GPS Wireless Standard

Publish Date: Mar 15, 2016 | 3 Ratings | 4.67 out of 5 | Print | Submit your review

Overview

The global positioning system (GPS) is a radio navigation standard created and maintained by the U.S. Department of Defense. It was originally designed for military applications, but it has been expanded for civilian use. This paper examines the use of GPS satellite signals for navigation.

This paper is part of the Wireless Standards White Paper Series.

Table of Contents

  1. GPS Signals
  2. Calculating Position
  3. Testing GPS
  4. Other Navigation Standards
  5. National Instruments Hardware Applicable to the Standard 

1. GPS Signals

Each GPS satellite transmits its signal on two frequency bands: L1 (1575.42 MHz) and L2 (1227.60 MHz). The L1 band contains standard positioning (SP) code available to all users and features a combination of the Course and Navigation (C/A) code (which is used as the signature code) and the Precision (P) code. The L2 band contains the modulated P code and can be used to measure ionospheric delays, which increase accuracy when precision position service is available. Modern commercial GPS signals feature L2 signals to take advantage of measuring ionospheric delays without needing the equipment required by the L2 band. 

GPS Data 

The GPS data signal is a series of data sets that are each 1500 bits long (frames) and are sent at 50 bits per second. After a total of 12.5 minutes, an entire data set has been sent. These frames are further divided into subframes (300 bits) and words (30 bits). Each subframe contains navigation information that is useful to the receiver for providing accurate location information. 

The satellite time information is a combination of the satellite transmission time and the data necessary for time correction. To ensure that the delay of each satellite’s signal is known, average ionospheric data is provided to give the receiver an approximate phase delay of the satellite signal at any location and time.

Each satellite also transmits ephemeris or precise orbital data. Received from the control stations, this data is updated every hour. Ephemeris data is valid for up to four hours without significant error. It is used to calculate the position of the satellite at a specific point in time. The almanac data on all of the orbiting satellites is useful for quick receiver startup time. This data is approximate orbital data for all GPS satellites. The difference between this data and the ephemeris information is the accuracy of the data. Note that you can simulate GPS signals using the NI GPS Simulation Toolkit for LabVIEW. For more information, see GPS Receiver Testing.

Signal Construction

Each satellite has a unique identifier (C/A code) that is made up of 1023 pseudorandom noise (PRN) bits that repeat every millisecond. This signature code signal modulates the data signal using an exclusive-or operation. The resulting signal is then modulated using binary phase shift keying (BPSK) to the L1 carrier. This signature code is used by the receiver to calculate the user’s position. 

SignalBlockDiagram.gif

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2. Calculating Position

Each GPS receiver generates the C/A for a specific satellite and correlates this data with the signal being received. When correlation between a particular satellite and the receiver is found, the delay of the signal is calculated (approximately 67 ms). You can multiply this time by the speed of light to determine the distance between the receiver and the satellite. 

Where tmeasured is the delay measured by the receiver, tactual is the actual signal transition time and terror is the error induced from imperfections in the clock. 

The atomic clocks onboard the satellites create precise and synchronized signals while the receiver contains a less accurate clock. Because there is a small error between the speed of the satellite clock and the receiver clock, the time delay measured by the receiver consists of the actual travel time plus the time error introduced by the receiver. This total time multiplied by the speed of light (c) is known as a pseudorange (PSR). Each pseudorange contains the actual distance from the user to the satellite plus some error introduced from the receiver clock. 

The user’s position (X, Y, and Z) can be found by finding the pseudorange for four satellites and solving the resulting four independent equations for X, Y, Z, and the time error. The position of the satellite is known from the recorded ephemeris and almanac data for each satellite that is within view of the receiver. 

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3. Testing GPS

Besides the error introduced from a nonideal clock in the receiver, other sources of error can be introduced that affect the precision of a GPS measurement. When testing GPS, several factors can affect the accuracy of a signal. 

Atmospheric Effects

You can assume that a GPS signal travels at the speed of light. Because the signals must travel through the atmosphere (ionosphere), their speed deviates and causes further time delays. The ionospheric data transmitted within the data code can account for about 70 ns of atmospheric delay, leaving about 10 m of residual error. Without accurate ionosphere information, the accuracy of position is reduced. 

Visibility

With reduced visibility, the power of the received GPS signal also can be reduced significantly. For the L1 band, the minimum signal power provided by the satellite at ground level is -130 dBm. Materials such as plastic can degrade the signal. 

Multipath Errors

Reflected signals from surfaces near the receiver can either interfere with or be mistaken for the signal that follows the straight line path from the satellite. Multipath is difficult to detect and sometimes hard to avoid. It usually results in about 0.5 m of reduced accuracy. 

Selective Availability

Selective availability is controlled from the monitoring stations on the ground. The C/A code is intentionally biased with a time-varying signal (very low frequency), reducing the correlation between the receiver generated C/A code and the received C/A code. By keeping the signal frequency low, the intentional error cannot be averaged out without averaging measurements for several hours. 

Human Error

Improper data sent from the control segment as well as receiver (both hardware and software errors) can result in position errors. These mistakes can decrease accuracy anywhere from several meters to hundreds of kilometers. 

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4. Other Navigation Standards

Currently both Russia and the European Union are developing alternatives to GPS. Both systems, GLONASS (the Russian standard) and GALILEO (the EU standard), are designed to complement the U.S. GPS standard but provide localized alternatives to the U.S.-dominated GPS. 

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5. National Instruments Hardware Applicable to the Standard 

National Instruments offers the NI PXI-5671 RF vector signal generator with digital upconversion, PXI-5661 RF vector signal analyzer with digital downconversion, and PXI-5690 RF preamplifier. You can use these devices together to record and later play back GPS signals in the laboratory or test environment. For more information on using NI GPS receiver test products, see GPS Receiver Testing.  

Case Studies

The Averna RF Signal Record and Playback System uses NI PXI modules and PXI stream-to-disk power to record RF signals (such as GPS). By using actual GPS signals, you can test GPS receivers for accuracy under nonideal conditions. 

Dynamic signal acquisition devices usually consist of high-channel-count monitoring systems that you can spread out by several hundred meters or more. Using GPS for time synchronization can provide an easy solution for this.

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