Space may be the final frontier to some, but for the United States Air Force Space Command (AFSPC), the space around Earth is familiar territory. It operates an amazing array of satellites, both in technological capabilities and sheer numbers. This network of man-made constellations link commanders from the staff level in Washington to the squad leader on the ground in a far-flung country. They can communicate and exchange information and in many cases see what the other is seeing. This ability is not by chance, but by careful design with satellites providing the eyes, ears and links for it all to come together.

Every satellite in the system has related capabilities, but each serves a niche for which it is custom designed. Military Aerospace Technology sorts out the different systems and looks at what each adds to the whole.

Defense Satellite Communications System

The Defense Satellite Communications System (DSCS) is the backbone of the government’s satellite communications system, providing both secure voice and high data rate transmissions in the Super High Frequency (SHF) band. DSCS provides unique and vital national security communications for global command and control, crisis management, intelligence and early warning data relay, treaty monitoring and surveillance information, and diplomatic traffic. DSCS serves all Department of Defense (DoD) branches in a worldwide network coordinated by the Defense Information Systems Agency (DISA).

Ground mobile forces (GMF) use DSCS spacecraft to fulfill multichannel SHF initial system worldwide operational requirements. The DSCS II and III satellites are positioned in geostationary orbits and are continuously available for use by earth terminals located within 5,000 nautical miles of their subsatellite points.

The DSCS system originated in DoD’s Initial Defense Communications Satellite Program, which was inaugurated in 1966. After launching 26 of these spacecraft, the Air Force renamed the program in 1971 and launched the first of 16 DSCS Phase II satellites.

Phase II satellites have two transponders, each having a bandwidth constrained by the nominal 2009 MHz bandwidth of the total usable bandwidth of 410 MHz. Each transponder is subdivided by filters to provide two operational channels; for example, four operational channels for each satellite. Four antennas are mounted on the despun platform. Two of them are Earth coverage horn (ECH) antennas, one for uplink and one for downlink. The remaining two are parabolic dish type antennas. A single biconical horn is used to support the dedicated S-band telemetry and control link and was mounted on the lower end of the spinning section.

The on-orbit DSCS constellation is comprised of five DSCS III satellites in geosynchronous or geostationary (these terms have slightly different definitions) orbit at an altitude of about 22,300 miles. B11 (DSCS III SLEP), launched in October 2000 and now operating in the Eastern Atlantic, is the newest DSCS satellite on orbit. Fourteen DSCS III satellites have been built, with 12 on orbit and two yet to launch. The MILSATCOM Joint Program Office of Space and Missile Systems Center (SMC), Los Angeles Air Force Base, CA, is responsible for development, acquisition and sustainment of the DSCS Space Segment.

The last two DSCS satellites are scheduled for launch in 2003 and will utilize the Evolved Expendable Launch Vehicle (EELV) whereas previous launches all took place using the Atlas Centaur booster.

The DSCS III satellites support globally distributed DoD and national security users. The final four satellites, such as B11, have been upgraded with Service Life Enhancement Program (SLEP) modifications. These provide substantial capacity improvements through higher power amplifiers, more sensitive receivers and additional antenna connectivity options.

The DSCS III communications payload includes six independent SHF transponder channels that cover a 500 MHz bandwidth. Three receive and five transmit antennas provide selectable options for Earth coverage, area coverage and/or spot beam coverage. A special purpose (AFSATCOM) single-channel transponder is also on board. DSCS III also carries a single-channel transponder used for disseminating emergency action and force direction messages to nuclear-capable forces.

Modulation techniques include time division multiple access (TDMA), frequency division multiple access (FDMA) and spread spectrum multiple access (SSMA). Future modulation techniques will include demand assigned multiple access (DAMA). The DSCS radio frequency portions are undergoing various upgrades to support its mission well into the 21st century.

Some of the upgrades include:

• Heavy/Medium Terminal Modification: This upgrade affects the AN/FSC-78 and AN/GSC-39 terminals. These terminals’ RF components will be replaced with solid state devices for the transmitters and receive amplifiers. Included will be computer monitor, test and measurement, and control of the terminal functions.
• Super High Frequency Tri-band Range Extension Terminal (STAR-T): This developmental terminal will replace AN/TSC-85B/93B terminals at echelons above corps (EAC). The terminal will be mounted on a HMMWV. Its primary mission is to provide multiband communications and interface with commercial and military assets (dial central and satellite); DGM; TRI-TAC terminals at EAC; and with MSE terminals and ECB.

