# Advice on measuring relative speed and position of an active transmitting and reveiving object

Using multiple transmitter/receiver stations on ground and the doppler shift to measure relative velocity and position of an airborne active receiver/transmitter in low altitude (0-2 km).

I want to measure the position and speed of an falling object with parachute (speed: 10-20 m/s). The object has about the size of a cola can.

My idea was to use a radio connection or an acoustic signal to measure the distance and using the doppler effect the velocity of the object. There would be multiple transmitter/ receiver stations on ground that would determine relative speed and position to triangulate the correct 3D velocity and position.

I have considered many possible solutions but my last idea is this: One after another the stations send a signal for a period of time. The object receives this signal and filters out anything except the expected range of frequencies that can occur when the signal is doppler shifted by the movement of the object. The signal would now be amplified and send back by the object. To avoid a feedback loop my idea was to use diodes to double the frequency of the signal that's being send and filtering this frequency on the receiver side of the object. Now this double-frequency signal is again doppler shifted by the moving object and received by the receiver of the active ground station. This signal gets filtered and mixed with the doubled frequency of the transmitted frequency. By applying a low pass filter on this mixed signal we get a lower frequency that corresponds to the doppler effect that effected the signal during travel and thus can be used to calculate the relative velocity of the falling object. Now u could also use frequency modulation on the transmitted signal to also measure the range of the falling object.

I am quite new to this topic and I had a hard time searching for related info. I guess because id don't have much experience and so don't now what exactly to search for. I would be very happy if you could give some advice or evaluate my idea or just give me some clever words to google.

• Why is measuring the speed from a device attached to the falling object not an option? Commented Aug 24, 2023 at 11:19
• The project is a prove of concept and on the other hand the speed measurement should not rely on the object landing softly. Also i dont know any sensor that is capable of measuring speed and can be installed on a tiny craft Commented Aug 24, 2023 at 11:29
• You could calculate falling speed (trivial) and relative position (less trivial) by means of on-board MEMS electronics. Optionally transmit the results wirelessly on regular basis - probably you can use the same technology and radio band as is used by commercial drones. That sounds like a project which should be perfectly possible to carry out. If you do some "proof on concept" thing, the first thing you need to do is to benchmark alternative solutions and list their advantages/disadvantages to whatever you are about to do. In order to prevent yet another "invention of the square wheel". Commented Aug 24, 2023 at 11:36
• Would you be able to use GPS and accelerometers to find the position and speed and transmit the data to a receiving station in real time? Commented Aug 24, 2023 at 11:37
• @GodJihyo Heh and here comes the next person proposing the very same thing as I just did, completely independently of each other :) Commented Aug 24, 2023 at 11:38

For simply finding the descent velocity, it is much simpler to use an inertial device on the falling object. It may well be easiest to use GPS on the falling device, with constant location transmission. This is a well-solved problem, I'd look for good telemetry modules and solutions from drone engineers.

But if you want to solve the problem as stated ...

Falling objects in the air can have an awful lot of noise, I'd recommend against anything acoustic. Your proposed solution involves bidirectional transmission, which isn't strictly necessary. Depending on how sophisticated you want your ground systems to be, you could consider GPS-like techniques:

• Object transmits the time
• Ground receivers receive object's Time and also know the current accurate time and their own locations
• Use GPS-like algebra to locate object

Alternatively,

• Transmit current time from multiple ground stations
• Receive multiple messages on falling objeect
• Calculate just like GPS

The first is much easier because:

• Falling object has transmitter only, but simpler than a receiver
• No data is lost if the object is destroyed on landing

You would do well to read the excellent HP Application Note 1272 GPS and Precision Timing Applications, which covers the whole mechanism extremely well, from the general concept of time-of-arrival ranging and the triangulation through to the difficulties of the actual GPS system.

From AN 1272