Alternative Approach to Tilt-to-Length Coupling Estimation for Laser Ranging Interferometers in Future Gravity Missions

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In this article, the authors investigate a new estimation method for the angular jitter-to-pathlength coupling (tilt-to-length coupling) that is commonly seen in the laser ranging interferometry of GRACE-type missions. The paper seems to be well organized and the results are clearly presented. Only a few points need to be addressed in a revised version.

 1. Line 135: “quandrant” should be “quadrant”.

 2. Line 203: Why is the TTL coupling correction not included in the practical light time correction?

 3. Line 248: The reviewer wonders if we need to take the light travel time into account the pointing angles used in the TTL estimation because of the difference between the nominal time of the two satellites.

 4. Line 460: What about other types of longitudinal offset?

 5. Line 561: Does the RMS value of GRACE-FO’s TTL error remain unchanged? Since the center of mass position in each satellite changes all the time, the RMS value of TTL may vary and deviate from 15 nm.

Author Response

Thank you very much. Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

Congratulation.Your proposal of an alternative approach to TTL coupling estimation for LRI in future gravity missions is very promising.

Author Response

Thank you very much for the very positive feedback.

Reviewer 3 Report

Comments and Suggestions for Authors

In the context of space geodesy mission using Low-Low satellite to satellite tracking by means of laser ranging interferometer, one of the dominant error source is due to tilt-to-length coupling. The article presents an approach to estimate this effect and improve the measurement principle without needs of calibration maneuver. The authors give a very good overview of the measurement principle of the LRI and derive the influence of the TTL coupling on the overall measurement performances. A very exhaustive and comprehensive characterization of the error sources, including TTL but also laser frequency noise, time delay,…, is provided. The numerical simulation of the TTL, based on data simulated but provided by GRACE type mission (e.g. GNSS, star tracker, accelerometer, ranging measurement and rotation measurement coming from the FSM) provide a solid base for estimation of the TTL. The very good estimation of the TTL, with the conclusion that the CM-VP offset can be relaxed to 1.5m provided pointing angle noise below 0.3 microrad/sqrt(Hz), gives a very promising tool for future space geodesy mission. The proposal to relax the position of the TMA with respect to centre-of-mass of the satellite opens also potential relaxation for the implementation of the accelerometer and the LRI.

The reviewer strongly recommends the publication of the article.

Author Response

Thank you very much for the very positive feedback.