RS platforms. Fabio Dell Acqua - Gruppo di Telerilevamento

Transcription

1 RS platforms

2 Platform vs. instrument Sensor Platform Instrument The remote sensor can be ideally represented as an instrument carried by a platform

3 Platforms Remote Sensing: Ground-based air-borne space-borne Different characteristics and cost structures

4 Ground based RS Platform is bound to the Earth surface Ex.: radar, weather radar, GPR, GB-SAR, hand-held spectrometers

5 airborne RS Flying platform: airplane, UAV, helicopter, kite, balloon... elevation: 100 m 30/40 km e.g. airborne radar

6 Airplane Usually modified to fit the RS instruments cruise speed between 140 and 600 km/h, often connected with the specific acquisition height from hundreds to thousand of m, connected with the specific acquisition wind bias, limited pilot correction GPS + IMU (=Inertial Measurement Units) for a-posteriori correction high costs (both maintenance and missions), ad-hoc acquisitions mission cost: tens of k s

7 Spaceborne RS mostly artificial satellites but also spacecrafts (e.g. Space Shuttle, for SRTM mission) km elevation periodic acquisition high design, production and launch costs low per-datum cost

8 spaceborne RS: two segments Space segment acquisition storage (temporary) downlink Ground segment mission control data reception correction processing

9 What is a satellite? a celestial body revolving around another body artificial satellites Earth Observation satellites

10 Structure of an EO satellite Complex, depending on addressed application Two main sections: - payload and - bus

11 Details

12 Communication systems up- and down-link. Command and control, downlink of RS data

13 Sensors Earth Observation sensors and related devices (e.g. temporary storage)

14 Navigation systems Exact determination of satellite location and attitude Some (limited) capabilities for adjustments

15 Power Normally supplied by sunrays through solar panels Excess energy stored in batteries (about ½ orbit)

16 Structure and envelope Mechanically keeps the entire device together, ensuring also thermal and radiation protection. Controlled dispersion of heat

17 Orbit Repetitive path of a satellite around the Earth, due to the equilibrium between weight and centrifugal force (due to its motion) Gravity force is inversely proportional to the squared distance from Earth, while the centrifugal force is proportional to tangential velocity orbits at higher elevation slower revolution

18 Circular orbit In a simplifying hypothesis of circular orbit and mass negligible wrt Earth, radius r and period T comply with: T 2 GM = 4π 2 r 3 where M=mass of the Earth, G= gravity constant The period can thus be computed as: T = 2π r 3 GM

19 Features of an orbit Height: distance between the satellite and the average Earth surface km or km (GEO) Inclination: angle between the equator and the orbit plane Period: time required to complete an orbit (tens of minutes, at the usual speeds of 7-8 km/s) Revisit time: time between two passages above the same point on the order of days or tens of days

20 RS orbits heliosyncronous orbits Low Earth Quasi-polar Orbits (LEO) orbits Geostationary Earth Orbit (GEO)

21 RS orbits Quasi-polar: next to the polar axis. Most common km LEO: km. Used for high resolution and short revisit times Sun-synchronous: to minimize sun bias and to ensure proper operation of solar panels GEO: km. Revolution=Earth rotation. The satellite appears idling on the equatorial plane. Weather satellites.

22 Nadir Nadir direction: towards the Earth centre Nadir point: intersection between the Earth surface and the line joining the sensor and the Earth centre

23 Swath the Earth surface strip as sensed by an EO satellite, and the width of the strip itself

24 Downlink RS satellites have limited onboard storage capabilities data is usually directed downlinked to the ground station in some cases a second satellite can provide support to link with non-visible stations