Who developed GPS?

The Global Positioning System is a space based military navigation system designed,
financed, deployed and controlled by the U. S. Department of Defense.

GPS was designed to replace the ageing TRANSIT Doppler Satellite navigation system and
other nav-aids for navigation and geodetic community for over 20 years. This is a microwave
satellite based system having many advantages.

The development work of GPS started in 1973 as a result of the merging of several R & D
Programs within the U. S. Department of Defense, namely, the Navy’s “ TIMATION” Project
and the Air Force’s “621 B” Project . The first GPS satellite NAVSTAR was launched in
1978.

The development and production programme for the GPS is managed by the U.S. Air Force
(USAF) Systems Command, Space Systems Division, Joint Program Office (JPO) at the Los
Angeles Air Force Base, California. The JPO is manned by personnel from the USAF, US
Navy, US Army, US Marine Corps, US Coast Guard, US Defense Mapping Agency, Nato
Nations and Australia.

The aim of the JPO was to develop an all-weather, 24 hour, truly global navigation system to
support the positioning requirements for the armed forces of the US and its allies. Though the
system was designed to replace the large variety of navigational system already in use, a
great change was made on system’s utility, reliability and survivability. A number of stringent
conditions therefore had to be met.

The GPS Satellites Constellation

These are groups of satellites in GPS Satellite constellation like stars constellations. Five
groups of GPS satellite constellations are present-

Block I Satellites – they were built by the Rock Well International Corporation. They were the
experimental satellites launched in between 1978 and 1985 to test the efficacy of the system.
Total eleven satellites were launched but at present are not functioning. The design life was 5
years.

Block II Satellites are also made by the Rock Well International Corporation, these are
massive and the weight being above 900 kg. The Block II satellites contribute first nine
satellites in the operational series. The design life was 7.5 years. These satellites are
maintained by once per day upload. The navigation message is up to 11 days in length.

Block II A Satellites are also made by the Rock Well International Corporation, contribute 19
satellites of the operational series. The design life is 7.5 years. The navigation message is
stored up to 180 days in length with the help of appropriate features.

Block II / II A Satellites –are total 28 in numbers out of which 24 satellites were launched and
activated. The rest of the satellites kept moth balled and to be used at the time of requirement.

Block II R Satellites – these are the replacement satellites and design life is 10 years. They
are built by the General Electric Corporation and now Martin Marieta Corporation. There are
20 satellites in this group. They have the features to store navigation message up to 180 days
in length.

Block II F Satellites – these are fourth generation satellites which will be used after the Block
II A Satellites. The space crafts have a design life of 12 years with an orbit period of 12 hours.
The programme delays and technical problems pushed the launch of satellites from 2006 to
May 2010.

Block I and Block II/IIA/IIR Satellites are not same in design, features and functions. The most
important difference is that Block I satellites were launched in the orbits having 63 inclination
whereas Block II/IIA/IIR satellites are having 55 inclination.

In the orbits there are 27 Block II/IIA and IIR satellites. Additional satellites remain
replenishment satellites. The GPS system was to be launched in the late 1980’s but due to
some reasons particularly Space Shuttle Challenger Disaster (28th January, 1986) the major
setback occurred. The full GPS system was deployed in 1995 with 24 Block II/IIA satellites
operational in the orbits. These 24 orbiting satellites ensure that at least four satellites will be
visited at all sites at all time on the globe. At some times of the day up to 12 satellites may be
visible simultaneously. Initial Operational Capability (IOC) was declared in July, 1993 for 24
Block I, IIA satellites and Full Operational Capability (FOC) was declared in 17th July, 1995
with 24 satellites of Block II or IIA satellites. US Department of Defense guaranteed 24 satellite
coverage for 70% of the time and 21 satellite coverage for 98% of time. There may have some
degraded satellites coverage even in the FOC conditions.

In Block II satellites, electrical power is generated through two solar panels. When the sun
rays are not available during the eclipses or when the satellites move into the earth’s shadow
region, the onboard battery backup gives them the energy to move. The satellites also have
the rocket booster to keep them moving into correct path.

Outages – When the satellites are not healthy and the satellite configuration geometry is poor
for navigation users, these periods of degraded geometry are known as outages.

Basic Functions of GPS – The basic functions of GPS are –

• To find out location – determining a basic position

• Navigation – going from one location to another

• Tracking - monitoring the movement of people, vehicles and things

• Timing – bringing precise timing to the world (required by Astronomers, Power

Companies, Computer Network, Communication Companies, Banks, Radio and
Television).

GPS System
The GPS System can be divided into three segments for better understanding –

• the Space Segment

 • the Control Segment, and,

• the User Segment

1. The Space Segment – It deals with the GPS satellites themselves, transmits the signals
necessary to operate the whole system.

The basic functions of the satellites are –

▪ to transmit the information and signals to users on one or both L-band frequencies.

▪ to maintain accurate time by means of several onboard atomic frequency standards.

▪ to receive and store data transmitted by the Control segment.

Information on NAVSTAR Satellite Constellation –

a) GPS Orbit Characteristics –

▪ divided in nearly 6 orbital planes

▪ 4 satellites are equally spaced within the plane ( 3 spare satellites in orbit are spread
across different orbital planes).

