OpenSky Radio Trunking System

gqrx capture

OpenSky is a proprietary trunking radio that is designed to carry both voice and data traffic. the protocol is marketed as to be secure and private. Opensky operates on the 700, 800, and 900 MHz bands.

OpenSky was originally developed by M/A-Com as part of the Monarch wireless voice and data system for FedEx in the 90s. Later M/A Com was purchased by Tyco Electronics who was then purchased by Harris RF Communications. Harris has now merged with L3 Technologies to become L3Harris. This protocol has gone on a wild ride of Mergers and Acquisitions for this protocol hasn't it? The original OpenSky protocol was upgrades in 2010 and named OpenSky2.

The integrated data capabilities in OpenSky allow for more features in one single base station than voice-only trunking systems. This integration has allowed for dispatchers to have location data for radios in the field and, the ability to send data to terminals in for example police car, and for users log into the handsets pulling down their profile with the various talk groups and other preferences.

OpenSky and OpenSky2 are TDMA based protocols they have been designed to operate using 25W micro repeaters. Opensky2 introduced support for the 900mhz band and a more narrow bandwidth. The table below lists them out.

OpenSkyOpenSky2
Number of Slots42
Raw bit rate (bps)19,200 9,600
Channel Width (khz)2512.5
Frequecy bands700 / 800700 / 800 / 900
Released19992010

Signal Harbor describes there are 3 major components to the OpenSky signaling protocols:

  • FMP (Federal Express Mobile Protocol) - Providing Digital Voice
  • OCP (OpenSky Communication Protocol)
  • OTP (OpenSky Trunking Protocol)

These protocols are based off a modified CDPD (IS-732) similar to a an IS-54 (D-AMPS) network. I could not find a lot of exact details on lower level protocol operations other then Each radio assigned an IP address.

Digital voice is encoded using the Advanced Multi-Band Excitation (AMBE) speech encoding standard. This a proprietary standard that was developed by Digital Voice Systems, Inc. Interestingly this standard has been using in the Iridium Network and XM Satellite radio. There are more details on this standard here and here.

This protocol came to my attention while visiting my parents in Oakland County Michigan that has adopted OpenSky as a Countywide standard in 2002 for radio communications. Oakland county like many other areas is now replacing their OpenSky systems with a P25 system. Many of the large deployments have run into issues, for example, the State of Pennsylvania has had many issues and is replacing the system with a P25 Phase II system. The State of New York has had an OpenSky deployment which has run into many issues which are detailed in the OpenSky Wikipedia article.

I found this protocol interesting because, like most technology, it's a product of it's time. In this case OpenSky comes from a time before the pervasive presence of 4G/LTE wireless. Today you can accomplish many of the same goals as a OpenSky system by utilizing the current carrier LTE networks.

Below is a list of resources I used when researching this protocol:

Author: Ben Mason

Technical Architect - Computer Networking - Security - Electronics Hobbyist - Sometimes Photographer - Spaceflight - Cat Enthusiast - HAM KC1GDJ

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