3.4.11. Errors in GNSS Observations

The process of transmitting, receiving and detecting the GNSS signals may contain certain errors. The GNSS positioning accuracy is affected due to (Garg, 2021): (i) satellite signal blockage owing to high-rise buildings, bridges, dense forest trees, etc., (ii) indoor or underground use of GNSS, and (iii) signals reflected-off buildings or walls ("multipath").

There are several ways GNSS signals can encounter errors. First, signals can be blocked by objects like tall buildings or dense trees, which prevents the receiver from obtaining a clear signal. If someone is indoors or underground, GNSS signals have difficulty reaching the receiver. Lastly, signals bouncing off of buildings or other structures before reaching the receiver can significantly alter the time it takes for the signal to reach the device, leading to inaccuracies.

Imagine trying to hear someone speak to you while standing in a crowded room with loud music and people talking. The voices of the people around you (buildings and trees) block the sound from your friend (GNSS signals). Just like how you might not catch everything your friend says because of the surrounding noise, GNSS signals can be obstructed and reflected, leading to inaccuracies.

Other causes may include: (i) radio interference or jamming, (ii) major solar storms, (iii) satellite maintenance/ manoeuvres creating temporary gaps in coverage, and (iv) improperly designed devices that do not comply with GNSS interface specifications.

Several external factors can disrupt GNSS signals. Radio interference occurs when other radio waves interfere with the transmission of GNSS signals, leading to errors. Solar storms can generate electromagnetic interference that disrupts satellite signals. During maintenance, satellites might be temporarily out of service, affecting signal availability. Additionally, devices not built to GNSS standards may have issues receiving signals properly.

Consider a radio that sometimes tunes into other stations instead of the one you're trying to listen to. This is similar to how radio interference can affect GNSS signals. If too many waves are competing, your intended signal might get distorted or lost, leading to incorrect positioning data.

Due to mapping software, the errors may include, such as, (i) incorrectly drawn maps, (ii) mislabelled businesses and other points of interest, and (iii) missing features, etc.

Errors can also occur in the software that interprets GNSS data. This can happen because the maps might be outdated or drawn inaccurately, businesses may be mislabelled, and important landmarks might be absent altogether. These errors can lead users to incorrect locations or mislead them about their surroundings.

Imagine trying to follow a map that hasn’t been updated since a new road was built or a store changed locations. You may find yourself lost or directed a long way from where you wanted to go. Similarly, inaccurate mapping software misguides users relying on GNSS data for navigation.

The major sources of errors and their magnitudes are given in Table 3.8. These are shown in Figure 3.29, and explained below.

Different categories of errors have specific magnitudes contributing to GNSS inaccuracies. For example, errors due to satellite ephemeris and clock issues have a magnitude of approximately 3 meters, whereas ionospheric delays can add up to 4 meters to the error. The magnitude of these errors shows how significantly they can affect GNSS observations.

Think of a game of darts. If you consistently throw with a slight aim off to the right, every dart will miss the bullseye by a similar amount. In GNSS, errors like satellite clock issues or atmospheric delays can consistently shift the calculated position, leading to a predictable bias and inaccuracies in navigation.

1. Receiver clock errors- A receiver's built-in clock is not so accurate as the atomic clocks on-board GPS satellites, therefore, the data may have slight timing errors.
2. Orbital errors- These are also known as ephemeris errors that are the inaccuracies in the reported locations of satellites.
3. Number of satellites visible- In general, lesser the number of satellites tracked, lesser is the accuracy of GNSS observations.
4. Interferences- Magnetic terrain, magnetic ores, electronic interferences, electric poles, etc., can cause positional errors in measurements.
5. Undercover- Typical GNSS units do not work indoors, underwater or underground.
6. Multi-path error- It occurs when the GNSS signals are reflected-off the objects falling in the path, such as tall buildings or dense forest or large mountain, before it reaches the GNSS receiver.

Each of these types of errors can critically affect GNSS performance. For instance, the receiver’s internal clock may not be as precise as the ones used in satellites, leading to timing errors. Orbital inaccuracies can misplace data regarding where satellites actually are. The accuracy of a GNSS observation is heavily dependent on having a good view of multiple satellites; fewer satellites usually mean less accuracy. Natural and man-made interferences can cause additional complications. For example, signals might be blocked by electromagnetic fields or physical barriers, and multi-path errors occur when signals bounce off surfaces before reaching the receiver, complicating the accuracy further.

Imagine if you were trying to catch a ball thrown by a friend but someone else threw several other balls at different angles and speeds at the same time. You might mistakenly catch the wrong one or not be able to catch it at all. This scenario is similar to how multiple interference sources can confuse a GNSS receiver, leading to positioning errors.

7. Ionospheric and tropospheric delays- The satellite signals slow down as they pass through the atmosphere. The GNSS system uses a built-in model that calculates an average amount of delay to partially correct this type of error.

As satellite signals travel through the Earth's atmosphere, they encounter different layers that can slow them down, particularly the ionosphere and troposphere. These delays affect the timing of the signals, and while GNSS systems can estimate these delays and make corrections, they may not fully compensate for them all the time.

Think of how your voice sounds different when you shout underwater versus when you shout in the air. The water slows down the sound waves just like the atmosphere slows down GNSS signals. The GNSS uses models to predict this slowing down, but it’s not always perfect.

8. Satellite geometry- This refers to the relative position of the satellites at any given time, which may not be desirable positions. In an ideal satellite geometry, the satellites are located at wide angles relative to each other. The dilution of precision (DOP) is an indicator of the geometrical strength of the satellites being tracked at the time of measurement.

The arrangement of satellites in relation to one another and the receiver affects the precision of the GNSS signals. Ideally, satellites should be spaced out at wide angles to minimize the chance of errors, allowing for more accurate triangulation of a position. If satellites cluster closely together, DOP increases, negatively impacting accuracy. DOP values can inform users about the quality of the satellite configuration.

Imagine trying to triangulate your position based on three landmarks. If two of them were very close together, it would be hard to figure out exactly where you are. It's like trying to pinpoint where you live using just two homes on the same block; you'd want homes spread out to get a clearer picture of your location.

--