Global Navigation
Satellite System (GNSS)
GPS - The Global Positioning System is owned and
operated by the United States. The official US
Department of Defense name for GPS is NAVSTAR.
BEIDOU - BeiDou (or BDS, formally known as COMPASS)
is a regional GNSS owned and operated by the People's
Republic of China.
GALILEO - Galileo is the European Union’s global GNSS.
The EU completed the system in 2020.
GLONASS - GLONASS (Global Navigation Satellite
System) is owned and operated by the Russian
Federation. The fully operational global system consists
of 24+ satellites.
QZSS - QZSS, or Quasi-Zenith Satellite System, is a
regional GNSS operated by QZS System Service Inc.
(QSS) and owned by the Government of Japan. QZSS
works with GPS to improve coverage in East Asia. The
constellation is operational with 4 satellites. By 2023,
Japan plans to have 7 satellites for autonomous
capability.
System BeiDou Galileo GLONASS GPS NavIC QZSS
Owner China European Union Russia United States India Japan
Coverage Global Global Global Global Regional Regional
Coding CDMA CDMA FDMA & CDMA CDMA CDMA CDMA
32,600 km (20,300 mi)
–
Altitude 21,150 km (13,140 mi) 23,222 km (14,429 mi) 19,130 km (11,890 mi) 20,180 km (12,540 mi) 36,000 km (22,000 mi)
39,000 km (24,000 mi)
[29]
Period 12.63 h (12 h 38 min) 14.08 h (14 h 5 min) 11.26 h (11 h 16 min) 11.97 h (11 h 58 min) 23.93 h (23 h 56 min) 23.93 h (23 h 56 min)
Rev./S. day 17/9 (1.888...) 17/10 (1.7) 17/8 (2.125) 2 1 1
BeiDou-3:
28 operational By design:24 active + 6
(24 MEO 3 IGSO 1 backup
24 by design
GSO) Currently: 4 operational (3 GSO, 1
24 operational 30,[32] 3 GEO,
Satellites 5 in orbit validation 26 in orbit GEO)
1 commissioning 24 by design 5 GSO MEO
2 GSO planned 20H1 24 operational 7 in the future
1 in flight tests[31]
BeiDou-2: 2 inactive
15 operational 6 to be launched[30]
1 in commissioning
1.57542 GHz
1.561098 GHz (B1) 1.559–1.592 GHz 1.593–1.610 GHz (G1) 1.563–1.587 GHz (L1)
(L1C/A,L1C,L1S)
1.589742 GHz (B1-2) (E1)1.164–1.215 GHz 1.237–1.254 GHz 1.215–1.2396 GHz 1.17645 GHz(L5)
Frequency 1.22760 GHz (L2C)
1.20714 GHz (B2) (E5a/b) (G2)1.189–1.214 GHz (L2)1.164–1.189 GHz 2.492028 GHz (S)
1.17645 GHz (L5,L5S)
1.26852 GHz (B3) 1.260–1.300 GHz (E6) (G3) (L5)
1.27875 GHz (L6)[33]
Operating since 2016
Status Operational[34] Operational Operational Operational Operational
2020 completion[30]
3.6m (Public) 1m (Public) 0.3m - 5m (no DGPS or 1m (Public) 1m (Public)
Precision 2m – 4m
0.1m (Encrypted) 0.01m (Encrypted) WAAS) 0.1m (Encrypted) 0.1m (Encrypted)
System BeiDou Galileo GLONASS GPS NavIC QZSS
GPS (Global Positioning System)
• The first GPS satellite was launched in 1978.
• A full constellation of 24 satellites was achieved in 1994.
• Each satellite is built to last about 10 years. Replacements are constantly
built and launched in orbit.
Why Did the Department of Defense
Develop GPS?
In the latter days of the arms race the
targeting of ICBMs became such a fine art
that they could be expected to land right
on an enemy's missile silos. Such a direct
hit would destroy the silo and any missile
in it. The ability to take out your
opponent's missiles had a profound effect
on the balance of power.
But you could only expect to hit a silo if
you knew exactly where you were
launching from. That's not hard if your
missiles are on land, as most of them were
in the Soviet Union. But most of the U.S.
nuclear arsenal was at sea on subs. To
maintain the balance of power the U.S.
had to come up with a way to allow those
subs to surface and fix their exact position
in a matter of minutes anywhere in the
world Hello GPS!
The result is the Global Positioning System,
a system that's changed navigation
forever.
