Arbitron obtained a court order last week forcing Spanish Broadcasting System – one of the coalition’s members – to resume encoding its content so it could be detected and measured by PPM devices.

The coalition and its members argue that there are flaws in the design of the PPM audience measurement system which means that audiences for minority-targeted radio stations are being undercounted, thus harming their ratings figures and advertising revenue.

SBS has long been a vocal critic of PPM. Reports claim its legal dispute with Arbitron started with SBS refusing to pay its licence fee after becoming dissatisfied with the service. In response Arbitron stopped providing SBS with ratings data and the broadcaster in turn hit back by ceasing to encode its broadcasts.

A temporary restraining order forcing SBS to resume encoding applies to nine stations in five US cities and is valid until 16 February.

In its letter to the FCC, the PPM Coalition asks the commission to investigate the accuracy and reliability of Arbitron’s PPM methodology. It claims its members were effectively forced to sign up to the PPM service because of Arbitron’s position as a monopoly in the radio ratings market and that they are provided with little recourse if they are unsatisfied with Arbitron’s performance.

Arbitron has declined to comment on the coalition’s letter. It has stated previously that differences between PPM ratings and those obtained from the diary-based system that preceded it are down to the fact that PPM measures actual exposure, while the diary relied on recall and loyalty.