Ground Station Status

Introduction

The ground station for ECOSat-II, responsible for communication between the ground and the satellite, is in its final stages of completion. The antenna being used is a large Yagi.  Yagi antennas have very directional radio propagation that we can use to our advantage by pointing at the satellite.

yagi

Yagi Propogation [1]

That is why the ground station is equipped with an elevation and azimuth rotor that will be able to track any satellite that passes overhead. Satellites do not pass slowly though, and in a low-earth orbit (which is what ECOSat-II will be in), they travel around 7km/s. While their orbit is far enough away (~800km) that our rotor can keep up with them, another problem arises. Communication with satellites at these speeds will be heavily affected by the doppler effect. As they approach the frequencies get higher and after they pass they get lower. This can shift the incoming frequencies from 3kHz in the 2m band to 9kHz in the 70cm band [2].

The Set Up

The Ground station will be located in the penthouse of the ELW with the tower being on top of the roof. As seen by the images below, the ground station is all wired and ready to begin testing. On the outside of the box are the RF connectors and switches that will switch antennas used for transmitting and receiving as well as two push to talk switches for the power amplifiers. Inside the box is the computer, connected to the USRP and rotor controller. the USRP connects to the RF switches and power amplifiers and then out of the box to the antennas. The power amplifiers are powered by each their own 13.8V power supply. The rotor controller is powered by a 19V power supply and connects through a 8 wires, 4 for each elevation and azimuth to a terminal bloc. A teck cable runs from this terminal block to the lightning arrestor on the tower and then to rotor itself. The full wiring plan is seen below.

wiring

Wiring Plan

 

ground_station_wired

The Ground Station Enclosure

lightning_arrestor

The Lightning arrestor

Tracking

The rotor in use is an Alfa-spid Big RAS, controlled by a controller inside the building. The controller can be operated manually or remotely by a computer and some software. We chose to use Gpredict and Hamlib as our Tracking software. Gpredict is a GUI of Predict, which provides real time satellite tracking and orbit prediction. Hamlib lets us interface with our rotor, thankfully the two programs are compatible and make for easy tracking.

gpredict

Gpredict with all NOAA Satellites being Tracked

Frequency Correction

For our transmitting/receiving software, we are using GNU Radio Companion (GRC). GRC is a free and open source software development toolkit that provides signal processing blocks that, when combined with our USRP, linear amplifiers and low noise amplifiers, create a software defined radio. To correct for the doppler frequency shift we use Predict. Predict provides us with our doppler information that we implement into our receiver in GRC. GRC works nicely with Linux but on Windows we use SDRsharp and Orbitron to do the correction when receiving. SDR# works well with add-ons and adding Orbitron is a simple process. Enabling the add on automatically keeps the frequency centred for the duration of the satellites pass.

 

Next Steps

The next steps for the ground station is going to be mounting the tower on the roof and securing it down, hooking up the antennas and rotor and testing the RF hardware. For the RF testing we will be using a signal generator and spectrum analyzer to test our linear amplifiers to find their 1dB compression point. This is an important attribute to analyze in linear amplifiers since it identifies where the amplifier becomes non-linear.

 

 

[1] http://www.cisco.com/c/en/us/products/collateral/wireless/aironet-antennas-accessories/prod_white_paper0900aecd806a1a3e.html

[2] http://www.qsl.net/vk3jed/doppler.html

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