Positions in RT3 are represented by the AgEntityPosition object. Point entities have a single position that always represents its last known point, while polyline entities have a collection of positions that indicate its last known geometry. Position in RT3 is always represented as latitude, longitude, and altitude in degrees and meters in Earth-Center-Fixed; if your position data is not in this format, it must be converted. If an entity is fixed in space - for example, a ground based tracking station - you should set the Static property to true.
Time is not tied exclusively to position, but rather represents the time of the last update to any aspect of the entity; it is always defined in Coordinated Universal Time.
PointEntity.Static = false;
PointEntity.Time = DateTime.UtcNow;
PointEntity.Position = new AgEntityPosition() as IAgEntityPosition;
PointEntity.Position.Set(Latitude, Longitude, Altitude);
STK 11.2.1 Programming Interface