Environmental Monitoring by the NADIR Satellite Mission
M. Perelli1, G. Perrotta1, G. Cucinella1, A. Jebril 2
(1) IMT, via Bartolomeo Piazza 8, 00161 Rome-Italy; e-mail: [email protected]
(2) University of Rome-Tor Vergata, Dept. of Electronic Engineering,
Via del Politecnico 1, 00133 Rome-Italy, e-mail: [email protected]
Abstract
In this paper, in introduction is made to the NADIR (Nanosatellite per l’Ambient, la Didattica e la Ricerca)
satellite project carried out by IMT; an aerospace Rome-based SME. NADIR is a LEO (Low Earth Orbit)
nanosatellite with the aim of providing environmental monitoring services in addition to becoming a key element
of a space-based and space-oriented educational and training system.
Keywords
NADIR, Educational satellite , Environmental satellite, Space missions, Environmental data
gathering, Platform , Data system, Buoys.
1. Introduction
The first pre-operative mission proposed for NADIR concerns environmental monitoring,
more specifically the remote gathering of data collected by remote terminals, either land or
marine-based, for the monitoring of environmental parameters. Besides the preoperational
mission aim at demonstrating the remote control of both the nanosatellite operative
configuration and of the remote terminals designed for data collection.
At present, non-profit organizations has expressed their initial interests for the IMT's NADIR
initiative, motivated by a need of gathering data collected by environmental terminals located
in hardly reachable sites, and of inexpensively and safely controlling these terminals. The
preoperational use of NADIR for environmental monitoring will be performed in time-sharing
with the Educational and Distance Learning Mission.
In brief, the twin-missions approach aims to pursue the following objectives:
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to show that a system dedicated to educational formation can also solve some problems
related to the environmental monitoring. regarding especially areas of important touristenvironmental value, or high risk, and generally of difficulty and/or expensive
accessibility and maintainability;
activation of regional establishments, although initially on small scale, related to the
realization of smart centers to carry out data collection and transmission via radio;
to include the regional technical Institutes in the realization and the following
management of the environmental part of the project, in addition to the proper activities of
the educational-formative part of the project;
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to emphasize the activities of the Associations and interested Corporate body to the
environmental monitoring;
to promote educational and formative activities concerning environmental themes,
focusing on the environmental monitoring technologies.
2. The Nadir Nanosatellite
The proposed initiative [1] can be considered a working tool serving, simultaneously and
somewhat interactively, to support and complement the educational and formative needs of
young students of the technical schools in view of their entrance in the labor market, and the
data collection needs – by both Researchers and Service Companies- concerning Remote
Sensing and Remote Data. The technological validity of this mixed educational-applicative
project can be better appraised looking at the key features of space and ground segments of
the nanosatellite-based system. The nanosatellite will be equipped with:
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VHF and UHF radio transceivers, operating in the amateur bands, for analogue and
digital, low data-rate transmissions;
two on-board computers organized in a very efficient and reliable manner;
a power system based on GaAs cells and li-ion rechargeable batteries.
The nanosatellite payload consists of the UHF and VHF transceivers and by other technology
experiments part of which can be contributed by Researchers willing to test their items. Since
the two radios are provided with a solid state memory, the nanosatellite can also operate in a
store- and-forward mode, for messaging purposes or platform data collection (Figure 1).
Figure 1 - The NADIR scenario
The nanosatellite signals are exchanged with simple ground stations operating as well in the
amateur bands. A single station per Country is presently envisaged (central regional station)
with the system operating in SCPC/TDMA or SCPC/RTMA access mode. The Central
Regional Stations are interlinked to the regional schools and/or to the Remote Sensing Data
Users ( where post-processing and data organization-distribution-archiving-post-products
dissemination may occur) via Internet, which is an optimum solution for distance-learning
and e-data exchange, allowing to link cooperating Entities in different countries.
