IR works well outdoors if excellent daylight blocking filters are used on receiver.
You need to measure range, azimuth for tracking then , altitude and centre offset error above bucket for dropping.
Amplitude will change with distance and offset to beam angle of each emitter as well as 'copter tilt, yaw and roll. You can also emit pulses at different frequencies with separate PLL optical receivers, spread apart from 30 to 50kHz. Clever might be one band for NS and another for EW, but avoid harmonic carriers like 24, 48 kHz. AGC is very effective but rate of tracking is uncertain, so tracking errors can be affected by rotation of moving copter. Time sequenced bursts to each emitter can also help determine location of each emitter using narrow vertical offset angles to help in centering before drop.
It is a geometry problem which can be solved using spacial and time resolution to very precise position if you need it. But I will let you figure this out.
As you know from recent history tracking one emitter with one antenna is ineffective.
If you can chose any method, you can make it beacon 2way with ground detected position error or 1way with bucket surrounded by a string of emitters with alternating wide angle and narrow angle , so you can compute tracking error from far and near field and overhead.
Photodiodes are very precise but amplifiers using single transistor have a wide spread in gain. Emitters can be calibrated to match by design but initially can vary widely 50%. Keep this in mind. Transconductance receivers have precise gain but suffer from wide bandwidth noise and thus filters are needed. Remote control receivers are cheap, accept a wide range of signal sources with AGC and PLL burst tracking filter but will only track the strongest signal, so several may be needed with synchronous time slots emitted for each frequency.
there are several things to keep in mind, when combining data from different systems.
Combining different Satellite Navigation systems
Typically it is possible to combine gps+glonass or any other navigation system. That is called GNSS (Global Navigation Satellite System). For a single GPS case you need 4 Satellites for the components [X,Y,Z, dt (the estimated receiver clock error)]. For each new systems you have to add one bias term, describing the difference between the transmitted time scales (since every system has it's own specific time scale) for each system relative to one time scale, which is kept as reference. This bias term is an additional unknown in the estimation equation to receive/determine the position solution. Consequently you need at least 3 Satellites for system 1 and 2 satellites for System 2 (for example...any other combinations are also possible). The additional bias term will describe the relative difference from one time scale to the other, (Petrowski, page 95f, 2014).
Example: 3 GPS satellites and 2 Glonass satellites will give a direct solution without redundance (and no control). The system will be [X,Y,Z, dt(GPS), bias(GLONASS_to_GPS)]. The more satellites the more redundance is possible - and you can control your estimation process weather by RAIM, Kalman filter, least squares (sequentially) or any other processor technique.
Further details for combining different systems
Be sure to get the correct and no corrupt data from the antenna and the receiver ensemble. Since you ask if a GPS Module would be able to use GLONASS - I suggest it isn't possible, since GLONASS uses a complete different access method than GPS. GLONASS Satellites are identified by their specific frequency (Frequency Division Multiple Access - FDM) and GPS Satellites are identified by different codes (Code Division Multiple Access - CDMA). The current new GLONASS-K Satellite generation will support CDMA Signals, but the current GLONASS constellation works completely with FDMA (current GLONASS constellation)
Additionally the antenna inside the mobile device must be able to see the GLONASS-Signals to give them to the baseband processor where the GNSS signals will be aquired. Both GPS and GLONASS use different bandwiths (see signal structure in Petrowski 2014, page 39) - so you better keep in mind to use a GNSS-capable module for processing not only GPS but GLONASS, COMPASS, Galileo (and any other derivative) Data. With a GNSS receiver unit you will be able to do this, but I'm not sure if you will be able to do this with a pure GPS module.
For further reading (below others):
- Petrowski, Ivan G. (2014): GPS, GLONASS, Galileo and Beidou for Mobile Devices, iP-Solutions, Cambridge University Press, Tokyo
- Hofmann-Wellenhof, Bernd, Lichtenegger, Herbert, Wasle, Elmar (2008): GNSS - Global Navigation Satellite Systems, Springer, Berlin
Best Answer
People have reported obtaining ~meter range precision combining data from a GPS unit - again as Dzarda suggested, make sure you use a GPS antenna sensitive enough to pick the signal obstructed by the forest - and an IMU (Inertial Measurement Unit), measuring 3D acceleration and 3axes rotation. I would personally recommend the 10DOF IMU from Pololu, which I have been using with Arduino and works nicely.
You will then have to implement the data fusion - kalman filtering using the data from the GPS and the IMU. You should be able to find a lot of documentation/tutorial/codes sample on the Arduino forum, on how to do that.