=== GPS ===
Introduction to GPS
This article will cover all the issues and questions you'll have concerning GPS.
Use Two GPS Units
Our team has flown 5 flights. The first flight SB1 had only one GPS-transmit system, and unfortunately took ~3 months to recover after an antenna failure. Flights SB2, SB3, and SB4 all had two GPS-transmit systems, and none took longer than 30 minutes to recover. SB3 and SB4 each had failures of one GPS system. SB5 had three GPS units, the third being a cell phone transmitter.
How Does GPS Work?
1) picks up these very weak signals
From there you have the choice of directly transmitting this info into the APRS system, as described in the space transmitter tutorial, forwarding that info to your cell phone transmitter, and/or logging it to a microSD card or EEPROM.
Receiving GPS String Data
It's a very non-demanding signal. This means that if your microcontroller does not have enough UART ports, you can use a software-based UART without issue.
The data will likely be in TTL format, or maybe RS-232 if it's an old unit. You store the string as an array, and then process that array looking for the variables you want. GPS strings are standardized, so you can likely find code for your favorite microcontroller online to copy/paste. I used WebbotLib software library with the Axon Mote microcontroller.
It's best to get a GPS that outputs normal TTL so that it can be directly interfaced with a microcontroller. Avoid those that connect by RS-232 or USB, as they require the additional complication of adaptors.
Does GPS have an altidue limit?
The COCOM limit exists to prevent GPS units from being used on ICBMs (intercontinental ballistic missiles). Any GPS that violates the COCOM limit would be subject to ITAR regulations, making it a huge pain for any GPS manufacturer to sell their product abroad.
The limit states GPS units cannot work if it exceeds above 18km AND travels faster than 515 m/s. GPS units are allowed to violate either limit, but not both simultaneously.
Our balloons had a max speed of about ~50 m/s and a max altitude of ~36km, so it does not violate both COCOM limits.
However some GPS units are programmed to shut down if either limit is violated anyway. It's important that you carefully read the datasheet and confirm with others who have already used that particular GPS unit that it will in fact work at your planned altitude.
Some GPS units have what's called 'ground mode', 'aviation mode', 'high speed mode', etc. If, for example, your GPS is factory set for 'ground mode', it may have poor accuracy in terms of altitude and/or may not work above some particular altitude. You'd have to reconfigure the firmware for 'aviation mode'.
The below image was obtained while testing our GPS unit using the GPS Viewer software.
Does Altitude Affect GPS?
So the answer is 'yes': altitude does appear to increase GPS locks. I suspect this is because of two reasons:
What Else Affects Number of Sat Links?
The first and most obvious is the signal quality. Your antenna (type, length), obstructions to the signal (are you inside a building?), and air humidity (rain, clouds, fog, etc) all affect signal quality. Obviously don't launch your balloon in bad weather, and make sure your GPS unit antenna(s) are not obstructed by metal objects (batteries, wiring, etc) as they are packed into the launch capsule.
Note that it might take minutes or longer after bootup of your GPS unit to get a good position fix - four satellite locks are required to get a position. The datasheet of your GPS will fully describe the cold boot and warm boot times required. Of course, it's not recommended to launch the system until you confirm it has at least 6 sat locks.
As best demonstrated by this below animation I 'borrowed' from the wikipedia article on GPS, the number of detectable satellites fluctuates throughout the day:
Determining Velocity and Acceleration Using GPS
note: a Knot is 0.514 m/s
What I find interesting about the above velocity data is that although the altitude is increasing at a rather steady rate, the velocity is still changing quite a bit. This would mean that vertical velocity is nearly constant, but horizontal velocity (ie. wind speed) varies quite a bit. This would mean that wind is generally horizontal throughout the vertical air column.
You will see three general regions of maximum velocity. The first and last represent the jet stream. But what I don't really understand is at max altitude the velocity is also quite high - despite the very low air pressure! Interesting . . .
Using the velocity data, we can get acceleration by using this equation:
In the acceleration data you can see the two regions of highest acceleration is during the passing through the jet stream.
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