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1. Vulnerability of GNSS and Needs of Pseudolites
GNSS provides accurate positions at cheap price to everyone on the Earth. For this reason, GNSS became a dominant infrastructure in navigations.
However, GNSS has some vulnerabilities. First, it is impossible to use GNSS signals in signal-blocked spaces. The reason for this is that the received power of GNSS signals is very weak because they are broadcasted from the satellites which are on the 20,000km altitude. Due to this problem, the availability of GNSS is significantly degraded in indoors, mountainous terrains, and urban areas. Second, vertical accuracy of GNSS is worse than horizontal because of the geometry of GNSS satellites. This problem limits the use of GNSS in aviation.
There were many studies to resolve the weak points of GNSS. In SNU GNSS Lab., we are working on the use of additional signal sources (Pseudolites) to make up for the vulnerabilities of GNSS.
Figure 1. Why do we need Pseudolites?
2. Pseudolites
Pseudolites are a GNSS-like signal transmitter. GNSS receiver can regard pseudolite signals as GNSS signals. Therefore, pseudolites can replace GNSS satellites, and user can easily utilize pseudolite signals with existing GNSS receivers with minor changes on its firmware.
Table 1. GNSS Satellites vs Pseudolites
3. Structure of Pseudolites
In this article, we focuses on the pseudolites for GPS L1 signal only. Fundamental structure of pseudolites is illustrated in Figure 2. It generates L1 frequency carrier signal, mixes this carrier with C/A code which is multiplied by navigation data bits, and broadcast the mixed signal through RX antenna.
Figure 2. Block diagram of Pseudolite
4. Pseudolite-Based Indoor Navigation System
Pseudolites can be utilized in various fields. Typical applications are indoor navigations, precise landing system of UAV (Unmanned Areial Vehicle) or airplanes, independent navigation systems in no-GNSS environments (eg. on the Moon or the Mars), E911 services, etc. Figure 3 shows the classification of pseudolite-based navigation systems.
Figure 3. Classification of Pseudolite Navigation System
In SNU GNSS Lab. we have been studying on indoor navigation system using pseudolites since 1999. Currently, we built a Pseudolite-Based Indoor Navigation System in our laboratory. Using this system, cm-accuracy positioning is obtained.
Figure 4. Pseudolite Indoor Navigation System in Seoul Nat'l Univ.
We developed unsynchronous and synchronous navigation system based on pseudolites. For the unsynchronous system, there are some errors due to the independent clock sources. Therefore differential GPS concept is used to compensate this clock error. In this case, user receiver needs a data link for the correction data from reference station. On the other hand, for the synchronous system, user can obtain cm-level accuracy without any correction data.
5. Performance Analysis of Indoor Navigation System
Some performance analysis results of the Pseudolite-Based Indoor Navigation System are shown below.
Figure 5. Static error of Pseudolite based Indoor Navigation System
Figure 6. Dynamic error of Pseudolite based Indoor Navigation System
Figure 7. Zigzag test of Pseudolite based Indoor Navigation System
6. Technique Situations
2000 : Pseudolite-Based Indoor Navigation System (World Second)
2002 : Synchronous Pseudolite-Based Indoor Navigation System
2005 : GPS Pseudolite System of Aviation
2007 : Smart Pseudolite Navigation System using Two-way Measuring Technique
2009 : Single AP-Based Positioning System (Patented)
2011 : Pseudolite Navigation Algorithm for Commercial GNSS receivers (Patented)
Figure 8. Automatic Control of Vehicle using Pseudolite based Indoor Navigation System
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