GPS World, February 2018

ANTENNAS for each azimuth and elevation angle and is shown in FIGURE 3 confirming the better behavior of the four feed antenna FIGURE 4 shows the group delay versus frequency and elevation with different azimuth values being represented by curves with different colors for the three typologies of antennas such typology of figure contains all information about DGD variation versus frequency and angle and is first introduced in this article For comparison in the RTCAs 2006 MOPS document for airborne antennas for the sake of simplicity either DGD variation versus angle at central frequency or DGD variation over frequency at zenith were considered hence not fully covering the complete space Frequency Azimuth Elevation While the single feed antenna in Figure 4 shows a big variation of the DGD when moving from zenith that is Elevation 90 to lower elevations a substantial decrease in the DGD spread is recorded for the four feed solution with the dual feed one having again intermediate results It is worthwhile noticing that the results obtained for the dual feed solution are in agreement with the current MOPS for L1 antennas RTCA DO 301 specifying a maximum value of 25 nansoseconds ns for the group delay spread at low elevations normalized to boresight El 90 The results show how angular variation of the DGD can be related to non uniformity along the aspect angle Az or El and frequency hence suggesting to use multiple feed solution for obtaining optimal performance A useful metric to quantify the uniformity of the group delay can be introduced as the Uniformity Indicator for Group Delay UIGD 2 with being the sum over frequency N f is the number of frequency steps considered and DGD zenith n being the value of the DGD at zenith for frequency n The UIGD expresses the maximum variation of the DGD over elevation and azimuth from a reference condition the DGD at zenith in the bandwidth of interest extending de facto the MOPS requirements by considering the whole bandwidth behavior in the whole upper hemisphere The UIGD for the one two and four feed antennas is respectively 418 103 and 005 ns hence effectively mirroring the better pattern uniformity of the four feed solution FEBRUARY 2018 WWW GPSWORLD COM GPS WORLD 29