The DSCS Operational Control System (DOCS) controls all DSCS user terminals operating on DSCS spacecraft. This system also is undergoing upgrades that will allow higher control capability using less equipment and manpower resources.

A secondary payload is a U.S. Air Force SATCOM UHF single transponder used to transmit emergency action messages between military command posts (CPs) and force elements. The communications system is supported by one receive multibeam antenna (MBA), two receive ECH antennas, two transmit MBAs, two transmit ECH antennae, and a gimballed dish antenna (GDA). Each of the six independent repeaters operates in the SHF region to relay telephone, data, wideband imagery and secure digital voice signals.

Defense Meteorological Satellite Program

Operational commanders require timely, quality weather information to effectively employ weapon systems and protect DoD resources. The Defense Meteorological Satellite Program (DMSP) is DoD’s most important and often the only source of global weather data to support U.S. military operations. It provides visible and infrared cloud cover imagery and other meteorological, oceanographic, land surface and space environmental data.

DMSP, originally known as the Defense System Applications Program (DSAP) and the Defense Acquisition and Processing Program (DAPP), is a long-term Air Force effort in space to monitor the meteorological, oceanographic and solar-geophysical environment of the Earth in support of DoD operations. The first DMSP satellite, F-8, was built by General Electric’s Astro-Space Division and was launched on June 18, 1987, from Vandenberg AFB, CA, using an Atlas E rocket.

All spacecraft launched have a tactical (direct readout) and a strategic (stored data) capacity. The USAF also maintains an operational constellation of two near-polar, sun-synchronous satellites.

While managed at the Space and Missile Systems Center, Los Angeles Air Force Base, CA, DMSP command and control is provided by a joint operational team at the National Oceanic and Atmospheric Administration, Suitland, MD. DMSP satellites provide meteorological data in real time to Air Force, Army, Navy and Marine Corps tactical ground stations and Navy ships worldwide. This data is also stored in recorders on the satellites for later transmission to one of four ground stations located near Fairbanks, AK; New Boston, NH; Kaena Point, HI; and Thule Air Base, Greenland. From these command stations, data is relayed to the Air Force Weather Agency at Offutt Air Force Base, NE, to the U.S. Navy’s Fleet Numerical Meteorological and Oceanographic Center at Monterey, CA, and to the Air Force’s 55th Space Weather Squadron at Falcon Air Force Base, CO, where this information is used to compile numerous worldwide weather and space environmental information.

There are currently four operational satellites in the DMSP constellation. F-13 coverage is for early morning with no secondary coverage. The mid-morning orbit primary is F-15 with secondary coverage provided by F-12 followed by F-14. F-8 is still in orbit but is not operational and utilized for testing only. The satellites circle Earth at an altitude of about 500 miles in a near-polar, sun-synchronous orbit every 101 minutes. Each scans an area 1,800 miles wide and covers the entire planet in about 12 hours. The combination of day/night and dawn/dusk satellites allows monitoring of global information every six hours.

F-16 is scheduled for launch in mid-2003, and will replace F-15 which will remain in orbit in a secondary role. F-16 is the first complete 5D3-3 design (F-15 is a hybrid 5D-2/5D-3 as it carries the 5D-3 satellite bus but not the new 5D-3 payload upgrades) which incorporates a full suite of four solid state recorders, a higher commanding rate, one additional S-band transmitter, a deployable UHF antenna with two frequencies, and three new sensors: the Special Sensor Microwave Imager Sounder (SSMIS), a Special Sensor Ultraviolet Limb Imager (SSULI) and the Special Sensor Ultraviolet Spectrographic Imager (SSUSI).

The primary sensor on board is the Operational Linescan System (OLS) that observes clouds via visible and infrared imagery for use in worldwide forecasts. A microwave imager (MI) and sounders cover one half the width of the visible and infrared swath. These instruments cover polar regions at least twice and the equatorial region once per day. Other space environment sensors record along-track plasma densities, velocities, composition and drifts.

