Collecting Data for NGS
This Monday I will get my first experience with static GPS. I will occupy an NGS point for at least 4:10:00, the boss
likes a ten minute buffer, then move on to another point. The data will be used by NGS for the 2022 datum. The boss
showed me the logging data from a short static session he ran at the office. Does anyone have a description of what all
the data means?
If you viewed the file itself in a text viewer then I would say you were looking at RINEX file.
NGS requires The Receiver Independent Exchange Format (RINEX) files. Depending on how your receiver collects data, each occupation set creates at 3 files (The observation data file , the navigation message file and the Meteorological data file).
The observation file header contains the basic information about the setup (antenna type & height, time, point name etc.). The main body of the file grouped by individual observation and contains information collected from each satellite.
Wiki has a link to the IGS documentation on the RINEX format if you want to really delve into it.
Sounds like you are very new at GPS. Things you want to make sure you have recorded, most of which should be in the RINEX file.
Site IPD number, Receiver type and serial number, antenna type and serial number, antenna height to ARP (Antenna Reference Point), Company name.
Record the antenna height twice at the beginning and twice at the end of the observation. Measure in feet and remeasure in meters, do not measure in one and convert to other. You can add information on the "comment" lines. The RINEX file should automatically include the RINEX converter type and RINEX file type.
Of interest in the data for basic GPS, the L1 and L2 are wave length specific information and are dependent on the receiver algorithms. i.e. two different receivers can give two different values for the exact same signal.
C1, P1, C2 or P2 are actual metric measurements from the satellite antenna to the receiver antenna, (20,000,000m to 24,000,000m typical)(a value like 19,730,000m means the satellite is almost directly overhead). They vary because the speed of light varies with the signal wave length and varies in different directions from the receiver antenna due to atmospheric conditions. C1 varies from P1 because of algorithmic differences, etc.
D1 and D2 are Doppler differences and the sign varies whether the satellite is approaching or departing from an overhead position.
The L1 and L2 values are to 3 decimal points and the 4th and 5th digits after the decimal point refer to other values of the quality of that signal. L1 is geater than L2 by a factor of 1.283333, the difference in wavelength. If you divide L1 by L2 and get a value near 1.283+/- that tells you the receiver is calculating the observations independently. Some receivers base both values on a intertwined calculation and the differences can be in the order of 1.2833xx to 1.2833yy.
Every epoch of data is preceded by the Julian date and GPS time of the signal, the number of satellites reported and the satellite PRN numbers in order of data, plus there is number included as to the number of GPS seconds added to the UTC clock.
A RINEX file from any manufacturers receiver can be used with any manufacturers post processing software, hence the term RINEX, Receiver INdependent EXchange format.
That is the quick version.
Paul in PA
Does anyone have a description of what all
the data means?
It means that you are going to have 4 hrs and 10 minutes of free time on your hands. Bring some reading material.