Opinion
BARROW, J.This appeal is from a speeding conviction resulting from radar surveillance. The appellant contends that (1) he was entitled to introduce the testimony of an expert witness to show that the radar did not accurately record his automobile’s speed and (2) calibration of the radar with tuning forks was insufficient to permit the. introduction of the radar measurement into evidence. We conclude that the testimony of the expert -witness was admissible and that the calibration of the radar by tuning forks alone was insufficient without proper corroboration. For these reasons, we reverse.
At the time of the alleged offense, the police officer was operating a stationary radar unit located in a church parking lot facing *165oncoming traffic. The officer testified that her radar unit recorded that the appellant’s automobile was traveling fifty-three miles per hour. The speed limit was forty miles per hour. The trial court convicted the appellant of speeding and imposed a $50 fine.
During the course of the trial, the appellant testified and attempted to offer the testimony of an expert witness. The appellant testified that he was driving a Pontiac Fierro, a small two-door sports car, made of “indura-flex material . . . essentially plastic.” Fie said that he saw the arresting officer parked in the church parking lot talking with another police officer in another car parked parallel to hers and that the arresting officer looked up when he passed. He said that the arresting officer’s automobile was parked at an angle to the highway of approximately twenty-five to thirty degrees. The traffic was light, and, although there was no traffic in his “immediate vicinity,” there were several cars in back of him, including a large tractor-trailer truck, that were closing on him.
The appellant also proffered testimony of an expert witness. After the witness described his credentials and appellant’s counsel offered him as an expert regarding the training and operation of speed measuring devices, his testimony was proffered outside of the presence of the jury. The purpose of the testimony was to show that the radar did not record the appellant’s vehicle but, instead, recorded that of another vehicle, possibly the tractor-trailer truck behind him. The witness testified that, because of its size and material, the appellant’s vehicle was extremely difficult to pick up on radar, that the thirty degree angle of the police officer’s car to the highway would have caused an inaccuracy in the speed recorded by the radar, that if the radar had skipped over the appellant’s vehicle it would have probably picked up the more reflective vehicle such as the tractor-trailer truck and that the likelihood of the radar picking up the appellant’s vehicle was further reduced by the fact that the police vehicle was parked on a lot approximately eighteen inches to two feet above the road. Finally, he would also have testified that the audio doppler from the radar should not have remained constant as the police officer testified that it did, but should have dropped off as the appellant’s vehicle got closer, unless the radar was reading a vehicle further away, such as the tractor-trailer truck.
*166The result of the use of radar to check the speed of a motor vehicle is prima facie evidence of the speed of that motor vehicle. Code § 46.2-882.1 This follows from the “natural and rational evidentiary relation” between the results of a radar check of speeding and the actual speed of the motor vehicle. Dooley v. Commonwealth, 198 Va. 32, 35, 92 S.E.2d 348, 350 (1956), appeal dismissed, 354 U.S. 915 (1957). Expert testimony is not admissible to contest the reliability of the use of radar for this purpose. Thomas v. City of Norfolk, 207 Va. 12, 14, 147 S.E.2d 727, 729 (1966). However, evidence may be introduced to show that a particular radar reading may have improperly recorded the speed of a vehicle which it had been used to measure. Dooley, 198 Va. at 35, 92 S.E.2d at 350. Such evidence is admissible to rebut the prima facie presumption created by the statute. Id.
In this case, the expert’s testimony was not offered to contest the legislative decision that radar readings provided prima facie evidence of a motor vehicle’s speed; it was introduced, instead, to rebut the prima facie case by showing that the particular radar reading in this instance was not a reliable indication of the speed of the appellant’s automobile. The reflective qualities of the appellant’s motor vehicle, the presence of the tractor-trailer truck behind the appellant’s vehicle, the angle of the radar unit with respect to the highway, and its height above the highway are not factors challenging the radar unit’s ability to measure motor vehicular speed. They are factors, however, that challenge the accuracy of the radar reading of the appellant’s speed on this occasion. For this reason, we hold that the trial court erred in refusing to admit the expert’s testimony into evidence.2
The appellant also contests the sufficiency of the evidence of the radar’s accuracy with the use of tuning forks. Since the proceeding must be remanded for a new trial, we address that issue.
The only evidence that the radar had been tested for accuracy was the officer’s testimony that she had tested it with tuning forks, which in turn had been certified to be accurate by “Southeastern *167Communications.” The officer testified that she used two tuning forks, one for thirty-five miles per hour and one for sixty-five miles per hour and, using standard procedure, the radar readings indicated that the unit was functioning accurately. In addition, the court admitted into evidence, over the appellant’s objection, certificates signed by a person on behalf of Southeastern Communications certifying that ten months earlier the radar and the tuning forks had been tested and were found to be operating properly.3
The General Assembly statutorily has exempted from the bar of the hearsay rule certain certificates used as evidence of a radar device’s calibration; however, the exemption extends only to certificates involving calibration by use of a speedometer, not tuning forks. A certificate is admissible over a hearsay objection when signed by an officer who has calibrated a radar device against the speedometer and when accompanied by a further certificate reflecting the accuracy of the speedometer used to calibrate the radar device. Code § Í6.2-882.4 The certificates must conform to the requirements of the statute, including one which limits their validity to six months. Id. There is no statutory authority overcoming the hearsay rule with regard to certificates of the accuracy of tuning forks. The certificates in this case, therefore, were not exempt from the hearsay rule and should not have been admitted into evidence.
Without the certificates, the evidence of the accuracy of the radar device was limited to the testimony of the officer that the radar device had responded appropriately to two tuning forks, the accuracy of which was not established. Thus, there was no evidence upon which to find that the radar device appropriately responded to the tuning forks. Biesser v. Town of Holland, 208 Va. 167, 169, 156 S.E.2d 792, 794 (1967); see also Crosby v. Commonwealth, 204 Va. 266, 268, 130 S.E.2d 467, 468 (1963); Royals v. Commonwealth, 198 Va. 883, 884, 96 S.E.2d 816, 817 (1957). The evidence of the accuracy of the radar device used in *168this case was, therefore, insufficient.
In summary, we hold that the appellant was entitled to present expert testimony tending to impeach the accuracy of the radar reading upon which the Commonwealth’s case was based, so long as it did not challenge the use of radar as a device for measuring the speed of a moving vehicle. In addition, we hold that a radar unit’s accuracy is not established simply by proof that the unit was tested by tuning forks without proper evidence of the accuracy of the forks.
The judgment of conviction is reversed and this proceeding is remanded for a new trial.
Reversed and remanded.
Willis, J., concurred.
Formerly Code § 46.1-198, repealed by 1989 Acts c. 727 effective October 1, 1989.
Having reached this conclusion, we need not address the appellant’s contention that Code § 46.1-198, if it prohibits the expert’s testimony, is unconstitutional because it creates a bar to the defendant’s introduction of relevant evidence on his own behalf. See Massey v. Commonwealth, 230 Va. 436, 442, 337 S.E.2d 754, 757-58 (1985).
We are not able to correlate the radar certificate of accuracy with the tuning fork calibrations. The radar certificated describe frequencies measured with the abbreviation “GHz,” while the tuning fork certificates refer to frequencies measured with the abbreviation “MHz.” Because of the conclusion we reach, we need not address resolution of the significance, if any, of these different abbreviations.
In 1990, the General Assembly amended Code § 46.2-882 to make admissible the certificate of accuracy of the tuning forks, subject to a six month limitation.