The GPS Spider
Time for a new KML animation!
The idea of this one is to plot out simulated GPS (Global Positioning System) satellite movements onto Google Earth, and simultaneously show how many are visible from a specific location at a particular time.
GPS receivers need to be able to see several of these to return accurate location fixes. This animation shows that with 24 satellites in orbit the average number visible at any one time is 8.
The visualization is pretty much a direct conversion of this GIF animation from Wikipedia’s GPS article – click on the image to view it in action. The only major difference is that I display the satellites at ground level rather than their real altitude – partly due to the display limitations in Google Earth, but mainly to give better geographic context.
Currently, the file shows satellites visible from the Googleplex – check out those fancy new roof tiles shown in GE’s recent hi-res update 🙂 – but my aim, and the main reason I decided to play around with this, is to eventually make the location completely user selectable and/or a moving target.
Download the current file here:
GPS Spider Animation 180kB
A few viewing suggestions:
- Switch off any time animations already open in GE.
- Put the time control on to a slower speed setting, and set the repeat option to wrap. Hit the play button.
- To simplify the animation, you can try turning off/on the KML features in the left side-panel, such as the orbit paths or individual satellites.
- You get some interesting effects if you slightly widen the time control in this.
To create the KML I used similar parameters to the Wikipedia GIF:
- 24 satellites, evenly spaced along six separate orbits. Each satellite completes an orbit in 12 hours. These are depicted using Icons, with varying <heading> values to point them in the right direction.
- Orbital Paths (shown as white lines using an image overlay) are spread around the Earth at 60 degree intervals, and inclined at 55 degrees to the equator. During the animation these rotate because they’re are fixed in orientation relative to distant stars, not the Earth.
- The green lines represent direct lines of sight.
- The counter shows the current number of satellites visible. In KML this is simply done by flipping through a series of different screenoverlay images. One for each number.
- The animation shows one day, which is exactly how long it takes for a satellite to return to the same geographic location. In real life this would be a sidereal day: just under 24 hours.