Psat - May 2015 - The next APRS satelite
US Naval Academy Satellite Lab,
Bob Bruninga, WB4APR
Midns Crawford, Lumsden, Guilfoyle, Randall, Ridge, Pollock, Schlottmann (09)
Midns Icard, Edirisinghe, Mayer, Papso, Phillips (08)
Midns Campbell, Dendinger, Lewis, Lindsay, Londono, Mayer, Okun (07)
Midns Paquette, Robeson, Koeppel, Piggrem, Lovick & Vandegriff (06)
Midn Edwards, Oceanography (05) and Midn Humberd, EE (04)
OVERVIEW: . PSAT is a Naval Academy student satellite project named in honor of one of our graduates, Dr Parkinson of GPS fame, which contains an APRS packet radio communications transponder for relaying remote telemetry, sensor and user data from remote users and amateur radio environmental experiments or other data sources back to Amateur Radio experimenters via a global network of internet linked volunteer ground stations. The data transponder also includes all telemetry, command and control for a complete cubesat. A secondary transponder supports multi-user PSK31 text messaging users via a Brno University trasnponder.
See the Psat paper at the 2010 AMSAT symposium
Operations in the Amateur Satellite Service: Both of the transponders on PSAT are operated in the Amateur Satellite Service to encourage amateur radio students, educators and experimenters around the world to contribute additional satellities to this constellation on 145.825 MHz or to build interesting self motivated remote sensors suitable for the uplink channel. See our ocean or bay oceanographic data buoys for examples. This kind of Amateur Radio experimentation fits well in the ITU rules (see Psat justification) for operating in this service and well serve our educational and outreach goals for student projects encouraging young people to be interested in Science, Technology, Engineering and Math.
APRS Packet Transponder: The APRS packet transponder is an AX.25 Packet Radio Relay similar to what is flying on PCsat and the ISS. This ongoing mission in space on the original PCsat is now over 12 years old and pioneered this very popular operating mode via the ISS since 2006. Both of these missions deliver packets to users worldwide via the global network of volunteer ground stations feeding the two downlink capture pages: psat.aprs.org and ariss.net. These pages display live maps such as the one above, of the most recent user position data and capture all message traffic between users. See the APRS link budgets. In addition there is also a PSK31 transponder as noted below.
PSK31 Transponder: The PSK-31 multi-user FDMA transponder experiment is similar to what we flew on RAFT and PCSAT2 missions. See the full PSK31 design concept here. This text messaging transponder allows messaging between up to 30 modest ground stations simultaneously. In the example waterfall display above, 10 users are clearly visible. Each user transmits a 31 Hz wide narroband transmission within the 3 kHz wide transponder, and all can be seen simultaneously via the UHF FM downlink. Uplink stations do not need gain antennas on the HF uplink but can use a vertical monopole antenna and 5 Watt SSB transmitter to give the transmit antenna profile as shown above. To calculate the PSK31 link budgets we combined the user TX antenna gain profile with the variation in range gain from the horizon to over 60 degree elevation as shown here at right. The combination gives an uplink power variation per user of less than +/- 4 dB over 90% of the duration of the pass.
CUBESAT MODEL: Back in 2008, we re-designed the original 1 cubic foot ParkinsonSAT to fit inside the a 1.5 unit cubesat so that two Psats could be launched from the same P-Pod launcher. That design used four deployable solar arrays in a sunflower configuration and pointed towards the sun with a 3 axis ADCS. Half of the 1.5 unit Psat was available for auxilliary payloads or experiments such as the Brno University PSK31 experiment. The current 2014 design remains a 1.5U cubesat but with higher efficiency solar cells, we no longer need the deployable side petal panels and can operate with fixed side panels as shown at the top of the page.
