Meridian 9 was successfully injected into orbit on February 20th, 2020. Unofficial reports indicated that the third stage of the Soyuz 2.1a rocket shut down prematurely and that the Fregat space tug needed to compensate to place the satellite into the desired orbit. Later it was noted the Argument of Perigee (AoP) for the satellite was not the typical Molniya value of 270 degrees. Was this an indication that the satellite was not placed into the correct orbit by the malfunction or something else?
Satellites are placed into Molniya orbits primarily to allow their payloads to view far northern latitudes for prolonged periods of time. The Molniya orbit uses a few properties to do this.
First, the mean motion of the orbit is synchronized with the Earth’s rotation by completing two orbits in a sidereal day.
Second, the orbit is highly eccentric so the velocity at perigee is relatively high compared to a relatively low velocity at apogee which allows the spacecraft to loiter over a spot for a prolonged period of time minimizing tracking requiurements.
Finally, the inclination of the orbit is chosen to take advantage of the equilibrium of the tidal forces from the Moon and the Sun to freeze the Argument or Perigee of the orbit over a desired spot and prevent it from drifting and minimizing (or eliminating) the need to perform station keeping manoeuvrers. The chosen inclination often called the ‘critical inclination’ is approximately 63.4 degrees.
Meridian 8 [2019-046A, 44453] orbit visualized to demonstrate the basic characteristics of a Molniya orbit. The apogee is usually high over the northern hemisphere with the perigee locked in position over the southern hemisphere. Although you can reverse this and place the apogee over the southern hemisphere if desired.
Historically most spacecraft launched into Molniya orbits use an Argument of Perigee of approximately 270 degrees. This locks the perigee due south of the apogee when the inclination is approximately 63.4 degrees. As we will see the Meridian constellation, which uses Molniya orbits, uses two initial different value of Argument of Perigee and these values are related to the spacecraft’s position in the constellation.
Meridian Constellation’s Use of AoP
After studying the recent launch of Meridian 9 it was noted that the initial Argument of Perigee was not the expected value of approximately 270 degrees as we noted from Meridian 8, rather it was approximately 295 degrees. It was first assumed that the third stage anomaly the mission encountered may have been the cause but after reviewing the history of the constellation’s TLEs an interesting pattern emerged placing doubt as the the malfunction theory.
Inspecting the operational characteristics of the constellation one will notice that there are always two spacecraft at or near apogee at any one time. Each of these two satellites occupies the North American and Russian apogee respectively at the same time. They arrive at apogee at roughly the same time and loiter for about six hours in this position.
The ground tracks of the Meridian 4 and 6 pair.
The ground tracks for Meridian 7 and 8 pair.
What we noticed is that the spacecraft within the pair have ‘opposite’ Argument of Perigee values. That is to say one has a value of approximately 270 and the other 295 degrees.
There are two satellites in the Meridian constellation at each of the two respective apogees at any given time. Each of these satellites in this pair have different Argument of Perigee values. The pattern becomes clear when examined in the table above and note the behaviour of Meridian 9.
Constellation History Confirms a Pattern
Meridian 1 was never really part of the constellation as it was launched in 2006 and failed before the arrival of a successful launch into the present constellation by Meridian 3. Meridian 2 failed to achieve the correct orbit and operates outside the constellation to this day.
Meridian 8 was launched to replace Meridian 3 that went silent a few weeks before the launch of Meridian 8. As you can see from these plots the initial value of Meridian 3’s Argument of Perigee is similar to that of Meridian 8’s, around 270 degrees.
Meridian 8 was launched into Meridian 3’s orbital plane. They share the same initial AoP value of ~270 degrees.
As noted earlier Meridian 9 appears to have been launched into the same plane as Meridian 6. The initial AoP for these two is approximately 295 degrees.
Meridian 9 was launched into Meridian 6’s orbital plane. They share the same initial AoP value of ~295 degrees.
What Does this Mean?
As the Meridian constellation has matured a distinct pattern has developed as to the choice of the specific spacecraft AoP based on its position within the constellation. This observation clearly indicates that Russian claims that Meridian 9 was placed into the correct orbit are true despite the third stage anomaly and the recovery using the Fregat tug.
While this observation is useful for future predictions of member orbits before launch what the purpose of this is unknown. I would like to speculate that there is some clever astrodynamics going on here worthy of more study to understand this further to unravel this riddle in the sky.