S-band ground station prototype for low-earth orbit nanosatellite missions
TELECOMUNICAŢII ● Anul LII, nr. 2/2009 67
RS-232, and to an antenna through a 50-Ohm MCX
connector – see Fig 3.1. The electrical interface of
MHX-2400 is simple and promotes rapid integration.
It accepts regulated 5V from the electrical power
subsystem and communicates with the Command
Data and Handling (CD&H) processor through a serial
port with selectable data rates of up to 115200 baud.
Hardware flow control is used to manage data flow.
Fig. 3.1. The MHX-2400 modem and the modem board
with RS 323 port.
In addition, this modem can be integrated in the
ground station without any separate radio or trans-
ceiver equipment, because the modem transparently
supports Doppler shift correction up to 60 KHz, auto-
matic channel selection, and frequency detection. The
MHX-2400 transmitted signal is approximately 200 kHz
wide. The receiver bandwidth is about 400 kHz. While
this can be considered an inefficient feature of the
receiver, due to the unnecessary increase in noise
entering the unit, it allows for Doppler signal shifts to
fall within the receiver bandwidth – see Table 3.3.
The table below [7] shows how Doppler shift
increases with frequency. The other issue with Doppler
shift is how it varies during the satellite pass itself. This
is highly dependent on the pass itself. Low elevation
passes have a fairly linear variation of Doppler shift,
spread out over the pass. On the other hand, a pass
directly over local Zenith has most of the Doppler shift
variation concentrated around the middle of the pass.
3.3. The satellite tracking module
The chosen satellite tracking program is Orbitron
[8]. This application computes the position of
satellites at any given moment (in real or simulated
time) using as input Two Lines Elements (TLE) data
and sends azimuth and elevation angle data to the
antenna rotor digital interface. Satellite position
predictions are based on the NORAD SGP4/SDP4
model and the theoretical accuracy is close to 100
ms or 1 meter at the Equator. Satellite TLE data are
provided on-line by USSPACECOM (United States
Space Command) for approved users. For low
orbiting objects (400-1000 km), TLE data must be
updated every few days. Because the ground station
is located in a remote rural area, the only option for
TLE data update was to use a VSAT terminal.
The accurate geographical position and time
reference for the tracking module is provided by a
Hopf 6039 GPS [9]. The Hopf 6039 GPS equipment is
composed of a PCI board which is connected through
a coaxial cable to an external Hopf FG449010 antenna
mounted on the roof of the station, in order to have a
visibility of 360°. Using NTP (Network Time Protocol)
it is possible to achieve PC synchronization of other
units’ clocks.
Table 3.3
Maximum Doppler shift vs frequency for an LEO satellite at 800 km altitude
Band Frequency [MHz] Max Doppler Band Frequency [MHz] Max Doppler
15 m 21.280 +/- 477 Hz 23 cm 1269.000 +/- 28.5 kHz
10 m 29.400 +/- 659 Hz 13 cm 2401.000 +/- 53.8 kHz
2 m 145.900 +/- 3.27 kHz 3 cm 10250.000 +/- 230 kHz
70 cm 435.070 +/- 9.76 kHz
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