4.1.2 GPS Receiver Design:

ANTENNAS

The role of the antenna is to filter, amplify and down-convert the incoming signals into an electric signal that can be processed by the receiver electronics wintin the RF section. There are a number of special considerations as far as GPS antenna design is concerned:

• The antenna must be able to pick and discriminate very weak signals (the signal strength is approximately the same as those from geostationary TV satellites).
• The antenna may need to operate at just the L1 frequency, or at both the L1 and L2 frequencies.
• As the signals are circularly polarised, the GPS antenna must also be circularly polarised.
• The antenna gain pattern design typically is intended to enhance the ability of the RF section to filter multipath and low elevation signals.
• An essential requirement is a stable electrical centre which is coincident with the geometric centre and insensitive to the rotation and inclination of the antenna, and if this cannot be achieved then the offset parameters should be known to a suitable accuracy (sub-centimetre in the case of survey grade GPS antennas).
• The construction of the antenna consists of: (a) the omnidirectional antenna element, (b) the antenna preamplifier electronics, and (c) a groundplane (though not always present).

Several types of GPS antenna element-groundplane assemblies have been (and still are) used:

• Monopole, or dipole, configurations.
• Quadrifilar helices.
• Spiral helices.
• Microstrip.
• Planar rings ("choke ring"), and other multipath-resistant designs.

The Figure below illustrates some of these basic antenna types.

GPS antenna types. (WELLS et al, 1987)

The basic functions of the antenna preamplifier are (NATO, 1991):

GPS surveying antennas are required to be rugged, simple in construction, have stable electronic phase centres, be resistant to multipath, and have good gain and pattern coverage characteristics. The following general comments can be made with regard to the antenna types identified above:

• Quadrifilar Helix:
  • single frequency
  • difficult to adjust for phasing and polarisation
  • not azimuthally symmetric
  • good gain pattern
  • no ground plane required
• Monopole:
  • single and dual-frequency (dipole construction)
  • requires extensive groundplane
  • very stable phase centre
  • simple construction
• Microstrip:
  • rugged and simple construction
  • single or dual-frequency
  • low profile (ideal for aircraft applications)
  • low gain (offset by appropriate preamplifier)
  • most common antenna available today
• Spiral Helix:
  • dual-frequency operation
  • good gain pattern
  • high profile
  • azimuthal asymmetry (requires ground orientation)

Present antennas are generally either microstrip or quadrifilar in construction. There is, however, intensive effort being invested in multipath-resistant antennas for both high precision geodetic use and the modern "rapid static", "stop & go" and "kinematic GPS" techniques (section 5.5.1). There is also a trend to integrating the antenna assembly with the receiver electronics.

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© Chris Rizos, SNAP-UNSW, 1999