Current Status of NGLR-1 - 3/10/2025

The MSFC objectives for the 19D Blue Ghost mission consisted of obtaining an initial range measurement. This was achieved on the day after the Blue Ghost landed on the Moon.

The objectives of the NGLR/LLR program at the University of Maryland are listed below. These are the objectives that were described in our proposal that was selected by NASA Headquarters.

Objectives

1. Successful Operation on the Moon

   The initial objective is that the NGLR-1 can survive launch and separation shocks, the transit to the moon, the issues in landing, and then operate in the thermal environment found on the lunar surface. As demonstrated by the lunar laser ranging returns obtained by the MeO at Grasse, France and WLRS at Wettzell, Germain, NGLR-1 was successful. Therefore, NGLR-1 has reached a Technical Readiness Level (TRL) of 9.

2. Provide a new retroreflector with improved location

   Many returns and measurements have been obtained by the Metrology and Optics Telescope (MeO) Lunar Laser Ranging Observatory (LLRO) in Grasse, France on the first day after the Blue Ghost landed. Returns and measurements were also obtained by the Wettzell Laser Ranging Station (WLRS) LLRO in Wettzell, Germany on the second day after the landing. These accomplishments demonstrate that this first objective has been accomplished. Note that Wettzell has obtained their returns with a telescope collecting area that is 22 times smaller than the telescope of the APOLLO LLRO.

   Advantages: The longer baselines with respect to the existing retroreflector arrays will result in greater accuracy for various lunar physics properties.

3. Provide greatly reduced dispersion limited only by LLRO

   The dispersion, that is, the temporal spread in time of laser pulse returns, is greatly increased for the existing retroreflector arrays due to the tilt of the arrays caused by the lunar librations.

   Advantages: A lower dispersion then requires fewer returns to reach a one mm one-way statistical uncertainty. It also allows one to reach sub-millimeter results in a relatively easy manner. It also allows Satellite Laser Ranging (SLR) Stations to participate, greatly increasing the number of potential LLROs. Show details.

   Figure 1

   Figure 2

   NGLR-1 supports a very small dispersion in the offsets of the magnitude of the Lunar Laser Ranging (LLR) return residuals. The dispersion supported by NGLR-1 is much less than the dispersion involved in the hardware systems currently used by the various LLROs. Figure 1 is from the initial LLR results of MeO at Grasse, France, illustrating the dispersion, or spread in LLR residual magnitudes, of their March 4th observations, using the green laser at 532 nm (red curve). It compares the observations with the dispersion of the data from their local calibration CCR (green curve). This comparison illustrates that the dispersion of the NGLR-1 returns almost exactly matches the dispersion from the calibration CCR. This illustrates that NGLR-1 is supporting the minimum dispersion that can be obtained with the MeO green LLR system. Figure 2, again from the initial March 4th results of MeO, compares the dispersion of the LLR residuals using the infrared laser at 1064 nm (red curve) with the dispersion of the data from the local calibration CCR (green curve). This again illustrates that the dispersion of the NGLR-1 residuals almost exactly matchs the best dispersion that can be obtained with the MeO infrared LLR system. The very small dispersion that the NGLR-1 supports means that as the dispersion of the LLROs improves over the decades, NGLR-1 will continue to support their improvement. For the Apollo retroreflector arrays, the contribution of the LLROs went from ~75 mm to a few mm over the past 50 years.

4. Provide Improved Dispersion as Compared to Existing Retroreflector Arrays

   As discussed, the existing retroreflector arrays participate in the lunar libration. This results in arrays being tilted with respect to the direction to Earth. This causes significant dispersion of the data at critical periods in the lunar cycle. The purpose of the NGLR-1 is to reduce the dispersion, so the librations are no longer a problem. As a result, each LLRO can provide data to the limit of their capacity.

   Advantages: Note again that there is no excess dispensation provided by NGLR-1. The statistical uncertainty for a single rang measurement in 532 nm for MeO is 1.6 mm. Addressing one of the ranging sessions in 532 nm results in a statistical uncertainty of 0.8 mm, obtained less than 24 hours after the landing of the Blue Ghost. Show details.

   Figure 3

   The blue data was obtained by MeO when the Moon was at a critical phase of the lunar librations. The blue curve is the best fit Gaussian curve to the blue data with an rms value of 267 ps (40 mm one way range). The red data is the result of MeO ranging to the NGLR-1 on March 3rd. The red curve illustrate the best fit Gaussian to the red data with an rms of 72 ps (11 mm one-way range). The green curve represents the data obtained by "ranging" to the local calibration CCR at MeO. This illustrates the least dispersion in the residuals that can be provide by the current hardware at the MeO LLRO. The white curve represent the "ranging" to a local calibration at the Wettzell LLRO. This represents the least dispersion that can be accomplished at the Wettzell LLRO. The ranging data to NGLR-1 for the Wettzell LLRO is currently being analyzed.