▪ the orbital planes are at inclination of 55 relative to the Equator (which means that
satellites cross the equator tilted at a 55 degree angle).

▪ the altitude is about 20,200 km above the earth

▪ the GPS satellites move in nearly circular orbits completing an orbit in approximately
12 hours. So they pass any point on the earth surface twice a day. The difference
between the rise and set of the satellites is about four minutes earlier each day.

b) Satellite ClockThe
satellites maintain accurate timing through three / four on board atomic
frequency standards, called as Hydrogen Masers. The installation of these four hydrogen
masers onboard is one of the main reasons for the atomic clocks becoming so expensive.

The atomic clocks generate noise and clock drifts which can be corrected and the estimates
of the accuracy of the atomic clock are uploaded through navigation messages. As these
corrections are based on observations only and not based on the clock’s current state, any
problem whatever small it is, may add some few meters of inaccuracy.

Each Block II or IIA satellite has two cesium atomic clocks and two rubidium atomic clocks,
while each Block IIR has three rubidium atomic clocks. The stability of these clocks is
estimated to be approximately 1 second per 300,000 years. Only one clock is in use on each
satellite at a time. The others are backups.

c) GPS Satellite Coverage

▪ from 4 to over 12 satellites above observers horizon

▪ satellites will be visible for many hours above on observer’s horizon

 ▪ short periods of degraded satellite coverage affect the navigation users but not the
surveyors

d) GPS Satellite Visibility

The availability of satellites is represented in satellite visibility graph and “skyplots” showing
the satellite tracks on a polar projection in relation to the site’s zenith.

e) GPS Satellite Health Monitoring

▪ There are different sources of information on GPS satellite health and the general
status of the system like notification of satellite manoeuvres, status of Selective
Avaibility (SA), launching programme etc. These sources are as follows –

▪ “Health Status” is transmitted as part of the Navigation Message. The GPS receivers
monitor these automatically and take necessary action. Even if the satellite becomes
“unhealthy” for navigation purpose, it is still suitable for phase tracking and for GPS
surveying.

▪ Different Bulletin Board Services and other electronic GPS information sources on the
Internet.

▪ Different Specialist Communication Services, for a certain section of users like real
time Differential GPS (DGPS) users, are served with health messages along with the
Standard Data Stream; aviation users receive NANUs (Notice Advisory to Navigational
Users) directly through radio links etc. It is expected that these specialist services will
increase in recent future and will be offered to surveying also.

Navigation Message is the information the satellites transmit to a receiver. It contains the
satellite orbital and clock information health status and general system status messages and
an ionosphere delay model.

2. The Control Segment – The Control System deals with the ground based time and orbit
control prediction and entirely controlled by the US Department of Defense (DoD). The tasks
of Control Segment are –

▪ to monitor and control the satellite system continuously

▪ to predict the satellite ephemerides and the behavior of satellite clock

▪ to send the corrected data to the satellites

▪ for updating of the navigation messages periodically for each satellite

This segment consists of facilities required for satellite Constructions, Launching,

maintenances, Satellite Health monitoring, Telemetry, Tracking, Command and Control,
Ephemeris Computations and up linking.

a) There are five Ground Control Stations with one Master Control Station (MCS) –

▪ Colorado Springs
▪ Hawaii
▪ Ascension Island
 ▪ Diego Garcia
▪ Kwajalein

The functions of these monitoring stations are -

❖ All five stations are monitoring stations of Department of Defense (DoD); they are
tracking the satellites and their signals, receiving these data continuously and sending
the tracking data to the Master Control Station at Colorado.

❖ Falcon Air Force Base, Colorado Springs, Colorado is the site of the Master Control
Station (MCS) at which the tracking data is processed to compute the satellite
ephemerides and satellite clock corrections. It initiates all operations of the Space
segment as for example, satellite maneuvering, signal encryption, satellite clock
keeping etc. (MCS is managed by the 2nd Operations Squadron, 50th Space Wing, US
Air Force).

b) There are 4 Ground Antennas (GA) for uploading data to the satellites. These are -

▪ Cape Canaveral
▪ Ascension Island
▪ Diego Garcia
▪ Kwajalein

❖ The data sent for up loading include the ephemerides and clock corrections information
transmitted with the Navigation Message as well as Command telemetry from the
MCS. Every upload station views all the satellites once in a day. Hence all the satellites
can be viewed by one upload station three times a day. So new Navigation Messages
and Command Telemetry can be transmitted to GPS satellites approximately every
eight hours if it is required. Presently, the uploading rate is once a day (sometimes
twice also).

c) A very important but latent function of the Control Segment is to maintain the WGS 84
reference system. This reference system is accessible to GPS users through the satellite
ephemerides computed by the MCS, from data collected at the monitor stations.

3. The User Segment – Both in the military and the civilian area, there has been tremendous
advancement in technology of the GPS user equipment. The user segment deals with different
types of advanced GPS receivers and their use. These receivers are combinations of –

▪ Hardware – signal tracking measurements

▪ Software – positioning algorithms
▪ Operational procedures (accuracy, functions etc)
▪ Antenna with pre-amplifier
▪ RF section with signal identification and signal processing
▪ Microprocessor for receiver control, data sampling and data processing
▪ Precision oscillator
▪ Power supply
▪ User Interface, Command and Display Panel
▪ Memory, data storage