GPS Wireless Standard

The Global Position System (GPS) is a US Department of Defense created and maintained radio navigation standard. It was originally designed for military applications, but has been expanded to be used in civilian applications as well. This paper will outline the use of the GPS satellite signals for navigation purposes.

This paper is part of the Wireless Standards White Paper Series

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 that is available to all users.  The L1 band contains 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 will increase accuracy when precision position service is available.  Modern commercial GPS signals have the availability of the L2 signal to take advantage of measuring ionosphere delays without needing the equipment required by the L2 band. 

GPS Data 

The GPS data signal is a series of sets of data that are each 1500 bits long (frames) that 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 into 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.  In order 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.  This data is received from the control stations and 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 is the accuracy of the data.   Note that GPS signals can be simulated using the NI GPS toolkit for LabVIEW.  For more information, see: GPS Receiver Test.

Signal Construction

Each satellite has a unique identifier (C/A code) that is made up of 1023 pseudorandom noise (PRN) bits that repeats 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. 

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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).  This time can be multiplied by the speed of light to determine the distance between the receiver and the satellite. 

Where t_measured is the delay measured by the receiver, t_actual is the actual signal transition time, and t_error 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.  Since 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 will contain 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 psuedorange 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. 

Testing GPS

Besides the error introduced from a non-ideal clock in the receiver, other sources of error can be introduced that will affect the precision of a GPS measurement.  When testing GPS, there are several factors that can affect the accuracy of a signal. 

Atmospheric Effects

A GPS signal can be assumed to travel at the speed of light.  Since the signals must travel through the atmosphere (ionosphere), their speed will deviate and cause further time delays.  The ionospheric data transmitted within the data code can account for about 70 ns of atmospheric delay leaving about 10 meters of residual error.  Without accurate ionosphere information, the accuracy of position will be reduced. 

Visibility

With reduced visibility, the power of the received GPS signal 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 sometime hard to avoid and 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) mistakes can result in position errors.  These mistakes can decrease accuracy anywhere from several meters to hundreds of kilometers. 

Other Navigation Standards

Currently both Russia and the European Union are in development of alternatives to GPS.  Both systems, GLONASS (the Russian standard) and GALILEO (the EU standard) are designed to be complimentary to the US GPS standard, but provide localized alternatives to US dominated GPS. 

National Instruments Hardware Applicable to the Standard 

National Instruments currently provides PXI-5671 RF Vector Signal Generator with Digital Upconversion, PXI-5661 RF Vector Signal Analyzer with Digital Downconversion, PXI-5690 RF PreAmplifier.  These devices can be used in conjunction to record and later playback GPS signals in the laboratory or test environment.  For more information on using NI products for GPS receiver test, please see GPS Receiver Test.  

Customer Solutions

The Averna RF Signal Record & Playback System currently uses NI PXI devices and the power of PXI to stream to disk to record RF signals (such as GPS).  By using actual GPS signals, GPS receivers can be tested for accuracy under non-ideal conditions. 

Dynamic Signal Acquisition devices usually consist of large channel count monitoring systems that can be spread out by several hundred meters or more.  Using GPS for time synchronization can provide an easy solution for this.  For more information on remote synchronization see GPS Synchronization Architecture for DSA Devices. 

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