Trajectory of LRO

Erwan Mazarico

In order to accurately place the data acquired by LRO on the lunar surface, and allow comparisons and analysis between LRO instruments or with other lunar missions, the trajectory and orientation of the spacecraft need to be known at all times. The orientation information is provided by monitoring star fields with star tracker cameras. The trajectory information is not as readily available, and need sto be reconstructed. This is done through the process of "orbit determination" (OD), which involves integrating the equations of motions in an iterative process designed to properly model tracking observations. Spacecraft tracking at radio frequency is the most common type, and the NASA Deep Space Network (DSN) is key to communicating and performing OD for most planetary missions (see DSN Now for what is being tracked right now). However, because the Moon is a relatively closer neighbor, other stations can do this task, and LRO has long relied on the NASA White Sands station in New Mexico, with support from the commercial network USN.

The LRO-LOLA team uses the GEODYN II software developed and maintained at NASA GSFC to perform OD for the LRO mission. The spacecraft navigation team produces rapid orbit reconstructions on a daily basis, for internal use, quick-look products, and data processing. The high-resolution instruments onboard LRO benefit however from higher-fidelity trajectory reconstructions. Improved accuracy can indeed significantly reduce the need for a human-in-the-loop to co-register images taken at different times for instance. In our processing, we can better account for perturbations on the spacecraft, for instance by making use of a higher-resolution gravity field from the GRAIL mission, by accounting for self-shadowing of spacecraft panels when considering solar radiation pressure forces, or by using our now-excellent knowledge of the lunar shape to compute the incident solar flux on the spacecraft.

For more on the orbit determination methodology and for additional LRO-specific details, please refer to these two publications:

- Mazarico, E., Rowlands, D.D., Neumann, G.A., Smith, D.E., Torrence, M.H., Lemoine, F.G., Zuber, M.T., 2012. Orbit determination of the Lunar Reconnaissance Orbiter. Journal of Geodesy 86, 193–207, doi:10.1007/ s00190-011-0509-4.

- Mazarico, E., Neumann, G.A., Barker, M. K., Goossens, S., Smith, D.E., Zuber, M.T.. Orbit determination of the Lunar Reconnaissance Orbiter: Status after Seven Years. Planetary and Space Science, under review.

The LRO trajectory is processed over short spans (typically 2.5 days) and combined into month-long batches. These are made available publicly through the "LOLA / LRO Radio Science" PDS Data Node, as NAIF/SPICE kernels (SPK format).

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