Making a RF based tracker

[chrisshortys]

Hi there, iv'e been asked by a friend to come up with a design for a paintball site he runs for experianced players. Therefore i thought i'd come on here and share my idea before commiting to the materials and finding out the idea is flawed etc.

The basic idea he gave me was each team is given a reciever that beeps more frequently as you get closer to the transmitter which will be deployed in advance.

To further this i have decided to attempt to use this project for my college project for my HNC in electronics.
I have come up with the idea of using a pre built off the shelf components for both the transmitter and reciever and then taking the RSSI output from the reciever and putting this into a PIC where i would sort out all the sounder frequencies etc.

I am also looking at having a sort of anti-tamper device on the transmitter so that when the transmitter is found it sends out a different signal which is read by both teams recievers and informs them that the item is found.

For the transmitters and recievers i was looking at using **broken link removed**

Thanks for any help

Chris

 

[Diver300]

I am not sure if you will be able to get that to work.

How are you going to measure how close the receivers are to the transmitter?

Signal level is fairly useless as it changes so much with aerial orientation and obstacles in the way.

That leaves you with transit time. The transmitters can turn on and off at about 10 kHz according to the data sheet, so the radio signal will have travelled 30 km during the period of one bit. That will limit your resolution to about 3 km if you are lucky.

There is also the problem of knowing the time that the signal was transmitted, and the time at the receiver. You could have a receive and transmit system, but that needs a receiver and a transmitter on both the fixed and the portable equipment. Other than that, you need a clock accurate to about 1 µs over the length of the game at both the fixed and the portable equipment.

Why don't you just use a GPS receiver for each team? You can buy small GPS engines that output the NMEA string. You read that, find the position and compare with the "target" position and use the distance to decide what sounds to output. That way all the difficult bit of measuring distance by time of radio signals is done by someone else.

You could add an RF link to announce when the target has been found, or just put a notice with the organiser's phone number at the target.

 

[RadioRon]

Chris, I think the idea is feasible and straightforward. The one negative, and I don't think it is a showstopper, is that the level coming out of RSSI will not be anywhere near a perfect indicator of exact range from the transmitter. You see, the RF has a tendency to bounce around a bit and as mentioned, the signal strength will also be strongly influenced by the orientation of each player's body. But I don't think this is a huge negative. In fact, it could add more complexity to the game and make things a little bit less predictable, which could be more interesting and perhaps add to the player's experience. The player would have to learn the personality of the tones that he hears and all of the quirks involved too. I think it could be quite cool actually.

You are wise to start with pre-made receivers and transmitters. Building these from scratch is beyond the scope of what you are trying to do. You have the right idea with the RSSI output. These are very linear with signal strength in dB and you should be able to put these into the PIC A/D converter without trouble. RSSI voltage will vary fairly quickly in tracking the signal level. But the RSSI can also be a bit noisy, that is, have higher audio frequency noise on the DC. So you should plan to include a simple low pass filter with a cutoff frequency high enough to let the level track changes due to fast movement, but low enough to get rid of as much noise as possible. Perhaps 1 KHz would be a good starting point as this will allow a slew rate on the order of about 1V per milliscond. A simple RC filter should be enough.

The antennas will be key to range and consistency around the player. You may want to consider the tradeoffs. On the one hand, at 430 MHz, the antenna should probably be at least 100mm long or so and preferably a bit longer. You can use off-the-shelf antennas, but most of them work best "in the clear", that is when not laid flat against the body. Can you consider a mounting method where the antenna is up on the shoulder or on the head of the player? That would be ideal.

 

[chrisshortys]

Chris, I think the idea is feasible and straightforward. The one negative, and I don't think it is a showstopper, is that the level coming out of RSSI will not be anywhere near a perfect indicator of exact range from the transmitter. You see, the RF has a tendency to bounce around a bit and as mentioned, the signal strength will also be strongly influenced by the orientation of each player's body. But I don't think this is a huge negative. In fact, it could add more complexity to the game and make things a little bit less predictable, which could be more interesting and perhaps add to the player's experience. The player would have to learn the personality of the tones that he hears and all of the quirks involved too. I think it could be quite cool actually.

You are wise to start with pre-made receivers and transmitters. Building these from scratch is beyond the scope of what you are trying to do. You have the right idea with the RSSI output. These are very linear with signal strength in dB and you should be able to put these into the PIC A/D converter without trouble. RSSI voltage will vary fairly quickly in tracking the signal level. But the RSSI can also be a bit noisy, that is, have higher audio frequency noise on the DC. So you should plan to include a simple low pass filter with a cutoff frequency high enough to let the level track changes due to fast movement, but low enough to get rid of as much noise as possible. Perhaps 1 KHz would be a good starting point as this will allow a slew rate on the order of about 1V per milliscond. A simple RC filter should be enough.

The antennas will be key to range and consistency around the player. You may want to consider the tradeoffs. On the one hand, at 430 MHz, the antenna should probably be at least 100mm long or so and preferably a bit longer. You can use off-the-shelf antennas, but most of them work best "in the clear", that is when not laid flat against the body. Can you consider a mounting method where the antenna is up on the shoulder or on the head of the player? That would be ideal.

I was thinking of just using simple hand held units where an antenna would be mounted at the front of the unit as they would be working in large teams.

Range wise i believe it was looking at around the 600m to possibly 1 km but im still waiting to get the full spec of the unit that they want and again i guess this will be largly down to the pre-made units.

I just picked up a PICKIT1 for £8 on ebay so im guna try and make some basic programes to incorporate into the device.
I mainly need help on decideding on which rf units to go for.
I understand the signal isnt going to be perfect but thats not such an issue as it will cause more intresting senarios than people running straight toward the objective!
Ill try the unit without the high and low pass filters to start with and proberly take a module with one built when i go to test the circuit and see how much of an advantage it gives the circuit.
Cheers Chris

 

[The Mad Professor]

Have you considered using infrasound in conjunction with an RF signal to give a range to target?

The time difference between the reception of a radio signal and that of the low frequency audio wave traveling through air behind it is far easier to measure with relatively cheap and simple electronics.

Of course propagation of the audio wave depends upon many factors such as temperature , humidity and terrain thus a woodland scenario may have a reduced detection range. An urban environment also may have some problems of echoes creating misleading results, but it all adds to the fun.

 

[Shax]

Try this :

http://www.loc8tor.com/

Not sure how well it fits your needs, but it's cheap enough to have a play!