Navigating in Challenging Environments - GPS Degraded Signals and Signals of Opportunity
Aerospace researchers continuously tackle the problem of exploring distant places while navigating challenging environments closer to home with greater precision and accuracy. This is the case in challenging terrestrial-based environments. To solve this, many navigation systems rely on the fusion of multiple sensors to compensate for degraded signal reception. Additionally, if there is knowledge of the surrounding environment, the degraded signals and even signals that are absent can be used for navigation. Similarly, the navigation and timing signals that have been beneficial to spacecraft navigating in low earth orbit can be leveraged in high earth orbits and beyond, if the GNSS signals that are degraded at these distances can be used in unconventional ways. To address novel ways of navigation in difficult environments, we have established a research group at the University of Minnesota within Aerospace Systems. This Advancing Technology in Localization and Satellite Navigation (ATLAS) lab primarily uses GNSS signals, signals of opportunity, and unconventional navigation algorithms in diverse settings.
Navigating the Urban Environment
One of my primary goals is to conduct research into navigating the urban environment. Many location-based services rely on GPS technology for personal and commercial navigation. Increasingly, users are depending on these services to work reliably in GPS signal-degraded environments. Specifically, in urban areas, conventional methods for GNSS positioning perform poorly. Due to tall, densely spaced buildings, many of the direct line-of-sight (DLOS) signals from GNSS satellites are blocked. When fewer than four satellites are visible along the DLOS, a user cannot form an instantaneous solution at all unless the solution uses non-line-of-sight (NLOS) signals that reach the receiver via a reflection off of the surrounding buildings (Figure 1).
To enhance GPS urban localization, methods utilizing weak and reflected signals and signals that do not appear to be present at all can be quite useful as those are often the only type of signals available. Our research methods are similar to Shadow Matching (SM), which is a technique for using visibility predictions in conjunction with signal strength for positioning. Similarly, predicting NLOS signal reception throughout the city, referred to as specular matching (SPM), can aid navigation. SPM is based on the likelihood of receiving a specular reflection from surrounding buildings. These reflections can be predicted by sampling a scene through ray tracing and determining locations that fit a criterion for a specular reflection. The formulation of this method of predicting specular reflections was designed to be similar to the visibility sky plots used in SM so that it could be easily implemented. The specular matching method outperforms the conventional SM scoring scheme.
Additionally, a direct positioning estimator method addresses utilizing weak GPS signals in a way that is analogous to SM and SPM. A GNSS receiver searches for and acquires each GNSS satellite signal in normal conditions. But in urban environments, where many of the received signals from satellites are degraded and weak, if the region of interest is sufficiently small, and the user can approximate the receiver clock error, the method of collective detection using a direct positioning estimator (DPE) can increase acquisition sensitivity by essentially combining acquisition results from multiple satellites (Figure 2).
The use and fusion of these various methods can significantly improve navigation in urban environments. There are many topics to explore within this line of research in the rapidly expanding field of urban navigation. For instance, there is value in developing methods to evaluate various city regions and predict the likelihood that using alternative GPS positioning methods will outperform a more conventional approach. Additional work is needed to validate 3D city models and catalog the GPS environment within the models. Using the GPS data from a transit system like a commuter rail network to study the properties of buildings and their materials as the GPS signal specular intersection points trace across the building surfaces could also be a major contribution to the urban navigation problem.
Similarly, another topic of interest related to urban navigation is research investigating the use of long-term evolution (LTE) signals, typically used for communication purposes, as signals of opportunity for navigation. Because there are currently many restrictions to flying MAVs in urban environments, all the preliminary work of characterizing the city environment and signal conditions will be essential to advancements in this area. There is substantial value in understanding the fusion of complementary sensors in urban environments.
Navigating Beyond the Constellation
The Global Positioning System has been useful for on-board navigation purposes for space-based applications, particularly for low Earth orbit (LEO) satellites. However, there is interest in the ability to use a GPS receiver on-board a high Earth orbit (HEO) satellite. In contrast to LEO satellite navigation, one difficulty in determining the position of a HEO satellite is the poor instantaneous geometry of the HEO receiver relative to the visible satellites in the GPS constellation. The typical GPS user is an Earth-based user and therefore the antenna on-board a GPS satellite points toward the center of the Earth and the main-lobe of the antenna beam does not extend far beyond the solid angle of the Earth (Figure 3).
A HEO satellite which is at a higher altitude than a GPS satellite will only be visible when the HEO satellite is near the opposite side of the Earth and therefore within the main lobe of the antenna pattern but simultaneously not occluded by the Earth. The number of GPS satellites visible to the HEO satellite is greatly reduced from those for an Earth-based user or LEO receiver. Additionally, the spatial distribution is limited because the GPS satellites that are visible are not evenly angularly distributed or geometrically diverse. Similar to the work in the urban canyon, the ATLAS lab is investigating the use of limited and weak signals for navigation by implementing ad space-based DPE navigation algorithm.
Even beyond the HEO satellite, there is interest in developing a lunar receiver to use GPS signals for lunar mission spacecraft and even for navigation on the surface of the moon. With work in both DPE and mapping, we are developing a research project addressing the weak GPS signals paired with creating maps from orbital imagery to help with navigation and positioning in this environment with particular interest in the use of a lunar GPS receiver (Figure 4).