What is SPS and what does it stand for?

The term SPS stands for Standard Positioning Service, and describes GPS position measurements that are based only on the C/A code (please see our GPS signal page for more on signal codes). The distinction is needed because not all GPS position measurements are made in the same way, and the method used to calculate the measurement affects the accuracy.

What is PPS and SPS in GPS?

GPS provides two positioning services, PPS and SPS. As described above, SPS is the Standard Positioning Service, whereas PPS, stands for precise positioning service. They differ on accuracy and in most cases who is permitted to use them. PPS is the more accurate service and is primarily used in military applications. SPS is less accurate and is used mainly for commercial purposes.

What is the position accuracy of SPS?

SPS provides the lowest accuracy GPS position measurements, normally in the region of 3–10 metres. To make SPS measurements the GPS receiver locks onto four or more satellites, and then uses the C/A code to estimate the distance to each satellite. These estimates are called pseudo-range measurements.

Using SPS, the accuracy of the measurement comes from the receiver’s ability to correctly align its internally generated C/A code with that received at the antenna. Although even if it were able to do this perfectly, the accuracy is still limited by the design of the system.

In most cases, the best alignment accuracy we can hope for is about 1% of the width of a bit. As bits are transmitted at 1,023 per millisecond, and light travels about 300,000 metres per millisecond, our 1% alignment gives us an accuracy of roughly three metres. However, some L1 receivers can achieve alignment to roughly 0.6%, giving them an accuracy of 1.8 metres.

While that level of accuracy is adequate for sat navs, greater accuracy is required in many other applications, so other measurement techniques such as DGPS (Differential GPS) and Real-Time Kinematics (RTK) are often required.

What is DGNSS?

DGNSS is an acronym of differential global navigation satellite system. It is a way of improving the accuracy of position, navigation and timing systems.

Inertial navigation systems take position measurements from GNSS satellites orbiting high above the earth. The updates can sometimes be negatively affected by atmospheric conditions – leading to inaccurate position information being relayed to the vehicle in question. To overcome this, a base station can be deployed, and its known position used as a reference to then more precisely update the vehicle’s position.

Using DGNSS corrections can vastly improve the vehicle position data.

What is C/A Code?

C/A code is one of two pieces of code sent down by a GNSS satellite. The C/A code is a binary string made up of 1,023 bits. Each satellite sends down a code unique to it and it takes only one milisecond for the complete code to be sent, however, it repeats constantly. Using the C/A code, the receiver is able to calculate the distance between it and the satellite.

You can learn more about the process behind it here – working out the range to a satellite.