Visible and infrared imagery from the OLS instruments are used to monitor the global distribution of clouds and cloud top temperatures twice each day. The archive data set consists of low-resolution global and high-resolution regional imagery recorded along a 3,000 km scan, satellite ephemeris and solar and lunar information. The OLS instruments, built by Westinghouse Corp., consist of two telescopes and a photo multiplier tube (PMT). The visible telescope is sensitive to radiation. The detectors sweep back and forth in a “whisk broom” or pendulum-like motion.

A second important sensor is the Special Sensor Microwave Imager Sounder (SSMIS), which provides all-weather capability for worldwide tactical operations and is particularly useful in typing and forecasting severe storm activity. The SSMIS is a seven-channel, four frequency, linearly polarized, passive microwave radiometric system which measures atmospheric, ocean and terrain microwave brightness temperatures. The satellite also carries a suite of additional sensors, which collect a broad range of meteorological and space environmental data for forecasting and analysis. The SSMIS replaces the functionality of three 5D-2 microwave sensors and adds upper atmospheric profiling.

The last scheduled launch of a DMSP satellite is planned for some time in 2011.

Milstar

Milstar is a joint service program to provide extremely high frequency (EHF) satellites; a satellite mission control segment; and new or modified Army, Navy, and Air Force communication terminals for survivable, jam-resistant, worldwide, secure communications to strategic and tactical war fighters. A key feature of the Milstar system is the interoperable terminals used by U.S. warfighters. For example, sea-based terminals can be used to upload new data onto cruise missiles carried aboard submarines and guided missile destroyers in real time. Land-based terminals, such as the Several Channel Anti-Jam Man-Portable (SCAMP) and the Secure Mobile Anti-Jam Reliable Tactical Terminal (SMART-T), provide communications and data exchange for the mobile, ground-based warfighter.

Milstar had a rough beginning as there were concerns over the high cost estimates and uncertainty over the requirements as the Cold War drew to a close. The satellite constellation was originally planned to consist of four active (and one spare) satellites in geosynchronous equatorial orbit, as well as three active (and one spare) satellites in geosynchronous polar orbit, with a tenth spacecraft procured as a ground spare in anticipation of a launch failure. Plans were revised calling for six satellites in a mixture of low- and high-inclination orbits. A low-inclination orbit would place the satellites in positions to cover the Atlantic, Pacific, and Indian Oceans as well as North and South America. Satellites in the high-inclination orbit would cover the polar regions, Europe, Africa and Western Asia.

The first launch of a first-generation Milstar Block I satellite was on February 7, 1994, with a second launch in November 1995. Block I satellites feature a Low Data Rate (LDR) payload built by TRW (now Northrop Grumman) Space and Electronics Group and two satellite crosslink antennae, built by Boeing Satellite Systems

The LDR payload offers nearly 200 user channels and relays coded teletype and voice messages at data rates of 75 to 2,400 bits per second. The crosslink antennae provide rapid, ultra-secure communications by enabling the satellites to pass signals to one another worldwide while requiring only one ground station on friendly soil. The crosslink payload provides V-band (60 GHz) data communications between Milstar satellites for both the MDR and LDR payloads. This includes modulation and demodulation of the data, upconversion, amplification for transmission and downconversion. Crosslink operations are performed at or near frequencies that are absorbed by Earth’s atmosphere, preventing detection by Earth-based ground stations.

Milstar has been specifically designed to overcome shortfall characteristics of existing satellite communications systems. Concepts for survivability in a hostile space environment (Cold War-era thinking) had shaped the design of this military communication system, especially with the first two Milstar satellites. As part of the program restructuring, requirements for a classified payload were deleted from the Block 2 satellites as were “heroic” survivability features, such as hardening against nuclear shock.

Milstar Block 2 satellites 3 through 6 have both Low Data Rate and Medium Data Rate (MDR) payloads with increased tactical capabilities, including two nulling spot antennae that can identify and pinpoint the location of a jammer and electronically isolate its signal, allowing Milstar users to operate normally and at full capacity with no loss in signal quality or speed, despite any attempt by hostile forces to jam or intercept its signal.

