US Military GPS

The GPS Block IIF satellites are the next generation of global positioning system (GPS) space vehicles built by Boeing for the US Air Force.

The US Airforce and Missile Systems Center initially placed an order for the development of 33 GPS Block IIF Satellites in 1996, but later the contract was modified and the number was reduced to 12.

The satellites were built to replace previous blocks GPS Block I, II, and IIA satellites, the earliest of which went into service in 1978. GPS is the largest satellite constellation of US Department of Defense (DOD) with 31 operational satellites on orbit. Delivery of the Block IIF satellites started in April 2001 and sustained the Global Positioning System constellation, which provides signals for continuous day and night, all weather, three-dimensional positioning for worldwide navigation.

The GPS IIF features significant improvements over previous blocks and offers flexibility when it comes to evolving user requirements, such as new military or commercial frequencies.

The 12th GPS IIF satellite was launched aboard United Launch Alliance Atlas V launch vehicle in February 2016. It also marked the launch of 50th GPS satellite delivered by Boeing on orbit to US Air Force.

GPS Block IIF satellite design and features

the GPS Block IIF satellite has a height of 2.51m, width of 2.06m, depth of 1.8m and weighs 1,633kg at launch. It is designed for a longer lifespan of 12 years which reduces operating costs.

The satellites are capable of carrying out autonomous operations up to 60 days. The flexible design of the satellite helps in upgrading to net-centric operations and advanced security.

The satellites provide enhanced accuracy through advanced atomic clocks, longer design life, new operational third civil signal (L5) and safety-of-life applications.

Navstar GPS radio navigation

Navstar GPS is a satellite-based radio navigation system developed and operated by the US Department of Defense (DOD). GPS allows land, sea, and airborne users to immediately determine their three-dimensional position, speed and time, 24 hours a day, under all weather conditions, anywhere in the world.

It consists of a constellation of orbiting satellites, a ground control system and thousands of GPS information receiver units in aircraft, ships, land vehicles and hand-held manpacks. The system achieved full operational capability in July 1995.

GPS uses GPS is used to aid land, sea and airborne navigation, geographical exploration, cartography and geodesy, surveying, vehicle location systems, search and rescue operations, aerial refuelling and rendezvous and a wide range of additional applications.

There are also numerous civilian users, such as trucking fleets, commercial airlines and law enforcement agencies. Hikers, farmers and fishermen also benefit from GPS. The military capabilities of GPS were demonstrated in the Gulf conflict, cited by US defence officials as a key factor in the successes of operations Desert Storm and Desert Shield.

Satellite constellation

The space segment of the GPS programme consists of 24 operational satellites in six circular orbits at an altitude of 20,200km (10,900nm), with a 12 hour period, at an inclination of 54.8°.

The satellites are positioned so that at any one point in time, six of them will be in view to users anywhere in the world. Position and time data are continuously broadcasted to users all over the world. The Block IIF satellites broadcast on 2 L-band frequencies.

Satellite control

The Control Segment consists of a master control station operated by the 2nd Satellite Control Squadron at Falcon Air Force Base, Colorado, with five monitor stations and three ground antennas dispersed globally.

The monitor stations track all GPS satellites in view and collect information from their broadcasts. The monitor stations send the information they collect from each of the satellites back to the master control station, which computes precise satellite orbits.

The information is then formatted into updated navigation messages for each satellite. The updated information is transmitted to each satellite through the ground antennas, using an S-band signal.

The ground antennas also transmit and receive satellite control and monitoring signals. Boeing GPS Block IIF’s partner Computer Sciences Corporation began assuming responsibility for the operational control segment in 2000.

GPS signal reception

The User Segment consists of the signal receivers/processors, antennas and control/display units that allow land, sea, or airborne operators to receive the GPS satellite broadcasts and compute their precise latitude, longitude, altitude, precise time and velocity at any time, in any weather. The system can accommodate an unlimited number of users without revealing their positions.

Satellite launch

The Delta II expendable launch vehicle is used to launch GPS satellites from Cape Canaveral Air Force Station, Fl. Block IIF spacecraft will use the Air Force’s planned Evolved Expendable Launch Vehicle (EELV).

Under management of a Joint Programme Office at the US Air Force’s Space and Missile Systems Centre, Los Angeles Air Force Base, Calif, Boeing Reusable Space Systems designed, built, and tested eleven developmental Navstar GPS satellites and developed and qualified a second-generation production prototype.