler Lock, SV clock regeneration, etc. in a completely digital fashion. Our discussion will focus on how the GP2021 obtains C/A code Lock, Doppler Lock, and 50 Hz Data demodulation. 9.2.1 Single-Channel Block Diagram
understand the lowpass approximation, examine Fig. 9.6. This figure shows the contents of the accumulator as a function of time for two sinewave inputs. For this example, assume that the sample rate is lower than actually used so as to show the “steps” in the accumulation and that the input resolution (in bits) is fairly high. In the first case, the sinewave has a frequency of 100 Hz. For convenience, the DUMP period is shown synchronous with the sinewave and starts at the beginning of the