This page is intended for people who are interested in the more technical aspects of the NAVSTAR GPS Signals. YOU DO NOT NEED TO READ THIS TO USE A GPS RECEIVER!
This document details the characteristics and content of the NAVSTAR GPS transmissions as described in the "Global Positioning System Standard Positioning Service Signal Specification" document. I have pretty much taken the outline for the actual signal specification part of the document but I do not intend to attempt to reproduce the whole thing. Among the things that are missing are the large numbers of graphics that accompany the document. What parts I have taken I have not taken exactly. I've paraphrased quite a lot to keep the page reasonably short.Readers requiring more detail are urged to get a copy of the original document. The document is available online in Adobe Acrobat format from the U.S. Coast Guard's Navigation Center (NAVCEN). Printed copies are available from the NAVTECH bookstore.
Frequency Plan
The L-band SPS ranging signal is contained within a 2.046 MHz band centered around L1. The carrier frequency for the L1 signal is coherently derived from a frequency source within the satellite. The nominal frequency of this source, as it appears to an observer on the ground, is 1.023 MHz. To compensate for relativistic effects, the output of the satellite's frequency standard is 10.23 MHz offset by a tiny amount. The frequency offset results in an output of 10.229999999543 MHz, which is divided to produce the carrier modulation signal of 1.0229999999543 MHz. The same output frequency source is used to generate the nominal L1 carrier frequency of 1575.42 MHz. The L1 signal is transmitted with enough power to ensure a minimum signal power level of -160 dBw at the Earth's surface.The L2 signal is transmitted with enough power to ensure a minimum signal power level of -166 dBw at the Earth's surface.
Correlation Loss
Correlation loss is defined as the difference between the satellite power received in a 2.046 MHz bandwidth and the sig power recovered in a nominal correlation receiver of the same bandwidth. On the L1 channel, the correlation is budgeted as follows:
- Satellite modulation imperfections - 0.6 dB
- Ideal user receiver waveform distortion - 0.4 db.
Carrier Phase Noise
The phase noice spectral density of the unmodulated carrier is such that a phase locked loop of 10 Hz one-sided bandwidth is able to track the carrier to an accuracy of 0.1 radians RMS.Spurious Transmissions
In-band spurious transmissions are at least 40 dB below the unmodulated L1 carrier over the allocated channel bandwidth.Equipment Group Delay
Equipment group delay is defined as the delay between the L-band radiated output of a specific satellite (measured at the antenna phase center) and the output of that satellite's on-board frequency source. The majority of this value is included in the clock correction parameters relayed in the navigation message.Signal Polarization
The transmitted signal is right-hand circularly polarized. The ellipticity for L1 will not exceed 1.2 db for the angular range of plus or minus 14.3 degrees from boresight.
For more information on satellites in general, check out these web sites:
The U.S. Coast Guard Navigation Center
The U.S. Coast Guard is the official civilian interface to the NAVSTAR GPS System. The original NAVSTAR GPS Signal Specification may be downloaded in Adobe Acrobat format from the Coast Guard's Navigation Center.