# Practical Use of Starlink Downlink Tones for Positioning
## 2 Signal Detection and Tracking
### Signal Recording  
The user's downlink signal is in the 10.7-12.7 GHz band, which is the same band used by geostationary television satellites. The signal is broken down into several sub-bands spaced 250 MHz apart [25], within which there are multiple tones in a bandwidth of nearly 1 MHz. In this work, we will use the tone band centered on Fo = 11,325 MHz. The reception of Ku-band signals can classically be done with a reflector-based or a parabolic dish antenna. These types of antennas offer the best performance in terms of bandwidth and gain, but they require motorization in order to track the satellites over the whole field of view, making it difficult if not impossible to mount them on a small vehicle or flying platform such as a UAV. An example of such an antenna is General Dynamics's M20-24M Ku Satcom On-The-Move antenna, whose volume and weight are approximately 0.3 cubic meters and 59 kg, respectively. Moreover, due to the typical beamwidth of these antennas, it is only possible to track a single satellite at a time. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Unit** & **A Typical Range Limit** \\ \hline Range & km & 400 to 1150 \\ Doppler shift & kHz & \(-\)230 to 230 \\ Doppler shift rate & kHz/s & \(<\)5 \\ Rate of the Doppler shift rate & Hz/s\({}^{2}\) & \(<\)70 \\ Number of satellites in view & - & 16–35 \\ Satellite visibility time & minutes & \(<\)4 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Typical range limits of range dynamics and visibility for a static observer located at mid-latitude and using an elevation angle mask of 25\({}^{\circ}\). The Doppler shift and the Doppler shift rates are given for 11,325 MHz, which is one of the Starlink signal frequencies. The used TLE file contains the orbits of nearly 3000 satellites. On the other hand, electronically scanned phased array antennas offer the smallest profile, at the expense of design and computational complexity, power consumption, and cost, which limits their application in most consumer products. They are also theoretically capable of tracking multiple satellites. Examples of commercially available user's phased arrays for the tracking of LEO signals are Hanwha Phasor's antenna, Requtech's REqRESA-S Ku antenna, and Starlink user's dish antenna [30]. The selection of the antenna and of the LNB has been carried out considering constraints of size, weight, and cost. A key feature is the antenna beamwidth, which should be sufficiently large to simultaneously detect most of the satellites in view. This choice is made at the expense of the antenna gain and therefore of the signal-to-noise ratio in order to capture many satellites and optimize the satellites' geometry. Universal LNB have reduced size and weight, and can be easily obtained at a lower cost. They also offer a reasonable compromise between beamwidth and gain. We have set up an acquisition tool to record the Starlink downlink tones around Fo (Figure 1a). A universal Ku-Band satellite LNB usually dedicated to satellite TV reception is employed to receive and down-convert the Ku band signals to a lower frequency (typically around 1-2 GHz). It has a local oscillator frequency of 9750 MHz. As it is supposed to be mounted on a parabolic dish antenna, it also directly integrates a linearly polarized choked-feed horn antenna with a half-power beam width of around 35\({}^{\circ}\). The LNB provides a 50 dB gain conversion. However, since the Starlink downlink signal would have a circular polarization (RHCP), a mismatched loss of 3 dB is expected to occur when using this LNB. A bias-tee and a DC power supply is used to power the LNB. It is then connected to the USRP X310. An external 10 MHz reference is provided by Keysight's 33,250. A waveform generator (whose TXCO have a frequency accuracy of 2 ppm) is employed to discipline both the digitizer and the LNB clocks. A photo of the experimental set-up is provided in Figure 1b. The recorder has a signal timestamp capability of a few seconds.