Build your own satellite:

the Sputnik Challenge

Howard Long

15 November 2007

Updated: 17, 29 November 2007

 

Abstract

 

This project was inspired by a BBC News article[i] available online published on 22 October 2007. It asked readers to have a go at building their own satellite, reflecting on fifty years since Sputnik became the first man made satellite in space to orbit the Earth. Presented is a demonstration satellite providing both data and voice capabilities using readily available parts and built on the author’s kitchen counter.

 

Figure 1: Kitchen table satellite building: the satellite with an example handheld groundstation terminal

 

Introduction

 

This satellite includes the basic subsystems for an orbiting satellite including power source, communications and basic command and telemetry features. It is built on a 130mm x 130mm x 130mm cube, incorporating three trays. The trays are the OBC (on board computer), the battery and BCR (battery charge regulator) and radio trays.

 

Figure 2: Block diagram of the satellite

 

The spaceframe is constructed from 10mm x 10mm right angle aluminium struts 1mm thick, sourced from B&Q. The solar panels were extracted from £10 Maplin solar battery chargers (part number L09BK), and these pretty much defined the satellite’s size.

 

4-Battery Solar Charger

Figure 3: Maplin's L09BK battery charger provides cheap solar panels

 

OBC Tray

 

The OBC is a Kantronics MT1200[ii] TNC (terminal node controller) that provides 1200bps AFSK AX.25 communications[iii] and basic secure command and telemetry functionality. Although limited in its capabilities with only two telemetry and two command ports, for rapid development it provides a ready-made platform to demonstrate basic satellite capabilities. Other Kantronics TNCs have already been launched into a number of space missions[iv],[v],[vi],[vii]. The MT1200 benefits from flashed settings rather than battery backed settings. On the missions mentioned, the manufacturer hard coded the settings into ROM.

 

The telemetry can be set to beacon at user defined intervals on the data channel.

 

Command and control uses the secure remote login capability of the MT1200 TNC, and allows two port bits to be set. In this demonstration, this allows control over two LEDs situated on the spaceframe. This also allows maintenance of all TNC configuration parameters.

 

The TNC RS-232 data port is available on the spaceframe for debugging purposes, so that the spacecraft does not have to be disassembled if the TNC needs to be accessed directly rather than over the remote command channel. It would also be possible to connect this to the radio’s command port and therefore allow remote control of the radio’s settings.

 

The TNC is set to operate as a digipeater allowing global APRS[viii] user coverage.

 

The TNC also includes a GPS unit although this is not used at present. As well as an external GPS antenna requirement, the GPS unit would not be able to operate in space due to the US government imposed altitude and velocity limits on commercial GPS devices.

 

Figure 4: View of the -X facet with the MT1200 TNC acting as OBC

 

Power consumption of the TNC is 60mW, but this could be reduced by removing the LEDs and RS-232 level translation device inside the TNC, or reducing the supply voltage to 4.5V with an efficient switch mode power supply (SMPS): presently it operates directly off the unregulated 6V bus, and it incorporates a linear voltage regulator.

 


Battery and BCR Tray

 

The satellite is surrounded by six 7V solar panels. These are estimated to provide about 1W each. Solar panel pairs on opposite sides of the spaceframe are operated together through Schottky diodes to a set of five 3.7AH NiMH rechargeable batteries. Thus, there are three sets of batteries, each charging from the three opposing facets, -X and +X, -Y and +Y, –Z and +Z. By using opposing paired solar panels in this manner, the power transfer is optimised by having up to three sun-facing panels charging the batteries and providing power at any one time depending on sun angle.

 

A simple 5Ω resistor limits current for charging purposes to each battery set. Each battery set is presented onto the unregulated bus via a Schottky diode.

 

For demonstration purposes, an external charging and powering jack is provided: indoor lighting is nowhere near sufficient to maintain power, although the NiMH rechargeable batteries will last many hours on a single charge if necessary.

 

The solar panels are also allowed to present power onto the unregulated bus via Schottky diodes in the case of a battery failure, so the satellite will operate when in sunlight even if all three battery sets fail.

