The configuration of the GPS Space Segment is well known. The satellites are on orbit at a nominal height of about 20,000 km above the Earth. There are three carriers L1 (1575.42 MHz), L2 (1227.60 MHz), and L5 (1176.42 MHz). A minimum of 24 GPS satellites ensure 24-hour worldwide coverage, but there are more than that minimum on orbit. There are a few spares on hand in space. The redundancy is prudent because GPS is critical to positioning, navigation and timing. It is also critical to the smooth functioning of financial transactions, air traffic, ATMs, cell phones, and modern life in general around the world. This very criticality requires continuous modernization.
GPS was put in place with amazing speed considering the technological hurdles and reached its Fully Operational Capability (FOC) on July 17, 1995.The oldest satellites in the current constellation were launched in the early 1990s. If you imagine using a personal computer of that vintage today, it is not surprising that there are plans in place to alter the system substantially. In 2000, U.S. Congress authorized the GPS III effort. The project involves new ground stations and satellites, additional civilian and military navigation signals, and improved availability. This chapter is about some of the changes in the modernized GPS, its inclusion in the Global Navigation Satellite System and more.
The configuration of the GPS space segment, 24 satellites for 24-hour worldwide coverage. There are six orbital planes that are inclined 55 degrees and the nominal altitude in the neighborhood of 20,000 kilometers above the earth. These facts are fairly well known. We've discussed that a little bit in the past. There are actually more than 24 available at any given time. This is so that there would be few spare satellites on hand in case some of them need to be moved or retired. The fact that GPS has become more and more critical to all kinds of navigation over the years is a testament to the need to have these spares and have this constellation be stable and constantly available.
The first of the 11 successful Block I satellites was launched in1978 from Vandenberg Air Force Base. The last of them was launched in 1985. One launch failed, Navstar 7. They were all retired by late 1995. None of the Block I satellites are on orbit now.
These satellites needed frequent help from the Control Segment. They could operate independently for only 3 1/2 days. The Control Segment handled the necessary momentum dumping for the satellites and maintained their attitudes using hydrazine thrusters. The inclination of these satellites relative to the equator was 63° instead of the inclination of 55° used for subsequent GPS blocks.
They had a design life of 4.5 years, though some operated for double that. They were powered by 7.25 square meters of solar panels and they also had three rechargeable nickel-cadmium batteries. In subsequent blocks of satellites design lives increased and dependence on the Control Segment decreased. However, some of the features of Block I were carried forward into the subsequent blocks of GPS satellites. They carried onboard nuclear detonation detection sensors a feature which has continued in future GPS satellite blocks.
It was clear from the beginning that atomic frequency standards, clocks, were necessary for the proper functioning of the system. Therefore the Block I satellites had cesium and rubidium frequency standards on-board, a feature that future GPS satellites share. The first 3 Block I satellites carried 3 rubidium clocks. Unfortunately they stopped working after about a year in space. The 3 rubidium standards were improved. Equipment was added to keep the frequency standards at a constant temperature during flight and 1 cesium frequency standard was added to subsequent satellites in this block.
The Block II satellites were about twice as heavy as the Block I satellites. The first of them was launched in 1989. The Block II satellites often exceeded their 7.3 design life. The last was decommissioned in 2007 after 17 years of operation. They could be autonomous, without contact with the Control Segment, for up to 14 days. The uploads from the Control Segment to the Block II satellites were encrypted unlike uploads during the Block I. The signals from the Block II satellites were periodically and purposely disrupted. Specifically the on-board clocks were intentionally dithered in a procedure known as Selective Availability (SA).
It is amazing the the Block II satellites lives extended so far beyond the expected mean mission duration.
Block IIA satellites are an improved version of the Block II. The first of 19 Block IIA satellites was launched in 1990. While none of the Block II satellites are functioning today, some of the Block IIA satellites are still healthy. There are 6 Block IIA satellites on orbit and operational. These survivors are now the oldest of the GPS satellites operating on orbit.
They are radiation hardened against cosmic rays, built to provide Selective Availability (SA), antispoofing (AS) capability and onboard momentum dumping. This SA continued until May 2, 2000 when it was discontinued.
Selective availability has been turned off. This is the dithering of the clocks that was done to degrade the C/A Code positioning. Anti spoofing is still available and turned on from time to time. This is the encryption of the P-Code into the Y-Code. The objective of anti spoofing is to prevent bad guys from broadcasting signals that masquerade as GPS signals to confuse GPS receivers.
Block IIA satellites can store more of the Navigation message than the Block II satellites could and can, therefore operation without contact with the Control Segment for 6 months. However, if that were actually done, their broadcast ephemeris and clock correction would degrade.
Two Block IIA satellites, SVN 35 (PRN 05) and SVN 36 (PRN 06), have been equipped with Laser Retro-reflector Arrays (LRA). The second of these was launched in 1994 and is still in service. The retro-reflectors facilitate satellite laser ranging (SLR). Such ranging can provide a valuable independent validation of GPS orbits.
Like the Block II satellites, the Block IIA satellites are equipped with two rubidium and two cesium frequency standards. They are expected to have a design life of 7.3 years. While the design life has obviously been exceeded in most cases, Block IIA satellites do wear out.
The first launch of the next Block, Block IIR satellites in January of 1997 was unsuccessful. The following launch in July of 1997 succeeded. There are 12 Block IIR satellites on orbit and operational. There are some differences between the Block IIA and the Block IIR satellites. The Block IIR satellites have a design life of 7.8 years and can determine their own position using inter-satellite crosslink ranging called AutoNav. This involves their use of reprogrammable processors onboard to do their own fixes in flight.
They can operate in that mode for up to 6 months and still maintain full accuracy. The Control Segment can also change their software while the satellites are in flight and, with a 60-day notice, move them into a new orbit. Unlike some of their direct predecessors, these satellites are equipped with three rubidium frequency standards. Some of the Block IIR satellites also have an improved antenna panel that provides more signal power. They are more radiation hardened than their predecessors, and they cost about a third less than the Block II satellites did.
Despite their differences, Block IIA and the Block IIR satellites are very much the same in some ways. They both broadcast the same fundamental GPS signals that have been in place for a long time. Their frequencies are centered on L1 and L2. As mentioned before, the Coarse/Acquisition code or C/A-code is carried on L1 and has a chipping rate of 1.023 million chips per second. It has a code length of 1023 chips over the course of a millisecond before it repeats itself. There are actually 32 different code sequences that can be used in the C/A code, more than enough for each satellite in the constellation to have its own. The Precise code or P-code on L1 and L2 has a chipping rate that is ten times faster than the C/A code at 10.23 million chips per second. The P-code has a code length of about a week, approximately 6 trillion chips, before it repeats. If this code is encrypted it is known as the P(Y) code, or simply the Y-code.
Nine of the Block IIR satellites carry Distress Alerting Satellite System (DASS) repeaters. These DASS repeaters are used to relay distress signals from emergency beacons and were part of a proof of the concept of satellite-supported search and rescue that was completed in 2009. Twelve additional IIR satellites will carry them too.