# Towards NGGM: Laser Tracking Instrument for the Next Generation of Gravity Missions
## 2 LTI Design Principle  
We investigated two concepts for the laser tracking instrument over several years under the ESA contract:  
* The so-called \"retroreflector concept\", in which the interferometer is configured very similarly to typical interferometer designs for terrestrial use, with most of the interferometer system mounted on one satellite and only a passive retroreflector placed on the second satellite;
* The \"transponder concept\", in which the received signal from the first satellite is amplified on the second satellite before being sent back to the first satellite and where both satellites are essentially identical. A detailed comparison of the two concepts, including requirements, implementation parameters and a noise performance analysis, can be found in [19]. In addition to the optical power limit necessary to achieve the required noise performance, the onset of cycle slips limited the acceptable carrier to a noise ratio of at least 70 dB-Hz and, thus, the feasibility of the two configurations. While the retroreflector concept allowed for a simpler implementation for distances of up to approximately 30 km and was feasible up to 100 km with increased laser power, only the transponder concept could cover the required inter-satellite distance range from 100 to 300 km, and was, therefore, the selected instrument concept for the NGGM. The transponder concept LTI consisted of five main units on each satellite, individually placed in the spacecraft:  
* The instrument control unit with the phasemeter (ICU);
* The laser head unit (LHU) as the light source;
* The laser stabilization unit (LSU), an optical cavity, to stabilize the laser in frequency;
* The optical bench assembly (OBA) to host the interferometer optics;
* The off-axis retroreflector unit (RRU) to route the beam to the other spacecraft. It was completed with a set of optical baffles to reduce straylight and ensure a clear optical aperture, optical fibers between the laser, the laser stabilization unit and the optical bench, as well as the electrical harness. A scale factor measurement system (SFMS) was under discussion to provide an absolute laser frequency reference. A detailed discussion of the individual LTI units is given in Section 3 below. The basic working principle was as follows: The laser head (\"master laser\") on satellite one, providing approximately 25 mW of single-mode single-frequency light to the optical bench, was locked to the cavity. The light was then sent to the other satellite via the optical bench and the retroreflector. On the second satellite, the laser frequency of the transponder laser was phase locked to the weak incoming light (\(\approx\)1 nW) with a fixed frequency offset of some MHz and sent back to the first satellite. At the first satellite, the phase shift between the master and transponder lasers, which contained two times the Doppler shift caused by the relative movement of the satellite, was recorded. Figure 2 shows the conceptual sketches of the laser tracking instrument in an off-axis transponder configuration with (a) partial redundancy and (b) full redundancy. In both variants, a cold redundancy for the laser heads was foreseen. The cavity did not need to be redundant on each satellite, because redundancy was achieved by flying one cavity on each spacecraft while needing only one in the constellation. There was the possibility to exchange the master and transponder operational modes. In the partially redundant implementation, each laser had a fiber connected to the single optical bench, providing cold redundancy, not only for the laser itself, but also the fiber and beam collimator. Redundancy switching on the optical bench was achieved via a 50:50 beam splitter, accepting a 3 dB power loss of each signal.