Inside GNSS Media & Research

JUL-AUG 2019

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Page 36 of 67 J U L Y / A U G U S T 2 0 1 9 Inside GNSS 37 • Time to disclosure of the secret data. is in an important feature that is actually a design parameter of the authentica- tion approach. e Delayed authentication service (KDA) is based on a delay for the implementation of the asymmetricity (e.g. the TESLA protocol [Perrig et alia, Additional Resources]). Consequently, a minimum time between the broadcast of the last chip of the Authentication-Code, and the disclosure of the data is necessary to reconstruct the same code. is minimum time must be carefully determined to guarantee suffi cient pro- tection against attacks that shi the receiver time. In fact, if the time advance of the attacker is suffi cient to decode the broad- cast reconstruction data, while the receiver is still receiving the chips, the attacker can eff ectively fake the signal in an unde- tectable way. • Impact to standard tracking. The TH approach has a signifi cant advantage: the local generation of the Secure Navigation Code has punctual error where the broadcast code has been watermarked. is happens when the receiver cannot reconstruct in run-time the Authentication-Code. In practice the punctual errors have an impact on the cor- relation value produced by the correlators: ideally the loss is exactly . is means that the tracking remains stable, likely suff ering more noise. Conversely, the TD approach necessarily introduces a gap in the spreading code. e width of this gap can be suffi cient to prevent proper accu- mulation in the tracking correlators. In this case, the receiv- er should support a costing time or external aiding is nec- essary to properly update the loops to prevent loss of lock when the Secure Navigation Code is broadcast. All the features presented here are reported to indicate the main parameters that should be considered for the fi ne confi guration of the authentication approach. It is assumed that the system designer will perform a detailed requirement analysis, in order to understand clearly which are the design drivers that should be considered and coherently the features to be implemented. Regardless the specifi c type of implementation, the two watermarking approaches share the same performance in terms of detection of the secret sequence. is, under the assumption that the coherent integration time and the num- ber of non-coherent accumulations are the same (minus the residual time in the last accumulation due to diff erent obser- vation windows). An insight into the expected performance in terms of feasibility of the detection test is presented in Figure 11 and Figure 12. Figure 11 shows the probability of detection for a false- alarm probability of 10 - 6 , with a chipping rate of 1.023 MChip/s. e fi gure indicates the particular case in which the base interval of the watermarking sequence is 10 mil- liseconds (ms) and within this interval the override of the

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