5. Investigating Ranging Errors in Light of The Higher NGLR Accuracy

   As determined by the initial measurements by MeO, the statistical error is at the level of 0.8 mm. However, this is the distance between MeO and the NGLR-1 on the Moon. For our science analysis, one needs the distance between the center of mass of the Earth and NGLR-1. To make these corrections, one needs to address more phenomena. Show details.

   Phenomena to address:

   • Horizontal Gradients in the Earth's atmosphere. This addresses the problem that data to address the excess delay in the atmosphere can be obtained only at the LLRO site. Due to the slant rang through the atmosphere for typical observation we need the meteorological data a few kilometers away from the LLRO site. This is not available. For a further discussion of the impact of the atmospheric effects¹.
   • Geophysical Effects. Correction to the statistical data must be made to correct for the lunar tide, the ocean tide, rainfall effects and other geophysical effects. The expected magnitude of these corrections have been described². However, the uncertainty in the magnitude of these effects (and thus the errors to be expected) are not currently available.
   • Systematic Biases. LLROs are subject to systematic biases. These can be detected by comparison to the data from other LLROs. While this has been accomplished very effectively for the Satellite Laser Ranging (SLR) program with ~40 stations, it is difficult with only a few LLROs. For this reason, we are reaching out to SLR stations that might have the capacity to become LLROs.
6. Science Program

   These initial observations of March 3 and March 4 will be submitted to the Crustal Dynamics Data Information Archive (CDDIS³). This is a public archive that contains all of the LLR data starting from our initial deployment of the Apollo 11 retroreflector array in 1969. This data is then available for all LLR Analysis Centers.

   Background: Our NGLR-1 science program is based upon our Apollo LLR program (ALLRP). The ALLRP over the past 55 years has produced many of the best scientific results in lunar physics, astrophysics, cosmology, and tests of General Relativity. For example, ALLRP discovered that the Moon has a liquid core 20 years ago⁴. It demonstrated that "Big G" is a constant in both time and local space, and it has provided the most accurate test of the Weak Equivalence Principle (WEP) extending the result to gravitational energy itself, a fundamental aspect of Einstein's General Relativity^5,6. Show details.

   To address the results that may be expected within the NGLR LLR Program (NLLRA), a simulation analysis has been performed at JPL^5,6. A wide range of different configurations was analyzed for a 6-year program. Both the minimum and the most aggressive configurations are expressed in Table 1.

   Case	Beta	Gam	h_2	l_2	cosD	<a>	Tau S 815 d
   SPL	92% 13x	74% 3.9x	87% 7.9x	70% 3.3x	98% 56x	99% 50x	7% 40x
   CRS WST SW SPL	99% 111x	99.8% 420x	99.5% 212x	99.8% 570x	99.4% 162x	99.3% 147x	99.7% 330x
   Table 1. Factors and percentages of improvement with the Artemis III mission and the assumption that ranging by the LLROs is performed at the ultimate levels of accuracy supported by the SBR package.

   Based upon the current capabilities of the existing LLROs, without taking into account any improvements over the next 6 years, we expect an intermediate result, that the accuracy of most of the scientific parameters that have been obtained within the ALLRP will be improved by an order of magnitude. Note that the NP accuracy of the ranging to the Apollo retroreflectors has gone from 150 mm to a couple of mms over the past 55 years.

²Liliane Biskupek, "Bestimmung der Erdorientierung mit Lunar Laser Ranging (Determination of Earth orientation with lunar laser ranging)" Ph.D Thesis, Leibniz University Hannover, 2015

³See https://cddis.nasa.gov/

⁴Discovery of Liquid Core of the Moon and Rotational Energy Dissipation, Lunar rotational dissipation in solid body and molten core, James G. Williams, Dale H. Boggs, Charles F. Yoder, J. Todd Ratcliff, and Jean O. Dickey, Journal Of Geophysical Research, Vol. 106, No. E11, Pages 27,933-27,968, November 25, 2001

⁵Analysis of lack of change in the Weak Equivalence Principle (WEP) as a test of General Relativity, Lunar Laser Ranging Science: Gravitational Physics and Lunar Interior and Geodesy, James G. Williams, Slava G. Turyshev, Dale H. Boggs, and J. Todd Ratcliff, http://arxiv.org/abs/gr-qc/0412049v1

⁶Another Analysis of lack of change in the Weak Equivalence Principle (WEP) as a test of General Relativity, Lunar Laser Ranging Tests of the Equivalence Principle With the Eearth and Moon, James G. Williams, Slava G. Turyshev, Dale H. Boggs, International Journal of Modern Physics D, World Scientific Publishing Company January 2, 2009 arXiv:gr-qc/0507083v2 2 Jan 2009