Aerospace and Electronic Systems - October 2018 - 17

Rodriguez et al.

Figure 3.

(A) PDOP during fuel cycle 1. Small blue dots depict points where the PDOP is less than 7 while larger red dots depict points where the PDOP is greater
than 7. (B) Percent of visible sky during fuel cycle 1. Small blue dots depict points where the percent of open sky was greater than 70% while larger red
dots depict points where the percent of open sky was less than 70%. Slope of the terrain is depicted from green to red (0° to 81°).

phase and code phase G7 positions, which has been designated as
the most accurate under these circumstances, used as the baseline.
Dilution of Precision (DOP) are reported here to describe the
effect of satellite constellation geometry on position measurement errors. When satellites are in close positions relative to the
receiver, the position error increases even though measurement
uncertainty is unaffected [31]. The DOPs, including the horizontal,
vertical and 3D DOP, are provided in the GNSS DOP and Active
Satellites NMEA sentence, allowing for recording of this statistic
in real-time [32].
Spatial interpretations were made with ArcGIS™ (version
10.1; ESRI® Inc., Redlands, CA) all data projected to North American Datum 1983 (PACP00) Universal Transmercator Zone 4N.
Percent skyline visibility was calculated using the Skyline Graph
tool in 3D Analyst with x, y, and z coordinate positions overlaid a
10 m Digital Elevation Model (DEM) of Maui created from USGS
7.5′ DEM Quads with 90 percent of data having a vertical accuracy
of 7 m RMSE or better and the remaining 10 percent with 8-15 m
[33]. The area visually searched was calculated from the collected
GNSS position and field of view (FOV) of the three-person flight
crew using the Viewshed Analysis tool. The azimuthal angles of
the FOV were offset from the aircraft vector at 240° port through
90° starboard, encompassing the front of the aircraft, and elevation
angles from 0° to −70° (relative to horizon), and radial distances
from 1 to 56 m.
OCTOBER 2018

To infer the obstructions to satellite constellations by the rotor
disk and terrain we created skyview projection plots for a single
location selected from the flight path of the first operation, representative of an area with extreme topographical obstruction. These
plots were prepared using satellite orbit and position information
determined using Trimble® GNSS Planning Tool, at 30 minute intervals spanning the operations described here. Main rotor obstruction was modeled as an 8 m diameter disk located 1.9 m above the
GPS receiver antenna inside the aircraft [34]. Obstruction of the
terrain was modeled based on the minimum zenith angle to the top
of the terrain from the Skyline Plot.

RESULTS AND DISCUSSION
The R10 experienced numerous interruptions in the carrier phase
tracking (also known as cycle slips), where interference by the helicopter's main rotor and proximity to steep terrain were likely the
main causes of the (somewhat expected) poor performance exhibited by this receiver recording from a dynamic platform. Unlike
the survey-grade R10, the G7 has the capability to record the lower
frequency code phase when the carrier phase was unavailable, allowing the position to be recorded regardless of cycle slips in the
carrier phase, albeit with lower resolution. With a more complete
(reliable) data track, as indicated by the residuals of the carrier
phase differential GPS (CDGPS) and differential GPS (DGPS) in

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Aerospace and Electronic Systems - October 2018

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