ISS Falling Body Gravity Test
David Ways

with input from Bob Bruninga, WB4APR

This project proposes to launch three spheres from a cubesat launcher from the ISS and explore the effects of the space environment on the spheres due to the earths magnetic field, gravity drag, solar wind and others. The constraints on the 4" spheres weighing no more than 1.3 kg each suggest the following challenges:

  • Antenna: There can be no external antennas to effect the perfect sphere
  • Power: There can be no solar cells on the exterior that would detract from the perfect sphere
  • Life: Thie mission life when deployed from the ISS will only be a few months.

    Prior Example - ANDE: The ANDE satellite has all of the same challenges noted above and can be used as a case study in how we addressed each one of the design challenges noted above. ANDE was a 19" sphere, with no external antennas nor solar power. These issues were solved by separating the halves of the sphere with an insulator so that the shells themselves could be the antenna, and the power pwas provided by 112 "D" cell lithium cells. Although these techniques can also be used to solve the challenges with the 4" cubesat spheres, there are notable challenges:

  • The Circumference of the spheres are 4.75 times smaller - this will impact the antenna characteristices and would suggest an increase in operating frequency to reduce the scale of the antenna. It looks like the 900 mHz band would be appropriate.
  • The Volume will be less than 1% of ANDE. This places challenges on battery capacity and electronics volume. Fortunately, the electronics has shrunk since 2006 from the 10" square tray shown below, down to a 3.5" diameter circuit card. In addition, ANDE carried two dual redundant systems and additional comm experiments that needed more power than these spheres would require.
  • The Area-to-mass ratio is larger meaning a shorter orbit life, maybe 3 months, which further reduces the volume needed for batteries.

    The remainder of this page details the ANDE project so that some of its design approaches may be applied to this new project.

    See the ANDE-Operations Page for operations which began the first week of Dec 2006 and ended one year later on Dec 25, 2007.
    See the Documentation Page for details on the spacecraft design.

    . . . .
    ANDE Telemetry/Communications Tray was 10" by 10" . . . . . . . Today, (2013) the entire system can fit on a 3.5" square card shown here.

    Comm Board Availability: The 3.5" telemetry comm board shown above is also going to fly on the PSAT mission though that specific card is no longer available. Similar comm systems are available from, Argent Data, and

    VOLUNTEER AMSAT GROUND STATIONS: Since ANDE had no onboard computer for whole-orbit telemetry capture it was dependent on volunteer ham radio ground stations (see typical photo - AA6RR) for telemetry feeds to the APRS Internet system for live distribution. You can use any APRS program to feed satellite data to the APRS-Internet system (APRS-IS) in the normal manner like any IGate. Two popular programs are APRSISCE32 and YAAC.

    ANDE MISSION: The Flight unit was launched on STS-116 planned for the first week of Dec 2006. The USNA comm systems for ANDE were complete and integrated in the spacecraft and the final tuning of the spacecraft as an antenna occured on 15 Aug 05 and got an amazingly good 1.2:1 SWR!

    Antenna Matching PI Circuit
    SWR Testing

    ANDE stands for Atmospheric Neutral Drag Experiment and is a 19" passive sphere with optical corner reflectors and 6 Lasers for precise orbit determination. The Naval Academy was given the opportunity to construct a digital communications transponder for use in the Amateur Satellite Service to fit inside the ANDE sphere similar to what it is flying on the PCsat and PCSAT2 missions. The PCsat-like follow-on packet communications mission continued the interest of students worldwide by letting them communicate via the satellite and capture telemetry relative to its temperature in the space environment. See the paper on Licensing of ANDE for operations in the Amateur Satellite Service

    COMMUNICATIONS DESIGN DETAILS: This web page addresses only the communications portion of the design which was our responsibility. The communications, telemetry, command and control wass all based on the off-the-shelf Kantronics KPC-3 TNC as shown below. What makes this design unique is the absence of any external antennas as required by the minimum drag needs of the science experiment. We solved this by cutting the sphere in half so that we can use it as a dipole antenna across the two halves.

    Lithium Primary Battery Power System: Another unique requirement of the comm system was to operate for up to 1.5 years on primary batteries without any solar cells or external charging. Thus, ANDE runs on 112 "D" cell Lithium thionylchloride cells arranged in 4 packs of 7 strings of 4 cells in series. To meet the man safety requirements for launch on the shuttle, extensive testing has been conducted.

  • See Higher resolution of Batteries in box
  • And Higher resolution Battery Box on disk

  • Battery Box: Cell Arrangement, Schematic.
  • Battery Life Management uses 3 battery packs separately to exhaustion for lasers to yield measurable life data.
  • Present design yields 1.5 year Power Budget using the Tadrian TL-5930 Cell.


