MOUNTING CONSIDERATIONS

This section discusses how to mount the GPSFlight Transmitters into a high power rocket in the optimum fashion for reception, GPS coverage, and elimination of negative effects of G-forces on the GPS.  As you know, the GPS is rated for 4 Gs only, so in many rockets the GPS will go 'silent' during the boost phase.  However,  mounted correctly, your ST900 or XT900 can provide continuous data during your entire flight even at more than 15Gs, and you will KNOW exactly where it is because it will be telling you the whole time.  If you seek to provide position data under these high thrust conditions is your objective, then read on.

There are several factors to consider when using a GPSFlight  flight telemetry unit in a high power rocket.  They are:

1. What to expect from your ST900

2. Mounting the unit to eliminate or reduce the effect of sharp G-force on the GPS unit.

16 things to EXPECT from your Transmitter:

1. On an ST900, expect on a fully charged 2000mah battery pack (AA) to get 5 hours of run time at 1-per-second update rates. Likely this will be more like 12 hours, but be conservative. Using the ST900e or XT900, be sure you charge the battery. The run time there should give you at least 4 hours.

2. NEVER REVERSE VOLTAGE OR YOU WILL FRY THE UNIT AND HAVE TO BUY A NEW ONE!

3. Expect to have a range of up to 20,000ft using only the internal 'wire' antenna, even farther with a higher gain antenna on the ground (like a Yagi).  We have data tracks from these units to over 58 miles line of sight using gain antennas on the RXBase.

4. Expect the barometric altimeter to provide pressure and temp data regardless of the GPS having lock, and to be able to update as fast as every 250ms with fresh data. This will be valid from sea level to 25,000ft, but not really good beyond that.

5. On the ST900, the temperature will be board temperature. With the ST900e, you can connect an optional probe which will be the temperature of whatever it is in contact with from -55 to +125C.

6. Expect the data to stop when the unit lands on the ground or goes over a hill.  This breaks the line of sight.  However, even if it looses sight at 500 ft, the last known coordinates will be very close.

7. Expect the GPS data position from last known coordinates on a flat area (brush, trees, etc) to be within 30'.  Set your second GPS and BELIEVE it. Sometimes you won't think you are in the right area. YOU ARE.

8. Expect that if you mount the GPS firmly to the airframe, YOU WILL LOSE LOCK during the boost phase of flight, but to recover during decent.  Use bubble wrap method for best results.  Expect the GPS to blank out during periods of high thrust, but it will come back online after they are over. You might lose lock ANYWAY.

9. Expect the unit to short out, and be damaged if you mount it with other metal touching the case, or the PCB.  If you choose to cram it in somewhere, you should protect the PCB and GPS case from connection with other metal.

10. Expect that if you mount the GPS using the 'bubble-wrap' technique, to maintain lock, even under 15G flights or possibly higher.

11. Expect the ST900 and ST900e to fit in a 54mm tube, but nothing smaller. The XT900 will not because of the enclosure.

12. Expect it to report over 1,000 mph speeds under 60,000 ft, and report over 60,000 ft while under 1,000 mph.  (Also expect that under extreme conditions like this the actual Max Speed values may be a bit off - esp if the GPS loses lock for a second or two)

13. Expect the GPS to SHUT DOWN if you exceed 5,150mph.

14. Expect the range of the radio to be 20 miles with standard antennas line of sight. Moving the 2db dipole RXBase unit antenna around a metal reflector like the side of a car can sometimes bring in far-off 40+mile signals - actually acting like a higher gain antenna.

15. Take care to be sure the power connection, battery pack, and radio components cannot come apart under thrust.

16. Face the GPS patch antenna sideways or up if you can. Remember tho - facing up going up, may be facing down coming down so think it through..

TRICKS IN MOUNTING THE TRANSMITTERS

The Bubble-wrap technique is the ONLY way to do this.  We have tested this over and over, and the GPS (rated to 6Gs) does not handle the 'jerk' of a rocket launch well if mounted to the airframe.  With flights of 6+ Gs, your GPS will stop reporting valid position data (or report just the last one from the pad) until the high G-forces stop.  If the G-forces are REALLY high, it may take 10 minutes for it to recover.  But don't worry!!!  Bubble-wrap works wonders!