GPS Navigation Toolbox
[GNT08.1.2]
5
1. Introduction
In the past several years compact radio navigation receivers have proliferated in the civilian market
place, generally under the name of GPS or Global Positioning System. Appearing as handheld units or
built into cars, mobile phones, and airplanes, these commercial applications originated from a Cold War
military application. GPS is a worldwide radio-navigation system formed from a constellation of 24
satellites, each in its own orbit 11,000 nautical miles above the Earth, and five ground stations that make
sure the satellites are working properly. The GPS satellites each take 12 hours to orbit the Earth. Each
GPS satellite continuously broadcasts a Navigation Message at 50
bit/s giving the time-of-day, GPS week
number and satellite health information (all transmitted in the first part of the message), an ephemeris
(transmitted in the second part of the message) and an almanac (later part of the message). The position
calculated by a GPS receiver requires the current time, the position of the satellite and the measured delay
of the received signal and then by using the triangulation rule the position of the receiver is determined.
The large sources of error in GPS are produced by the atmospheric effect (ionospheric, tropospheric),
clock errors of the satellite, multipath effect, satellite orbits errors and calculation-rounding errors. The
errors of the GPS system are summarized in the following table. The individual values are no constant
values, but are subject to variances. All numbers are approximate values [1]:
Table 1. Errors of GPS
Error Source Error
Ionospheric effects ± 5 meter
Shifts in the satellite orbits ± 2.5 meter
Clock errors of the satellites' clocks ± 2 meter
Multipath effect ± 1 meter
Tropospheric effects ± 0.5 meter
Calculation- und rounding errors ± 1 meter
This report include the review of
• Principle of Radio Navigation: navigation base on trilateration has been investigated
and the over determined Eq. for navigation has been solved.
• GPS Ephemeris Data: for GPS navigation, the position of GPS satellite is very
important so the by receiving ephemeris data the position of the satellite is determined.
• GPS Errors: Three different sources of errors in GPS navigation has been investigated
(Ionosphere, Troposphere and Satellite clock). Ionospheric Error model is generated base
on Parkinson [1] and the Tropospheric Error Model based on Hopfiel model [2].
• Simulation & GPS Toolbox: One of the targets of this work is generating Matlab GPS
Toolbox and in one case study the performance of generated toolbox will be verified.