A Satellite Based Earth Observation System  

Umair Naeem Mehran University of Engineering & Technology, Jamshoro, Pakistan Email: freshup85@gmail.com

Abstract: In this scientific project report/paper, a satellite based earth observation system is proposed. This project aims at climate monitoring by observing global circulation in the ocean and in the atmosphere. The purpose of the project is to monitor earth oceans’ properties by spreading a large number of small platforms over all oceans which will take different readings based on ocean water properties and the idea is to collect that data from these platforms through a satellite network. The proposed satellite network consists of MEO satellites which would use FDMA technique together with turbo channel coding to gather all the data from these platforms (called buoys). This paper provides details of its accomplishment by focusing on certain constraints. The proposed system meets a target Bit Error Rate of 1e-3 both at uplink and downlink.

1. Introduction

The project is inspired by the ARGO project [1]. A large number of deep sea buoys will be deployed in the world oceans. They sink and drift for several days with the ocean currents, take and store measurements of water temperature, pressure, and salinity, amount of oxygen and pH values and rise up to the ocean surface again. The recorded data is transmitted to the research laboratory in Singapore (Singapore is chosen as it is has the worst climate to deal with and if the proposed system works in this climate then it will work in other regions as well) by means of the proposed satellite system. The buoys also take GNSS measurements which shall be transmitted to the satellite system, too. The buoys stay on the sea surface for approximately 1 hour between two deep sea drifts. During this time they receive GPS signals with 1Hz sampling that are stored with 500 KB size. The system designed works with very high channel availability (99.9%). This paper will present how the satellite orbit parameters, buoy parameters, frequency and link budget were selected/calculated to fulfill the requirements.

2. Constraints And Requirements

Since buoys are battery powered and cannot carry large weights, they are limited to low power transmissions such as 0.5-5W [11] and low weight compact antenna. These problems/constraints are the source of all the assumptions that will be discussed later in this report. Furthermore, since all the buoys’ movement is of random nature, it is very difficult to time assign and gather data from buoys using TDMA scheme. 3. Frequency And Polarization

Selection of a particular frequency band depends upon many factors like Path loss, ionospheric (scintillation) losses, antenna gain and beamwidth, rain Attenuation and atmospheric absorption. By comparing different frequency bands in context of these factors, C-band is selected as an optimum solution. Circular polarization is chosen to immunize the effect of Faraday rotation and to minimize rain attenuation. For that purpose buoy antenna uses helix antenna, satellite uses circular feed horns with parabolic reflector(s) for uplink and helix antenna for downlink and earth station with parabolic reflector antenna.

 

 

4. Satellite Subsystem

There are two possibilities to gather data through a satellite network. Inter-satellite links (complex network) or onboard processing (complex transponder) within each satellite. The 2nd case is chosen for the project. The subsystem is portrayed in figure 1.

Figure 1: Satellite Subsystem

5. Modulation/Multiplexing/Coding Scheme

There is a multitude of schemes that can be applied for modulation/multiplexing/channel coding. QPSK is suggested as the modulation scheme as it has a relatively good spectral efficiency and same error probability as that of BPSK and less complexity for buoy point of view. FDMA is chosen for its simplicity, however if a lot of carriers/channels are to be used, inter-modulation noise should be taken into account at the transponder amplifiers (backing off from saturation limit). Since the channel behavior is unknown between any buoy and any satellite, it would be fruitful to use certain channel coding method to account for errors. For bursty channels, interleaving is often used with some coding method to improve the Bit Error Rate. 1/2 rate turbo coding has been selected as it gives very low BER values even for lower received signal-to-noise ratios (see figure 2).

6. Data Rate And Bandwidth

Each buoy is assumed to transmit GPS data (for location identification) and measurements taken under/on the water. Total data is assumed to be 524KB+500KB = 1MB. A retransmission is also suggested for every data stream. If both the transmitted and retransmitted (duplicate) measurement is acquired by satellite(s), it will be rectified on the Singapore earth station. Let’s assume that a buoy will take about 4 minutes to transmit its measurement (1MB),

• Data Rate Rb = 33.33 kbps • Rate After (1/2) Turbo Coding Rc = 66.66kbps • Symbol Rate (QPSK) Rs = 33.33kbps • Bandwidth BW = 50kHz (with excess bandwidth factor of 0.5 for Nyquist pulse

shaping) Now the buoy will take 14 mins (usually it requires 5-12 minutes) to gather GPS

information, 4 mins to transmit 1st measurement, wait for 4 mins, 4 mins to re-transmit 1st measurement, 5 mins to gather GPS info again, 4 mins to transmit 2nd measurement, wait for 4 mins and finally 4 mins to re-transmit 2nd measurement making a total time of about 43 mins. For FDMA, the total bandwidth required for 4000 buoys each requiring a bandwidth of 50 kHz plus 2 kHz of guard band will be 208 MHz. For a 36 MHz C-band transponder, it would require 6 transponders per satellite for all the buoys.

