Global Positioning System (GPS) satellites transmit at least 2 low-power radio signals. The signals transmit by line of sight, meaning that they can transmit and receive data only where the stations of transmitter and receiver are in view of each other without any sort of obstacles between them. They will pass through clouds, glass and plastic but won’t go through most solid objects, such as buildings and mountains. However, modern receivers are more sensitive and can usually track through houses.
A GPS signal contains 3 different types of information:
- Pseudorandom code is an I.D. code that determines which satellite is transmitting information. You can see which satellites you are getting signals from on your device’s satellite page.
- Ephemeris data is needed to locate a satellite’s position and gives important information about the performance of a satellite, current date and time.
- Almanac data tells the GPS receiver where each GPS satellite should be at any time throughout the day and shows the orbital information for that satellite and every other satellite in the system.
GPS Signal Errors and Accuracy
There are few factors that can affect GPS accuracy and signal consist of the following:
- Gravity can pull the GPS satellite slightly out of orbit, so the satellites reported location’s may not be accurate.
- Ionosphere and troposphere delays, satellite signals may slow as they pass through the atmosphere. The GPS system uses a built-in model to partially correct for this type of error.
- The GPS signal may reflect off objects such as tall buildings or large rock surfaces before it reaches the receiver, which will increase the travel time of the signal and cause errors.
- Parts of the Earth’s atmosphere sometimes distort the satellite radio signals.
- A receiver’s built-in clock may have slight timing errors because it is less accurate than the atomic clocks on GPS satellites.
- The more satellites a GPS receiver can “see,” the better the accuracy. Trees, buildings, mountains and other structures can also block the signals. When a signal is blocked, you may get position errors or possibly no position reading at all. GPS units usually won’t work underwater or underground, however new high-sensitivity receivers are able to track some signals when inside buildings or under tree-cover.
- Rather than in a line or tight grouping, satellite signals are more effective when satellites are located at wide angles relative to each other.
- Selective availability such as The U.S. Department of Defense once applied Selective Availability (SA) to satellites, making signals less accurate in order to keep ‘enemies’ from using highly accurate GPS signals. The government turned off SA in May of 2000, which improved the accuracy of civilian GPS receivers.