GPS Q&A

Q.What is multipath?

A.Multipath is one of the major error sources in GPS. Radio sources such as GPS satellites transmit signals in many directions. This signal travels away from your transmitter in various paths and usually encounters various natural and man-made phenomena such as buildings, trees, bodies of water etc., prior to reaching the receiving antenna. Whenever these phenomena are encountered, the GPS signal is absorbed, attenuated, refracted or reflected. The receiving antenna then becomes the point of convergence for the direct and multipath signals. Depending on the situation, the multipath signals can reach the GPS antenna at the same time or after the direct signal. The direct or true signal is the one that is transmitted from the GPS satellite and received at the GPS receiver without any multipath.

Q.How does multipath affect GPS?

A.As GPS is a radio ranging and positioning system, it is imperative that the GPS receiver signal reception from each satellite be direct line of sight. Anything other than the "true" signal will bias the range measurements and thus induce an error in the calculated position. In effect, multipath signals give a false impression of where you are. The true magnitude of the error varies and is dependent on how much multipath is present.
    Multipath will induce errors in the tens of meters in your measurement. If only one receiver is used (autonomous positioning), the affects of multipath will be amalgamated with the other errors and will not be significant. However, in DGPS mode, these errors will be significant. In DGPS mode where a base and one or more rover GPS receivers are used, typical accuracies range from centimeters to meters. Any multipath present at the base station will be transferred to the rover stations(s). The rover(s) then accumulate the base station's multipath errors along with its own multipath errors.

Q.How do you avoid the affects of multipath?

A.To insure your measurements are as clean as possible, NovAtel Inc., a leading supplier of GPS technology, has developed several software techniques that help in minimizing the effects of multipath. These include:

¥ Site Selection
As previously mentioned, multipath is caused by several phenomena (natural and man-made). Some of these are controllable. It is possible to reduce multipath just by choosing a different antenna location. The best possible site would have an absolutely clear view of the sky from horizon to horizon at all bearings without any obstructions. However, locations like this are few and are far between. If this type of location is not possible, choose a site that has minimal obstructions. Some surfaces such as buildings, bodies of water, and trees can cause severe multipath errors. It is best to place the antenna as far as possible from any obstructions, especially reflective surfaces. Obstructions that are closer tend to cause more problems. Other recommendations include placing the antenna as high as possible to improve visibility and reduce reflected signals.

¥ Antenna Design
Multipath signals often change polarization while they are refracted/reflected. Good antennas incorporate some sort of rejection technology to minimize reception of multipath signals. There are several different types of GPS antennas currently available. Most antennas are designed for specific applications. That is, antennas to be used in a forestry environment are designed differently than those used in a marine environment. The antenna that you choose should be the one that is appropriate for your application.

¥ Antenna Ground Plane/Choke Ring
A ground plane is a metallic, usually flat surface that is either already part of the GPS antenna or can be attached to a GPS antenna. Typically the GPS antenna resides on top of the ground plane. The purpose of this ground plane is to act as a surface which stops reflected signals-usually transmitted from the ground. Ground planes are very effective and usually inexpensive.
    A choke ring consists of a series of cylinders, which sit on top of a metallic flat surface. Again, a GPS antenna usually comes with the choke ring (already assembled) or can be attached to the choke ring. The purpose of the choke ring is similar to the ground plane. A choke ring stops reflected signals from the ground and from nearby reflective surfaces. A choking ring is typically used in applications such as GPS base stations or high precision surveying. Most applications do not require choke rings.

¥ Software
There are several software techniques available which help in minimizing the effects of multipath errors. These include: 1) Narrow Correlatorâ-initially designed to improve pseudo-range measurement accuracy-it also inherently reduces multipath reception by establishing a "narrow" tracking path and keeping a lock on your GPS signal. 2) Multipath Elimination Technology (METâ)-enhancement of Narrow Correlator technology. It provides the ability to model out the multipath signal. 3) Multipath Estimation Delay-Lock-Loop (MEDLLâ) Technology-combining hardware and software techniques. Further enhancement to Narrow Correlator and MET technology. Several GPS cards are used in conjunction to predict multipath.

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