GRACE XINGXIN GAO, LIANG HENG, TODD WALTER, AND PER ENGE
Arctic navigation is becoming increasingly important, because oil exploration and normal shipping are both on the rise. Navigation integrity is particularly important, because an accident could be very damaging to the sensitive Arctic environment. Thus, this paper investigates the Arctic extension of space-based augmentation systems (SBAS, e.g., WAAS [Wide Area Augmentation System], EGNOS [European Geostationary Navigation Overlay Service], and MSAS [Multifunctional Satellite Augmentation System]). More specifically, it analyzes new reference stations for the far North; Iridium satellites to broadcast the integrity information to the users; and multi-constellation GNSS to improve vertical performance.
The Arctic houses an estimated 90 billion barrels of undiscovered, technically recoverable oil and 44 billion barrels of natural gas liquids according to the U.S. Geological Survey (2008). These potential oil reserves represent 13 percent of the undiscovered oil in the world. Russia, Canada, and the United States plan to explore the Arctic for extensive drilling soon. At the same time, the Arctic is becoming more accessible to normal shipping because of global climate change. New summer sea lanes have already opened up, and projections of sea ice loss suggest that the Arctic Ocean will likely be free of summer sea ice sometime between 2060 and 2080. The combination of undiscovered oil and climate change are driving a dramatic increase in the demand for navigation in the Arctic.
MORE REFERENCE STATIONS FOR SBAS COVERAGE
The Arctic is a sensitive environment, and thus navigation should have high integrity. For this reason, we are interested in extending the SBAS to serve this region. At present, none of the three operational SBAS provides meaningful service in the far North. In fact, Figure 1 shows the current SBAS availability coverage with vertical alert limit (VAL) equal to 35 meters, and horizontal alert limit (HAL) equal to 40 meters. Figure 1 is based on two of the currently operating SBAS: the U.S. WAAS and the European EGNOS. The locations of the WAAS and EGNOS reference stations are shown in Figure 2. The indicated lack of integrity coverage in the Arctic is due to too few reference stations.
Of course, we can extend integrity into the Arctic by adding reference stations to those shown in Figure 2. We include the reference stations of the Russian SDCM (System of Differential Correction and Monitoring) and Japanese MSAS systems, and add five new reference stations, whose locations are shown in Figure 3. We assume all these references stations provide the same measurement quality as current WAAS reference stations. We also assume that there is continuous user connectivity, that is, the user is always able to receive the SBAS corrections. Although SBAS GEO coverage is limited in the Arctic, there are other ways to maintain the connectivity, such as using low Earth orbit (LEO) satellites. This topic is addressed in more detail in the next section.
Figure 4 shows the horizontal and vertical availability with user connectivity and the reference stations in Figure 3. Again, we set VAL to 35 meters and HAL to 40 meters. Availability has been improved from no availability coverage to greater
FIGURE 1 Current integrity availability in the Arctic with VAL = 35 m and HAL = 40m.
FIGURE 2 Locations of current WAAS and EGNOS reference stations.
FIGURE 3 Reference stations from WAAS, EGNOS, MSAS, SDCM, and five additional stations. Source: Tyler Reid, Stanford University. Reprinted with permission.
than 90 percent coverage in the Arctic region. In the case of seafaring navigation (as opposed to airborne navigation), we can relax the vertical requirement. Figure 5 shows the availability when there is only the horizontal requirement of 40 meters. As shown, the horizontal availability exceeds 99.9 percent throughout the Arctic.
IRIDIUM SATELLITES FOR BROADCAST TO USERS
The second issue with ensuring integrity in the Arctic is continuous connectivity, in other words, how the SBAS messages are delivered seamlessly to the users. Currently, WAAS uses GEO satellites to broadcast error corrections to users. Because the GEO satellites orbit directly above Earth’s equator, WAAS GEO coverage does not include the Arctic. As shown in Figure 6, there is lack of SBAS connectivity.
Iridium satellites are a promising alternative for communicating and broadcasting SBAS messages to the Arctic. The constellation of Iridium satellites is shown in Figure 7. This constellation includes 66 active satellites in low Earth orbit at an altitude of approximately 781 km with an 86.4° inclination (Evans, 1998). The orbiting period from pole to pole is about 100 minutes. The over-the-pole design of Iridium orbits ensures very good high-elevation satellite coverage in the Arctic. Because Iridium satellites already provide voice and data services to satellite phones and integrated transceivers all over Earth, Iridium is a strong candidate for enabling SBAS linkage to the Arctic users.
