A New Fast Acquisition Algorithm for GPS Receivers
Abstract
In many GPS applications, such as high dynamics vehicles or an appliance which uses sleep mode to save power consumption, a fast acquisition of signal is strongly required. In this paper, a new fast method to acquire GPS signal is proposed. The proposed methods find the candidates of Doppler shift using Squared-D algorithm, which is based upon a signal squaring method and dramatically reduces the search space of Doppler shift. This paper also proposes XMC (eXtended Multi-Correlator) to efficiently search C/A code. It is distinguished from the multi-correlator in that the combined replica of several codes is used. The expected performance of XMC method is equal to that of the multi-correlator while computational burden is reduced to 90%.
FFT Acquisition of Periodic, Aperiodic, Puncture, and Overlaid Code Sequences in GPS
Abstract
This paper presents a study of FFT-implemented circular correlation and its application to fast direct acquisition of GPS codes. This includes the periodic C/A-codes, practically non-periodic P(Y)-codes, never-repeating cryptographic M-code, and puncture acquisition (PA) codes which have been proposed to aid direct M acquisition as well as overlaid codes which are created by surface-reflected GPS signals extended beyond one code chip. FFT operates on blocks of incoming and replica code samples, thus providing simultaneous search over the entire block of code phases. It is straightforward to work with periodic codes for circular correlation. However, it is not obvious for puncture codes and long codes in particular. One major concern is how to ensure that the incoming and replica code samples contained within the working block could be correlated. In addition, it is quite possible that the data bit sign may reverse in the middle of an integration interval. Furthermore, how to efficiently make use of complex FFT when the data length is not a power of two or highly composite is critical for practical implementation. These design and computation issues are properly formulated in this paper and pertinent acquisition schemes are suggested.
Rapid Parallel GPS Signal Acquisition
Abstract
A GPS acquisition system which achieves extremely short
search times is presented. The system requires no prior knowledge of position or
time. Using a 50MHz clock frequency, 5 satellites can be searched in parallel.
Under nominal signal conditions, two-second search times have been demonstrated.
The acquisition unit performance has been verified both with live satellites as
well as with a GPS simulator. With the GPS simulator the performance with weaker
signal conditions was also evaluated. The acquisition threshold was found to be
−134 dBm with a RF noise figure of 4 dB. The developed acquisition system is
based on parallel
matched-filter implementation, which is capable of searching code offsets of a
number of satellites simultaneously. The frequency domain is searched by
sweeping a common local oscillator frequency. The search process is controlled
by a state machine, which has a number of parallel channels, each capable of
performing a search of its corresponding PRN code. This implementation is
considerably more effective than performing sequential search of each satellite
signal. The modest hardware complexity increase and the search speed improvement
compared to the single-satellite search unit makes the parallel architecture
attractive. The advances in current CMOS manufacturing technologies make fully
digital implementation of a matched filter based acquisition unit economically
possible even for low-cost GPS receivers. The acquisition system has been
implemented in 0.35 µm CMOS process. A sensor GPS receiver containing the
acquisition unit, 12 channel correlating receiver together with the DSP and SRAM
memory for running the tracking algorithm fits in a single chip and is packaged
in a 64-pin LQFP package.
Integration of GPS with a Rubidium Clock and a Barometer for Land Vehicle Navigation
Abstract
An integrated navigation system consisting of GPS with a rubidium clock and a barometer is developed and investigated for land vehicle navigation. In order to evaluate the performance of the integrated navigation system, a field test in an ideal environment with excellent satellite coverage and geometry was conducted. Post mission precise satellite orbits and clock corrections were used to reduce the effects of SA. The test results have shown that the integration of GPS with a rubidium clock and a barometer provides significant improvement in the vertical accuracy. A virtual wall satellite rejection algorithm was implemented to simulate urban areas, where GPS outages occur frequently. The integrated navigation system was proven capable to navigate using three or two satellites with acceptable accuracy for vehicular navigation.