3. The Nanosatellite Characteristics
The characteristics the nanosatellite are as follows:
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cubic shape with side length of around 25 cm;
constructive technology: aluminum plate with reinforcements;
maximum payload weight of 10 kg
thermal control: exclusively passive;
orbital control: not foreseen;
satellite attitude control: initially, the nanosatellite will not be equipped with attitude
control means, though experimentally it may carry equipments for attitude measurement;
autonomous orbit determination: the first satellite will not carry any GPS receiver;
power generation: with GaAs cells on 5 of the 6 faces of the cube
primary installed power: around 20 W in sunlight;
energy storage: rechargeable Li-ion batteries, in 3S2P configuration; having a capacity of
5.5 Ahs;
a non rechargeable battery is foreseen as backup system to supply the priority circuits;
power supply of the utilization circuits: through DC/DC micro-converters;
Data management : microprocessor based , inclusive of solid state memory for data and
messaging storage ;
radio communications: in the amateur bands VHF (around 150 MHz) and UHF (around
450 MHz);
communication functions: a half duplex transmitter/receiver in VHF band and a half
duplex transmitter/receiver in UHF band with an output power around 1 W at both
frequencies. The system is compatible with both analogue and digital modulation types;
Concerning the operation of the two-way communications payload with data collection
platform, we summarize here below the key points
a) operation principle based on store-and-forward.
b) the data is temporarily saved in a mass storage rewritable memory
c) a memory capacity of 4 Mbytes minimum;
The NADIR satellite proposed architecture is mainly composed of 3 electronic sub-systems to
which the mechanical parts of the vehicle are added (such as the carrying structure and lockup
and release mechanisms from the rocket). The main three sub-systems electronic are the
following:
• The Command and Data Handling sub-system (C&DH) that manages the command and
control functions of the vehicle and of the payload in nominal conditions. It is composed
of two modules microcontroller-based, each with a different function: the Core module
and the Mission module. The different performed operational combinations of the two
models are such as to allow the realization of a completely redundant scheme having any
of the two modules capable of ensuring the operation of the satellite.
• The Communications (COM) sub-system, that allows to manage the communications
from and toward the Earth’ stations. The configuration of base of the satellite foresees a
RTX VHF for narrow band (300-3000 Hz, 1200 bps) analogue and digital
communications; and a RTX UHF for digital communications in UHF with higher
bandwidths and data rates (9600 or 19200 bps).
• The Power sub-system (PWR), that converts the energy of the solar radiation in electric
energy and which also manages the battery charging / discharging and power distribution
to the users;
Figure 2 illustrates the basic architecture of the nanosatellite NADIR.
Figure 2 - Basic NADIR nanosatellite architecture
Figures 3 and 4 illustrate the design architecture of the data collection center and data
collection terminals that will use NADIR for environmental monitoring applications.
Figure 3 - Scheme of the data collection and management Centre
Figure - 4 Scheme of the UHF data collection Terminal
4. Specifications of the Data Transmission System
4.1. Data Size and Rates of the System
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Daily time interval useful for the overall data-dump towards every satellite of the
constellation: a passage every approximately 12 hours for one effective duration T_pl of
approximately 360 sec.;
Data transmission speed in the direction platform-satellite R=19.2 kbps, modulation
BFSK,
Number of platforms included in System: N_pt, variable typically between 5 and 25;
Overall transferable data size from N_qt platforms to the satellite at every passage:
Vol_tot = T_upl × R = 6912× 106
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Transferable data size for each platform: Vol_pt = Vol_tot / N_pt
Number of sensors for each platform: N_sens (typically 3-8)
Digital Quantization the analog data: Q (typically 8-12 bit)
Sampling rate: S_rate (typically 1/sec. to 1 every one 10 sec.)
Number of second in 12 hours: H_time = 12 x 3600 = 43200 sec.
