Guidance and Navigation

The primary objective of the Guidance and Navigation (G&N) subsystem is to determine the position and velocity vectors of the satellite as a function of time. These data can be used to calculate the orbit of a satellite. 

One reason the UW Dawgstar will need to know its orbit is so that it can determine when it is in the right position to communicate with the ground stations. Another reason is that a magnetometer requires data indicating the location of the satellite in the magnetic field of the Earth in order to calculate accurate attitudes.  

The TechSat21 mission places special requirements on the G&N subsystem of the UW Dawgstar. The requirements include low mass and low power consumption. These requirements come from the TechSat21 requirement of designing a nanosatellite with a mass of less than 10 kg. Specific mass and power requirements were defined by the Structure and Power subsystems of the UW Dawgstar. 

The initial mass allocation to the G&N subsystem was 0.1 kg. The initial power allocation for the combined Guidance, Navigation and Communication subsystem was 7 W. The G&N will use 2 W. Additional requirements on the G&N subsystem are created by the formation-flying mission. These requirements include:
  • Provide position and velocity inputs to the formation flying controls
  • Relative position and velocity calculations for each satellite in ION-F
  • A desired relative position accuracy of 10 meters

The formation flying experiment is dependant on accurate relative position information. As this accuracy is increased, more complicated formation flying experiments are possible.

A Global Positioning System (GPS) receiver is the only autonomous navigation system that can fit the size requirements with present technology. To determine the relative positions of the satellites there must be communication between the satellite’s navigation subsystems through the use of crosslinks, which are discussed in the Communications Section. 

There must also be software onboard that can calculate relative positions of the satellites in ION-F from the absolute positions and velocities of the satellites. The absolute positions are the positions calculated by the GPS receivers on board each satellite, which are then transmitted to the other satellites using the crosslinks.

The navigation subsystem will consist of three components. These include the GPS antenna, the GPS receiver, and relative position software. The hardware chosen provides a demonstration of how the G&N subsystem design will fit within the given requirements.

The GPS antenna will collect range signals from GPS satellites at 1575.42 MHz. The antenna will be a patch antenna located on the top of the satellite. The antenna needs to be on the top of the satellite because the bottom-side of the satellite will always be Earth pointed for the communication downlinks and uplinks to and from Earth. 

The GPS antenna receives signals from the GPS satellite constellation (which orbits at an altitude of 20,200 km). For the antenna to receive incoming GPS signals at all times, it must be placed on top of the satellite. A patch antenna will be used because it is the smallest and lightest option available. The height of 1.4 cm will cause significantly less solar cell shadowing than other types of antennas that are larger. 

One of the requirements on the G&N subsystem for the UW Dawgstar is the ability to calculate position accuracy within 10 m. The accuracy of the GPS receiver corresponds to the accuracy of our absolute position.  Using range tones to determine the distances between the satellites can improve the relative position measurements of the satellites. 

The relative position accuracy of the navigation subsystem depends directly on the accuracy of the GPS receiver and the accuracy of the range calculations using the range tones. This is the accuracy that is required to be within ten meters for the formation flying experiment.