GPS

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 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 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

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. 

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 

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. 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.