GLONASS Satellites In View

Articles about GPS, GLONASS, GALILEO, WAAS and other satellite navigation systems
jimh
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GLONASS Satellites In View

Postby jimh » Thu May 11, 2017 1:54 pm

With most modern GNSS receivers now being designed with the capability of using the Russion GLONASS global positioning satellite navigation system, boaters may be interested in knowing what satellites of the GLONASS constellation will be in view. The GLONASS system provides guidance about satellites in view at their website INFORMATION AND ANALYSIS CENTER FOR POSITION, NAVIGATION, AND TIMING. At that top page you will find a real-time plot of satellite footprints shown on a mercator map projection.

You can also get a daily summary of satellite positions on the VISIBLE ZONES CALCULATOR page. Using the calculator, I was able to generate a graph that illustrates the GLONASS satellites in view for a particular day, a particular location, and with an elevation mask of a particular angle. Here is the output generated:

GLONASS_SatelliteInView.gif
GLONASS Satellites In View
GLONASS_SatelliteInView.gif (20.67 KiB) Viewed 6971 times


The y-axis of the graph gives the GLONASS SLOT NUMBER for the satellite. You can convert SLOT NUMBER to NMEA NUMBER in most representations on chart plotters by using the relationship NMEA NUMBER = SLOT NUMBER + 64.

The X-axis of the graph gives the TIME OF DAY in Moscow, which it UTC +3. Local time at the location specified is UTC-4. Thus subtract 7-hours from the time shown to get local time.

The COLOR of the horizontal lines represents the satellite being in view ABOVE the horizon mask angle (10-degrees) at the latitude and longitude specified (45N, 82W). RED lines indicate the satellite will be in view.

A BLUE line across the bottom shows times when there are at least FOUR satellites in view. (Four satellites being the minimum number to get a position fix in three dimensions with the GLONASS constellation.) For this day and location, four satellites are always in view, 24-hours per day.

As I am writing this at approximately 2 p.m. local time (1400), I must add 7-hours to get Moscow time. Thus at 2100 Moscow time on the chart, I see that the following GLONASS satellites should appear in my sky with at least 10-degree elevation: 21, 20, 19, 11, 10, 9, 2, an 1. That is eight GLONASS satellites in view for the location, date, and day of this calculated ephemeris. You can make an inference about the maximum elevation: it will occur at the midpoint of a red line. Also, the longer the red line, the higher the satellite will appear to you. For about 2100 Moscow time on the chart, the GLONASS satellite that is probably the highest in my view would be SLOT NUMBER 10, as that satellite has a long period of being in view, almost six hours, and at 2030 it is just about in the middle of that period of availability.

Also on the top page is a near-real-time plot of a metric identified as INSTANT AVAILABILITY. This is a bit difficult to understand on the top page. See a more detailed explanation at

https://www.glonass-iac.ru/en/GLONASS/MomentaryAvailability.php

The presentations maps the regions of the globe at which the GLONASS constellation geometry results in various levels of Position Dilution of Precision (PDOP). The best level is 1 to 2, shown in white. Yellow regions have a PDOP between 2 and 3. Orange regions have PDOP 3 to 4. Position solutions at locations mapped in white should be highly accurate. Here is a typical map presentation:

Image

GLONASS satellites are in orbits with higher inclination than GPS satellites, and this tends to improve availability in higher latitudes. Note how mother Russia is almost always covered in the white region.