From The RadioReference Wiki
OpenSky is L3Harris implementation of AMBE digital modulation over a TDMA channel format.
AMBE is another vocoder developed by DVSI Inc. OpenSky systems do not use a control channel. Open Sky cannot be monitored by any scanner, past or future.
OpenSky technology is currently owned by L3Harris
Here is the Harris Link for this product. Signal Harbor has a list of pending and implemented OpenSky systems. No new OpenSky Systems have been deployed in nearly 10 years.
OpenSky is a digital radio system sold by L3Harris (formerly a product of M/A-COM, a wireless products unit of TYCO Electronics which was later sold to Harris in 2009 for 675M). OpenSky was introduced in 1999 as an expansion of the Monarch wireless voice and data system built for FedEx on 900mhz. It was originally intended to handle voice on a limited basis for Fedex driver emergencies.
OpenSky uses a four-slot TDMA transmission method (four-level GMSK modulation) in 25 kHz bandwidth channels with an aggregate data rate of 19.2 kbps (symbol rate of 9600 baud). This is on both the forward and reverse links.
There are three primary OpenSky protocols: FMP (Federal Express Mobile Protocol), OCP (OpenSky Communication Protocol), and OTP (OpenSky Trunking Protocol). FMP supports digital voice. The airlink protocols are based on modified Cellular Digital Packet Data (CDPD) IS-732 and carry voice using DVSI's AMBE (Advanced MultiBand Excitation) vocoder. M/A-COM modified the physical layer and Media Access layers to support trunking and mixed voice and data traffic.
Harris has since introduced Opensky 2. This version supports 2 slot TDMA at a raw bit rate of 9600 bps in a 12.5 kHz channel using the AMBE+2 vocoder. Harris sells Opensky equipment for 770 MHz, 851 MHz and 935 MHz.
Many Opensky systems have already reach EOL and have been deprecated and replaced with P25 Phase 1 or 2 system provided by L3Harris or Motorola.
3 Major OpenSky statewide systems, Pennsylvania, New York State and City of Las Vegas have all be deprecated and replaced due to performance issues.