GPS Facts and References

---

Signal Direction of Arrival Measurements

• At the receiver location, the GPS signal arrives like light from the radiating satellites and illuminates an effective aperture for the GPS antenna
• Buildings and foliage reflect and scatter the signal away from line of sight between satellite and receiver and cause inaccurate distance calculations.
• Receiver processing eliminates the effect of ground-reflections at significant flight altitudes and GPS functions a near line of sight communication link.
• The directional gain of even simple GPS patch antennas is sufficient to indicate the general direction of arrival (DOA) for both satellite and important interference signals.

Antenna Orientation Relative to Satellites

• There is a natural geometric equivalence between the orientation of an antenna aperture, and the direction of arrival of signals relative to the antenna. 
• While this mathematical fact has nothing to do with GPS, it did enable us to design and patent a generic system architecture that uses DOA measurements to determine platform attitudes.
• In the particular case of GPS, the application of the patent allows us to use the GPS antenna's directional capabilities to determine the platform's 6DOF position and attitude.

Redundant Sensors

• There are alternative means for most platforms to obtain and maintain their attitudes, since autonomous platforms exist - GPS6D does not compete with any of these.
• A modern Inertial Navigation System (INS) already integrates data from multiple sensors to produce an accurate navigation estimate. 
• GPS6D naturally complements the tendency of inertial sensors to drift due to integration over long periods of time, and it provides an independent attitude referenced to the GPS constellation.

Reliability and Safety

• A business enterprise has to plan for the inevitable failure of equipment, and consider the consequences thereof for customers as well as internal operations.
• The probability of system failure depends mathematically on whether there is backup functionality in the case of component malfunction.
• Redundant sensors increase the likelihood that an autonomous system can recognize a component failure and react to that event in a manner that optimizes safety for all involved.

Antenna Baselines

• Two antennas may be separated by some distance defining a line segment using the two end points.
• The direction of the line segment may be estimated with increasing accuracy as the separation between antennas increases.
• Refinements of this technique, such as dual compassing, that use cooperating receivers and multiple baselines are able to estimate attitudes on larger platforms 

Angles of Arrival

• Most antennas are sensitive to the angles at which signals arrive at the aperture of the antenna, providing a stronger Carrier to Noise (CN0) ratio as the source enters the field of view.
• If the antenna were mounted on a rod, and rotated around this axis through 360 degrees, the CN0 would fluctuate as the angles aligned or not with the satellites.
• We presume  that there will be one optimum alignment angle at which the antenna is pointed in the direction of the satellite source.
• The requirement to separate two antennas is gone if we can physically or electronically rotate the antenna.

 A draft of a paper for the August 2022 Institute of Navigation Joint Navigation Conference may be found by clicking the link below.