Abstract
Satellite systems are a fast-developing and broad field of study. The use of global navigation satellite systems for relatively autonomous spacecraft navigation holds a lot of interest for researchers. It is extremely expensive to research space applications as live experiments. Therefore, computer modelling comes in handy when there is a need to analyze important factors in space environment. The chapter describes the radionavigation field model that uses the off-nadir satellites. This model allows estimation of the availability and accuracy characteristics of autonomous satellite navigation in space up to the geostationary orbit in order to provide the necessary research data.
TopBackground
The idea to extend the range of satellite navigation from the surface up to the near-Earth space has appeared somewhere in mid-20th. This mostly were attempts to formalize the service volume in space and to produce a viable solution for the lack of satellite signals in space above Earth. The researchers produced different ideas like the use of back-lobe signals of the navigation satellites’ antennas radiation patterns, signals passing through main-lobes and side-lobes of the antennas radiation patterns of the off-nadir satellites and surface-based “pseudo”-satellites. After the 1997 and AMSAT-OSCAR-40 (Moreau et al., 2002) launch, the opinions shifted towards the signals that are transmitted through main-lobes and side-lobes of the navigation satellites’ antennas from behind Earth, as it was proven that a sensitive receiver can receive the weak GPS signals.
According to (US Department of Defense [DOD], 2008a, 2008b, 2008c), the near-Earth space is divided into terrestrial service volume (TSV) and space service volume (SSV). The TSV covers the near-Earth space from the surface and up to 3000 km for GPS, while the SSV covers the volume from TSV up to geosynchronous orbit altitude that is approximately 36000 km. For visual information see Figure 1.
Figure 1. The near-Earth space as defined by (DOD, 2008)
The SSV is divided into medium and High Earth orbit altitudes including the geosynchronous orbits. The navigation satellites are orbiting at about 20000 km, so how can they be available at 36000 km? The traditional navigation is still partially possible in the medium altitudes, but, with the increase of altitude, it becomes evident that the number of available satellites is not enough to find the user's location. Therefore, the need for a new source of signal arises. The signals from the off-nadir satellites have become such a source, they are shown in Figure 2.
Figure 2. The main-lobe and side-lobe signals of the off-nadir satellites
Key Terms in this Chapter
Navigation Satellite Constellation: A group of navigation satellites that is synchronized in time and can be used to obtain the position solution.
Accuracy: The difference between the real and calculated user position.
Dilution of Precision Factors: The measure of how the satellite geometry influences the accuracy of the navigation solution.
Radio Navigation Field (RNF): This is a field formed by radio signals of navigation satellites.
Availability: The percent of the time when the navigation characteristics meet the requirements of given operation.
Off-Nadir Satellites: The satellites that are located behind with respect to the receiver’s position.
Space Service Volume (SSV): The special volume in the near-Earth space where navigation satellites can be used to determine the user’s position.