Future Satellite for IMD SAT
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Transcript of Future Satellite for IMD SAT
13. SATELLITE SYSTEM PARAMETERS
BACK OFF LOSS
High power amplifiers used in earth station transmitters and the travelling wave tubes typically used in satellite transponders are non-linear devices; their gain is independent of input signal. The amount of output level backed off from rated levels is equivalent to a loss and is appropriately called back off loss.
TRANSMIT POWER AND BIT ENERGY
A power amplifier should be operated as close to saturation as possible for efficient operation. The saturated output power is designated as Po(sat) or simply Pt. The output power of a typical satellite earth station transmitter is much higher than the output power from a terrestrial microwave power amplifier. Consequently, when dealing with satellite systems, Pt is generally expressed in dBm.
Most modern satellite systems either use phase shift keying (PSK) or quadrature amplitude modulation (QAM) rather than the conventional frequency modulation (FM). With PSK and QAM, the input base band is generally a PCM-encoded, time division multiplexed signal that is digital in nature. Also, with PSK and QAM, several bits maybe encoded in a single transmit signaling element. Consequently, a parameter more meaningful than carrier power is energy per bit, Eb.
Mathematically, Eb is,
Eb = Pt x Tb
Where,
Eb = energy of a single bit
Pt = total saturated output power
Tb= 1/fb = time of a signal bit
EFFECTIVE ISOTROPIC RADIATED POWER
Effective Isotropic Radiated Power (EIRP) is defined as an equivalent transmit power and is mathematically expressed as
EIRP = Pin x At
Where,
EIRP = Effective Isotropic Radiated Power
Pin = antenna input power
At = transmit antenna gain
Expressed as a log,
EIRP (dBW) = Pin (dBW) + At (dB)
EQUIVALENT NOISE TEMPERATURE
With terrestrial microwave systems, the noise introduced in a receiver or a component within a receiver is commonly specified by the parameter, noise figure. In satellite communication systems, it is necessary to differentiate or measure noise in increments as small as a tenth or a hundredth of a decibel. Noise figure, in its standard form, is inadequate for such precise calculations. Consequently, it is common to use environmental temperature (T) and equivalent noise temperature (Te) when evaluating the performance of a satellite system. Total noise power is mathematically expressed as,
N = KTB
Rearranging and solving for T,
T = N/KB
Where,
N = total noise power (Watts)
K = Boltzmann's constant (Joules per Kelvin)
B = bandwidth (Hertz)
T = temperature of environment (Kelvin)
NOISE DENSITY
Noise density (No) is the noise power normalized to a 1-Hz bandwidth, or the noise power present in a 1-Hz bandwidth. Mathematically, noise density is
No = N/B = (KTeB)/B = KTe
Where,
No = noise density (Watts per Hertz) = 1 Joule/cycle
N = Total noise power (Watts)
B = bandwidth (Hertz)
K = Boltzmann's constant (joules per Kelvin)
Te = equivalent noise temperature (Kelvin)
CARRIER TO NOISE DENSITY RATIO
C/No is the average bandwidth carrier power to noise density ratio. The widEband carrier power is the combined power of the carrier and its associated sidEbands. The noise density is the thermal noise present in a normalized 1-Hz bandwidth. The carrier to noise density ratio may also be written as a function of noise temperature. Mathematically, C/No is
C/No = C/KTe
ENERGY OF BIT TO NOISE DENSITY RATIO
Eb/No is one of the most important and most often used parameters when evaluating a digital radio system. The Eb/No ratio is a convenient way to compare digital systems that use different transmission rates, modulation schemes or encoding techniques. Mathematically, Eb/No is
Eb/No = (C/fb)/(N/B) = CB/Nfb
Eb/No is a convenient term used for digital system calculation and performance comparisons, but in the real world, it is more convenient to measure the wideband carrier power to noise ratio and convert it to Eb/No. Rearranging the above equation,
Eb/No = (C/N)x(B/fb)
The Eb/No ratio is the product of carrier to noise ratio (C/N) and the noise bandwidth to bit ratio (B/fb).
