On April 1st GPS users experienced an unprecedented event; the full operational loss of the Russian GNSS constellation commonly referred to as GLONASS. As detailed in the GPS World article “GLONASS Gone . . . Then Back” the GLONASS system experienced a total disruption.
Frontier Precision received many calls from users around the country stating they were unable to connect to the Russian Constellation. As the GPS World article explains, this was the result of a bad ephemeris upload to all satellites. We wanted to take advantage of this event to explain what ephemeris data is and how it will affect the use of GPS on the ground.
Unlike the Almanac, which has approximate location information of all satellites, ephemeris data contains the exact orbital position of each satellite. The ephemeris data is critical to positioning on the ground because if your receiver doesn’t know the exact location of each satellite in the sky, it will never be able to calculate your position on the ground.
Ephemeris is defined by Merriam Webster as “a tabular statement of the assigned places of a celestial body for regular intervals.” For GPS, ephemeris data is stored in each satellite. This information is then transmitted from the satellites to receivers on the ground through each satellite’s navigation message. The navigation message for each satellite is comprised of three parts: GPS date and time, plus the satellite’s status and an indication of its health, the orbital position data (ephemeris), and the almanac. This message is modulated (or carried in) both the C/A and P(Y) carrier codes.
Since the information carried in the ephemeris data is highly detailed and considered valid for only 4 hours, periodic manual updates from the ground to the satellite vehicles are required. This is where the issue arose. The Russian Aerospace Defense Forces that operate the GLONASS system was in the process of updating the ephemeris data. There happened to be an error with the data they were uploading. This error caused the system to fault and took the GLONASS system offline for about 12 hours until a new message could be uploaded to each satellite. This failure is a testament to how important the ephemeris data truly is.