Acquisition Schemes for Software GPS Receiver
Abstract
This paper discloses two new acquisition approaches to improve the efficiency of conventional acquisition methods used in GPS software receivers. The first single side band approach will reduce the process time and search time of the conventional approach by more than half. The second approach is an innovative real number implementation of the delay-and-multiply concept which was developed under complex numbers. Since this new approach uses real numbers instead of complex numbers, it reduces the processing time by more than half. Also, it makes the delay-and-multiply concept more feasible and practical, This paper also introduces a methodology to estimate the noise statistics and develop the threshold for detection. Based on this methodology, the performances of the two new approaches is analyzed, estimated by simulation, and compared against the performance of the conventional approach. The paper concludes with a demonstration of two new approaches with field collected data and recommends a potential application area where the new approaches can be applied.
A 12-Channel Real-Time GPS L1Software Receiver
Abstract
A GPS receiver has been developed that runs 12 tracking channels in real-time using a software correlator. This work is part of an effort to develop a flexible receiver that can use new GPS signals as they become available without the need for new correlator hardware. The receiver consists of an RF front-end, a system of shift registers, a digital data acquisition (DAQ) card, and software that runs on a 1.73 GHz PC. The commercial RF front-end down converts the signal into a 2-bit digital data stream at 5.714 MHz. The shift registers parallelize the magnitude and sign data bit streams into separate words, which the DAQ reads into the PC's memory using direct memory access. The PC performs base-band mixing and PRN code correlations in a manner that directly simulates a hardware digital correlator. It also performs the usual signal tracking and navigation functions, under the control of a real-time Linux operating system. The software correlator receives frequency commands for simulated carrier and code NCOs and, in effect, uses these to reconstruct carrier and code replicas which it mixes with the input data stream. The resulting signals are summed to produce the standard in-phase and quadrature, prompt and early-minus-late accumulations. These, along with the phases of the 2 NCOs, are sent back to the part of the code that executes the tracking loops and the navigation functions. The contributions of this work are a set of special high-speed algorithms for doing the correlations in software. They make use of bit-wise parallelism so that a single C-code command (partially) processes 32 samples at a time. This system has been tested using a roof-mounted antenna. When operating with 12 channels, the entire receiver uses less than 50% of the capacity of the 1.73 GHz processor and navigates to an accuracy of 10 meters.
Acquisition to Tracking and Coasting for Software GPS Receiver
Abstract
The goal the software GPS receiver is to do acquisition and
real-time tracking of GPS C/A codes in software
using a receiver front-end, an Analog-to-Digital converter (ADC) and a PC to do
the processing. To start tracking two parameters need to be obtained from the
acquisition process: the Doppler frequency of the signal, and the sample on
which the C/A code sequence for the satellite begins. Software based acquisition
normally uses 1 msec of data (5 msec for fine frequency resolution), but the
process can take 1 sec or more for weak signals. The C/A code position shifts
over time, and for stationary receivers there is a linear relationship between
the rate of this shift and the Doppler frequency of the signal. One way to deal
with this shift in the C/A code is to store all samples collected during the
acquisition process so as to start tracking at a point immediately after
acquisition, but this would be prohibitive in both processing time and storage
requirements. A better way is to develop a method to estimate on which sample
the C/A code sequence begins after a period of time. The linear relationship
between the Doppler frequency and the C/A code shift can be used to predict the
code position, and this method is affective for a period of up to 30 seconds
after acquisition. This linear relationship can also be used to allow C/A code
tracking to coast and immediately resume after a drop out due to interference or
jamming. Since A/D converters clocks often have small variances from their
specified sampling frequencies and this variance leads to a bias in the amount
of C/A code shifting relative to the Doppler frequency. Because of this, one
must test each A/D converter to find this bias.
Extended Multiple Correlator for GPS Receivers
Abstract
This paper tries to analyze a signal detection probability of a GPS receiver under more realistic environments. The result shows that lower detection probability can be obtained when the phase differences among the codes in the correlation arms of a multiple correlator are larger. Furthermore, this paper proposes a new correlation scheme, named XMC(eXtended Multiple Correlator), which reduces the rate of degradation for the detection probability. The proposed XMC is different from the general multiple correlator in that a combined form of the locally generated codes is used for despreading. The performance of the proposed XMC is shown by the evaluating the detection probability and the mean acquisition time in a general SNR environments.
