Sources of GPS Error

Introduction

We have seen that the entire system of GPS is dependent on a network of 24 satellites orbiting the earth. While research and development work is still going on to develop more and more accurate systems, it would be a good idea to understand what the external sources of error are.

What's a GPS signal?

There are two frequencies of low power radio signals that GPS satellites transmit. These are called L1 and L2. The L1 frequency at 1575.42 MHz in the UHF band is what comes into play for civilian applications. These signals can pass through clouds, glass, plastic and such light objects, but cannot go through more solid objects like buildings and mountains.

Every GPS signal packs three bits of information- these are the pseudorandom code, ephemeris data and almanac data. The pseudorandom code is the identification code of the individual satellite. The ephemeris data identifies the location of each GPS satellite at any particular time of the day. Each satellite transmits this data for the GPS receivers as well as for the other satellites in the network. The almanac data has information about the status of the satellite as well as current date and time. The almanac part of the signal is essential for determining the position.

Possible Causes of Error

Apart from the inaccuracy of the clock in the GPS receiver, there can be other factors that affect the quality of the GPS signal and cause calculation errors. These are:

• Ionosphere and troposphere disturbances: These cause the satellite signal to slow down as it passes through the atmosphere. However the GPS system has a built in model that accounts for an average amount of these disturbances.
• Signal reflection: Here the signal hits and is reflected off objects like tall buildings, rocks etc. This causes the signal to be delayed before it reaches the receiver.
• Ephemeris errors: Ephemeris errors are also known as orbital errors. These are errors in the satellite’s reported position against its actual position.
• Clock errors: The built in clock of the GPS receiver is not as accurate as the atomic clocks of the satellites and the slight timing errors leads to corresponding errors in calculations.
• Visibility of Satellites: The more the number of satellites a GPS receiver can lock with, the better its accuracy. Buildings, rocks and mountains, dense foliage, electronic interference, in short everything that comes in the line of sight cause position errors and sometimes make it unable to take any reading at all. GPS receivers do not work indoors, underwater and underground.
• Satellite Shading: For the signals to work properly the satellites have to be placed at wide angles from each other. Poor geometry resulting from tight grouping can result in signal interference.
• Intentional degradation: This was used till May 2000 by the US Department of Defense so that military adversaries could not use the GPS signals. This has been turned off since May 2000, which has improved the accuracy of readings in civilian equipment.