Satellite/Space
Segment
Control
Segment
The control segment of the Global Positioning System
is a network of ground stations that monitors the
shape and velocity of the satellites' orbits. The
accuracy of GPS data depends on knowing the
positions of the satellites at all times
Monitor Stations are very precise GPS receivers
installed at known locations. They record discrepancies
between known and calculated positions caused by
slight variations in satellite orbits. Data describing the
orbits are produced at the Master Control Station at
Colorado Springs, uploaded to the satellites, and finally
broadcast as part of the GPS positioning signal. GPS
receivers use this satellite Navigation Message data to
adjust the positions they measure.
User Segment
• The Global Positioning System (GPS) employs trilateration to
calculate the coordinates of positions at or near the Earth's
surface. Trilateration refers to the trigonometric law by which the
interior angles of a triangle can be determined if the lengths of all
three triangle sides are known. GPS extends this principle to three
dimensions.
• A GPS receiver can fix its latitude and longitude by calculating its
distance from three or more Earth-orbiting satellites, whose positions
in space and time are known. If four or more satellites are within the
receiver's "horizon," the receiver can also calculate its elevation and
even its velocity. The U.S. Department of Defense created the Global
Positioning System as an aid to navigation. Since it was declared fully
operational in 1994, GPS positioning has been used for everything
from tracking delivery vehicles, to tracking the minute movements of
the tectonic plates that make up the Earth's crust, to tracking the
movements of human beings. In addition to the so-called user
segment made up of the GPS receivers and people who use them to
measure positions, the system consists of two other components:
a space segment and a control segment. It took about $10 billion to
build over 16 years.
1. Satellite clock: GPS receivers calculate their distances from satellites as a function of the difference in time
between when a signal is transmitted by a satellite and when it is received on the ground. The atomic clocks on
board NAVSTAR satellites are extremely accurate. They do tend to stray up to one millisecond of standard GPS
time (which is calibrated to, but not identical to, Coordinated Universal Time). The monitoring stations that
make up the GPS "Control Segment" calculate the amount of clock drift associated with each satellite. GPS
receivers that are able to make use of the clock correction data that accompanies GPS signals can reduce clock
error significantly.
2. Upper atmosphere (ionosphere): Space is nearly a vacuum, but the atmosphere isn't. GPS signals are delayed
and deflected as they pass through the ionosphere, the outermost layers of the atmosphere that extend from
approximately 50 to 1,000 km above the Earth's surface. Signals transmitted by satellites close to the horizon
take a longer route through the ionosphere than signals from satellites overhead, and are thus subject to greater
interference. The ionosphere's density varies by latitude, by season, and by the time of day, in response to the
Sun's ultraviolet radiation, solar storms and maximums, and the stratification of the ionosphere itself. The GPS
Control Segment is able to model ionospheric biases, however. Monitoring stations transmit corrections to the
NAVSTAR satellites, which then broadcast the corrections along with the GPS signal. Such corrections
eliminate only about three-quarters of the bias, however, leaving the ionosphere the second largest contributor
to the GPS error budget.
3. Receiver clock: GPS receivers are equipped with quartz crystal clocks that are less stable than the atomic
clocks used in NAVSTAR satellites. Receiver clock error can be eliminated, however, by comparing times of
arrival of signals from two satellites (whose transmission times are known exactly).
4. Satellite orbit: GPS receivers calculate coordinates relative to the known locations of satellites in space.
Knowing where satellites are at any given moment involves knowing the shapes of their orbits as well as their
velocities. The gravitational attractions of the Earth, Sun, and Moon all complicate the shapes of satellite orbits.
The GPS Control Segment monitors satellite locations at all times, calculates orbit eccentricities, and compiles
these deviations in documents called ephemerides. An ephemeris is compiled for each satellite and broadcast with
the satellite signal. GPS receivers that are able to process ephemerides can compensate for some orbital errors.
5. Lower atmosphere: (troposphere, tropopause, and stratosphere) The three lower layers of atmosphere
encapsulate the Earth from the surface to an altitude of about 50 km. The lower atmosphere delays GPS signals,
adding slightly to the calculated distances between satellites and receivers. Signals from satellites close to the
horizon are delayed the most since they pass through more atmosphere than signals from satellites overhead.
6. Multipath: Ideally, GPS signals travel from satellites through the atmosphere directly to GPS receivers. In
reality, GPS receivers must discriminate between signals received directly from satellites and other signals that
have been reflected from surrounding objects, such as buildings, trees, and even the ground. Some, but not all,
reflected signals are identified automatically and rejected. Antennas are designed to minimize interference from
signals reflected from below, but signals reflected from above are more difficult to eliminate. One technique for
minimizing multipath errors is to track only those satellites that are at least 15° above the horizon, a threshold
called the "mask angle."
