The University of Nevada, Reno
Nevada Space Grant
Student
High-Altitude Ballooning

NV SGC
This program is generously supported by the Nevada Space Grant Consortium


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This is our PTT, It has a battery Case, GPS receiver and a ARGOS transmitter.  The battery is a 15Ahr Li D cell which powers the device for a minimum of 68 days. Here is our PTT in it's insulating and protective case
Both GPS and ARGOS antennas face up because they are both designed to transmit to satellites. The PTT goes in the same case as the backup communication payload.  It makes up less than half the weight but it is very dense because of the battery case.

The PTT (Platform Terminal Transmitter) is more commonly known as an animal tracker. Typically used to track land-based animals, we use it as the ultimate (last chance) backup tracking system.  It contains a small GPS system (Xemics RGPSM002) that transmits its location a the ARGOS satellite constellation periodically for several weeks.  We learned through experimentation that the GPS does not function above 60,000 feet. However, when the payload descends below 60k, the GPS reacquires position and continues to transmit. The GPS is not very accurate when the payload is in motion (often resulting in errors of several miles). On the ground (motionless) the position is very accurate. Even if the PTT does not get a GPS lock, the ARGOS system will attempt to triangulate the location of the PTT using Doppler shift measurements taken from several satellite locations. The error in the Doppler shift location is usually several miles (okay for tracking an animal, but not sufficient for locating a payload). The PTT transmits its location every 90 seconds for the first 6 hours and then once every 6 hours after that.

Weight:

    Total: 393g

    Battery:87g

    Battery + Case:220g

Current draw: 6.5 mA (first 5 hrs)

                      3.6 mA (until recovery)

That means at the 6.5 mA rate of current use the device will stay on and transmit for 95.5 days!!

We've only had to rely on the PTT once in the 3 years we've been flying it. During NBS-08-01, both the primary and secondary communication systems failed (for separate reasons). The PTT did not report it's position until about 12 hours after the payloads reached the ground, but it worked flawlessly. All payloads would have been lost if not for the PTT. All payloads were recovered the day after the launch.

UPDATE: the PTT has been replaced by the SPOT personal GPS trackers. The SPOT costs a fraction of the PTT and is less costly per year to use.