  It is well known that GNSS receivers are vulnerable to jamming and spoofing
attacks, and numerous such incidents have been reported in the last decade all
over the world. The notion of participatory sensing, or crowdsensing, is that a
large ensemble of voluntary contributors provides measurements, rather than
relying on a dedicated sensing infrastructure. The participatory sensing
network under consideration in this work is based on GNSS receivers embedded
in, for example, mobile phones. The provided measurements refer to the
receiver-reported carrier-to-noise-density ratio ($C/N_0$) estimates or
automatic gain control (AGC) values. In this work, we exploit $C/N_0$
measurements to locate a GNSS jammer, using multiple receivers in a
crowdsourcing manner. We extend a previous jammer position estimator by only
including data that is received during parts of the sensing period where
jamming is detected by the sensor. In addition, we perform hardware testing for
verification and evaluation of the proposed and compared state-of-the-art
algorithms. Evaluations are performed using a Samsung S20+ mobile phone as
participatory sensor and a Spirent GSS9000 GNSS simulator to generate GNSS and
jamming signals. The proposed algorithm is shown to work well when using
$C/N_0$ measurements and outperform the alternative algorithms in the evaluated
scenarios, producing a median error of 50 meters when the pathloss exponent is
2. With higher pathloss exponents the error gets higher. The AGC output from
the phone was too noisy and needs further processing to be useful for position
estimation.
