3.4.10. Accuracy of GNSS observations

The GPS is a pervasive and wonderful technology. It is the first positioning system to offer very high accuracy in most surveying and navigational applications at low-cost and with high efficiency.

Global Navigation Satellite Systems (GNSS), including GPS, are essential tools for precise positioning and navigation. They provide high accuracy across various uses, making them invaluable in multiple sectors such as surveying, aviation, and mapping. This high efficiency also means that users can access these technologies without incurring significant costs.

Think of a GPS system as your personal navigator for a road trip. Just as your navigator can help you find the fastest route to your destination, GNSS provides accurate positioning information that can guide surveyors and navigators in all sorts of applications.

There are two main factors that determine the accuracy of a GNSS position (Garg, 2021): (i) error on range measurement (Noise+Systematic), and (ii) geometry of the satellites.

The accuracy of GNSS systems is influenced by two primary factors. First is the error in range measurements, which can stem from noise and systematic errors in the signals received from satellites. Second is the geometry of the satellites in the sky at the time of positioning. A better satellite arrangement can enhance accuracy, while poor positioning can lead to inaccuracies.

Imagine trying to pinpoint your location on a map with only two landmarks visible. If those landmarks are very far apart or poorly positioned, your estimated location could be significantly off. Now, imagine you have many landmarks (satellites) that are evenly spread out. This helps you to determine your location more accurately.

Accuracies routinely achieved in measurement of baseline lengths in relative mode, using high precision geodetic instrumentation, with many hours of observations and scientific data processing, are as follows: (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), and (iii) 1-2 cm in global surveys (1000-10000 km baseline lengths).

Using high-precision instruments, surveyors can achieve very accurate measurements. In local surveys, they can get measurements with accuracies of just a few millimeters. For larger regional surveys, this accuracy might reduce slightly, while global surveys can still achieve centimeter-level accuracy. This allows for precise mapping and data collection across various distances.

Consider this like measuring the height of a building with different tools; a laser rangefinder might give you millimeter precision, while a tape measure from a distance might only get you close. In surveying, as the distance increases from your reference point, the margin of error also grows, making precision tools crucial for extensive measurements.

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

In standalone mode, standard GPS receivers typically have an accuracy fluctuation of about ±10 meters. However, Differential GNSS (DGNSS) enhances this significantly, often to within a meter or less, by utilizing ground-based reference stations that can correct satellite data in real-time.

Imagine using a standard kitchen scale to weigh an ingredient; it might be off by a few grams. Now, if you used a calibrated scale that adjusts based on known weights in the kitchen (like how DGNSS uses reference stations), your measurements would be far more precise, ensuring your recipe turns out perfectly.

For standalone users, the extent 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.

To achieve high accuracy, GNSS receivers can use phase data from the signals, which is more precise than the traditional code-based measurements. This phase data can have noise levels of only 1 mm, allowing users to attain positioning accuracy of between 10 cm and 1 meter.

Think of it like tuning a musical instrument. The more precisely you can adjust the notes (phase data), the better the sound quality (accuracy of position). Just as musicians rely on fine-tuning to play in harmony, surveyors rely on phase data for precise location measurements.

The multipath also significantly reduces the phase measurement. In point positioning mode, accuracy within meter with 1 epoch solution and 24 hours of observations may be possible (depends on Selective Availability).

Multipath errors occur when signals from GNSS satellites bounce off surfaces like buildings or mountains before they reach the receiver. This can distort the timing and thus the calculated position. In ideal conditions with multiple observations over time, GNSS can achieve meter-level accuracy, but factors like Selective Availability still apply.

Imagine trying to hear someone shouting to you in a crowded room. If the sound reflects off walls, it makes it harder to pinpoint where they are. Similarly, GNSS signals can become distorted when they bounce off objects, affecting the receiver's ability to accurately determine its position.

In Differential mode, accuracy in cm (in P Code) and 1-5 m (in CA code) may be obtained, while accuracy of 5 mm ± 1 ppm may be obtained on Differential phase mode.

Advanced techniques like Differential GNSS allow for even greater accuracy. Depending on the coding used – such as the precise P Code or the civilian CA code – users can achieve centimeter-level accuracy or maintain substantial accuracy improvements with phase measurements.

It’s akin to taking a photo with different cameras; a high-end camera (like P Code) provides crisp, clear images (high accuracy), while a standard smartphone camera (CA code) gives decent photos but at a lower quality. In surveying, the choice of technology impacts the final result.

Special techniques (called Kinematics) make use of the phase to achieve higher accuracies. However, they are normally used to range 25 to 50 km baselines.

Kinematic techniques utilize moving GNSS receivers to measure position dynamically, allowing for accuracy improvements. However, these techniques are usually efficient only within certain baseline distances, typically ranging from 25 to 50 kilometers.

Think of kinematic surveying as tracking the movement of a car on a highway while it's moving; the closer you are (within 25-50 km), the better the GPS can track its location accurately. If you were several miles away, the estimates might lose precision.

On May 1 2000, the US government made the SA available to all civil and commercial users world-wide (Garg, 2019).

The US government's decision to eliminate Selective Availability (SA), which intentionally degraded GPS accuracy for civilian users, marked a significant milestone. This change allowed all users, including commercial and civil applications, to access higher precision navigation without restrictions.

It’s like turning on the high-resolution feature on a camera for everyone; suddenly, every user can benefit from clearer, sharper images rather than limited quality. With this change, the capabilities of GPS for farming, navigation, and surveying have advanced greatly.

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