GNSS constellations are coordinated groups of satellites maintained by different governments and organizations. These constellations operate separately, but many receivers can receive signals from multiple constellations. There are several in operation:

Every GNSS satellite is constantly sending out signals that include the exact time of the signal and the satellite's known location. The receiver does the work with that information to determine its own location in relation to the satellites. It needs to be in view of at least 4 to calculate an accurate location.

GNSS receivers use a mathematical technique called trilateration to determine the exact position of a receiver. All signals between satellites and receivers travel at the speed of light, and each signal includes the exact time of transmission. The receiver uses the information in each satellite's ping to compute how far away it is, as the satellite-receiver distance is equal to the distance light travels between the timestamp on the satellite's signal and the exact time the receiver gets the signal.

There are several sources of inaccuracies in GNSS positioning. Some of the common sources include:

• Atmospheric Delays: GNSS signals can be delayed or distorted as they pass through the Earth's atmosphere. Factors such as ionospheric and tropospheric conditions can introduce errors in the signal propagation, leading to inaccuracies in positioning.
• Multipath Interference: Multipath interference occurs when GNSS signals reflect off surfaces such as buildings, trees, or other obstacles before reaching the receiver. These reflected signals can interfere with the direct signals, causing errors in the position calculation.
• Satellite Orbit Errors: GNSS satellites have predictable orbits, but there can be slight deviations from their ideal paths. Inaccuracies in satellite orbit determination or clock synchronization can introduce errors in the positioning calculations.
• Receiver Errors: GNSS receivers themselves can introduce errors due to imperfections in the receiver hardware or software. These errors can arise from signal processing, noise, or calibration issues within the receiver system.
• Geometric Dilution of Precision (GDOP): GDOP is a measure of the geometric configuration of satellites relative to the receiver. Poor satellite geometry, such as having satellites clustered in one area of the sky, can result in higher positioning errors.
• Signal Obstruction: Buildings, trees, and other physical obstructions can block or weaken GNSS signals, leading to reduced signal quality and increased positioning errors. However, GNSS signals are not affected by clouds, fog, or even rain. The frequencies were specifically chosen to propagate through these weather events and provide accurate data in any weather or climate.
• Ephemeris and Clock Errors: The precise information about satellite positions (ephemeris) and satellite clocks is continuously transmitted to receivers. However, errors in the ephemeris data or satellite clock synchronization can impact the accuracy of GNSS positioning.

Differential corrections involve comparing the measurements from a reference station with those from a roving receiver. By calculating the differences in location data between the two sets of measurements, correction data is generated and applied to the roving receiver's measurements, resulting in improved accuracy.

RTK is a type of differential correction that involves measuring the carrier phase of GNSS signals instead of the positional data within them. By analyzing the carrier phase, RTK can mitigate errors caused by atmospheric delays and other sources, leading to centimeter-level positioning accuracy in real time.

Precise Point Positioning (PPP) is a technique used in GNSS positioning to achieve high-precision and accurate positioning solutions. Unlike traditional differential techniques that rely on correction data from nearby reference stations, PPP leverages a global network of reference stations distributed worldwide. By utilizing precise measurements from multiple GNSS satellites and sophisticated algorithms, PPP enables centimeter-level positioning accuracy without the need for a nearby reference station.

PPP operates by processing the raw GNSS measurements from the user's receiver along with the measurements from the global network of reference stations. The reference station measurements are used to compute correction parameters, such as satellite orbit and clock errors, atmospheric delays, and other systematic errors. These correction parameters are then applied to the user's measurements to improve the accuracy of the positioning solution. PPP takes into account factors such as satellite geometry, atmospheric conditions, and signal propagation delays to achieve precise positioning results.

SBAS is a regional or global system that utilizes additional satellites and ground-based infrastructure to augment and enhance the accuracy, integrity, and availability of GNSS signals. SBAS systems are designed to provide improved positioning accuracy, integrity monitoring, and availability of GNSS signals for various applications.

The primary purpose of an SBAS is to mitigate errors and enhance the performance of GNSS by providing correction and integrity information. These systems are typically operated by governmental or collaborative entities and cover specific geographical regions. Different regions have their own SBAS systems; Wide Area Augmentation System (USA), European Geostationary Navigation Overlay Service (Europe), Multi-functional Satellite Augmentation System (Japan), and the Indian Satellite-Based Augmentation System (India) are a few. These are all operated by their respective region's governing body for aviation.

SBAS systems use a network of reference stations, satellite links, and ground-based master stations to collect GNSS measurements, compute correction data, and broadcast them to user receivers via geostationary satellites. GNSS receivers equipped with SBAS capability receive these correction messages, which enable them to enhance their accuracy and reliability.

We sometimes think of ourselves living in a 3D world, but time is another dimension to our space we don't always remember. An event happens at a location that can be defined in x, y, and z coordinates, but cannot be pinpointed in spacetime unless t is also defined. Time is an integral part of the way we experience space, which makes it the fourth dimension to our universe.

This means that precise time measurement is as necessary to science as measuring three-dimensional space. Keeping time is just as important as understanding it, and humans have been inventing increasingly complex ways to measure time since the sundial, all the way up to the atomic clock and beyond.

A disciplined oscillator is an ulta-precise clock or frequency reference that combines GNSS technology with an internal stable oscillator. The oscillator is controlled to agree with time signals received from GNSS satellites, whose on-board clocks are incredibly accurate. Since the oscillators are so stable, resolution between satellite pings can approach a picosecond, or one trillionth of a second.

What types of industries and projects need timing accuracy this precise?