Tech Primer
GPS stands for Global Position System, and can be used to determine position, time or any combination there of (speed, acceleration, altitude etc) .There are three main components in the system satellites, ground stations and receivers. The 24 satellites are evenly spaced on 6 orbits (each orbit is deployed with 4 satellites) at an altitude of 20, 200km away from earth. Each satellite contains a highly accurate atomic clock and constantly sends information on its location and the current time. A receiver determines it's distance from the satellite, by comparing the time data received from the satellite to that held in it's own internal clock, it can then estimate it's position as being somewhere on a sphere at X distance from the satellite. By comparing signals received from multiple satellites, the receiver can determine its position in 2 dimensions as the point at which three spheres intersect or for 3 dimensions as the point where 4 spheres intersect. A Receiver needs signals from multiple satellites to initially and periodically synchronize their internal clocks. However even though synchronized, receiver clocks lack the accuracy and stability of an atomic clock and errors cause inaccuracies in the calculation of distance to satellites. However since the error is common to all satellites being used to make the measurement, the receiver can compensate to some degree and improve its reading.
The ground stations, which consists of worldwide monitors and control stations, are responsible for the collection of satellite data, adjustment of orbit and synchronization of the satellites internal Clocks.
The positioning precision that a GPS receiver can provide relies on the quality of the received signal and the distribution of the satellites in range. GPS receivers commonly have 12 channels (capable of communicating with 12 satellite at the same time), and some provide 16-channel and even 20-channel receivers. The main components of a GPS receiver are the antenna, a multichannel RF receiver unit, a microprocessor, memory and power supply. Using a GPS receiver module with externally attached active antenna and Low Noise Amplifier can increase the sensitivity of the receiver.
Other Resources
- Introduction of Glogbal Positioning System
- What is GPS
- How GPS Receivers work
- Basics of the GPS Technique:Observation Equations
- GPS Receiver design
- GPS TTFF and Startup Modes
- Commercial GPS Receiver Design for high Dynamic Launching Vehicles
- Receiver Sensitivity Analysis and Results
- Signal Analysis using RHCP and LHCP Antenna
Glossary:
| 2D Operating Mode | A two-dimensional GPS position fix that includes only horizontal coordinates (no GPS elevation). It requires a minimum of three visible satellites. |
| 3D Operating Mode | A three-dimensional GPS position fix that includes horizontal coordinates, plus elevation. It requires a minimum of four visible satellites. |
| Accuracy | A measure of how close an estimate of a GPS position is to the true location. |
| Active Antenna | An antenna that amplifies the GPS signal before sending it to the receiver. |
| C/A Code | Coarse/Acquisition Code ,the standard positioning signal the GPS satellite transmits to the civilian user. It contains the information the GPS receiver uses to fix its position and time, and is accurate to 100 meters or better. |
| Cold Start | The power-on sequence where the GPS receiver downloads almanac data before establishing a position fix. |
| L1 Frequency | One of the two radio frequencies transmitted by the GPS satellites. This frequency carries the Coarse Acquisition Code (C/A code), P-Code, and the nav message, and is transmitted on a frequency of 1575.42 MHz. |
| L2 Frequency | One of the two radio frequencies transmitted by the GPS satellites. This frequency carries only the P-Code, and is transmitted on a frequency of 1227.6 MHz. |
| Navigation | The act of determining the course or heading of movement. This movement could be for a plane, ship, automobile, person on foot, or any other similar means. |
| P-Code | The precise code of the GPS signal typically used only by the U.S. military. It is encrypted and reset every seven days to prevent use from unauthorized persons. |
| RHCP | Right Hand Circular Polarization |
| TTFF | Time To First Fix (TTFF) is a specification detailing the time required for a GPS receiver to acquire satellite signals and navigation data, and calculate a position solution (called a fix). |