Multipath error

 This is caused by the satellite signals being reflected off local surfaces such as buildings pr the ground resulting in a time delay and consequently a greater apparent range to the satellite ( Figure 9.18 ) 

At the frequencies used in GPS they can be of considerable amplitude due to the fact that the antenna must be designed to track several satellites and cannot therefore be more directional. 

Manufacturers have developed a number of solutions to minimize the effects of multipath 

A ground plane fitted to the antenna will shield the antenna from signals reflected from the ground but may induce surface waves reflected from buildings onto the ground plane which are then fed to the antenna 

This problem can be solved if a choke ring antenna is used 

A series of concentric circular troughs each at a depth of a quarter of the GPS signal wavelength are placed on the ground plane 

These troughs have high impedance to surface waves 

However choke ring antennae are much larger heavier and more costly than ordinary antenna and they cannot attenuate signals arriving from above 

Phased array antennae are designed to have high gain from the direct path to the satellite but attenuate signals from other directions 

However different satellites have different multipath geometry so there will be several simultaneous direction patterns required and of course the antenna must respond to changing satellite positions 

Such an antenna will be very expensive 

There are a number of software solutions called multipath mitigation approaches 

These include:

-Those associated with narrow pre-correlation of the signal ( narrow correlation ) 

- Using only the first part of the signal arriving at the antenna before it is corrupted by the reflected components ( leading edge ) 

- Modelling the shape of the total signal including the direct signal and its reflected components ( correlation function shape )

- Modifying the C/A code waveform ( modified waveforms )

However these methods only mitigate against the effect of multipath they do not remove it entirely 

Whatever hardware and software