Education and Outreach - Tutorial: Selective Availability and the GPS Receiver

### Selective Availability, Accuracy and the GPS Receiver

You are probably wondering what effect the reference ellipsoid shape used by the GPS receiver has on your elevation measurement. Usually this effect is expressed in terms of accuracy. You might have an idea what accuracy is, but below is an example.

The effect of the GPS receiver using the reference ellipsoid to calculate elevation is LESS ACCURACY. This means that sometimes the reference ellpsoid Model causes the GPS receiver to be less accurate. Another thing that makes the receiver less accurate is Selective Availability (S.A.).

### Selective Availability and Accuracy

Selective Availability is a term used to describe the way signals from the GPS satellites in orbit around the Earth are masked. The U.S. Government controls the satellites, and uses Selective Availability to confuse the GPS receiver so it can't find your exact position.

Each satellite broadcasts a unique signal that tells the receiver the time at the satellite in space. Each satellite also has a unique position in space. The GPS receiver knows where every satellite is supposed to be, knows what time it is on the ground, and hears the signal from each satellite about what time it was in space when the signal was sent. The receiver uses all of this information to calculate the time it took for at least 4 satellite messages to travel to the Earth, and then uses the known position for those 4 (or more) satellites to figure out your latitude, longitude, and elevation.

With Selective Availability on, the GPS receiver doesn't know what time it really is at the satellites, because the S.A. makes the satellite send the wrong time. The time the satellite sends is usually pretty close to the real time, but not exact. Without knowing the exact times at the satellites when they create their time message, the receiver cannot tell you the exact location you are trying to measure. This means the GPS receiver gives you a less accurate position because of S.A too!

### Using Averages to Increase Accuracy

Let's say you took GPS measurements for a study site and you have taken 15 readings, 1 each minute for 15 minutes. As you did this exercise, the GPS readings probably seemed to wander, and so you used an average of all 15 measurements to get the position. This averaging increased the accuracy of your GPS coordinates from 100 meters to 30 meters. What does this mean?