FMCW radar with refined measurement using fixed frequencies

1. A method for operating a radar altimeter, the method comprising:

generating a linear frequency ramp of a radar signal from a first frequency to a second frequency for one modulation period;

receiving reflection of the frequency ramp of the radar signal;

processing the reflection by determining an approximate distance to a target from which the reflections are received based on an amount of time between transmission of the radar signal having the frequency ramp and reception of the reflection of the radar signal having the frequency ramp, wherein the approximate distance is proportional to frequency difference between received reflections of the frequency ramp and generated linear frequency ramp, and wherein the approximate distance to a target is determined with a resolution based on a distance separating center points of neighboring fast fourier transform (FFT) bins in the radar altimeter; and

when the approximate distance is less than or equal to a maximum unambiguous distance:

transmitting a first radar signal having a first frequency for a first length of time;

receiving a reflection of the first radar signal at the first frequency;

transmitting a second radar signal having a second frequency for a second length of time;

receiving a reflection of the second radar signal at the second frequency; and

processing the reflections of the first radar signal and the second radar signal at the first and second frequency by:

determining a first phase of a reflection of the first radar signal transmitted at the first frequency, wherein the first phase is the difference in phase between phase of a sample of transmitted first radar signal at the first frequency and phase of the reflection of the first radar signal as received at the first frequency;

determining a second phase of a reflection of the radar signal transmitted at the second frequency, wherein the second phase is the difference in phase between phase of a sample of transmitted second radar signal at the second frequency and phase of the reflection of the second radar signal as received at the second frequency;

calculating a phase difference between the first phase and the second phase; and

refining the approximate distance to the target based on the phase difference between the first phase and the second phase, wherein refining the approximate distance includes calculating a refined distance based on the phase difference divided by 360 degrees and multiplied by the maximum unambiguous distance, where the maximum unambiguous distance equals the speed of light divided by two times the frequency difference between the first frequency and the second frequency.

2. The method of claim 1 , further comprising:

modulating the radar signal at the first frequency; and

modulating the radar signal at the second frequency;

wherein generating a frequency ramp starts immediately after modulating the radar signal at the first frequency and wherein modulating the radar signal at the second frequency occurs immediately after reaching the second frequency during generating a frequency ramp.

3. The method of claim 2 , comprising:

repeatedly transmitting the radar signal at the first frequency, generating a frequency ramp from the first frequency to the second frequency, and transmitting the radar signal at the second frequency.

4. The method of claim 1 ,

wherein refining the approximate distance includes refining the approximate distance if the reflection of the frequency ramp corresponds to a distance equal to or less than the distance of the center of the first FFT bin and not refining the approximate distance if the reflection of the frequency ramp corresponds to a distance greater than the center of the first FFT bin.

5. The method of claim 1 , wherein determining a first phase includes calculating an inverse tangent function on an imaginary value divided by a real value in the zero IF FFT bin of the reflected signal of the first frequency; and

wherein determining a second phase includes calculating an inverse tangent function on an imaginary value divided by a real value in the zero IF FFT bin of the reflected signal of the second frequency.

6. The method of claim 1 , wherein the first length of time is at least 0.5 ms and the second length of time is at least 0.5 ms.

7. A frequency modulated continuous wave (FMCW) radar comprising:

a transmitter configured to:

transmit a first radar signal at a first frequency for a first length of time;

generate a frequency ramp to ramp the frequency of a radar signal from the first frequency to a second frequency;

transmit a second radar signal at the second frequency for a second length of time; and

a receiver configured to:

receive reflections of the radar signals; and

process the reflections by determining an approximate distance to a target from which the reflections are received based on an amount of time between transmission of the radar signal having the frequency ramp and reception of a reflection of the radar signal having the frequency ramp, wherein the approximate distance is proportional to frequency difference between received reflections of the frequency ramp and generated linear frequency ramp, and wherein the approximate distance to a target is determined with a resolution based on a distance separating center points of neighboring of fast fourier transform (FFT) bins in the radar altimeter; and

when the approximate distance is less than or equal to a maximum unambiguous distance, processing the reflections further by:

determine a first phase of a reflection of the first radar signal transmitted at the first frequency, wherein the first phase is the difference in phase between phase of a sample of transmitted first radar signal at the first frequency and phase of reflection of the first radar signal as received at the first frequency;

determine a second phase of a reflection of the radar signal transmitted at the second frequency, wherein the second phase is the difference in phase between phase of a sample of transmitted radar signal at the second frequency and phase of the reflection of the second radar signal as received at the second frequency;

calculate a phase difference between the first phase and the second phase; and

refine the approximate distance to the target based on the phase difference between the first phase and the second phase, wherein refine the approximate distance includes calculate a refined distance based on the phase difference divided by 360 degrees and multiplied by the maximum unambiguous distance, where the maximum unambiguous distance equals the speed of light divided by two times the frequency difference between the first frequency and the second frequency.

