HSGEM – The Hughes GeoMobile Satellite System Story—Andy Ott

In the early 1990’s, Hughes Space and Communications Group (HSCG) teamed with Hughes Network Systems (HNS) to develop a satellite based cellular communications system.  This was to be a total end-to-end system. HSCG was responsible for the Space Segment (spacecraft, spacecraft on-orbit as well as launch operations, including the facilities, software for both spacecraft bus and payload, and launch vehicle procurement). HNS was responsible for the user and ground segments (ground hardware infrastructure, network management, gateway stations, as well as cell phones and the billing system). Project management, including overall “Big-S” Systems Engineering, was the responsibility of HSCG as the prime, requiring formation of a GeoMobile Business Unit within HSCG.

The spacecraft did not fit into either the existing HS601 product line nor the under development at the time HS701 product line, necessitating a unique spacecraft, labeled HSGEM. There were many new, unique requirements for HSGEM space segment, the following is a list of a few of the major challenges:

  1. 13 KW Spacecraft Bus with dry weight 5,500 – 7,000 lbs. A modular Xenon Ion Propulsion System (XIPS) addition, if required due to launch vehicle selection. Payload weight 3500 – 4000 lbs.
  2. A single L-Band 12.25-meter aperture antenna to provide both transmit and receive communications. The Astromesh reflector is 18 ft in length by 44 inches in diameter stowed for launch and when fully deployed is a 52.5 ft by 40 ft ellipse with a 12 ft depth. A 128-element feed array provides in excess of 200 individually controllable spot beams.
  3. Elimination of potential Passive Intermodulation Products (PIM) sources for the spacecraft bus and payload. The diplexer was a special challenge due to the single antenna and the significant difference between receive and transmit power at L-band.
  4. Digital Signal Processor (DSP) to provide channelization, routing and beamforming; all functions previously performed by analog and passive hardware. The DSP included a mobile-to-mobile switch to allow for direct routing of mobile terminal to mobile terminal calls, thereby reducing round trip delay to a single hop. The DSP utilized state of the art at the time ASICs jointly designed and qualified by Hughes and IBM and manufactured by IBM. Flexible digital beamforming was a special challenge.
  5. Common software for payload, spacecraft system test and launch plus on-orbit operations integrated from Commercial, off the shelf (COTS) products and HSCG developed DSP command and control.
  6. Unique approach to North-South station keeping using the power of the payload to perform electronic beam steering vs chemical station keeping while operating in inclined orbit.

A development vehicle and the first two spacecraft were manufactured by HSCG, the Satellite Control Center by Raytheon and the Network Control Center and ground infrastructure by HNS.  The first launch of a HSGEM spacecraft, however, occurred in the year 2000 after HSCG was bought by Boeing. Although Boeing activities are not discussed on this website, it is public information that the first HSGEM was successfully launched by Sea Launch and met or exceeded all requirements (space and ground), resulting in a very successful and happy customer. The satellite and ground systems are still operational today (2018) and revenue creating, exceeding the 12-year life requirement of the contract.

Fig I: HSGEM spacecraft in launch configuration at HSCG High BayFig 2: HSGEM On-orbit

Key to the commercial success of this project was its efficient use of very valuable and much in-demand L-Band frequency spectrum. Ability to control more than 200 individual spot beams allowed for reuse of the same frequency spectrum more than 40 times and tailoring the coverage area to meet needs of specific customers. A comprehensive article, “The Hughes Geo-Mobile Satellite System”, was co-authored by HSCG (John Alexovich and Larry Watson) and HNS (Anthony Noerpel and Dave Roos) with major support from the rest of the “Big S” Systems Team and presented at the 1997 International Mobile Satellite Conference held in Pasadena California. The article is an excellent description of the end-to end system. Some of the key points are as follows (full article appears immediately following key points).:

  1. HSGEM is sized to provide 16,000 voice circuits for 2 million subscribers, including presence of up to 10 dB of shadowing.
  2. The maximum coverage area with over 200 beams, each approximately 0.7 degrees in diameter or 450 km across, is 12 degrees as viewed from geosynchronous altitude.
  3. Dual mode terminals provide the ability to communicate with either the HSGEM or with local terrestrial cellular systems (GSM) for voice, data, facsimile, and supplementary services.
  4. The HSGEM accommodates many features that support flexibility and reconfigurability as technology further advances, which has been demonstrated over 17 years (so far).


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