Transitioning the NASA SLR network to Event Timing Mode for reduced systematics, improved stability and data precision

Abstract

NASA’s legacy Satellite Laser Ranging (SLR) network produces about one-third of the global SLR data to support space geodesy. This network of globally distributed stations has been using Time Interval Units (TIU) for range measurements for the last 25 + years. To improve the reliability of the SLR network and satisfy the need for stable millimeter precision data, a phased replacement of the TIUs in the network with picosecond-precise Event Timer Modules was initiated in 2015. This scheme allowed the time of flight and laser transmit epoch measurement to one picosecond resolution. For a network with global scientific impact, transitioning to a new data generation metrological scheme requires significant data scrutiny and long-term science data validation. Any long-term testing/measurement has the potential to interrupt the station’s daily operational data flow to the International Laser Ranging Service (ILRS) as the station under test will have to put its test data into quarantine. We have demonstrated a very effective way to test and implement the new device without removing the old hardware and without the need for the orbit analysis. This operationally noninvasive scheme performed concurrent test measurements enabling uninterrupted operational data flow to the users, while allowing simultaneous test data capture for short- and long-term systematics and stability analysis. Extensive analysis of the test data was performed by the NASA SLR engineering team and the ILRS Analysis Standing Committee, to uncover biases and any dependencies on the satellite ranges (for nonlinear scale issues). Multi-ETM comparison was also performed at two of the SLR stations through the interchange of hardware to establish the inter-device range biases and stability. Such benchmarked hardware was subsequently sent to the remaining stations to allow traceability and normalize the network performance. The range bias intercomparison performed using the multiyear SLR data analysis agreed well with the engineering changes, thus validating the approach to flush out station-specific ranging systematics affecting precise orbit determination. Such an improvement and rebalancing of the current network will allow an orderly transition of the current NASA SLR network operating at a maximum rate of 10 Hz to the NASA next generation Space Geodesy Satellite Laser Ranging (SGSLR) network operating at 2 kHz (McGarry et al. in J Geod, 2018. https://doi.org/10.1007/s00190-018-1191-6; Merkowitz et al. in J Geod, 2018. https://doi.org/10.1007/s00190-018-1204-5).