Rangefinder with sparkfun RX/TX modules

Development Boards Arduino
1

Greetings friends.

Laser rangefinders calculate the phase shift between TX and return RX, giving distance. Radars are similar using radio frequency. Correct me if I’m wrong here, I’m not an expert, but I’m thinking of using the sparkfun modules to calculate distance by sending a ping to the surface and reading the phase shift. I can modulate the TX to a frequency suitable for distance, and listen to the echo. I need about 1000 ft. I’m not sure if the sparkfun pair can do the job?

That would give me a rangefinder under $10… not bad.

2

What modules ? Have you calculated the optical power needed or the time measurement needed to get whatever resolution you desire ?

3

Radars don’t use phase shift. They use time of travel for pulsed radars and frequency shift (which actually gets you the speed of the target) for dopplers.

4

KeithB:
Radars don’t use phase shift. They use time of travel for pulsed radars and frequency shift (which actually gets you the speed of the target) for dopplers.

They measure doppler for speed. I don’t need speed, I need range, so I send a radio signal at say 5mhz, and listen to the bounce back, then measure the phase shift to get distance, just like the laser rangefinder does. Both are EM waves. Should work just the same.

PS: I don’t need “optical” power since I don’t use a laser. I do not want to use too much power since I want to stay within allowed limits for RF transmissions.

5

How much optical power is needed at the receiver to produce a signal sufficiently above the noise floor such that it’s phase re: the Tx waveform can be measured ?

6

Mee_n_Mac:
How much optical power is needed at the receiver to produce a signal sufficiently above the noise floor such that it’s phase re: the Tx waveform can be measured ?

Well the sparkfun transmitter has a 500 ft range under ideal condition, enough for me to experiment with, to see what range I get. I also want to use a directional antenna at the receiving end. That would help. I’m not an expert, and this is my first foray into RX/TX circuitry. And BTW, radars use phase shift for distance as well as speed in aircraft radars. So definitely, it is possible.

7

OK I take it you’re making a RADAR, not a LIDAR. What RF modules ? What resolution ?

Have you assigned some numbers to the basic radar range equation ?

http://en.wikipedia.org/wiki/Radar#Radar_equation

http://hackaday.com/2012/04/28/build-yo … ar-system/

8

What if your target shifts the reflected phase? A short or an open reflect different phases.

(Aircraft radars use time of flight for distance since there are many many wavelengths during the round trip. They do use phase to steer the beam, which gives the bearing, not the distance.)

9

Here is what I have in mind…

Quote:

Phase Shift Method for Distance Measurements

Definition: a method for optical distance measurements

Laser range finders are often based on the phase shift method, a technique for measuring distances in the following way. A laser beam with sinusoidally modulated optical power is sent to a target. Some reflected light (from diffuse or specular reflections) is monitored, and the phase of the power modulation is compared with that of the sent light. The phase shift obtained is 2π times the time of flight times the modulation frequency. This shows that higher modulation frequencies can result in a higher spatial resolution.

Although the phase shift is directly proportional to the time of flight, the term time-of-flight method should be reserved to case where one really measures a delay time more directly.

As for an interferometer, the phase shift method involves an ambiguity regarding the measured distance, because with increasing distance the phase will vary periodically. However, the periodicity is much larger than in an interferometer, since the modulation frequency is much lower than the optical frequency. Also, the ambiguity can easily be removed, e.g. by measuring with two different modulation frequencies.

Compared with interferometers, devices based on the phase shift technique are less accurate, but they allow unambiguous measurements over larger distances. Also, they are suitable for targets with diffuse reflection from a rough surface.

− Different Realizations

The power modulation may be obtained with an electro-optic modulator, acting on a continuous-wave laser beam. Modulation frequencies of many megahertz or even multiple gigahertz are easily obtained. A resonant type of modulator can be operated with relatively low input voltage, but only with a small range of modulation frequencies, making the removal of the mentioned ambiguity more difficult.

Some kinds of lasers, particularly laser diodes, can be modulated directly via the drive current at frequencies in the megahertz domain or even higher.

A special kind of power modulation is achieved by using a mode-locked laser. Advantages are the high modulation frequency (allowing for high accuracy) and (for passive mode locking) that no optical modulator is required.

The use of a laser beam allows to realize a laser radar, where an image is formed by scanning the laser beam direction in two dimensions. However, imaging systems can also be made with one or several current-modulated light-emitting diodes (LEDs) illuminating the whole object area. The spatial resolution is then obtained via imaging detection. There are optoelectronic chips with two-dimensional sensor arrays, being able to measure the phase shift for each pixel.

http://www.rp-photonics.com/phase_shift … ments.html

10

Mee_n_Mac:
OK I take it you’re making a RADAR, not a LIDAR. What RF modules ? What resolution ?

Have you assigned some numbers to the basic radar range equation ?

http://en.wikipedia.org/wiki/Radar#Radar_equation

http://hackaday.com/2012/04/28/build-yo … ar-system/

Tx for the link… :slight_smile:

11

KeithB:
What if your target shifts the reflected phase? A short or an open reflect different phases.

(Aircraft radars use time of flight for distance since there are many many wavelengths during the round trip. They do use phase to steer the beam, which gives the bearing, not the distance.)

Actually most targets have several dominant reflectors. As the aspect angle changes so do their relative contributions and hence the reflected phase and that's in addition to the phase changing w/path length. But the OP seems to be talking low freq and static targets. He'll still have to use either long sequences or multiple PRFs (like most PD radars)

to avoid range ambiguities.

12

Ah, A CW radar. I haven’t worked with many of those. Here is what Brookner states, which is still relevant today:

"A CW … waveform can be used for clutter rejection. It provides Doppler velocity unambiguously but has the disadvantage of not giving target range information. By modulating the carrier frequency, range information can be obtained. For example, transmitting up- and down- ramped carriers provides range and Doppler velocity information on the target. The Hawk AN/MPQ-39 and the Nato Sea-Sparrow director/illuminator are examples of CW radars. Because CW radars must receive the weak target echo signal while still transmitting, means must be provided for preventing the strong transmitted signal from saturating the weak one being received. "

13

Wave guides separates the signals… two tin cans.

But thanks for your help, I’m obviously not in your league.

14

Tin cans is basically what the Hawk had:

http://en.wikipedia.org/wiki/MIM-23_Hawk

Scroll down to the HPI radar. 8^)

Even still, you have to very carefull about letting the high power RF leak into the receiver path.

15

Thanks Keith, it’s definitely a learning experience for me. I know aircraft radars sell for about 10Gs, they’re like fish finders, with good graphics. I don’t need anything fancy, maybe just a light signal indicating the presence of another aircraft in the range.

16

Whoa, wait a minute, you aren’t really thinking of using this on an aircraft are you?

The FCC and FAA - or the equivalent in your neck of the woods - would be very unhappy about using an unlicensed transmitter in an aircraft. In any case, as was alluded to in the Radar equation above, nothing sparkfun sells would have enough power to detect anything more than about 10 feet away. Most of the units I am familiar with needed 10s to 100s of watts.

17

One other thing the OP has to ponder over … spatial resolution. That is when you say you have the range to object X, that it is X and not Y and Z as well. At a minimum that means an antenna w/directivity and that antenna size and directivity are related to frequency. The lower the freq, the larger the antenna has to be.