Note that the current practice of Iridium satellites is to shut off redundant beams when near the poles to avoid inter-satellite interference. Because of the low Earth orbit and thus short orbital period, the satellites are in view for only about 10 minutes. Therefore, the challenge of using Iridium satellites for communication in the polar region is to accommodate fast switching of transmitted satellites.
As a bonus, Iridium satellites could improve the vertical dilution of precision (VDOP) if the Iridium satellites also broadcast ranging signals. VDOP is a measure of how well the positions of the satellites are arranged to generate the vertical component of the positioning solution. Higher VDOP values mean less certainty in the solutions and can be caused if the satellites are at low elevations. Figure 8 shows the VDOP improvement in the Arctic using Iridium. We simulate two scenarios with and without additional Iridium satellites: using 31 GPS satellites as the healthy satellites in current GPS constellation, and 24 GPS satellites as in the original design for GPS. If we have GPS constellation only, the VDOP values in the Arctic are about 2.1 and 1.8 for 24 and 31 GPS satellites, respectively, worse than those elsewhere on Earth (1.7 and 1.5 for 24 and 31 GPS satellites, respectively) as shown in Figure 8(a,b). With added Iridium satellites, the VDOP values increase to 1.6 from 2.1 for 24 GPS satellites, and to 1.3 from 1.8 for 31 GPS satellites. Moreover, the VDOP values are more even over the Earth surface. For both 24 and 31 GPS satellites scenarios, adding Iridium satellites improves VDOPs in the Arctic. In the next section, we investigate using multi-constellation navigation to improve VDOPs.
FIGURE 6 WAAS GEO satellite coverage with minimum elevation angle of 6.35°.
FIGURE 7 Constellation of Iridium satellites.
MULTIPLE CONSTELLATIONS FOR HIGH AVAILABILITY OF INTEGRITY
A third issue, although not as critical as the first two, is the VDOP degradation in the Arctic. Because GPS satellites are in an orbital plane of 55° inclination, there are not enough high-elevation satellites in the Arctic. For this reason, VDOPs in the Arctic are worse (i.e., higher) than those close to the equator. See Figure 9 for a comparison of VDOPs at the North Pole and at Stanford, California. In contrast, horizontal dilutions of precision (HDOPs) in the Arctic may be better than elsewhere because of this special satellite geometry.
Besides Iridium, another approach is using multi-constellation (i.e., GPS, Compass, Galileo, and GLONASS). Figure 10 shows the significant VDOP improvement in the Arctic using two or more constellations. The VDOP values reduce to below 1.3 with the help of multiple constellations. If using only two constellations, adding GLONASS to GPS is the most helpful combination. GLONASS satellites orbit at 19,100 kilometer altitude with a 64.8° inclination. Compared to the 55° inclination of the GPS orbital planes, the GLONASS constellation has better coverage in high latitudes. The VDOP improvement in the Arctic is more dramatic using three or even all four constellations.
FIGURE 8 VDOP improvement in the Arctic with the help of Iridium satellites. (a) 24 GPS satellites; (b) 31 GPS satellites; (c) 24 GPS + Iridium satellites; (d) 31 GPS + Iridium satellites.
This paper identifies a need for high-integrity navigation in the Arctic and analyzes techniques to extend SBAS coverage to this critical area. We show that the current reference stations network can be augmented to provide Arctic integrity with high availability. Iridium satellites could provide a broadcast channel to the SBAS users. Multiple GNSS constellations significantly improve VDOPs and thus reduce vertical positioning errors in the Arctic.
Evans, J.V. 1998. Satellite systems for personal communications. Proceedings of the IEEE 86(7): 1325–1341.
U.S. Geological Survey. 2008. 90 Billion Barrels of Oil and 1,670 Trillion Cubic Feet of Natural Gas Assessed in the Arctic. Available online at http://www.usgs.gov/newsroom/article.asp?ID=1980.
FIGURE 9 VDOP at the North Pole and Stanford, California. Source: Tyler Reid, Stanford University. Reprinted with permission.
FIGURE 10 VDOP improvement in the Arctic using two or more constellations. (a) 31 GPS + 30 Compass MEO satellites; (b) 31 GPS + 24 GLONASS satellites; (c). 31 GPS + 30 Compass + 30 Galileo satellites; (d) 31 GPS + 30 Compass + 30 Galileo + 24 GLONASS satellites.