In a news alert published earlier this year, we outlined the development of the "Mosaic Theory" in Taiwan's privacy law. As in many countries, installing a GPS tracker on a suspect's vehicle for prosecution purposes is against the due process of law and can amount to a crime in Taiwan. A similar and perhaps less interfering tracking technique is the use of cell site simulators by the police to locate mobile phones. Contained in metal cases easily loaded in common sedans, cell site simulators are dubbed "M-Cars" and have proved instrumental in hunting down wily criminals whose whereabouts were only traceable through signals from mobile phones they used. However, here lies the problem: is use of M-Cars lawful?

In late May 2020, this question was tackled in a court decision for the first time, and the answer was "no". Taoyuan District Prosecutors Office v. Chen et al., 106 Yi 164 (Taoyuan District Court, May 2020). The three-judge panel led by Judge Yu-Chieh Shih held that the investigative use of cell site simulators is in violation of the principle of legal reservation, because it interferes with the right to privacy and is not mandated by any Congress-made law. Location evidence obtained through a cell site simulator was held inadmissible in this phone extortion scam case.

According to the decision, cell site simulators used by Taiwan's police are able to catch mobile phone signals within a certain radius and hence detect the identity-related information including International Mobile Equipment Identities (IMEIs) and International Mobile Subscriber Identities (IMSIs). By comparing the strengths of phone signals collected from different sites as the M-Car moves, the simulator is able to identify the location of a target mobile phone with such precision that, as a news article disclosed in 2012, the error can be less than one meter. Of course, in real cases, M-Car patrols are not conducted aimlessly. The patrolled area is selected based on locations of real cell sites where the target mobile phone had been logged into. Such login information as well as the IMEI and IMSI linked to the target phone is held by telecommunication companies but is disclosed to the police with a warrant issued by a court under the Communication Security and Surveillance Act (CSSA).

However, the investigative use of cell site simulators is not regulated in any laws including the CSSA. Instead, for years it was merely guided by a "Workflow on Conducting Mobile Positioning Tasks" issued by the national police bureau and, as the reported judgment found, M-Cars and cell site simulators might have never been used under a warrant. Although a privacy protection protocol had long been implemented to check the use of these devices,1 the constitutionality issue was not addressed, as the court panel said:

Yet only the legislature has the constitutional power to enact a law permitting an investigative agency to launch an action that interferes with fundamental rights. The judicial branch, namely the judges, is not allowed to shape the law in a way as it finds convenient and apply it by analogy against a suspect in a criminal action. What is at stake is not just the present case, but also the fundamental principle of rule of law. As such, it is up to the legislature's wise deliberation to strike a balance, hopefully as soon as possible, between criminal prosecution and human rights protection in terms of investigative use of new technologies including "M-Cars."

Like other Taiwanese courts, the panel adopted the "reasonable expectation of privacy" test originated from US court opinions. After citing Katzv. United States, 389 U.S. 347 (1967) and Kyllo v. United States, 533 U.S. 27, 35-36, 40 (2001), the panel held:

It is true that M-cars and GPS trackers operate with different technologies and are deployed in different manners (most notably, a cell site simulator is not attached to a target vehicle). However, M-Cars and GPS trackers are both able to identify surveillance targets' locations and trace them continuously. In this context, when an M-Car is used to gather the locational data about a target phone, interference with the said fundamental right (i.e. right to privacy) does occur, ... not to mention the locational data obtained through use of an M-Car is not discernible by an ordinary person anytime and anywhere. A person shall not be deprived of his/her reasonable expectation of privacy regarding his/her whereabouts and the location of his/her communicative equipment simply because he/she was reported or has been considered as a suspect.

In a dictum, the decision further discussed whether the use of cell site simulators can be justified by the "Third Party Doctrine":

The fact is: when an M-Car is used, the target phone's location is identified through direct detection on the strength variations of signals from the target phone, but these signals are not held by a third-party. Therefore, the "third party doctrine" does not apply here. United States v. Miller, 425 U.S. 435 (1976).

Further, the evidence presented before the court lacked some critical pieces in proving the accused were indeed the criminals the police chased after, even if the excluded location evidence had been considered. The decision indicated:

While the phone frauds took place on April 9, May 15, and May 19, 2015, the search was conducted on June 9, 2015 and the searched premises were frequented by other people whose identities are still unknown. There lacks positive evidence for the conjecture that the searched premises had been subject to strict entry controls excluding any persons other than the gang committing the phone frauds in dispute... The physical evidence [including a victim list and fingerprint samples] collected from the searched premises indicates the possibility that the premises might have been used by multiple fraud gangs/fraudsters committing different types of frauds. Also, the evidence cannot even prove the accused were present on the premises when the fraud at issue were committed.

What is more, the IMEI and the IMSI numbers of the mobile phones, SIM cards, pre-paid SIM cards held by the accused and discovered on the searched premises did not match those linked to the phones making the scam calls.

Although the accused might have more knowledge than anyone else about who committed the frauds, it was their right to remain silent before the question of "whodunit" and even retort with an "it-was-someone-else" argument, especially in a case where the police had to rely heavily upon digital evidence to infer the criminal's identity and whereabouts (see our report published in July 2019 for a copyright case where digital footprint evidence was questioned by the court).


1. According to the testimony of police officers involved in this case, the cell site simulators do not save mobile phone information gathered and are not linked to any personal information database. While location evidence obtained through the simulator plays an important role in the police's application for a search warrant, it was accompanied with other evidence including on-site surveillance report and car plates search results, as well as locations of the real cell sites.

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