Expressed as a log,
Eb/No (dB) = C/N (dB) + B/fb (dB)
GAIN TO EQUIVALENT NOISE TEMPERATURE RATIO
Gain to equivalent noise temperature ratio (G/Te) is a figure of merit used to represent the quality of a satellite or an Earth station receiver. The G/Te of a receiver is the ratio of the receiving antenna gain to the equivalent noise temperature (Te) of the receiver. Because of the extremely small received carrier powers typically experienced with satellite systems, very often an LNA is physically located at the feed point of the antenna. When this is the case, G/Te is a ratio of the gain of the receiving antenna plus the gain of the LNA to the equivalent noise temperature.
Mathematically, gain to equivalent noise temperature ratio is
G/Te = {Ar + A(LNA)}/Te
Expressed as a log, we have
G/Te (dB/K) = Ar (dB) + A(LNA) (dB) - Te (dBK)
G/Te is a useful parameter for determining the Eb/No and C/N ratios at the satellite transponder and earth station receivers. G/Te is essentially the only parameter required at a satellite or an earth station receiver when completing a link budget.
14. SATELLITE SYSTEM DOWN LINK EQUATION
When evaluating the performance of a digital satellite system, the uplink and downlink parameters are first considered separately, then the overall performance is determined by combining them in the appropriate manner. In this project, we are only considering reception from satellites, hence only downlink equation is used which considers the ideal gains and losses and effects of thermal noise associated with earth station receiver.
DOWNLINK EQUATION
C/No = EIRP (dBW) - Lp (dB) + G/Te (dB/K) - K (dBWK)
Where,
EIRP = effective isotropic radiated power
Lp = free space path loss
G/Te = gain to noise temperature ratio
K = Boltzmann's constant
15. SYSTEM PARAMETER GRAPHS
To determine the system parameters, the following graphs have been used which are as follows:
Fig 1: Determination of C/N ratio
P(e) performance of M-ary PSK, QAM, QPR and M-ary APK coherent systems. The rms C/N is specified in the double sided Nyquist bandwidth
Fig 2: Determination of Eb/No ratio.
Fig 3: Determination of antenna gain.
Antenna gain based on the gain equation for a parabolic antenna:
A (dB) = {10 log η (3.14 D/λ)2}
Where,
D- Antenna diameter
λ- Wavelength
η- Antenna efficiency = 0.55
Fig 4: Determination of free space path loss (Lp)
Free space path loss (Lp) determined from:
Lp = 183.5 + 20 log(f) (GHz)
Elevation angle= 90 degrees
Distance = 35930 km
16. DOWN LINK BUDGET ANALYSIS
INSAT-3A
(V.H.R.R.)
1. Satellite EIRP : 18.0 dB W
2. Free Space Loss (4.5 GHz) : 197.0 dB
3. Data rate : 526.5 Kbps
4. Eb/No (required at bit error of 10) : 10.8 dB
5. DEMOD implementation margin : 2.0 dB
6. Eb/No : 12.8 dB
7. C/No : 70.0 dB Hz
8. G/Te (6.1 m antenna 46 dB, T system 100K, 5 deg elevation) : 46-20 = 26 dB/K
9. C/No = EIRP - Lp + (G/Te) - K : 75.6 dB Hz
10. Required C/No : 70.0 dB Hz
11. Margin : 5.6 dB
INSAT-3A
(CCD)
1. Satellite EIRP : 22.0 dB W
2. Free Space Loss (4.5 GHz) : 197.0 dB
3. Data rate : 1.28875 Mbps
4. Eb/No (required at bit error of 10) : 10.8 dB
5. DEMOD implementation margin : 2.0 dB
6. Eb/No : 12.8 dB
7. C/No : 73.9 dB Hz
8. G/Te (6.1 m antenna 46 dB, T system 100K, 5 deg elevation) : 46-20 = 26 dB/K
9. C/No = EIRP - Lp + (G/Te) - K : 79.6 dB Hz
10. Required C/No : 73.9 dB Hz
11. Margin : 5.7 dB
17. CALCULATION OF DOWN LINK BUDGET
1. Effective Isotropic Radiated Power (EIRP):
EIRP = 22.0 dB w (for CCD of INSAT-3A)
The value is given for each payload of satellite.
2. Free space path loss (Lp):
Lp = 183.5 + 20 log(f) (GHz)
Given that,
f = 4.5 GHz
So, Lp = 183.5 + 20 log (4.5)
Lp = 196.56 dB
Lp = 197 dB
The calculated power of Lp can be verified using graph no. 4 which gives the characteristics of relation between free space path loss and frequency in GHz.
