This paper surveys the signal properties of 18 commercially available GPS jammers based on experimental data. The paper is divided into two distinct tests. The first characterizes the jamming signals, and the second test determines the effective range of 4 of the jammers. The first test uses power spectra from discrete Fourier transforms (DFTs) of the time series data to show that all the jammers employ approximately the same jamming method, i.e. linear frequency modulation of a single tone. The spectra also show that there are significant jammer-to-jammer variations, including between jammers of the same model, and that a given jammer’s signal may vary over time. The first test also includes measurements of signal power within frequency bands centered at the L1 and L2 frequencies, along with the sweep periods and the sweep range at both frequencies. The second test presents measurements of the attenuation of the jamming signal necessary to allow a commercially available GPS receiver to acquire and track signals from a GPS simulator. From the attenuation levels and some assumptions about the antennas used, upper limits on the effective jamming ranges are calculated for 4 of the jammers, with a resulting maximum range of 6–9 km.