Field Astronomy and GPS

A GPS (Global Positioning System) is a satellite-based radio navigation system, owned by the USA, which gives three-dimensional position (latitude, longitude, height) and time anywhere on or near the Earth in all weather, day and night.

Components (segments)

     Space segment (24+ satellites, ~20,200 km)
            |  signals
     Control segment  <-->  User segment
     (master & monitor        (receivers)
      stations, uplink)

1. Space segment: at least 24 satellites in six orbital planes inclined at 55°, at an altitude of about 20 200 km, orbital period about 12 h. Each satellite sends signals on carrier frequencies L1 (1575.42 MHz) and L2 (1227.60 MHz), carrying the ranging codes and navigation message (satellite position and clock).
2. Control segment: a master control station, monitor stations and ground antennas; they track the satellites, compute orbits (ephemeris) and clock corrections and upload them to the satellites.
3. User segment: the GPS receivers (handheld, geodetic or surveying types) and the users, which receive the signals and compute the position.

Working principle

GPS works by trilateration using the travel time of the signal. The satellite sends a signal stamped with the time of transmission and its own position. The receiver notes the time of arrival, and the distance (pseudo-range) is

$$\rho = c\,(t_r - t_s)$$

where $c$ is the speed of light, $t_s$ the transmission time and $t_r$ the reception time. Each range puts the receiver on a sphere centred on the satellite. Three satellites would fix the position (three unknowns $X, Y, Z$), but the receiver clock is not accurate like the atomic clocks in the satellites, so a fourth unknown (clock error $\delta t$) is added. At least four satellites are required to solve for $X, Y, Z$ and $\delta t$:

$$\rho_i = \sqrt{(X_i - X)^2 + (Y_i - Y)^2 + (Z_i - Z)^2} + c\,\delta t,\quad i=1,\dots,4$$

The accuracy of a point depends on the geometry of satellites (DOP), atmospheric delay, multipath and receiver quality. Differential GPS (DGPS) or relative (static/RTK) positioning, which uses two receivers, removes most common errors and gives centimetre to millimetre accuracy.

A GPS (Global Positioning System) is a satellite-based radio navigation system, owned by the USA, which gives three-dimensional position (latitude, longitude, height) and time anywhere on or near the Earth in all weather, day and night.

Components (segments)

     Space segment (24+ satellites, ~20,200 km)
            |  signals
     Control segment  <-->  User segment
     (master & monitor        (receivers)
      stations, uplink)

1. Space segment: at least 24 satellites in six orbital planes inclined at 55°, at an altitude of about 20 200 km, orbital period about 12 h. Each satellite sends signals on carrier frequencies L1 (1575.42 MHz) and L2 (1227.60 MHz), carrying the ranging codes and navigation message (satellite position and clock).
2. Control segment: a master control station, monitor stations and ground antennas; they track the satellites, compute orbits (ephemeris) and clock corrections and upload them to the satellites.
3. User segment: the GPS receivers (handheld, geodetic or surveying types) and the users, which receive the signals and compute the position.

Working principle

GPS works by trilateration using the travel time of the signal. The satellite sends a signal stamped with the time of transmission and its own position. The receiver notes the time of arrival, and the distance (pseudo-range) is

$$\rho = c\,(t_r - t_s)$$

where $c$ is the speed of light, $t_s$ the transmission time and $t_r$ the reception time. Each range puts the receiver on a sphere centred on the satellite. Three satellites would fix the position (three unknowns $X, Y, Z$), but the receiver clock is not accurate like the atomic clocks in the satellites, so a fourth unknown (clock error $\delta t$) is added. At least four satellites are required to solve for $X, Y, Z$ and $\delta t$:

$$\rho_i = \sqrt{(X_i - X)^2 + (Y_i - Y)^2 + (Z_i - Z)^2} + c\,\delta t,\quad i=1,\dots,4$$

The accuracy of a point depends on the geometry of satellites (DOP), atmospheric delay, multipath and receiver quality. Differential GPS (DGPS) or relative (static/RTK) positioning, which uses two receivers, removes most common errors and gives centimetre to millimetre accuracy.

Principle of GPS

GPS measures the distance (pseudo-range) from the receiver to at least four satellites by the signal travel time, $\rho = c(t_r - t_s)$. As the positions of the satellites are known from the navigation message, the receiver position $(X, Y, Z)$ and its clock error are obtained by trilateration from four equations.

Components of GPS

1. Space segment: 24+ satellites in six orbital planes at about 20 200 km, sending L1 and L2 signals.
2. Control segment: master control and monitor stations that track the satellites and upload orbit and clock data.
3. User segment: the receivers used by surveyors and other users.

Applications of GIS

• Land use planning and urban planning; cadastral and land records.
• Route selection and site suitability analysis for roads, canals and dams.
• Utility mapping and management (water supply, sewers, power lines).
• Disaster management, flood and landslide hazard mapping.
• Water resources and watershed management; environmental impact assessment.
• Transport and traffic planning; infrastructure asset management.

GPS (Global Positioning System) gives the three-dimensional position of any point on the Earth from satellite signals. GIS (Geographic Information System) is a computer system for capturing, storing, analysing and presenting spatially referenced data.

