User's Manual

HE910 Hardware User Guide

1vv0300925 Rev.28 – 2015-06-24

However, the GPS signal is heavily influenced by attenuation due to foliage such as tree canopies, etc.,

as well as outright blockage caused by building, terrain or other items in the line of sight to the specific

GPS satellite. This variable attenuation is highly dependent upon GPS satellite location. If enough

satellites are blocked, say at a lower elevation, or all in a general direction, the geometry of the

remaining satellites will result is a lower accuracy of position. The HE910 reports this geometry in the

form of PDOP, HDOP and VDOP.

For example, in a vehicular application, the GPS antenna may be placed embedded into the dashboard

or rear package tray of an automobile. The metal roof of the vehicle will cause significant blockage,

plus any thermal coating applied to the vehicle glass can attenuate the GPS signal by as much as 15 dB.

Again, both of these factors will affect the performance of the receiver.

Multipath is a phenomena where the signal from a particular satellite is reflected and is received by the

GPS antenna in addition to or in place of the original line of sight signal. The multipath signal has a

path length that is longer than the original line of sight path and can either attenuate the original signal,

or if received in place of the original signal add additional error in determining a solution because the

distance to the particular GPS satellite is actually longer than expected. It is this phenomena that makes

GPS navigation in urban canyons (narrow roads surround by high rise buildings) so challenging. In

general, the reflecting of the GPS signal causes the polarization to reverse. The implications of this are

covered in the next section.

7.3.2 GPS Antenna Polarization

The GPS signal as broadcast is a right hand circularly polarized signal. The best antenna to receive the

GPS signal is a right hand circularly (RHCP) polarized antenna.

Remember that IS-GPS-200E specifies the receive power level with a linearly polarized antenna. A

linearly polarized antenna will have 3 dB loss as compared to an RHCP antenna assuming the same

antenna gain (specified in dBi and dBic respectively).

An RHCP antenna is better at rejecting multipath than a linearly polarized antenna.

This is because the reflected signal changes polarization to LHCP, which would be rejected by the

RHCP antenna by typically 20 dB or so. If the multipath signal is attenuating the line of sight signal,

then the RHCP antenna would show a higher signal level than a linearly polarized antenna because the

interfering signal is rejected.

However, in the case where the multipath signal is replacing the line of sight signal, such as in an

urban canyon environment, then the number of satellites in view could drop below that needed to

determine a 3D solution. This is a case where a bad signal may be better than no signal. The system

designer needs to make tradeoffs in their application to determine which is the better choice.