3.4.10.2. Achievable accuracies

Accuracy of a GNSS position is primarily determined by two factors: (i) error on range measurement (Noise + Systematic), and (ii) geometry of the satellites.

The accuracy of GNSS positions is influenced by two key factors. First is the error in range measurement, which can be a combination of noise and systematic errors. Noise refers to random errors while systematic errors can occur from equipment malfunctions or environmental conditions. Second is the geometry of satellites, referring to their arrangement in the sky. A greater angle between satellites generally leads to improved accuracy.

Think of satellite geometry like the position of three people holding a triangle of string connected to a spot you are trying to find. If they are spread out wide, it’s easy to pinpoint the location. However, if they are all close together, it’s much harder to determine exactly where that spot is.

Accuracies routine achieved in measurement of baseline lengths using high precision geodetic instrumentation are: (i) 0.1-4 mm in local surveys (10 m-100 km baseline lengths) (ii) 4-10 mm in regional surveys (100-1000 km baseline lengths) (iii) 1-2 cm in global surveys (1000-10000 km baseline lengths)

Different types of surveys yield different levels of accuracy. Local surveys that cover shorter distances (10 m to 100 km) can achieve remarkable precision ranging from just 0.1 mm to 4 mm. For larger regional surveys, the accuracy is slightly less, ranging from 4 mm to 10 mm over distances of 100 to 1000 km. Finally, global surveys that span distances of 1000 to 10,000 km may see accuracies between 1 cm and 2 cm.

Imagine you are measuring the length of a room with a very precise measuring tape. You can get very accurate readings in short distances. Now, if you're trying to measure the distance from your city to another city that is far away, the precision may not be as tight. The measurement can still be accurate, but it would be less so compared to the shorter room measurement.

Most GPS units in standalone mode may have an accuracy of about ±10 m. The DGNSS technique improves that accuracy better than ±1 m by adding ground-based reference station.

When using GPS units without any additional corrections (standalone mode), the accuracy can be quite poor, typically around ±10 meters. However, when you employ Differential GPS (DGNSS), which uses ground-based reference stations to provide corrections, the accuracy improves significantly to better than ±1 meter. This is because the reference stations can monitor errors in the satellite signals and correct them accordingly.

Consider trying to find a specific address in a large city. If you just use your phone's GPS without internet (standalone mode), it might lead you to within 10 fine blocks of your target. But if you are connected to a local Wi-Fi or cellular network (DGNSS) that updates your location more accurately, you can arrive within one block or even right at your destination.

The use of phase is a major factor in achieving the accuracy. The noise on the phase is typically 1 mm to provide the range 10 cm to 1 m.

In GNSS systems, the phase of the signal can provide more precise measurements than just the time of signal receipt. Noise is very low (around 1 mm), which means that the phase data can be incredibly accurate. By using this phase data, users can achieve a measurement accuracy within a range of 10 cm to 1 m. This aspect is particularly useful for users who need high precision in their applications.

Imagine you're trying to measure the distance across a river with a fishing line. If you simply toss a rock to the other side, you may land close, but without precision. Now, if you use a zig-zagging fishing line across multiple points, you can measure much more precisely by noting where the line passes with the least amount of wiggle.

--