Aerospace and Electronic Systems - October 2018 - 18

Performance Analysis of GNSS Units in Manned Helicopter Operations

Figure 4.

PDOP vs percent of open sky from with elevation angles from 0°-90° ((A) including area obstructed by rotor) and 0°-14° ((B) only area unobstructed by
rotor). Red points are the averages of data over a 2° increment.

Table 1, the postprocessed G7 data was instead used as the baseline. Almost half, i.e., 49%, of the total data points collected from
the G7 in all three operations (n = 10,539) were recorded from the
carrier phase, with 86% of those being float points. During float
point calculations, the solution algorithm calculates the number of
radio wavelengths between the satellites and the base station antenna using a decimal rather than a constrained integer resulting
in some ambiguity [35]. The 2D and 3D RMSEs of G7U data was
within 5 m of the postprocessed baseline (Figure 2).
In comparison to the postprocessed baseline G7 reference positions, the postprocessed J5 data had the smallest mean 2D and 3D
RMSE at 2.5 and 4.7 m, where the consumer grade GNSS units exhibited ∼4-fold greater 2D and 3D deviations, respectively (Figure
2). The first operation had frequent and large deviations due to lag
in the baseline G7 data. The recording software for the Geo 7 has a
built-in function to discard data that does not meet a preset position
dilution of precision (PDOP) threshold. This resulted in portions
of flight where the position solution recorded by the Geo7 was not
updated while the other receivers continued to update despite also
having large PDOP values. Figure 3 shows where the positional
DOP substantially increased in a topographically confined location, which coincided with areas with reduced sky visibility.
Percent skyline visibility plots for elevation angles from 0-90°
and 0-14° (only area below rotor blades) are shown in Figure 4.
Only the first two operations are considered due to portions of the
third cycle having altitude above ground level (Altagl) below the
DEM, likely due to the coarseness of the DEM combined with the
large slopes in these areas. At high percentages of open sky, the
18

Figure 5.

Two-dimensional projection of locations and orbits of GNSS satellites
with respect to a topographically occluded location along the flight
path (20.678°, −156.057°, 500 m) during the first fuel cycle. Satellite locations at the specified times during the fuel cycles are depicted
as circles with outlines and trajectories shown in green (GPS) or red
(GLONASS). Inclination angle of the satellite relative to the receiver
is depicted by the radial distance from the center where the center represents 90° (overhead) to 0° (in the same horizontal plane). Azimuth is
the polar angle where North is 0°. Interference by the rotor disk (blue)
and obstruction by terrain (orange) substantially diminish number of
satellites in clear line of sight, without any obstructions, (indicated by
numbers at the centers of the plots) and spatial bias of the remaining
satellites results in a large increase in PDOP.

IEEE A&E SYSTEMS MAGAZINE

OCTOBER 2018

Aerospace and Electronic Systems - October 2018

Table of Contents for the Digital Edition of Aerospace and Electronic Systems - October 2018