Unique Power System: This model with the 4 large and efficient solar cells per side is designed around a unique battery system. Since the total solar voltage per side is only 3.2 volts (0.8 volts per multijunction cell) we cannot charge the complete 9.6v battery string, but can charge two NiCd cells per solar panel. These 4 side-panel/NiCd-pairs are operated in series for the 9.6v bus, but are charged in parallel. This parallel charging of no more than two NiCd's in series per solar panel reduces one of the primary problems with spacecraft battery charging systems and that is the uneven charge balance that accumulated when entire strings are charged in series.
Attitude Control: The primary attitude control requirement is to evenly expose the four side panels to the sun so that the NiCd cells are equally charged and to even-out the thermal load on the panels. A very slight spin about the Z axis is maintained by the unbalanced solar radiation pressure on each side. A highly reflective strip on the clckwise edge receives greaterr radiation pressure than the momentum transfer to the mostly black solar panels. This should create a fractional RPM spin (the Spin on PCsat now, 12 years in orbit is maintained between about 0.6 and 0.8 RPM by this method). Secondly, an active Z-Align algorithm using a single Z coil activated at appropriate locations in the Earths Magnetic Field will be used to keep the Z axis aligned approximately with the Earths poles so that the side panels are always within +/- 23 degrees or so of the Sun. This can maintain more than 90% power budget through the seasons.
Global Experimental Data Channel: PSAT is the space segment of this initiative to encourage both new satellite construction in support of this experimental data channel and lower cost buoy and sensor experimentation at other schools as shown below. Today, the only AMSAT that is available for no cost to schools with such experiments are the Naval Academy's PCSATs. But with ParkinsonSAT we hope to commence an ongoing full time presence in space to continue this support of the 145.825 data uplink channel for future experiments. To this end we hope other schools to either build additional 145.825 MHz relay satellites and/or to build experimental sensors. The complete comms system including data transponder fits on one 3.4 inch square card shown below. We hope that ParkinsonSAT will be the Egg in this chicken-egg conundrum.
The 2014 Final Design: When the 2014 launch opportunity becamse available, and since all the original design students were long gone and the design had not been thoroughly vetted, the system was redesigned and simplilfied. Here are the final docs for the PSAT delivered for launch.
CUBESAT DEPLOYMENT: The two 2008 cubesat designs would come out of the P-POD launcher upside down so that the deployment of the panels is not a violent event. The arrays only begin to deploy in the last few percent of travel, and then they are constrained by the p-pod opening. At this point, the forces of the deploying panels tend to add to the acceleration of the cubesat and assure the separation from the launcher as shown below.
BACKGROUND: The Naval Academy's PCsat , PCSAT2 , ANDE and RAFT satellites launched in 2001, 2005 and 2006 provided links back to the APRS internet system from simple student projects anywhere in the world. These satellites can relay position/status and telemetry about 2 to 4 times a day back to shoreside observers as a small part of the overall APRS system used by 10's of thousands of terrestrial users and vehicles.
2008 DESIGN DOCUMENTATION: . The following section provides on-line links to all of the PSAT design documentation. In Summer 2008, a significant design change is being considered in re-packaging the ParkinsonSAT into a standard 4" cubesat design. By using a pair of 1.5 Unit cubesats in a single 3 Unit Deployer system, most of the original objectives of the ParkinsonSAT can be met but with a much greater probability of finding a launch.
2008 CUBESAT Design Details:
2008 3 AXIS ATTITUDE CONTROL SYSTEM MODEL: .
2008 ELECTRICAL POWER SYSTEM: .
2008 COMMS SYSTEM: .
POSSIBLE PROPULSION SYSTEM Project for future work: (does not apply to the cubesat model). . If PSAT was able to obtain a ride on the Space Shuttle, it would need a propulsion system to carry it from the Shuttle's low orbit, to a higher, more long term communications orbit. There is no way that a student built propulsion system wouild be able to get past all the Man Safety requirements for the Space Shuttle unless it was propelled by water. . Which is exactly what we propose. See our proposed H2O Propulsion System
Historical files, supporting documents and obsolete ideas: