LECTURE 10- GPS - COMPONENTS AND FUNCTIONS
1.Introduction
Space-based geodetic observations can be categorized into four basic techniques:
positioning, altimetry, interferometric synthetic aperture radar (InSAR), and gravity
studies.
Precise positioning is the fundamental geodetic observation required for surveying and mapping. Instead of
the traditional triangulation and levelling networks that require line of sight (LOS) between measurement
points, space geodetic methods use LOS between the measurement points and celestial objects or satellites.
Building on this idea, scientists have developed advanced positioning techniques through
Global Navigation Satellite Systems (GNSS). GNSS encompasses the various satellite
navigation systems, such as the United States’ GPS, Russia’s Globalnaya Navigatsionnaya
Sputnikovaya Sistema (GLONASS), Japan’s Quazi-Zenith Satellite System (QZSS),
India’s Indian Regional Navigation Satellite System (IRNSS), China’s Beidou and
Europe’s Galileo. Although these satellite systems were designed mainly for navigation,
they were found to be very useful for precise positioning, with accuracy levels of less than
a centimeter. GNSS also provides very high temporal resolution measurements (second
by second, or even faster), yielding key observations of time-dependent processes in the
lithosphere, atmosphere, and ionosphere.
2 GNSS Architecture
A GNSS basically consists of three main segments: the space segment, which comprises
the satellites; the control segment (also referred to as the ground segment), which is
responsible for the proper operation of the system; and the user segment, which includes
the GNSS receivers providing positioning, velocity and precise timing to users.
2.1 Space Segment
The main functions of the space segment are to generate and transmit code and carrier phase signals, and
to store and broadcast the navigation message uploaded by the control segment. These transmissions are
controlled by highly stable atomic clocks onboard the satellites. The GNSS space segments are formed by
satellite constellations with enough satellites to ensure that users will have at least four satellites in view
simultaneously from any point on Earth's surface at any time.
The GPS (US NAVSTAR) satellites are arranged in six equally spaced orbital planes
surrounding Earth, each with four `slots' occupied by baseline satellites. This 24-slot
arrangement ensures there are at least four satellites in view from virtually any point on
the planet. The satellites are placed in a Medium Earth Orbit (MEO) orbit, at an altitude
of 20 200km and an inclination of 55° relative to the equator. Orbits are nearly circular,
with an eccentricity of less than 0.02, a semi-major axis of 26,560km and a nominal period
of 11 hours, 58 minutes and 2 seconds (12 sidereal hours), repeating the geometry each sidereal day.
The nominal GLONASS constellation consists of 24 MEO satellites deployed in three
orbital planes with eight satellites equally spaced in each plane. The orbits are roughly
circular, with an inclination of about 64.8°, and at an altitude of 19,100km with a nominal
period of 11 hours, 15 minutes and 44 seconds, repeating the geometry every eight sidereal
days. Due to funding problems, the number of satellites decreased from the 24 available
in 1996 to only 6 in 2001. In August 2001, the Russian government committed to recover
the constellation and to modernise the system, approving new funding. A total of 24
operational satellites plus 2 in maintenance were again available in December 2011,
restoring the full constellation.
The Galileo constellation in Full Operational Capability (FOC) phase consists of 27
operational and 3 spare MEO satellites at an altitude of 23,222 km and with an orbit
eccentricity of 0.002. Ten satellites will occupy each of three orbital planes inclined at an
angle of 56° with respect to the equator. The satellites will be spread around each plane
and will take about 14 hours, 4 minutes and 45 seconds to orbit Earth, repeating the
geometry each 17 revolutions, which involves 10 sidereal days. This constellation
guarantees, under nominal operation, a minimum of six satellites in view from any point
on Earth's surface at any time, with an elevation above the horizon of more than 10°.
The Beidou (Compass) constellation (Phase III) will consist of 35 satellites, including 5
Geostationary Orbit (GEO) satellites and 30 non-GEO satellites in a nearly circular orbit.
The non-GEO satellites include 3 Inclined Geosynchronous Satellite Orbit (IGSO) ones,
with an inclination of about 55°, and 27 MEO satellites orbiting at an altitude of 21,528km
in three orbital planes with an inclination of about 55° and with an orbital period of about
12 hours and 53 minutes, repeating the ground track every seven sidereal days. The GEO
satellites, orbiting at an altitude of about 35 786 km, are positioned at 58.75°E, 80°E,
110.5°E, 140°E and 160°E, respectively, and are expected to provide global navigation
service by 2020. The previous Phase II involves a reduced constellation of four MEO,
five GEO and five IGSO satellites to provide regional coverage of China and surrounding
areas. The initial Phase II operating service with 10 satellites started on 27 December 2011.
