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.
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.
This is one of the articles in our ‘What is GNSS?‘ series.
