GPS from getting started to giving up (XVIII), multipath effect

One 、 Concept

stay GPS When the receiver receives the satellite signal , In addition to receiving a signal that reaches the receiver through a straight path , It is also possible to receive Reflection The satellite signal transmitted again , These reflected signals change the direction of propagation 、 The amplitude 、 Polarization, phase, etc , Superimposed with the linear signal , Thus, the observed value deviates from its true value and produces error , This phenomenon is called multipath effect （Multi-Path Effect）, Sometimes it's also called Multipath effect .

Two 、 characteristic

• Compared with direct wave , Multipath signal propagation path is long , Therefore, it will delay reaching the receiving antenna .
• In general , The reflected wave power will be weaker than the direct wave power ; Of course, it does not rule out the situation that the direct wave attenuation is serious or even blocked .
• Generally speaking, smooth ground 、 Surface of the water 、 Saline alkali beach 、 The reflection coefficient of glass curtain wall is very large , Strong reflex .
• The multipath effect error is greatly affected by the environment around the antenna , Another place may be completely different .
• Multipath effects at both ends of the baseline are not correlated , Therefore, it cannot be eliminated by differential technology .

3、 ... and 、 influence

In general reflective environment , The influence of multipath effect on PN code ranging can reach meter level , The influence on carrier phase ranging can reach centimeter level .
In a highly reflective environment , Its impact will increase significantly , It often leads to the loss of lock of the received satellite signal and the cycle jump of the carrier phase observation .

• The influence of multipath effect , It can be divided into constant part and periodic part . Where the constant part , It will repeat day after day in the same place ; The periodic part appears periodically .
• The reflected wave with long time delay has little influence , The reflected wave with short delay has great influence .
• Generally, the multipath effect of low elevation satellites is greater than that of high elevation satellites .
• Multipath effects can only be attenuated , Cannot be completely eliminated .

Four 、 Restraining measures

The influence of multipath effect can be suppressed from many aspects ：

• Choose the environment where the receiver antenna is placed , Avoid highly reflective environments , Or set up facilities to shield reflected waves . For example, in order to prevent the satellite signal reflected from the ground or floor from entering the antenna to produce multipath error , The diameter suppression plate can be configured under the receiver antenna .
• Choose an antenna with appropriate shape and good shielding , Such as choke antenna .
• Increase the satellite cutoff elevation , But doing so will also shield the low elevation satellite signal .
• Improve receiver signal processing technology , Such as narrow correlation technology 、 Multipath reduction technology and delay phase locked loop for multipath reduction MEDLL etc. .
• In static measurement and positioning , The observation time can be extended appropriately , Weaken the periodic influence of multipath effect .
• Improve the hardware circuit design of the receiver , Such as selecting the appropriate RF front-end analog filter .