6GHz to 4GHz DC IF Filter BankQPSK Demod + Turbo 

Decoder Bank LNA 

Storage QPSK Modulator IF to 4GHz UCSSPA

4GHz to IF DCRX 

TX 

 

FigureTurbo C

7. O7.1 O

Due toas it wand larwith heangle ((Nsat, w

Wh

satellit22.6o, ε

As

buoy l

e 2: BER vs Eb/Code gives a BER

Orbit, CovOrbit Sele

o power limitawould give higrge no. of sateeight of 1200(δ), the half bwith 21% cov

here Re is Rae (18878 Kmε = 56.71o, θ

shown in figies at the edg

Fi

/No curve for TuR of 1e-3 at arou

verage Anction

ations at buoygh path loss. ellites would 0 km is selecbeamwidth (θverage overlap

adius of earthm). Using abo= 10.68o, Nsa

gure 4 belowge of the sate

igure 3: Relation

δ 

Buoy 

urbo ½ rate codinund 0.65 dB Eb/

dB

nd Link D

ys, it would nFor LEO orbbe required tted as the optθ), the area copping) and no

,

,

h (6378 Km) ove equationst = 32, Porbit =

w, these calcuellite coverag

n between differ

ε 

ng [8] and QPSK/No. QPSK moduB Eb/No.

esign

not be a goodbit, the covero cover wholtimum solutioovered by a s

o. of orbital pl

and Ro is Diss with Area A= 5.3 ≈ 6.

ulations yield ge which is -

rent angles used

θ 

K Modulation [9ulation without

d idea of usinage area on ele earth. For ton. The elevatsatellite (At), lanes (Porbit) c

stance from cAt = 1.9635e

values for w3 dB down.

for coverage ca

Satellite

1e‐3 BER at 8

9]. Note that for coding gives BE

ng geostationaearth would bthese reasons,tion angle (ε)no. of satelli

can be found

center of the e13 m2 [1], I

worst conditioThe elevation

lculation

e

8 dB SNR 

10 iterations, ER of 1e-3 at 8

ary satellites be too small , MEO orbit ), the central ites required as,

earth to the found, δ =

on when the n calculated

 

 

determines the angle for the buoy from its main lobe to the satellite. This elevation will certainly give higher pointing mismatch loss and rain attenuation.

Now some of the orbital parameters and satellite timings will be calculated as,

2 ______ 5 , 2

______ 6  

 

______ 7 ,      ______ 8  

Where, a = semi-major axis of the orbit = R0 μ = 3.9861352×105 km3/s2 Hence above equations yield,

• Orbital Time Period T = 25812.9612 sec = 7hr 10min 13sec • Satellite’s angular velocity η = 2.434*10-4 rad/sec • Satellite’s linear velocity v = 4.595 km/sec • Satellite’s visibility time tvis = 3241.12 sec = 54 min • Orbital Eccentricity = 1, Orbital Planes = 6 (placed at 60 degree step) • Each orbit will have approximately 5 satellites each with True Anomaly of 360/5 =

72 degree steps.

Figure 4: Misalignment due to Elevation Angle

7.2 Uplink Parameters (From Buoy to Satellite)

7.2.1 Antenna Since buoy is power-and-space-limited, it is assumed that a buoy can have a maximum transmitted power of 4W (6 dB) and uses a helix antenna (Circular Polarization) with a gain (Gt) of 11.8 dBi and 46.5 degrees of beamwidth (θ3db) [2]. According to the elevation found previously the pointing mismatch loss at the coverage edge would then be,

12 1290

6.15 __________ 9

 

 

An extra 3 dB loss will also be added in above to account for satellite coverage edge which is 3 dB down from the centre of the coverage.

