GNSS/GPS Positioning, Applications, and Accuracy

Global Navigation Satellite System (GNSS)

is a satellite-based navigation system used for positioning, navigation, and timing.

position;

orbit or position

For a receiver to determine its ___, it must not only receive signals from satellites, the receiver must also be able to determine and predict the ___ of the satellites from which it receives signals.

highly stable clocks

GNSS also relies heavily on ___ because positioning is based on measuring the travel time of electromagnetic signals.

Rubidium and cesium atomic frequency standards;

hydrogen masers;

10⁻¹³ to 10⁻¹⁴

GNSS satellites therefore use extremely stable frequency standards. ___, as well as ___in some high-precision applications, are used for highly accurate time and frequency control. These clocks can have frequency deviations on the order of only ___.

Satellite surveying

refers to the use of artificial satellites and their signals to determine positions on or near the Earth's surface.

Land surveying

involves surveying and mapping features on the Earth's surface.

Marine surveying

includes hydrographic surveying, oceanographic surveying, and exploration geophysics surveying.

Hydrographic surveying

deals with the measurement and mapping of features associated with bodies of water.

Oceanographic surveying

involves observations and measurements related to oceans.

Exploration geophysics surveying

involves investigating the physical characteristics and subsurface structure of the Earth.

Airborne surveying

uses aircraft or other airborne platforms to collect surveying information.

space segment

consists of the satellites that transmit navigation signals.

control segment

consists of monitoring and tracking stations distributed around the world.

user segment

consists of GNSS receivers, antennas, data-processing software, and other equipment used by the users.

electromagnetic waves

GNSS satellites transmit ___ that propagate at approximately the speed of light.

L-band;

1.2 to 1.6 GHz;

19 to 25 cm

The main GNSS signals used by GPS occupy frequencies in the ___, roughly within the ___ range. The wavelength of these signals is approximately ___.

1575.42 MHz;

19.03 cm

The GPS L1 frequency is ___, with a wavelength of approximately ___.

1227.60 MHz;

24.42 cm

The GPS L2 frequency is ___, with a wavelength of approximately ___.

1176.45 MHz;

25.48 cm

The GPS L5 frequency is ___, with a wavelength of approximately ___.

C/A code or Coarse/Acquisition code

is a PRN code used for signal acquisition and civilian GPS positioning.

P code or Precision code

is a more precise ranging code.

navigation message

contains information needed by the receiver, including satellite information used for positioning and satellite orbit determination.

PRN or Pseudo-Random Noise

allows the receiver to distinguish one satellite's signal from another.

1.003 MHz;

1023 chips;

1 millisecond

PRN code has a frequency of approximately ___, contains ___, and the sequence lasts approximately ___.

GPS satellite → electromagnetic signal → GPS receiver

GPS measurements process

Ranging

is the process of determining the distance between a satellite and a receiver.

pseudorange

the measured range is called a

trilateration

Positioning is accomplished through

Pseudorange

is based on the measured travel time of the satellite signal.

Pseudorange

It is relatively simple to process and is the basic measurement used by many GPS receivers.

Carrier phase

determines the phase of the carrier wave received from the satellite.

Carrier phase

can provide much higher precision than ordinary code-based measurements.

Doppler

uses the frequency shift of the received signal.

Doppler

It is particularly useful for determining the motion and velocity of the receiver.

Pseudorange = geometric distance + clock effects + atmospheric effects + measurement errors

A simplified way to understand pseudorange is

Differencing

is a GNSS processing technique in which observations are combined by taking differences.

Differencing

The purpose is to reduce or eliminate errors that are common between observations.

static positioning

the antenna remains stationary during the observation period.

static positioning

This type of positioning is commonly used when accurate coordinates are required.

kinematic positioning

the antenna is moving while observations are being collected.

absolute positioning

a receiver determines its position using satellite observations without relying on simultaneous observations from another nearby receiver.

differential positioning

observations from two or more receivers are used together to reduce common errors and improve positioning.

Measurement errors and biases

can affect the calculated position.

positioning mode

The ___ also affects accuracy. Absolute and differential positioning do not have the same error characteristics.

Satellite-receiver geometry

___ affects the quality of the position. The arrangement of the satellites relative to the receiver influences the resulting uncertainty.

processing algorithm

The ___ can also affect the final result. Different algorithms and processing methods handle observations differently.

operational mode

is important because a receiver may operate in real time or use post-processing.

moving or stationary

Whether the receiver is ___ also affects the positioning method.

Measurement noise

introduces uncertainty into the observations.

Redundancy

provides additional observations that can help detect errors and improve the reliability of the solution.provides additional observations that can help detect errors and improve the reliability of the solution.

Kalman filter

can be used to estimate the state and movement of a receiver using observations collected over time.

Accuracy

refers to how close the determined position is to the true or accepted position.

Availability

refers to whether the positioning service is available and usable when required.

Reliability

refers to the ability of the system to consistently provide usable positioning information.

Integrity

refers to the ability of the system to detect problems or provide warnings when the positioning information should not be trusted.

Cost

is also an important consideration because equipment, observation, processing, and operational expenses affect the use of GNSS.

Random errors

vary unpredictably between observations. Repeated observations and statistical processing can help reduce their effect.

Systematic errors

are consistent or predictable biases. These may be modeled or corrected.

Gross errors

are major mistakes or blunders. They can result from incorrect observations, equipment problems, or operator mistakes and should be detected and removed.

Accuracy

refers to how close the measured position is to the true or accepted position.

Precision

refers to how closely repeated measurements agree with each other.

circle or confidence ellipse

For horizontal positioning, uncertainty can be represented using a

sphere or three-dimensional error ellipsoid

For three-dimensional positioning, uncertainty can be represented using a

CEP or Circular Error Probable

epresents a 50% probability level under the assumptions used in the model.

95% confidence level

represents a higher probability that the true position lies within the specified confidence region.

CEP (Circular Error Probable) → 50% DRMS (Distance Root Mean Square) → 68.3% 95% confidence → 95% 2DRMS → 98.2%

approximate probability relationships

Selective Availability (SA)

was a historical GPS policy that intentionally degraded the positioning accuracy available to civilian user