Small Aperture Telescope Observations of Co-located Geostationary Satellites

Co-location is a geostationary orbit formation strategy where two or more satellites reside within one station keeping box. As geostationary orbit continues to be populated, satellite operators are increasing usage of co-location techniques. Colocation causes the participants to move in relative motion ellipses about each other with typical separations varying from 1 to 100 kilometres. This paper focuses on correlation effectiveness on co-located geostationary satellites as their close proximity to one another is a challenge for ground-based space surveillance sensors. During the course of this study we identify two unique observational events where co-located satellites’ close proximity causes problems for ground based sensors. The satellites sometimes appear to conjunct which makes discrimination by automated space surveillance systems difficult. During these conjunctions, if one of the satellites is more optically reflective than the other, the possibility exists that it will glint-mask the fainter satellite under small phase angle conditions, further making its detection difficult. 1. I&TRODUCTIO& An issue encountered with automated optical space surveillance systems is satellite identity discrimination while observing closely-spaced geostationary (GEO) satellites. These systems can unintentionally mistag the identity of the objects they are tracking due to uncertainties in the predicted locations of the satellites. A more acute case is satellite orbital co-location which is gaining popularity with satellite operators to populate more capacity in geostationary orbit. Co-location is a GEO orbit formation strategy where two or more satellites reside within one geostationary longitude box [1]. As of this writing there are 48 satellites in 22 co-located groupings in GEO. Canada currently co-locates 6 satellites as 3 co-located pairs. The Luxembourg operator, SES, once co-located six satellites [2] in one longitude slot and currently has 3 co-located clusters. The satellites are often separated by less than 0.05 degrees of longitude and this close proximity between the satellites strains the accuracy of the general perturbation orbital elements used to predict their orbits and to correlate the identities of the satellites in optical systems. In a more extreme case, co-located satellites sometimes appear to conjunct. The actual physical separation may be tens of kilometers or more (more than enough to safeguard against collision) but the apparent angular extent of the satellite separation is small when observed by a ground-based observer. Under certain conditions it can become impossible to detect two individual objects on the field of view as the two satellites appear to merge on the image plane. For large geostationary satellites, CCD (Charged Couple Device) imager saturation is a possibility as the combined visual magnitude of the objects can sometimes exceed the well depth of limit of an optical CCD sensor performing metric measurements. In another case, two co-located satellites with largely different optical cross sections can cause glint-masking effects which can effectively mask the presence of the other satellite on a CCD imager. This paper seeks to characterize correlation effectiveness of a Ground Based Optical Sensor on co-located satellites using the DRDC Ottawa Space Surveillance Observatory [3]. The following sections provide a brief overview of geostationary orbital parameters and how geostationary co-location is performed. Relative motion of co-located satellite motion is described, and an analysis of measurements taken by a small ground based telescope is then presented. A check of the effectiveness of the correlation approach used to differentiate the satellites is investigated and results presented. We show that correlation is practical with relatively current orbital element sets on these objects but changes to the existing correlation algorithm in use by DRDC is required to further enhance its accuracy. Special cases are also shown where colocated satellites perform unique space surveillance events when detected by CCD imagers. Co-located satellites result in unique observational events as well. A visual conjunction (objects appearing to merge on the CCD imager) and glintmasking event where one object overwhelms the other due to one objects’ high reflectivity were both detected on closeformation co-located satellites. 2. GEOSTATIO&ARY SATELLITES A&D CO-LOCATIO& The International Telecommunications Union (ITU) assigns geostationary orbital longitude slots to satellite operators to maintain frequency management of the GEO orbit resource [4]. The ITU typically assigns a station keeping box of ~0.1 degrees in longitude and latitude, with approximate dimensions being 74 x 74 x 35 km. In practice, new geostationary satellites maintain station within 0.05 degrees of longitude and latitude and limiting dead bands are used to help prevent orbital box violation. Fig. 1 Geostationary satellite station keeping boxes with imagery showing Galaxy 11 (left on image) and the co-located *imiq-1, Direct TV-1 group (center of image) Geosynchronous orbital elements are linearized [1] Keplerian elements used for describing near circular geostationary satellite orbits. The primary parameter of the geostationary orbit is the semi major axis parameter (aGEO) which is equivalent to 42,164.5 km. The offset of the geostationary satellites orbital semi major axis size (a), above or below aGEO is expressed as equation 1. ( ) km 5 . 164 , 42 ≡ − = GEO GEO a a a a δ (1) Since the orbital inclination and eccentricity are near zero, the eccentricity and inclination vectors can be expressed as projections onto Earth’s equatorial plane as equations 2 and 3 respectively. [ ] [ ] y x T e e e e e , ) sin( ), cos( = Ω + Ω + = ω ω [ ] [ ] y x T i i i i i , ) sin( ), cos( = Ω Ω = (2)(3) Where e and i are the eccentricity (dimensionless) and inclination (degrees) parameters for the geostationary orbit respectively. The parameter ω is the argument of perigee and Ω is the orbit’s right ascension of the ascending node. The eccentricity vector points towards the location of orbital perigee and the inclination vector is a projection of the geostationary satellite's inclination vector onto the equatorial plane. The longitude drift rate (D) of a geostationary satellite with a semimajor axis above of below that of aGEO is expressed as equation 4. GEO a a D δ 5 . 1 − = (4) The co-location of two or more satellites in the same geostationary longitude slot requires management of the risk of collision between the co-located GEO peers. This encourages a separation approach for the formation. Geostationary satellites with suspended solar panels that experience an impact with as little as 1 m/s velocity could be very destructive [1]. The separation strategy normally used for co-located satellites uses combined offsets between the pair’s eccentricity and inclination vectors [1][5]. A relative motion ellipse is then formed where in-track position uncertainties of the satellites are aligned nearly perpendicular to the relative motion of the two spacecraft (fig. 2). This elegantly places the much higher precision components of radial and cross-track uncertainty in a direction that helps maintain separation between the satellites. The ellipse's shape can be estimated by determining the differences between each satellite's geostationary elements. The eccentricity and inclination vector differences, eij and Iij are shown as equations 5 and 6 respectively. These parameters establish the geometric shape of the relative motion ellipse. j i ij e e e − = j i ij i i I