    The communications system consists of a Kantronics KPC-3+ TNC, a Hamtronics transmitter and receiver and a custom interface board to hook it all together. These are all mounted in a 1" tall box on top of the battery boxes.

  • Communications Tray Layout, Interface Board PCB Design, and Parts Locations.
  • Comm system Schematic
  • Using the KPC-3+ for Teemetry TNC Mods.
  • TelemetrySensors, Schematic, and Conditioning Circuits.
  • Text-to-Speech Schematic.... with the supportingText description and a link to the MFR's documentation.
  • Antenna Matching Circuit.
  • Debugging problems
  • TNC ROM defaults.

    Background notes and supporting justification used in the design:

  • Terminal Node Controller (photo)
  • VHF 3 Watt Transmitter (photo)
  • VHF receiver (photo) (pots are removed for flight model).
  • ANDE Frequency Selection (2m downlink) and the actual IARU Frequency Request
  • ANDE User Expectations
  • Link Budget
  • Click here to download EXPRESSPCB.pcb file. Use the free ExpressPCB software to view it.
  • Telemetry Multiplexer , RunTime Counter, and Interface Board changes required .
  • Telemetry format and equations .

    Other Documents and drawings:

  • Prelimiinary Antenna testing connections.
  • Two man lift system.
  • GSE Test Equipment and Test Plan.

    MULTI-PACK BATTTERY DESIGN: Since the lithium discharge profile is totally flat to 97% of battery life, the only energy usage data will be obtained by using each pf the 4 battery packs to exhaustion to run the LASERs. This will give 4 energy benchmarks during the mission. Also, by using the LASER-ON switch to toggle the telemetry channels, we gained 4 more telemetry channels. Just prior to CDR the requirement for additional temperature data has extended the multiplexer to a total of 16 channels.

    The photo below is NOT ANDE, but it does give a representative view of a 19" spherical Spacecraft.

    In the spring semester 2002, Midn Patterson (EE Dept) in conjunction with the USNA Small Satellite program under the guidance of LTCL Billy Smith and Satellite Lab Engineer, Bob Bruninga, WB4APR did initial investigations into conformal antennas for such low-drag spherical spacecraft. These showed that the natural resonance of the 17" sphere was near to the amateur satellite band on 146 MHz and if this frequency was used, that the sphere could be used as the antenna itself with no external protrusions. The FIRST SEMESTER objectives below were accomplished in the Spring of 2002.

    Click here to see the SWR resonance achieved at 145 MHz.

    The ANDE Communications mission, can provide basic Telemetry Command and Control for ANDE in addition to supporting general communications in the Amateur Satellite Service. This includes:

  • Battery and Solar voltages (attitude)
  • Battery and Solar array charge currents
  • Temperatures
  • ON/OFF command switches
  • 1200 or 9600 baud RS-232 serial data telemetry channel for other Payloads
  • Digital Communications Relay support of the PCsat/APRS mission.
  • A text-to-speech experiment for downlink communications to mobile and handheld users without digital packet capability.

    The digital communications relay will operate within the ITU regulations for operations in the Amateur Satellite Service, to provide digital communications for amateur satellite operators, educators and possible remote environmental sensors worldwide. This mission will augment the communications mission of PCsat by adding a second and third satellite to the constellation for comms support of remote travelers Naval Academy Boats at sea, cross country travelers, expeditions, or any other travelers far from any existing APRS terrestrial communications infrastructure. The PCsat and ANDE downlink from such users is fed into the existing worldwide internet linked APRS system by a few permanent ground stations. ANDE would join ISS, MIR, PCsat and several other on-orbit experiments (ASTARS) that have been conducted over the years leading up to this exciting capability.

    The TNC COMM SYSTEM (this is the KPC-9612. The KPC-3 is slightly smaller).

    Here are several additional PHOTOS, DRAWINGS and Documents reorganized by topic:

  • The Prototype
  • Preliminary Internal Structure Sketch
  • TNC mods for telemetry

    The Space segment of the ANDE communications mission has been demonstrated a number of times in space via PCsat, MIR School tests, the Shuttle SAREX, and the SPRE mission. . It is also a spin off of a previous launch opportunity that we had in 1998 called NATSweb that almost got launched on SEA-LAUNCH.. The ANDE Communications mission is a project to produce a viable payload in a very short time frame using off the shelf components where possible. Here are the background topics of design:

  • The Mission and ANDE Team

    APRS is the Automatic Packet Reporting System that the Naval Academy uses for tracking its boats and a variety of other vehicles and networks using the APRS Automatic Packet Reporting System . The ANDE Communications transponder would be operated under the rules of the Amateur Satellite Service and the rules of the FCC. .