 

Figure 5: View of the trays from the -Z facet with the three 3.7Ah NiMH battery sets visible

 

The use of an efficient SMPS to produce a regulated bus to 4.5V will benefit power budget. This would introduce additional complexity, and with that potentially additional risk of system failure.

 

As the power supply stands, the simplicity inherently implies reliability, although maintenance of the NiMH cells without proper care may limit their life. Some care may be provided by groundstations deliberately commanding on or off various spacecraft components in order to ensure that over charging does not occur, and that regular conditioning in the form of occasional discharge to a set level extends the useable life of the batteries.

Figure 6: The simple BCR

 

Radio Tray

 

The radio tray incorporates a commercial Kenwood TH-D7 transceiver. This radio provides VHF and UHF communications for both data and voice. For demonstration purposes, the voice downlink, and data uplink and downlink are set to 145.825MHz. The voice uplink is set to 435.300MHz with a CTCSS tone of 67.0Hz. These settings may be modified either from the 2.5mm stereo jack radio control port (brought out to the spaceframe), or, if the satellite is so configured, remotely via the MT1200 TNC.

 

By default the TH-D7 does not include a voice repeater function. By extracting the CTCSS decoder output from within the transceiver and feeding it through the TNC’s external PTT (push to talk) input, the transceiver can be made to operate as a voice repeater. An additional modification is to provide a level control using two trimpots to level both the voice uplink to be retransmitted, and the TNC’s modulated output. Connecting in this way means that the satellite is only transmitting on the voice channel when a valid uplink exists, thus reserving power budget.

 

The internal TNC of the TH-D7 is not used currently, although it is planned to add a dsPIC33 microcontroller in conjunction with the TH-D7’s TNC to provide digipeating and video imaging as well as extended command, control and telemetry functions.

 

It is possible to adjust the transmit power of the TH-D7 to 50mW, 0.5W or 2.5W, thus allowing some power budget constraints to be taken into account. Power consumption on receive is 500mW with both voice and data uplinks available. With only the data uplink available this decreases to 270mW. Transmit at 50mW, 0.5W and 2.5W uses 1.8W, 3.0W and 7.2W of power respectively. It should be noted that for data purposes, the transmit duty cycle is much lower than for voice.

 

Practical tests have already demonstrated successful full duplex voice at 50mW of downlink power at 145MHz when in a LEO (low Earth orbit). In general, more power is required for data when using 1200bps AFSK AX.25.

 

Figure 7: The radio tray showing the TH-D7 transceiver and the dual band coaxial power splitter in the +X facet

 

One feature missing in this satellite is that there is no attitude control. This means that at any one time it is not possible to know which facet is Earth pointing. In order to attempt to eliminate fading due to a tumbling satellite, two dual band antennas are fed in phase and mounted orthogonally at opposing apices. Mounted in this manner (an idea originally from Freddy de Guchteneire), there is always at least one antenna visible from any angle of attitude. Because the antennas are orthogonally mounted, fading due to antiphase signals in the same polarisation cannot occur, although the groundstation will need to compensate for polarisation mismatches.

 

Using dual band antennas however means that typically a wide band splitter would be needed. However, as one band is almost exactly the third harmonic of the other, it is possible to use a power splitter based on a simple coaxial transformer, using two electrically ¼λ length 75Ω coaxial cables measured at the lower frequency band. On a network analyser the results showed return losses of over 20dB at both bands of interest.

 

The antennas used operate as ¼λ at 145MHz and as ½λ end-fed dipoles at 435MHz.

 

Conclusion

 

Within three weeks, a working demonstration has been achieved. Although the considerations of thermal, materials and radiation have yet to be considered, many other elements have already been fully demonstrated with this project, including secure command and control, telemetry, and redundant power systems. Once the current TNC based OBC is replaced with a custom programmed microcontroller, a camera[ix] may be fitted allowing SSTV (slow scan TV) pictures to be sent. Development has already begun on this aspect of the project.

 



[iii] AX.25 Link Access Protocol http://www.tapr.org/pdf/AX25.2.2.pdf