# LO and Calibration Signal Distribution in a Multi-Antenna Satellite Navigation Receiver +
## 3 Distributed GNSS Receiver Concept with Two 2 \(\times\) 1 Antenna Sub-Arrays
### System Calibration
#### 3.2.2 CAL Signal Distribution  
For calibration, an artificial GNSS signal is generated in the digital baseband and applied to all receiver chains. The calibration signal is generated in the IF domain at \(\mathrm{f_{IF}}\approx 70\) MHz and must be up-converted to the RF frequency bands with the synthesized LO signals. This guarantees phase coherence between the real GNSS receiver signals and the artificial GNSS calibration signal. Upon down-conversion, all signals share the same IF frequency band. Similar to the LO signal distribution, two options also exist for the CAL signal distribution. Either the calibration signal is distributed to the subarrays and up-converted there (CAL signal distribution at IF), as illustrated in Figure 5a, or it is up-converted centrally and then distributed to the subarrays (CAL signal distribution at RF), as shown in Figure 5b. Actually, the hardware effort on a sub-array level is larger for the CAL signal distribution at IF because an additional up-converter is required for each frequency band at each sub-array; in a future re-design, the up-converters could be integrated into the ASIC. In contrast, for the CAL signal distribution at RF, two LO signals have to be generated and distributed, with similar implications for the hardware and software efforts, as discussed, for the LO signal distribution. For a comparison of the phase imbalance between the CAL signals at both sub-arrays, similar considerations are applicable as for the LO signal distribution, as the calibration signal is connected to the sub-arrays by cables in both cases. Therefore, the same constraints on length differences and temperature changes apply. The lower frequency of the CAL signal at \(\mathrm{f_{IF}}=70\) MHz in comparison to the more than 20-fold higher RF signal in the L-band results in a correspondingly lower phase imbalance between the sub-arrays for a given change in the electrical length of the cables. This advantage, however, is lost upon up-conversion with the LO signal at each subarray. Additionally, it turns out that the calibration concept originally applied to the \(2\times 2\) URA cannot be applied to the distributed array in the same way. Rather, calibration with an artificial GNSS signal can be achieved at sub-array level only. There, the phase balance between the calibration signals for each receiver channel can be guaranteed by the use of a passive signal distribution network. \begin{table}
\begin{tabular}{l l l} \hline \hline  & **REF-Signal Distribution** & **LO-Signal Distribution** \\ \hline Hardware and & (+) internal ASIC PLLs & (\(-\)) two additional PLL modules \\ software effort & (+) programming via ASIC & required & (\(-\)) additional control software and \\  & control software & lock-detect monitoring \\ Number of cables & (+) 1x twisted pair & (\(-\)) 2x coaxial cable \\ Phase imbalance due to cable length & distribution & (o) depends on cables \\ Cable loss & (+) lower & (\(-\)) higher \\  & (\(-\)) higher, due to usage of the & (+) lower, due to usage of an \\  & internal frontend ASIC PLL & advanced external PLL IC \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison between REF-signal and LO-signal distribution. Table 2 compares the different properties of both CAL signal distribution concepts. Because of the lower effort and implementation costs, we selected the CAL distribution at IF for the compact distributed GNNS receiver. The full calibration of the distributed array must be conducted in the digital domain, resulting in a blind or hybrid calibration concept [18]. In [19], we proposed a solution to determine the accurate position and attitude of a multi-antenna system without the explicit use of a dedicated calibration signal. The capability to mitigate interference sources with the distributed array receiver was presented in [3; 20].