3. Data rate:
Data rate is the rate at which the signals are being received at the earth station unit. This rate varies with the payload.
e.g. data rate of CCD is 1.28875 Mbps
4. Eb/No : Energy of one bit to noise density ratio at an error rate of 10-6
Eb/No = (C/N)x(B/fb)
Where,
b- Bandwidth
fb- bit rate
C/N can be calculated ny using graph no. 1 and for BPSK,
B = fb = 20 MHz
Taking 10 log on both sides
Eb/No (dB) = C/N (dB) + 10 log (20x106/20x106)
So, Eb/No = 10.8 dB (at bit error of 10-6)
5. Demodulation Implementation Margin:
This value is pre-determined and is a constant value. Here, we have taken this margin as 2.0 dB.
6. Eb/No ratio:
Now, Eb/No ratio would be the sum of the calculated value and demodulation margin.
Hence,
Eb/No = 10.8 + 2
So, Eb/No = 12.8 dB
7. C/No ratio : Carrier to noise density ratio
The required C/No ratio is pre-determined and has a value of 73.9 dB Hz
8. Calculated C/No ratio :
C/No = EIRP - Lp + (G/Te) - K
Here, C/No = 22.0 - 197.0 + 26 -(-228.6)
So, C/No = 79.6 dB Hz
Where,
K- Boltzmann's constant
G/Te- Antenna gain to noise temperature ratio
Here, Margin = 79.6 - 73.9
So, Margin = 5.7 dB
9. G/Te ratio
Where,
G- Antenna gain
Te- noise temperature
Now, G (dB) = 10 log η (Pi D/λ)2
For antenna with diameter 610 cm
G (dB) = 10 log[(0.55)x((3.14x610)/6659)2]
G (dB) = 46.5 dB
G/Te = G- Te (in log)
G/Te = 46.5 - 20
Therefore,
G/Te = 26.5 dB
For antenna with diameter 710 cm
G (dB) = 10 log[(0.55)x((3.14x710)/6659)2]
G (dB) = 45.58 dB
G/Te = G- Te (in log)
G/Te = 45.58 - 20
Therefore,
G/Te = 25.58 dB
Future Satellite for IMD SAT-MAT
It is an exclusive designed for enhanced meteorological observations and monitoring of land and ocean surfaces for weather forecasting and disaster warning. The three axis stabilized geostationary satellite is to carry two meteorological instruments: a six channel imager and an IR sounder along with the channel in visible, middle infrared, and water vapor and thermal infrared bands, the imager includes a SWIR channel for wider applications. The sounder will have eighteen narrow spectral channels in three IR bands in addition to a channel in visible bands.
INSAT-3D is configured around standard 2000 kg 12k spacecraft bus with 7-years life. Several innovative technologies like on-the-fly correction of scan mirror pointing errors, biannual law rotation of spacecraft, micro stepping SADA, star sensors and integrated bus management unit have been incorporated to meet the stringent payload requirements like pointing accuracies, thermal management of IR detectors and concurrent operation of both instruments.
Payloads of INSAT-3D are:
6-channel imager 19 channel sounder for obtaining data on vertical temperature profiles.
Services which would be provided by INSAT-3D:
Rainfall Sea surface temperature Clouds classification Cloud motion vectors(visible & infrared) and water vapor winds Vertical profiles of temperature, humidity and ozone. Objective technique for estimation of intensity & position of tropical cyclones Assimilation of satellite data in global and regional numerical weather prediction
models. WESDIS interests include rainfall, aerosols, vegetation, health, sea surface
temperature, clouds and winds.
Provision of source program along with the algorithm will result in easy implementation of generating the product from INSAT-3D which can be quickly made operational.
Once INSAT-3D is launched, algorithms will run on a test sample data sets to derive products. Testing of products using in situational data is expected to provide a diagnostic understanding on the performance of the algorithms and their limitations. Further collaboration is envisioned in constant updating of research and derivation of products from future similar satellites.
Important features of INSAT-3D
Nation IndiaType/application MeteorologyOperator INSATContractor ISROEquipment 19-channel sounder, 6 channel imager,
DRT & SAR payloadsConfiguration INSAT-2/3 busPropulsion 440 Newton thrust liquid spogee motorLife time 7 yearsOrbit Geostationary