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

Systematic errors in SLR data and their impact on the ILRS products

“…This bias, never explicitly solved-for, had been absorbed into station height and remained undetected. Similar findings have been obtained during recent upgrades of the timing devices of NASA stations (Varghese et al 2019), with the Greenbelt, MD system MOBLAS-7 (7105) showing a − 4 mm systematic difference between TIU and ET referenced ranges (Fig. 1).…”

“…In the past 2 years, technologically outdated time-interval counter units (TIU) have been replaced by modern event timers (ET) at most MOBLAS stations of the NASA network. Analysis of the data collected simultaneously with the TIU and ET systems during the validation stage showed excellent agreement between the old devices and their replacements in all but one station (at MOBLAS 7 at GGAO, #7105), where a consistent 4 mm bias was detected in the observations to all satellites (Varghese et al 2019).…”

ETAM: Next Generation Event Timer for Picosecond-Precision Time and Amplitude Measurements

“…Existing ET such as A033-ET and A040-ET are characterized by 3 to 4 ps root-mean square error (RMSE) for single-shot pulses [ 1 ], in stable operating conditions. These timers are used in scientific applications such as satellite laser ranging (SLR) [ 2 , 3 , 4 , 5 ] and optical time domain reflectometry (OTDR) [ 6 , 7 ]. For example, in SLR, the Event Timers are used to measure the time delay between the emission of the laser pulse and its reception at the ground station.…”

Satellite laser ranging to GNSS-based Swarm orbits with handling of systematic errors

“…However, the nature of tropospheric biases is similar to elevation-dependent biases, which originate from the limitations of the detectors in the case of different returning signal strength at various elevation angles or other errors, such as the interval counter or event timer bias. A time bias is caused by errors in time measurements, clock stability, event timer resolution, and station clock synchronization with respect to the UTC system (Exertier et al 2017;Otsubo et al 2019;Varghese et al 2019). Barometer biases have been discovered at SLR stations, e.g., Wettzell (7827), Graz (7839), and Borowiec (7811), and can reach the level of 1 to even 5 hPa (Wang et al 2020;Celka and Schillak 2003), resulting in elevation-dependent systematic errors (Drożdżewski and Sośnica 2021).…”