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====== Lesson #8 ======
Up until this point we have interfaced with the flight controller using a direct connection via USB cable. This is fine for adjusting parameters of the flight controller, but we have not been able to monitor the vehicle while it is in flight. In flight monitoring has not been necessary so far, but for GPS based flight modes, it is very beneficial to have quick and easy access to the flight controller. With this connection we can do things like view the aircraft's position on a map, or upload new way point missions remotely.

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===== Step 1: Adding the Telemetry Radio =====
Adding a telemetry is easy, as they should come in pairs so communication out of the box will not be an issue Connect one radio to the USB port of your computer, and another to the "TELEM 1" port on the flight controller. Once this is completed, the process for connecting your ground control station is the same as if it were via direct connection. If multiple vehicles are in flight using telemetry radios, it is necessary to make sure they are paired correctly, via the radios ID number. TO check the ID number of your radios, open Mission Planner, and without connecting to your aircraft go to the Initial Setup menu, and under the Optional Hardware select "3DR Radio." Press "Load Settings" to see what the current Net ID of each radio is. You can then switch to whichever ID you please and load the setting by pressing the "Copy Required Items to Remote" button.

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===== Step 2: Adding and Calibrating the GPS =====
For our applications using the GPS is crucial for autonomous missions. The GPS is provided and should be plugged into the "GPS" port on the Pixhawk. When mounting the GPS be sure to mount it in a position that wont drastically affect mag sensor readings. This includes preventing the GPS from rotating during flight.
\\ To calibrate simply go into the mandatory hardware section of mission planner's initial set up and recalibrate the UAS's compass.
\\

For this lesson the first three introduced GPS flight modes are Loiter, Position Hold, and Return to Launch/Home (RTL/RTH).
\\ First two of the three is loiter and position hold. Both flight modes have the same function in it that they both use GPS to maintain their altitude and position. To control the aircraft in both of these modes is as usual. The throttle/left stick is used to control altitude. Your right stick is still functional for pitch, roll, and yaw. The main difference is that in the loiter flight mode it requires much more stick engagement to move the aircraft.

\\ HDOP will also determine how accurate the position holding capability will be.
Additional information and tuning guides for these modes are here:

http://ardupilot.org/copter/docs/loiter-mode.html#loiter- mode
The FC can be armed when in Loiter or Position Hold mode. It is recommended to take off in Stabilize
mode and then only select Loiter or Position Hold mode while in flight and at the desired location.

\\ Return to launch (RTL) is another flight mode/ flight option that can be programmed to one of your switches. Your launch spot is always wherever the aircraft was last armed (determined by your GPS). What return to launch does is, allow you to command the multirotor to return to its arming location at any given time. This function is very important and can be a real life saver in emergency situations.
With that in mind, it is important that the drone is always armed away from people.

\\ RTL/RTH can be set up as an individual flight mode, but it is most commonly assigned to either the

Ch7 or Ch8 switch as a quick on or off activation. In essence, when activated, RTL/RTH will stop any

flight movement, climb to a preset altitude, return the GPS coordinate the FC was armed at, then

descend to either a set altitude or land. There are some unique settings to be aware of for proper

RTL/RTH operation. All these settings can be found in Mission Planner/Configuration/Full Parameter

List. They are:

RTL_ALT: The set altitude it will climb to before returning to home. 0 will return at the current

altitude. Any other desired set altitude is entered in centimeters. It will climb if the craft is below the

set altitude. If will return at its current altitude if it is above the set altitude when RTL/RTH is

activated.

RTL_ALT_FINAL: The altitude will descent to after returning to the home position. A setting of 0 will

land and disarm the motors if the throttle stick is fully down. It will descend to then hover at any other

set altitude, set in centimeters.

RTL_LOIT_TIME: The time, in milliseconds, the craft will hover above the home position before it

starts its descent.

WPNAV_SPEED_DN: How quickly the initial descent in cm/s once it returns to the home position and

the RTL_LOIT_TIME as expired

LAND SPEED: The speed in cm/s of the final stage of the descent.

More information and settings on RTL/RTH can be found here: http://ardupilot.org/copter/docs/rtl-

mode.html#rtl-mode

RTL/RTH will not engage until the craft is above and beyond a certain distance from home. The craft

will simply land if RTL/RTH is activated and has not yet flown far enough away from home. The FC

will not ARM if RTL/RTH is engaged.