Features of GPS Signals:
The GPS satellites transmit signals on two carrier frequencies.
- The L1 carrier is 1575.42 MHz and carries both the status message and a pseudo-random code for timing.
- The L2 carrier is 1227.60 MHz and is used for the more precise military pseudo-random code.
What is Pseudo Random Code?
The Pseudo Random Code (PRC) is a fundamental part of GPS. Physically it’s just a very complicated digital code, or in other words, a complicated sequence of “on” and “off” pulses.The signal is so complicated that it almost looks like random electrical noise. Hence the name “Pseudo-Random
There are two types of pseudo-random code
The first pseudo-random code is called the C/A (Coarse Acquisition) code. It modulates the L1 carrier. It repeats every 1023 bits and modulates at a 1 MHz rate. Each satellite has a unique pseudo-random code. The C/A code is the basis for civilian GPS use.
The second pseudo-random code is called the P (Precise) code. It repeats on a seven day cycle and modulates both the L1 and L2 carriers at a 10 MHz rate. This code is intended for military users and can be encrypted. When it’s encrypted it’s called “Y” code.
Since P code is more complicated than C/A it’s more difficult for receivers to acquire. That’s why many military receivers start by acquiring the C/A code first and then move on to P code.
What are Errors Of GPS System?
GDOP(Geometric Dilution of Precision)
The idea of Geometric DOP is to state how errors in the measurement will affect the final state estimation. Geometric Dilution of Precision Estimate of satellite conditions for a given location & time, Given in distance units (meters or feet), When visible navigation satellites are close together in the sky, the geometry is said to be weak and the DOP value is high; when far apart, the geometry is strong and the DOP value is low. Consider two overlapping ring of different centres . If they overlap at right angles, the greatest extent of the overlap is much smaller than if they overlap in near parallel. Thus a low DOP value represents a better positional precision due to the wider angular separation between the satellites used to calculate a unit’s position.
Ideal GDOP: One Satellite directly overhead w/an abundance of additional satellites spaced evenly around the sky