'You are Here' to be More Precise than Ever
Have you ever met a person who can never tell where they are without checking a GPS device? Before they can know where they are, the satellites have to know, and NASA is working to improve this information greatly. Though a regular person will not need the sub-millimeter precision the project will allow more, many science experiments do need to know exact distances.
At the moment there are four primary methods to determining a position; GPS, Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR), and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS). A brief description of each method is at the end of the post.
At the moment though, only two sites use all four methods to determine their positions; one at the Goddard Space Flight Center and the other in Johannesburg, South Africa. NASA hopes to create or upgrade multiple stations to use all four independent methods. Along with enhancements to the individual methods to increase precision, NASA has developed a vector tie system to further improve results. Though there are stations that can collect data using different methods, the data is not combined to come up with a single result with superior accuracy to the methods. To accomplish this, not only will the data be combined, but also lasers will be used to measure the distance between the ground components of each method.
Numerous agencies across the planet will take advantage of the information these proposed stations will produce, and many of these agencies will also be involved in the development and deployment.
Descriptions of the different positioning methods:
- GPS works by measuring the time it takes a signal to reach a minimum of four satellites. The time measurement is then used to determine a distance from each satellite, so you need to know where they are for it to work.
- VLBI measures the time it takes for a signal detected at one telescope to be detected at others. This allows the geometry between the observatories to be determined.
- SLR exploits the fact that satellites rotate about Earth’s center of gravity and measures their altitude by bouncing signals off of them.
- DORIS also measures the distance satellites are from Earth, but relies on the Doppler Effect instead of time-of-flight.