OneWeb Collision Avoidance Manoeuvre Analysis

While waiting for the publication of the semi-annual Starlink constellation status report from SpaceX, which is expected in the next two to three weeks, I thought I'd take a look at some data published by OneWeb in December 2023 relating to the collision avoidance manoeuvres performed by spacecraft from the OneWeb constellation in the period February 2021 to March 2023. These data were published as part of a broader look at the challenges associated with the operation of the OneWeb constellation in a paper by Yoke Yoon and colleagues from the Wood Lane office of OneWeb in London.

In their paper, Yoon et al. describe the conditions leading to a red conjunction - one that triggers the automated collision avoidance manoeuvre planning procedure:

• A conjunction with collision probability (Pc) >= 2E-5 (or 1-in-50,000), or
• A miss distance <= 2 km

and with Time to Closest Approach (TCA) <=48 hours.

This collision probability threshold is five times lower than the level recommended by NASA but two times higher than the level that was used by SpaceX for its Starlink constellation in the same time period (SpaceX changed the probability level in 2024 to 1E-6, or 1-in-1 million). As a result, one would expect fewer manoeuvres by OneWeb spacecraft compared with Starlink, but perhaps more compared with other space systems, if they were operating in a similar environment.

Yoon et al. provided a visual representation of the collision avoidance manoeuvre frequency for the OneWeb spacecraft over a two-year period from February 2021 (reproduced here in Figure 1). This graphic shows the average number of collision avoidance manoeuvres per spacecraft per month in 100 km bins covering the altitudes traversed by the OneWeb spacecraft during their ascent from injection to their target orbit at 1200 km.

As well as offering a valuable insight into the manoeuvre burden for OneWeb, this graphic might also help us to understand the same burden for other operators with spacecraft in this low Earth orbit (LEO) region. In a sense, the OneWeb spacecraft are like the canaries in the mine, providing some clues about the collision risks in what is a highly congested part of LEO. Figure 2 shows the average monthly manoeuvre rates calculated for the period January to December 2022 for all altitude bins between 400 km and 1200 km. It reveals that the greatest manoeuvre burden - and hence the greatest collision risk - is associated with altitudes between 700 km and 1100 km. Whilst this is mostly consistent with our knowledge of the spatial density at these altitudes (see Figure 3), it does highlight some interesting differences, particularly at 700 km (lower than expected manoeuvre rate), 1000 km and 1100 km (both higher than expected manoeuvre rate).

It's important to understand how the OneWeb spacecraft move through this region. They ascend using Hall Effect Thrusters, which are low-thrust propulsion systems. This leads to to altitude profiles for OneWeb resembling those shown in Figure 4 (taken from Jonathan McDowell's incredible website). Ignoring pauses in the ascents, which usually occur to align the orbit right ascension of the ascending node with the destination orbit plane in the OneWeb constellation, the rate of ascent is about 10 km per day. Hence, traversing an altitude range of 100 km would usually take about 10 days.

Hence, the collision avoidance manoeuvres shown in Figure 1 and Figure 2 for altitudes below 1200 km will have been accumulated not over monthly intervals, but over intervals typically lasting about 10 days - the time taken for the spacecraft to traverse each altitude bin. Of course, spacecraft that pause their ascent will likely see a higher manoeuvre rate than those that do not pause. These instances will produce an enhancement of the average monthly manoeuvre rate, simply by virtue of the longer dwell time in the same bin. Setting aside this caveat, it's possible to imagine that other spacecraft residing wholly within a single altitude bin would see monthly manoeuvre rates that are about three times higher than shown in Figure 1 and Figure 2 (as one month is approximately 30 days). For example, the average monthly manoeuvre rate reported for OneWeb spacecraft by Yoon et al. for the year 2022 at altitudes 800-900 km is 3.46 (Figure 2), but one might expect the true rate to have been about 10 per month for a OneWeb spacecraft remaining at that altitude for a whole month. With a relatively simple calculation, it's possible to estimate what the monthly manoeuvre rate would be for a red collision probability threshold of 1E-4 (or 1-in-10,000), which is the level recommended by NASA. It's about two manoeuvres per spacecraft per month, or about 24 per spacecraft per year.

The other takeaway from the data published by Yoon et al. is the manoeuvre rate for OneWeb spacecraft at the service altitude of 1200 km, shown for January 2022 to March 2023 in Figure 5.

The average monthly manoeuvre rate for each spacecraft at the service altitude over the period in Figure 5 was 0.25, which equates to 2.96 manoeuvres per spacecraft per year. For a constellation of 648 spacecraft at the service altitude, one might therefore expect about 1900 collision avoidance manoeuvres per year for the whole constellation, ignoring any spacecraft ascending or descending. Over the same period, Starlink spacecraft were performing collision avoidance manoeuvres at a rate of about 8 per spacecraft per year, with about 20,000 manoeuvres performed by the Starlink constellation as a whole.

So, a OneWeb constellation spacecraft operating at an altitude of 1200 km, where the debris spatial density is relatively low (Figure 3), might have expected to experience a proportionally lower manoeuvre burden because of this and its chosen red collision probability level. Instead, the OneWeb spacecraft appear to be experiencing a relatively high manoeuvre burden at the service altitude, compared with the same for Starlink.

Every manoeuvre represents a meaningful risk to a OneWeb spacecraft. Hence, the manoeuvre rates are an indication of the risks faced by this constellation (just as the Starlink manoeuvre rates are an indication of the risks faced by Starlink). It is therefore of huge value and importance for operators like OneWeb to provide information about their manoeuvre burden on a frequent and regular basis. Only through such data reporting is it possible to gain an understanding of the impact of these space systems on the safety and sustainability of the space environment. I continue to be grateful for such reporting from SpaceX (admittedly, mandated by the FCC). I hope that other operators soon follow suit.

See you soon for a look at the data from SpaceX!

Hugh

18 December 2024