Importance of GPS

• Gives accurate position, height and time quickly, in all weather, day and night, with no need for intervisibility between stations.
• Reduces the time and cost of control surveys, topographic surveys and setting out.
• Useful for navigation, tracking, and monitoring deformation of dams, bridges and landslides.
• Gives positions in a global datum (WGS84), so surveys can be linked together.

Importance of GIS

• Stores large amounts of spatial and attribute data in one database that can be updated easily.
• Overlay and spatial analysis help in site selection, route alignment, and planning of utilities.
• Produces maps and reports quickly, and helps decision making in disaster management, environment and urban planning.
• Together, GPS supplies accurate locations for data collection and GIS analyses them, so the two are used together in modern surveying and planning.

The field procedure of a GPS survey (static or kinematic relative positioning) is:

1. Reconnaissance and planning: study the project area, existing control points and the sky visibility. Plan the network, receivers, observation sessions and baselines. Check the satellite availability and DOP (Mission planning), choosing a time with good geometry (PDOP below about 6).
2. Selecting stations: points should have a clear view of the sky above about 15° elevation, and be away from reflecting surfaces (multipath), power lines and transmitters.
3. Setting up the receiver: the tripod and antenna are centred over the station mark with an optical plummet and levelled; the antenna height is measured (before and after observation) in two or three places, and the antenna is oriented to north if required.
4. Observation: switch on the receiver, set the recording interval (e.g. 15 s) and elevation mask, enter the point name and antenna height, and log data for the session length (static: 30 min to several hours depending on baseline length; RTK: a few seconds).
5. Record keeping: field sheets record station name, receiver and antenna numbers, date, start and end times, antenna height and weather.
6. Data download and processing: the data are downloaded to a computer, baselines are processed, and loop closures and quality checks are done. The network is adjusted and the coordinates are transformed to the local datum.
7. Checking: repeat some baselines for check and compare with known points.

A GPS (Global Positioning System) is a satellite-based radio navigation system, owned by the USA, which gives three-dimensional position (latitude, longitude, height) and time anywhere on or near the Earth in all weather, day and night.

Components (segments)

     Space segment (24+ satellites, ~20,200 km)
            |  signals
     Control segment  <-->  User segment
     (master & monitor        (receivers)
      stations, uplink)

1. Space segment: at least 24 satellites in six orbital planes inclined at 55°, at an altitude of about 20 200 km, orbital period about 12 h. Each satellite sends signals on carrier frequencies L1 (1575.42 MHz) and L2 (1227.60 MHz), carrying the ranging codes and navigation message (satellite position and clock).
2. Control segment: a master control station, monitor stations and ground antennas; they track the satellites, compute orbits (ephemeris) and clock corrections and upload them to the satellites.
3. User segment: the GPS receivers (handheld, geodetic or surveying types) and the users, which receive the signals and compute the position.

Working principle

GPS works by trilateration using the travel time of the signal. The satellite sends a signal stamped with the time of transmission and its own position. The receiver notes the time of arrival, and the distance (pseudo-range) is

$$\rho = c\,(t_r - t_s)$$

where $c$ is the speed of light, $t_s$ the transmission time and $t_r$ the reception time. Each range puts the receiver on a sphere centred on the satellite. Three satellites would fix the position (three unknowns $X, Y, Z$), but the receiver clock is not accurate like the atomic clocks in the satellites, so a fourth unknown (clock error $\delta t$) is added. At least four satellites are required to solve for $X, Y, Z$ and $\delta t$:

$$\rho_i = \sqrt{(X_i - X)^2 + (Y_i - Y)^2 + (Z_i - Z)^2} + c\,\delta t,\quad i=1,\dots,4$$

The accuracy of a point depends on the geometry of satellites (DOP), atmospheric delay, multipath and receiver quality. Differential GPS (DGPS) or relative (static/RTK) positioning, which uses two receivers, removes most common errors and gives centimetre to millimetre accuracy.

Applications

• Control surveys and geodetic networks; establishing benchmarks.
• Topographic and cadastral surveys; setting out of roads, bridges and buildings.
• Navigation of vehicles, aircraft and ships; fleet tracking.
• Deformation monitoring of dams, bridges and landslides; crustal motion and earthquake studies.
• Photogrammetry (camera position and ground control) and GIS data collection.
• Disaster management, search and rescue, and precise timing.

Field procedure of GPS

1. Planning: study the area, locate existing control, check satellite visibility and DOP, and plan the observation sessions.
2. Station selection: a clear sky view above 15°, away from reflecting surfaces and electrical interference.
3. Set-up: centre and level the antenna on the tripod over the station mark; measure the antenna height.
4. Observation: start the receiver, enter the station name and antenna height, set the logging interval and observe for the required session time (several minutes for RTK, 30 min or longer for static).
5. Recording and processing: note the details in the field book, download the data, process the baselines, adjust the network and convert to the local datum.

Components of GPS

1. Space segment: 24+ satellites at about 20 200 km in six orbital planes, transmitting L1 and L2 signals and the navigation message.
2. Control segment: master control station, monitor stations and ground antennas that track the satellites and update orbit and clock data.
3. User segment: GPS receivers and antennas used by the surveyor for positioning.