The MDR payload provides secure, jam-resistant communications services through unique onboard signal and data processing capabilities. It sends real-time voice, video and data to military personnel in the field at rates up to 1.5 megabytes (MB) per second (Mbps). The payload uses a 32-channel EHF uplink and an SHF downlink. The MDR payload dynamically sorts incoming data and routes them to the proper downlinks to establish networks and provide bandwidth on demand. If necessary it passes the data on to another satellite via crosslink. The MDR antenna coverage subsystem consists of eight narrow spot beam antennae: two narrow spot beams with nulling capabilities (nuller antennae) and six distributed user coverage antennae (DUCAs), each supporting two-way communications.

In contrast to commercial communications satellites whose beams can cover entire continents, Milstar’s beams are very narrow, providing less opportunity for enemy detection and penetration. The nuller antennae resist jamming from within their respective coverage areas by changing their gain patterns when a jamming signal is detected.

Satellite 3, the first Block 2 satellite, did not reach its proper orbit and the satellite was placed in its final non-interference “parking” orbit and shutdown in April 1999. Milstar satellite 4 was launched successfully February 27, 2001, and satellite 5 was launched successfully on January 15, 2002. Milstar satellite 6 will launch in fiscal year 2003.

Navstar

Navstar Global Positioning System (GPS) is a space-based radio positioning, navigation and time distribution system. GPS User Equipment (UE) consists of standardized receivers, antennae, antenna electronics, etc., grouped together in sets to derive navigation and time information transmitted from GPS satellites. The Navstar Global Positioning System (GPS) is a constellation of 27 orbiting satellites in an 11,000-mile circular orbit that provides navigation data to military and civilian users all over the world. This exceeds the operational requirement of 24 satellites and provides robustness and overlapping capability should any one satellite fail. The system is operated and controlled by the 50th Space Wing, located at Schriever Air Force Base, CO.

GPS satellites orbit Earth every 12 hours, emitting continuous navigation signals. With the proper equipment, users can receive these signals to calculate time, location and velocity. The signals are so accurate, time can be figured to within one millionth of a second, velocity within a fraction of a mile per hour and location to within 100 feet.

The GPS Master Control Station, operated by the 50th Space Wing, is responsible for monitoring and controlling the GPS satellite constellation. The GPS-dedicated ground system consists of five monitor stations and four ground antennae located around the world. The monitor stations use GPS receivers to passively track the navigation signals on all satellites. Information from the monitor stations is then processed at the master control station and used to update the satellites’ navigation messages.

The master control station crew sends updated navigation information to GPS satellites through ground antennae using an S-band signal. The ground antennae are also used to transmit commands to satellites and to receive state-of-health data (telemetry).

GPS provides two levels of service, Standard Positioning Service and the Precise Positioning Service.

The Standard Positioning Service (SPS) is a positioning and timing service that will be available to all GPS users on a continuous, worldwide basis with no direct charge. SPS will be provided on the GPS L1 frequency that contains a coarse acquisition (C/A) code and a navigation data message. SPS provides a predictable positioning accuracy of 100 meters (95 percent) horizontally and 156 meters (95 percent) vertically and time transfer accuracy to UTC within 340 nanoseconds (95 percent).

The Precise Positioning Service (PPS) is a highly accurate military positioning, velocity and timing service which will be available on a continuous, worldwide basis to users authorized by the U.S. P(Y) code. Capable military user equipment provides a predictable positioning accuracy of at least 22 meters (95 percent) horizontally and 27.7 meters vertically and time transfer accuracy to UTC within 200 nanoseconds (95 percent). PPS will be the data transmitted on the GPS L1 and L2 frequencies. PPS was designed and made available primarily for U.S. military to use as well as the federal government. Access will be denied to unauthorized users by the use of cryptography.

The satellites transmit on two L-band frequencies with three pseudo-random noise (PRN) ranging codes. The coarse/acquisition (C/A) code has a 1.023 MHz chip rate, a period of 1 millisecond (ms) and is used primarily to acquire the P-code. The precision (P) code has a 10.23 MHz rate, a period of 7 days and is the principal navigation ranging code. The Y-code is used in place of the P-code whenever the anti-spoofing (A-S) mode of operation is activated.