8. The FMCW radar of claim 7 , comprising:

a single antenna coupled to the transmitter and the receiver.

9. The FMCW radar of claim 8 wherein the single antenna is integrated in a housing with the transmitter and receiver and there is less than a 2 cm signal distance between the single antenna and a circulator that couples signals between the single antenna, the transmitter, and the receiver.

10. The FMCW radar of claim 7 , wherein generate the frequency ramp starts immediately after transmit the radar signal at a first frequency and wherein transmit the radar signal at a second frequency occurs immediately after reaching the second frequency during the ramp of the frequency.

11. The FMCW radar of claim 10 , wherein the transmitter is configured to:

repeatedly transmit the radar signal at the first frequency, ramp the frequency from the first frequency to the second frequency, and transmit the radar signal at the second frequency.

12. The FMCW radar of claim 7 ,

wherein refine the approximate distance includes refine the approximate distance if the reflection of the frequency ramp corresponds to a distance equal to or less than the distance of the center of the first FFT bin and not refine the distance if the reflection of the frequency ramp corresponds to a distance greater than the center of the first FFT bin.

13. The FMCW radar of claim 7 , wherein determine a first phase includes calculate an inverse tangent function on an imaginary value divided by a real value in the zero IF FFT bin of the reflected signal of the first frequency; and

wherein determine a second phase includes calculate an inverse tangent function on an imaginary value divided by a real value in the zero IF FFT bin of the reflected signal of the second frequency.

14. The FMCW radar of claim 7 , wherein the first length of time is at least 0.5 ms and the second length of time is at least 0.5 ms.

15. A method for operating a radar altimeter, the method comprising:

transmitting a radar signal;

generating a frequency ramp of a radar signal from a first frequency to a second frequency;

receiving a reflection of the radar signal having the frequency ramp;

determining an approximate distance to a target from which the reflections are received based on an amount of time between transmission of the radar signal having the frequency ramp and reception of a reflection of the radar signal having the frequency ramp, wherein the approximate distance is proportional to frequency difference between received reflection of the frequency ramp and generated linear frequency ramp; and if the approximate distance is less than or equal to a distance corresponding to a center of a first FFT bin, then:

transmitting a first radar signal at the first frequency for a first length of time;

transmitting a second radar signal at the second frequency for a second length of time;

receiving reflections of the first and the second radar signals transmitted at the first frequency and the second frequency; and

processing the reflections by:

determining a first phase of a reflection of the first radar signal transmitted at the first frequency, wherein the first phase is the difference in phase between phase of a sample of transmitted first radar signal at the first frequency and the phase of the reflection of the first radar signal as received at the first frequency;

determining a second phase of a reflection of the second radar signal transmitted at the second frequency, wherein the second phase is the difference in phase between phase of a sample of transmitted radar signal at the second frequency and the phase of the reflection of the second radar signal as received at the second frequency;

calculating a phase difference between the first phase and the second phase; and

refining the approximate distance to the target based on the phase difference between the first phase and the second phase, wherein refining the approximate distance includes calculating a refined distance based on the phase difference divided by 360 degrees and multiplied by the maximum unambiguous distance, where the maximum unambiguous distance equals the speed of light divided by two times the frequency difference between the first frequency and the second frequency.

16. The method of claim 15 , wherein determining a first phase includes calculating an inverse tangent function on an imaginary value divided by a real value in the zero IF FFT bin of the reflected signal of the first frequency; and

wherein determining a second phase includes calculating an inverse tangent function on an imaginary value divided by a real value in the zero IF FFT bin of the reflected signal of the second frequency.

17. The method of claim 15 , wherein the first length of time is at least 0.5 ms and the second length of time is at least 0.5 ms.