Accumulated Platform Data Size for every 12 hours (interval between consecutive datadumps):
Vol_pt_cum = N_sens × Q × S_rate × H_time
The required binding condition:
Vol_pt_cum =Vol_pt
Therefore, the project variables can be calculated from:
6.912×106 / N_pt × 43200 = N_sens × Q × S_rate
that is:
160 / N_pt = N_sens × Q × S_rate
For example; consider the case where:
N_sens = 8, Q =12, S_rate = 0.1
the resulting number of terminals would be:
N_pt = 16
Thus, the assigned time interval to each platform including guard-times for data burst
transmission towards the satellite would be:
T _upl_pt = 360/16 = 22.5 sec.
4.2. Link budget
Given the satellite parameters, it is possible to estimate the EIRP required for each terminal to
establish the link and carry out data transmission, as a function of the data rates to be
transmitted. An initial link budget calculation to establish the satellite link is evaluated
assuming a scenario with the following characteristics:
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Satellite antenna beamwidth: sufficient to cover the national territory during two passes
daily;
• The useful access time for the terminals to the satellite: 2 intervals daily of about 6
minutes each (360 sec./pass.);
• Number of simultaneously active user terminals in the two pass-over time intervals: 20
typical
• number of environmental sensors per each terminal: typically 6
• sampling frequency (assuming slow variations of the environmental variables): 1 every 10
sec.
• Quantization: 12 bit;
• Number of generated bits/min.: 6 × 0.1 × 12 × 60 = 432 bit/min.
• Number of bit generated every 12 hours (average interval between two data-dumps
towards the satellite): 312000 bit
• Assigned time slot for data relay (in the terminal-to-nanosatellite direction) from every
terminal: 18 sec on each satellite pass ( mean value)
• transmission Data rates in each assigned timeslot:
312000/18 = 17.3 kbps (which is compatible with the transmission maximum data rate of
19.2 kbps);
Table 1 shown below outlines the terminal station link budget for determination of the earth
station antenna EIRP assuming data rate of 19.2 Kbps and BFSK modulation:
Table 1 - NADIR preliminary link budget
EIRP earth station
10 dBW
On-ground losses
Atmospheric and polarization losses
- 0.5 dB
- 3 dB
Path loss
- 145.5 dB
Satellite antenna gain
- 5dB
on board losses
- 0.5
Received power
-144.5 dBW
300 K°
- 161 dBW
16.5 dBW
7 dB
System noise temp.
KTB
Eb/No
margin to Eb/No = 9.5 dB
437 MHz, for an elevation angle of
30° and altitude of 600 km
Worst case
Bit rate= 19.2 Kbps
An EIRP of 10 W can be obtained in several ways, but it is suitable to choose an antenna that
has a wide enough beam to cover the arc of the satellite passage: an angle in the order of 120°.
Therefore, a Quad-Helix or equivalent antenna can be used with a gain of the order of 4-5 dB
max., and approximately 2 dB at the edges of the beam.
5. Conclusion
An operational data gathering space system -able to provide frequent passes over the sites
where are located the platforms to be remotely monitored- will be constituted by a small
constellation of 4 to 8 nanosatellites injected in LEOs with orbit plane inclination in the 45° to
60 ° range. Fig. 5 shows the plot of 4 nanosatellites injected in 50° inclined orbital planes,
to provide a mean repass interval of 6 hours of the land or sea-based platforms. The repass
interval will further decrease by increasing from 4 to 8 the nanosatellites of the constellation.
Figure 5 - Coverage of a 4-satellite constellation for an operational platform data
collecting service
Since the NADIR constellation will initially have only one satellite, it is of a major
importance to realize a project that will guarantee to obtain satisfying results even in case of
malfunction. This has been considered in the design of the remote control and power supply
subsystems to provide the main system functionality in extreme conditions. The project also
has a demonstration plan of the nanosatellite that matches and supports the architectural
approach to achieve a good probability of success for the NADIR Mission.
6. References
1] IMT WEB-Site
http://www.imtsrl.it