Satellite, will be equipped by one or two parabolic reflector antenna(s) with Gain (G) of 39.85 dB and beamwidth (θ3db) of 2o (assumed/calculated). The required width of the satellite main lobe calculated from the coverage area was θw = 2* θ = 2*10.68o = 21.36o but our antenna has only 2 degrees of opening angle. This problem can be solved by using multiple feed horns (called potter horns [3]) in a beam forming network with one or two dish antennas, each horn for separate beam of width of 2 degrees. It is found that approx. 143 feed horns are required.

7.2.2 Uplink Losses Free Space Path Loss:

204

189.58 1 ___________ 10

Rain Attenuation: P-region is taken as a worst region with 0.1% of channel unavailability as required. For uplink frequency (6GHz), circular polarization and minimum elevation of 56.71, attenuation from [5][6] using SAM model is as follows,

2.36 __________ 11

Where, k and α are parameters depending on wave frequency and polarization Le = Effective path length of rain R = Rain rate Scintillation Loss: The scintillation loss (Lsc) for 6 GHz frequency is about 6 dB as given in [5][6]. Polarization Loss: For circular polarization, the polarization loss due to Faraday’s rotation is negligible but the polarization mismatch between transmitter antenna and receiver antenna can be there which is usually within 0.2-0.85 dB max [6]. I will assume Lpol= 0.85 dB. Waveguide Losses: Assuming waveguide loss at satellite receiver to be Lw= 0.2 dB. System Margin/Link Margin/Miscellaneous Losses: This loss contains all other losses that might not be covered (system degradation etc.) but are low in magnitude. It is assumed to be Lm= 5 dB.

7.2.3 Satellite Receiver Noise As described above and shown in figure 1, the satellite receiving system consists of Antenna, waveguide, LNA, Down converters and IF amplifiers. Table 1 shows their noise and gains and the total noise. These values were taken from [4] and [7].

 

 

Table 1: Satellite Receiver Noise and Gain Parameters

Property Antenna Waveguide LNA 6 GHz to

4 GHz DC

4 GHz to IF DC IF

filter

Gain (dB) 39.85 -0.2 40 24.5 45 -2 Noise Figure (dB) 2 0.2 1.45 20 11 NA

Noise Temperature (K) Ta = 290K Tw =13.67 TLNA = 114.94

TDC= 28710

TIF = 3360.88 NA

Total System Noise Temperature (K) 407.8

Total System Noise Gain (dB) 0.2 40 24.5 45 2 107.3

Noise for One buoy 48.2

7.2.4 Link Budget Now that we have all the gains, losses and noise we can find Carrier-to-Noise and finally Bit Energy-to-Noise ratio. Power Received Pr at demodulator will be

50.8 ___ 12 Carrier-to-Noise Ratio C/N and Eb/No is,

50.8 48.2 2.6 ___ 13 , 10 0.824 ___ 14

It can be seen from figure 2 that received Eb/No is greater than required so that our link works in worst assumed conditions and gives 99.9% channel availability. Sometimes the sun transit outage destroys the link but it is within our limit i.e. sun outage time is around 0.01%.

7.3 Uplink Parameters (From Singapore Station to satellite)

Earth station at Singapore will contain multiple antennas (one for each orbit) to track satellites in each orbit. Earth stations’ beacon signals enable satellite to know that it is passing over the earth station and starts transmitting its data after acknowledging that beacon. The only difference from buoy uplink is transmitting antenna gain which will be around 52 dBi [10] for 6 GHz frequency, antenna pointing loss which will be very low (due to steerable parabolic dish) i.e. about 0.2 dB and data rates/modulation/bandwidth/coding technique which will be 33.33kbps/QPSK/25 KHz/no coding.

7.3.1 Link Budget 3 52 39.85 107.3 190.19 6 4 0.2 2.36 3.2 0.85 4.65 . For

25 KHz bandwidth, noise will be 51.21 . Hence the C/N and Eb/No is, / 4.45 51.21 46.56 , / 45.31 . Since the Signal-to-Noise is very high,

the saturation or addition of spurious (nonlinear) components should be taken into notice. We see that for IF stage, there will be very less spurious components at -30 dB of output power so we restrict our out power of IF stage to be less than -30 dB i.e. less than -32 dB output of filter stage. Making this assumption we will get, 24.35 .

 

 

7.4 Downlink Parameters (From Satellite to Singapore Earth Station)

Assuming worst case of downlink transmission in which a satellite has gathered all the transmissions from all the buoys we will get,

• Total data = 4000 buoys*1MB*4 transmissions = 15.625 GB • Data Rate Rb = 52.08 Mbps (Assuming total transmission time of 40 minutes)

So, a 54 Mbps link can be assumed which would take around 39 mins. For QPSK

modulation (without any channel coding), • Symbol Rate Rs = 27 Msps • Bandwidth BW = 35.1 MHz (with excess bandwidth factor of 0.3)

It means a separate transponder is required for downlink.