The Indian Regional Navigation Satellite System (IRNSS) consists of a constellation of
seven satellites (IRNSS-1A, IRNSS 1-B, IRNSS 1-C, IRNSS 1-D, IRNSS 1-E, IRNSS 1-
F and IRNSS 1-G). IRNSS 1-A was launched in 2013 and the last one of the series IRNSS
1-G was launched on April 28, 2016. This is an independent Indian Satellite based
positioning system for critical National applications. The main objective is to provide
Reliable Position, Navigation and Timing services over India and its neighbourhood, to
provide fairly good accuracy to the user. The IRNSS will provide basically two types of
services, viz., Standard Positioning Service (SPS) and Restricted Service (RS). Space
Segment consists of seven satellites, three satellites in GEO stationary orbit (GEO) and
four satellites in Geo Synchronous Orbit (GSO) orbit with inclination of 29° to the
equatorial plane. This constellation of seven satellites was named as "NavIC" (Navigation
with Indian Constellation)
2.2 Control Segment
The control segment (also referred to as the ground segment) is responsible for the proper operation of the
GNSS. Its basic functions are to:
1. control and maintain the status and configuration of the satellite constellation;
2. predict ephemeris and satellite clock evolution;
3. keep the corresponding GNSS time scale (through atomic clocks); and
4. update the navigation messages for all the satellites.
2.3 User Segment
The user segment is composed of GNSS receivers. Their main function is to receive GNSS signals,
determine pseudoranges (and other observables) and solve the navigation equations in order to obtain the
coordinates and provide a very accurate time. The basic elements of a generic GNSS receiver are: an
antenna with preamplification, a radio frequency section, a microprocessor, an intermediate-precision
oscillator, a feeding source, some memory for data storage and an interface with the user. The
calculated position is referred to the antenna phase centre.
Various GNSS receivers are available in the market, from chips on watches and mobile
phones, to tracking devices, amateur receivers with small antenna, mapping receiver with
single or dual frequency capable antenna, survey grade dual or triple frequency receivers,
geodetic survey receivers with special antenna and high data rate, mentioned in increasing
order of price and accuracy. They may cost from about Rs. 3,000 to about Rs. 30,00,000.
3 GNSS SIGNALS
GNSS satellites continuously transmit navigation signals at two or more frequencies in L band. These signals
contain ranging codes and navigation data to allow users to compute both the travel time from the satellite to
the receiver and the satellite coordinates at any epoch. The main signal components are described as
follows:
Carrier: Radio frequency sinusoidal signal at a given frequency.
Ranging code: Sequences of zeros and ones which allow the receiver to determine the travel time of the
radio signal from the satellite to the receiver. They are called PRN (Pseudo Random Noise) sequences or
PRN codes.
Navigation data: A binary-coded message providing information on the satellite ephemeris (pseudo-
Keplerian elements or satellite position and velocity), clock bias parameters, almanac (with a reduced-
accuracy ephemeris data set), satellite health status and other complementary information.
The current `legacy' Navigation Message (NAV) is modulated on both carriers at 50 bps.
The whole message contains 25 pages (or `frames') of 30 s each, forming the master frame
that takes 12:5 min to be transmitted. Every frame is subdivided into five subframes of 6s each; in turn, every
subframe consists of 10 words, with 30 bits per word (figure above of NAVSTAR GPS). Every subframe
always starts with the telemetry word TLM, which is necessary for synchronisation. Next, the transference
word (HOW) appears. This word provides time information (seconds of the GPS week), allowing the
receiver to acquire the week-long P(Y) code segment.
4 The Position Fix By Trilateration
As soon as a receiver is powered on it starts searching for satellites. However, ignorance
of satellites’ approximate position delays the time taken for the first position fix. Therefore
an almanac is needed to speed up this process. The almanac is a small file that provides
the positions of the GNSS satellites to a certain degree of accuracy for a 48 hours period.
The tracking stations monitor the satellites and pass the information to the master control
station where the information is used among other things to generate the almanac file and
upload them to each satellite. The user receivers while powered on can download this file
from the satellite in a matter fo12.5 minutes of through the internet.
Then receivers then lock on to each satellite and receive the ephemerides from each
satellite. The ephemerides provide the current information about the satellites. The
receiver must then align signals sent from the satellite to an internally generated version
of a pseudorandom binary sequence, also contained in the signal. Since the satellite signal
takes time to reach the receiver, the two sequences do not initially coincide; the satellite's
copy is delayed in relation to the local copy. The receiver generates the pseudorandom
sequence, but they do not match. By increasingly delaying the local copy, the two copies
can eventually be aligned. The correct delay represents the time needed for the signal to
reach the receiver, and from this the distance from the satellite can be calculated (Figure
10.1).
Propagation Time = Time Signal Reached Receiver - Time Signal Left Satellite.
Multiplying this propagation time by the speed of light gives the distance to the satellite. Distance or Pseudo
Range ‘D’ = Speed of light in vacuum × Propagation Time
Figure 10.1 Distance calculation
Knowing the position of the satellites from their ephemerides, the receiver calculates its
position. The receiver knows that the reason the pseudoranges to the three satellites are
not intersecting is because its clock is not very good and apply an ingenious techniques to
correct its clock error. The receiver is programmed to advance or delay its clock until the
pseudoranges to the three satellites converge at a single point as seen in the following
figure.