The C/A code is available on the L1 frequency and the P-code is available on both L1 and L2. The various satellites all transmit on the same frequencies, L1 and L2, but with individual code assignments.

Due to the spread spectrum characteristic of the signals, the system provides a large margin of resistance to interference

The first GPS satellite was launched in 1978. The first 10 satellites were developmental satellites, called Block I. Twenty-one of the original 27 Block II and Block IIA satellites launched between 1989 and 1996 are still operational, which is remarkable considering their estimated life space was 7 1/2 years.

There are seven Block IIR satellites on-orbit with six operational. There are three launches planned in 2003 with the first Block IIR-M scheduled for July 2004. Expectations are for 21 Block IIR total.

Between 12 and 16 Block IIF satellites are planned (there are currently none in orbit). Satellites 1 through 6 are being modernized to carry additional civil signals (L2C and L5), a civil-only safety-of-life signal and an M-Code military-only signal. Satellites 7 through 9 will be procured in fiscal year 2005 and the last three in fiscal year 2006.

The GPS Block III program is currently under review.

Wideband Gapfiller

The Wideband Gapfiller Satellites (WGS) will provide near-term continuation and augmentation of the services currently provided by the DSCS and the Global Broadcast Service (GBS). WGS will complement the DSCS III Service Life Enhancement Program (SLEP) and GBS payloads, and offset the eventual decline in DSCS III capability.

Together these assets will provide wideband services during the transition period between today’s systems and the advent of the Objective X/Ka wideband system or Advanced Wideband System (AWS) in 2008. All said and done, the Boeing 702 satellite that is the foundation of the WGS offers 10 times the capacity of the DSCS III SLEP platforms.

The WGS constellation will consist of three commercial DoD-owned satellites. This program is the first near-commercial acquisition of a satellite by the DoD with almost 95 percent of the satellite content from commercial off-the-shelf (COTS) products. As of October 2002 the contract value surpassed $500 million (total potential for the contract is $1.3 billion) and included non-recurring engineering, WGS satellites F1 and F2, long-lead elements for F3, and four payload control elements.

This combination of the Wideband Gapfiller Satellites, DSCS satellites, GBS payloads, wideband payload and platform control assets, and earth terminals operating with them has been referred to as the Interim Wideband System (IWS).

WGS will support wideband military satellite communications services beginning around 2004. It will provide services to the DoD and the Ministry of Defense for Canada as well as other government and allied users under unstressed conditions. The Gapfiller System will support continuous 24 hour per day wideband satellite services to tactical users and some fixed infrastructure users. Limited protected services will be provided under conditions of stress to selected users employing terrestrial modems capable of providing protection against jamming. The combined wideband satellite communications system consists of space vehicles of multiple types, control terminals and facilities and user terminals.

The space segment will support communication services in two military frequency bands: X-band and Ka-band. The WGS payload will be capable of supporting at least 1.2 Gbps aggregate simplex throughput. The Gapfiller satellites will operate in X-band (8 steerable/shapable) and in WGS Broadcast Ka-band (10 steerable), similar to the Phase II GBS in service today. This capability will ensure interoperability with existing and new X-band and GBS terminals. The Gapfiller satellites will also provide a new two-way military Ka-band capability to support the expected military mobile/tactical two-way Ka-terminal population with greatly increased system capacity.

X-band services will augment services provided by DSCS III satellites. The Ka-band services will supplement broadcast service provided by GBS payloads on UHF Follow-On satellites; the Ka-band services will also support two-way network services besides broadcast. The Gapfiller satellites also will support services that require cross-banded connectivity: X-band uplinks to Ka-band downlinks and Ka-band uplinks to X-band downlinks.

Each Wideband Gapfiller Satellite orbital configuration will provide services from 65 degrees north (objective is 70 degrees north) latitude to 65 degrees south latitude and for all longitudes accommodated within the field of view of the satellite. Worldwide coverage will be maintained by a combination of WGS and DSCS capabilities.

The satellite system is planned for three geosynchronous satellite configurations and ground equipment and software associated with Gapfiller payload and platform control. The Gapfiller satellites in pre-launch configuration are designed to be launched from a medium launch vehicle (MLV)-class Evolved Expendable Launch Vehicle (EELV).

The ground control segment lever