7.4.1 Antenna Earth Station is assumed with Parabolic Antenna having Circular Polarization and a gain of 49 dBi [10] with narrow beamwidth at 4GHz. The pointing loss would be minimum (0.2 dB) as the earth station is able to track satellite(s) with its main lobe directing towards the satellite. Since Satellite is not so much power-and-space-limited, any suitable power transmitted can be assumed like for SSPA, 16 dBW is suitable power assumption. Satellite will be equipped by same helix antenna that was used at buoys for uplink but with gain of 18.6 dBi and beamwidth of 21.2 degrees. This antenna at the satellite will cover same area on earth as that of receiving antenna.

7.4.2 Downlink Losses Free Space Path Loss: Using (10), we get 186 1

Rain Attenuation: Since Singapore comes in the P-Region or climate zone the highest rain attenuation can be found by the same method and values except for frequency difference. For downlink frequency 4GHz, circular polarization and minimum elevation of 56.71 degrees, rain attenuation from [5][6] using SAM model is , 0.4

Other losses include scintillation loss (Lsc) which is about 8 dB at 4 GHz (as in [5]), polarization mismatch loss (Lpol) which is 0.85 dB [6], waveguide loss (Lw) at satellite receiver which is assumed to be 0.4 dB (0.2 dB each at both satellite and earth station), system margin/link margin/miscellaneous loss (Lm) which is assumed to be 4 dB (nominal value).

7.4.3 Earth Station Receiver Noise As described above and shown in figure 5, the earth station receiving system consists of Antenna, waveguide, LNA, 4GHz to IF Down converter (+ IF amplifier). Table 2 shows their noise and gains and the total noise. These were taken from same [4] and [7].

Figure 5: Block Diagram of Earth Station (Singapore)

4GHz to IF DC  QPSK Demod Storage Other EquipmentLNA  ProcessingRX 

 

 

Table 2: Earth Station Receiver Parameters

Property Antenna Waveguide LNA 4 GHz to IF DC

Gain (dB) 40 -0.2 40 45 Noise Figure (dB) 2 0.2 1.45 11

Noise Temperature (K)

32

273 11

56

Tw =13.67 TLNA = 114.94 TIF = 3360.88

Total System Noise Temperature (K) 181.81

Total System Noise Gain (dB) 0.2 40 45 84.8Total Noise (dB) 45.75

7.4.4 Link Budget Now that we have all the gains, losses and noise we can find Carrier-to-Noise and finally Bit Energy-to-Noise ratio. Power Received Pr at demodulator will be,

16 18.6 49 84.8 187 8 4 0.4 0.4 3.2 0.85 35.45 Carrier-to-Noise Ratio C/N and Eb/No is, 35.45 45.75 10.3 , 8.4

Again our link works in worst possible conditions (see Fig. 2). 8. Conclusion

A satellite network with all aspects were discussed and an optimum solution based on calculations, realizations and complexities was chosen, got optimum BER of 1e-3 for uplinks and downlinks. A comparative study of using different techniques is also made describing their pros and cons and it comes out that proposed solution works with least complexity. Work can be further expanded by simulating satellite orbits and coverage using STK or other advanced softwares. 9. Keyword

Satellite Communication, Earth observation, Remote Sensing, Climate monitoring, Satellite Network References

[1] http://www-argo.ucsd.edu [2] http://www.daycounter.com/Calculators/Helical-Antenna-Design-Calculator.phtml [3] http://www.aticourses.com/antennas_tutorial.htm [4] http://www.nec.com/global/solutions/space/satellite_communications/index.html [5] G.O Ajayi, S.Feng, S.M. Radicella, B.M. Reddy, “Handbook on Radio propagation related to satellite communications in tropical and subtropical countries”. [6] Timothy Pratt, Charles W. Bostian, “Satellite Communications”. [7] www.comtechefdata.com [8] Matthew Valenti, Assistant Professor, West Virginia University, “Inserting Turbo Code Technology into the DVB Satellite Broadcasting System”. [9] http://en.wikipedia.org/wiki/File:PSK_BER_curves.svg [10] www.ascsignal.com [11] http://www.sevin.ru/seminars/argos/Argos-3.pdf