The accuracy of the resulting range measurement is essentially a function of the ability of the receiver's
electronics to accurately process signals from the satellite, and additional error sources such as non
mitigated ionospheric and tropospheric delays, multipath, satellite clock and ephemeris errors, etc.
5 Errors in Position
5.1 Clock Errors
Fundamental to GNSS is the one-way ranging that ultimately depends on satellite clock
predictability. These satellite clock errors affect both the C/A- and P-code users in the
same way. This effect is also independent of satellite direction, which is important when
the technique of differential corrections is used. All differential stations and users measure
an identical satellite clock error. The ability to predict clock behaviour is a measure of
clock quality. The GPS uses atomic clocks (cesium and rubidium oscillators) which have
stability of about 1 part in 10E13 over a day. If a clock can be predicted to this accuracy,
its error in a day (~10E5 s) will be about 10E- 8 s or about 3.5 m.
5.2 Ephemeris Errors
Ephemeris errors result when the GNSS message does not transmit the correct
satellite location. Because satellite errors reflect a position prediction, they tend to
grow with time from the last control station upload. These errors were closely
correlated with the satellite clock, as one would expect. Note that these errors are
the same for both the P- and C/A- codes. Each satellite has a unique Precision (P) and Coarse
Acquisition (CA) codes that distinguish between the different satellites comprising the GNSS.
Table 10.1 The Various sources of Error
5.3 Multipath errors
Multipath is the error caused by reflected signals entering the front end of the receiver and masking the
real correlation peak. These effects tend to be more pronounced in a static receiver near large reflecting
surfaces. Monitor or reference stations require special care in siting to avoid unacceptable errors. The
first line of defense is to use the combination of antenna cut-off angle and antenna location that
minimizes this problem. A second approach is to use so-called "narrow correlator” receivers, which tend
to minimize the impact of multipath on range tracking accuracy.
5.4 Ionospheric errors
Because of free electrons in the ionosphere, GPS signals do not travel at the
vacuum speed of light as they transit this region. The modulation on the signal is
delayed in proportion to the number of free electrons encountered and is also
(to first order) proportional to the inverse of the carrier frequency squared (1/f2).
The phase of the radio frequency carrier is advanced by the same amount because
of these effects. Carrier-smoothed receivers should take this into account in the
design of their filters. The ionosphere is usually reasonably well-behaved and stable
in the temperate zones; near the equator or magnetic poles it can fluctuate
considerably. Due to the above the delays range from a few meters at night to a maximum
of 10 or 20 m at about 1400 hrs.
5.5 Troposphere errors
Another deviation from the vacuum speed of light is caused by the troposphere. Variations in
temperature, pressure, and humidity all contribute to variations in the speed of light and radio waves. Both the
code and carrier will have the same delays.
5.6 Dilution of Precision
The geometry formed by the observed positions of satellites by a receiver at a point in
time can present an estimate of the achievable accuracy. Any receiver will try to use signals
from satellites in a manner that reduces the DOP value. A value of 6 or less is regarded
acceptable. DOPs can change with time and space. The DOP can be further defined as
separate elements as Horizontal DOP (HDOP), Vertical DOP (VDOP) and Position
DOP (PDOP).
6 Differential Correction
Standalone GNSS receivers are prone for the errors discussed above. Hence a DGNNS receiver is
positioned at a known location (reference/base station) and coordinates computed and errors determined.
This error can then be applied as a correction to nearby rover stations surveyed in the project area within a
vicinity of about 50 km. It should be noted however that the farther the rover from the base, more the error.
It is assumed that environmental factors are similar at base and rover locations.
Figure 10.3 Differential Correction
Data collected at Rover stations should overlap in both TIME and GNSS Satellite Vehicle so that corrections
for the exact same satellites at the exact same time can be applied. Data from rovers can be brought to the
office at the end of the survey day and processed in a software along with the base station data. This is
referred as the classical DGNSS operation the Static Post Processed, and gives the best accuracies.
However, it requires longer observation times than Real Time Kinematic discussed below.
Where a project dictates the availability of corrected position value is real time, the
corrections can be broadcast from the base over a radio link and rovers receiving them in
real time for applying the corrections. This is referred as Real Time Kinematic as the
corrections are applied on the go. The method takes advantage of the slow variation with
time and user position of the errors due to ephemeris prediction, residual satellite clocks,
ionospheric and tropospheric delays. Starting from the reference station, the system
computes and broadcasts either correction to the GNSS position or to the pseudorange
measurements to the DGNSS users. Other uncorrelated errors (e.g. multipath) cannot be
corrected by this method and specific techniques have to be applied to mitigate them.
The difficulty in making an RTK system is properly aligning the signals. The navigation
signals are deliberately encoded in order to allow them to be aligned easily, whereas every
cycle of the carrier is similar to every other. This makes it extremely difficult to know if
you have properly aligned the signals or if they are "off by one" and are thus introducing
an error of 20 cm (approximate wave length of the carrier), or a larger multiple of 20 cm.
This integer ambiguity problem can be addressed to some degree with sophisticated
statistical methods that compare the measurements from the C/A signals and by
comparing the resulting ranges between multiple satellites. However, none of these
methods can reduce this error to zero.
