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Understanding GPS/GNSS RINEX Files and Relevant Parameters

Friday, July 31, 2020 - 20:32 | Author: wabis | Topics: Knowlegde, Science, Geodesy

In the field of geodesy, Receiver Independent Exchange Format (RINEX)

is a data interchange format for raw satellite navigation system data. This allows the user to post-process the received data to produce a more accurate result, usually with other data unknown to the original receiver, such as better models of the atmospheric conditions at time of measurement.

Purpose of RINEX Files

The final output of a navigation receiver is usually its position, speed or other related physical quantities. However, the calculation of these quantities are based on a series of measurements from one or more satellite constellations. Although receivers calculate positions in real time, in many cases it is interesting to store intermediate measures for later use. RINEX is the standard format that allows the management and disposal of the measures generated by a receiver, as well as their off-line processing by a multitude of applications, whatever the manufacturer of both the receiver and the computer application. [1]

RINEX Observation File

Click into a field of the following example of a RINEX observation file to show a description of the field. Click anyhere in the image to hide the description. You can also select a field description by clicking on the tabs at the bottom of the image.

Example RINEX Observation File

Approximate Position of Receiver (Antenna Marker)

Observation Data

Observation Types

Number of Observations

Epoch (Receiver Time) and List of Satellites

RINEX Version

RINEX File Creation Data

RINEX Name: Antenna, Observer, Agency

Receiver Data

Antenna Data

RINEX
Receiver Position
Observations
Observation Types
Number of Observations
Epoch
Version
Created
Names
Receiver Data
Antenna Data

Observables Parameters

Excluding the approximate position APPROX POSITION XYZ in the header, RINEX files do not contain position estimates. RINEX simply provides the raw observables from each satellite that are used to compute positions. [2]

The basic GNSS observable is the travelling time ΔT of the signal to propagate from the phase centre of the satellite antenna (at the emission time) to the phase centre of the receiver (at the reception time). This value multiplied by the speed of light gives us the apparent distance D = c · ΔT between them. This measurement D = c · ΔT is what we know as pseudorange or pseudodistance and it is an "apparent range" between the satellite and the receiver that does not match with its geometric distance due to synchronism errors between receiver and satellite clocks and atmospheric delays. [3]

Further GNSS observables are code pseudoranges and carrier phases as well as Doppler measurements. [4] [5] [6]

The carrier-phase measurement at one or both carriers (L1, L2), actually being a measurement on the beat frequency between the received carrier of the satellite signal and a receiver-generated reference frequency.
The pseudorange measurement (P1, P2, C1, C2), equivalent to the difference of the time of reception (expressed in the time frame of the receiver) and the time of transmission (expressed in the time frame of the satellite system) of a distinct satellite signal.
The observation time (Epoch) being the reading of the receiver clock at the instant of validity of the carrier-phase and/or the code measurements.

PRN = Pseudo Random Noise

Pseudorandom noise (PRN) codes are an important element of code division multiple access (CDMA) based satellite navigation systems. This code allows any receiver to identify exactly which satellite(s) it is receiving. [7]

APPROX POSITION XYZ = Approximate marker position (WGS84) ECEF coordinates

According to the RINEX 2.11 specification provided by NASA, APPROX POSITION XYZ refers to the position of the antenna marker. The coordinates are defined by WGS84. Typically the position is of psuedorange quality, i.e. not corrected for time and atmospheric errors. [2]

OPUS (Online Positioning User Service) uses the approximate position to select CORS (Continuously Operating Reference Stations). [8]