GPS World, February 2009

Antenna Technology INNOVATION equals the RHCP antenna gain at a boresight angle divided by the sum of the RHCP and LHCP antenna gains at that angle Multipath signals from reflections against vertical objects such as buildings can be suppressed by having a good AR at those elevation angles from which most vertical object multipath signals arrive This AR requirement is readily visible in the MPR formula considering these reflections are predominantly LHCP and in this case MPR simply equals the co to cross polar ratio LHCP reflections that arrive at the antenna at high elevation angles are not a problem because the AR tends to be quite good at these elevation angles and the reflection will be suppressed LHCP signals arriving at lower elevation angles may pose a problem because the AR of an antenna at low elevation angles is degraded in real world antennas It makes sense to have some level of gain roll off towards the lower elevation angles to help suppress multipath signals However a good AR is always a must because gain roll off alone will not do not it Phase Center A position fix in GNSS navigation is relative to the electrical phase center of the antenna The phase center is the point in space where all the rays appear to emanate from or converge on the antenna Put another way it is the point where the electromagnetic fields from all incident rays appear to add up in phase Determining the phase center is important in GNSS applications particularly when millimeter positioning resolution is desired Ideally this phase center is a single point in space for all directions at all frequencies However a real world antenna will often possess multiple phase center points for each lobe in the gain pattern for example or a phase center that appears smeared out as frequency and viewing angle are varied The phase center offset can be represented in three dimensions where the offset is specified for every direction at each frequency band Alternatively we can simplify things and average the phase center over all azimuth angles for a given elevation angle and define it over the 10 to 90 elevation angle range For most applications even this simplified representation is over kill and typically only a vertical and a horizontal phasecenter offset are specified for all bands in relation to L1 For well designed high end GNSS antennas phase center variations in azimuth are small and on the order of a couple of millimeters The vertical phase offsets are typically 10 millimeters or less Many high end antennas have been calibrated and tables of phase center offsets for these antennas are available Impact on Receiver Sensitivity The strength of the signals from space is on the order of 130 dBm We need a really sensitive receiver if we want to be able to pick these up For the antenna this translates into the need for a high performance low noise amplifier LNA between the antenna element itself and the receiver We can characterize the performance of a particular receiver element by its noise figure NF which is the ratio of actual output noise of the element to that which would remain if the element itself did not introduce noise The total cascaded noise figure of a receiver system a chain of elements or stages can be calculated using the Friss formula as follows The total system NF equals the sum of the NF of the first stage NF 1 plus that of the second stage NF 2 minus 1 divided by the total gain of the previous stage G 1 and so on So the total NF of the whole system pretty much equals that of the first stage plus any losses ahead of it such as those due to filters Expect to see total LNA noise figures in the 3 dB range for high performance GNSS antennas The other requirement for the LNA is for it to have sufficient gain to minimize the impact of long and lossy coaxial antenna cables typically 30 dB should be enough Keep in mind that it is important to have the right amount of gain for a particular installation Too much gain may overload the receiver and drive it into non linear behavior compression degrading its performance Too little and low elevation angle observations will be missed Receiver manufacturers typically specify the required LNA gain for a given cable run Interference Handling Even though GNSS receivers are good at mitigating some kinds of interference it is essential to keep unwanted signals out of the receiver as much as possible Careful design of the antenna can help here especially by introducing some frequency selectivity against out of band interferers The mechanisms by which in band an out of band interference can create trouble in the LNA and the receiver and the approach to dealing with them are somewhat different An out of band interferer is generally an RF source outside the GNSS frequency bands cellular base stations cell phones broadcast transmitters radar etc When these signals enter the LNA they can drive the amplifier into its non linear range and the LNA starts to operate as a multiplier or comb generator This is shown in FIGURE 8 where a 30 dBm strong interferer at 525 MHz generates a 78 dBm spurious signal or spur in the GPS L1 band Through a similar mechanism third order mixing products can be generated whereby a signal is multiplied by two and mixes with another signal As an example take an airport where radars are operating at 1275 and 1305 MHz Both signals double to 2550 and 2610 MHz These will in turn mix with the fundamentals and generate 1245 and 1335 MHz signals Another mechanism is de sensing as the interference is amplified further down in the LNAs stages its amplitude increases and at some point the GNSS signals get attenuated because the LNA goes into compression The same thing may happen down the receiver chain This effectively reduces the receivers sensitivity and in some cases reception will be lost completely RF filters can reduce out of band signals by 10s of decibels www gpsworld com February 2009 GPS World 45