Gants, J.
Acting on a petition for relief brought by the Commonwealth under G. L. c. 211, § 3, a single justice of this court reserved and reported the question “whether a judge has the authority to impose a global positioning system (GPS) monitoring device as an additional condition of probation in a probation [12] modification proceeding” where there was no finding of a violation of a condition of probation. We conclude that, unless a judge finds a violation of a condition of probation, a judge does not have the discretion to impose GPS monitoring as an additional condition of probation where there is no material change in the defendant’s circumstances and where GPS monitoring, paired with geographic exclusions, is so punitive as to increase significantly the severity of the original probationary terms. Therefore, the judge was correct to deny the Commonwealth’s request to modify the conditions of the defendant’s probation to add GPS monitoring and geographic exclusion zones.
Background. On September 13, 1990, the defendant pleaded guilty in the Superior Court to three indictments charging rape of a child by force and one indictment charging kidnapping. On September 28, 1990, the judge sentenced the defendant to two concurrent State prison sentences of from ten to fifteen years on two of the rape convictions and a concurrent sentence of from nine to ten years on the kidnapping conviction. On the third rape conviction, the defendant received a sentence of from thirty to forty years in State prison, suspended for ten years, to be served from and after the committed sentences, with a special condition of probation that the defendant undergo psychiatric treatment. The defendant appealed from his sentence as to the third rape conviction and, after transferring the case from the Appeals Court, we concluded that the sentence was properly imposed. See Commonwealth v. Goodwin, 414 Mass. 88 (1993).
In our review, we summarized the facts of the defendant’s case as outlined by the prosecutor before the sentencing judge:
“The defendant had taken the victim, a seven year old boy, away from a social function the victim was attending with his parents by threatening to kill the victim if he refused to go along. The defendant first took the victim to an area underneath a bridge where an act of fellatio occurred. The defendant then brought the victim to the cellar of his (the defendant’s) parents’ house where other acts of fellatio were performed, and the defendant attempted to sodomize the victim. The victim was confined in the cellar overnight. The next morning the defendant committed an additional act of fellatio, after which the defendant [13] surreptitiously removed the victim from the house in a large cardboard box and sent him home in a taxicab.”
Id. at 89.
Before the defendant’s release from State prison in December, 2003, the Commonwealth filed a petition for his civil commitment as a “sexually dangerous person,” G. L. c. 123A, and he remained in custody until he was found sexually dangerous on July 7, 2005, and was civilly committed. On June 9, 2009, after a jury found him no longer sexually dangerous under G. L. c. 123A, § 9, he was released from the treatment center and began serving the probationary term on his third rape conviction.
On June 18, 2009, the probation department sought modification of the defendant’s probation conditions. Over the defendant’s objection, the judge, who was not the original sentencing judge,1 added probation conditions that the defendant have no contact with children under sixteen years of age and stay away from the victim.2
At a hearing on July 8, 2009, before another judge, the probation department requested that the defendant’s probation be further modified to require him (1) to remain in the care of the Department of Mental Health (DMH) and comply with its treatment plan, including taking all prescribed medications and participating in sex offender treatment; and (2) to wear a GPS monitoring device and not enter certain exclusion zones that would include all parks, playgrounds, schools, and libraries. At a subsequent hearing on August 19, one day after the issuance of our decision in Commonwealth v. Cory, 454 Mass. 559, 572 (2009) (Cory), where we held that GPS monitoring is “punitive in effect” because of its “substantial burden on liberty,” the judge modified the original probationary condition that the defendant receive psychiatric treatment to include compliance with all programs deemed appropriate for him by DMH, including any recommended sex offender treatment.3 The judge, [14] however, declined the probation department’s request that the defendant wear a GPS device and be prohibited from entering the specified exclusion zones. The judge concluded for two reasons that she had “no legal authority” to add these conditions. First, she found “there is no change in circumstances . . . which would support the addition of GPS as a condition.” Second, because GPS monitoring was declared to be “punitive in effect,” id., she indicated that, even if there were such a change in circumstances, it would “amount to ex post facto punishment” to add this condition where the defendant had not violated the terms of his probation.
The Commonwealth moved for reconsideration of the judge’s refusal to add probation conditions concerning the GPS device and geographic exclusion zones. On October 9, 2009, the judge denied the motion in a written memorandum and order. The Commonwealth did not file a notice of appeal, but on November 17, 2009, it sought relief, under G. L. c. 211, § 3, from the judge’s memorandum and order. The case having been reserved and reported by the single justice, we first address the defendant’s contention that it is not properly before us, and then proceed to the merits.
Discussion. 1. Appropriateness of relief under G. L. c. 211, § 3. The defendant argues that relief under G. L. c. 211, § 3, is inappropriate because the Commonwealth could have, but did not, file a direct appeal in the Appeals Court of the judge’s denial of its motion to add GPS monitoring and exclusion zones as a condition of probation, or of the judge’s denial of its motion for reconsideration. Our discretionary power of review under G. L. c. 211, § 3, is extraordinary and may not be sought “merely as a substitute for normal appellate review.” McMenimen v. Passatempo, 452 Mass. 178, 184 (2008), quoting McGuinness v. Commonwealth, 420 Mass. 495, 497 (1995). A party seeking review under G. L. c. 211, § 3, must demonstrate both a substantial claim of violation of its substantive rights and an error that cannot be remedied under the ordinary review process. McMenimen v. Passatempo, supra at 185. Here, the Commonwealth presents a substantial claim that the judge erred in concluding that she had no legal authority to add GPS monitoring and exclusion zones as a condition of probation where there was no violation of probation. We need not decide whether this [15] alleged error could have been remedied under the ordinary review process because, “[w]here the single justice has, in [her] discretion, reserved and reported the case to the full court, we grant full appellate review of the issues reported.” Martin v. Commonwealth, 451 Mass. 113, 119 (2008).4
2. Modification of the defendant’s probation conditions. Probation is a legal disposition that conditions the release of a defendant on his compliance with conditions deemed appropriate by the sentencing judge. See Commonwealth v. Durling, 407 Mass. 108, 111-112 (1990). If a defendant violates one or more conditions of probation, a judge may revoke his probation and sentence him to a term of imprisonment for his underlying conviction, or return the defendant to probation, with new or revised conditions.5 See id. Therefore, for a defendant, any condition of probation poses the risk that its violation may result in the revocation of probation and the end of his conditional release from imprisonment.
The two principal goals of probation are rehabilitation of the defendant and protection of the public. See Commonwealth v. Pike, 428 Mass. 393, 403 (1998); Mariano v. Judge of Dist. Court of Cent. Berkshire, 243 Mass. 90, 93 (1922) (“purpose [of probation] . . . is to enable the person to get on his feet, to become law abiding and to lead a useful and upright life under the fostering influence of the probation officer”). While these goals are intertwined, because a defendant who is rehabilitated is not committing further crimes, they remain distinct, because [16] a probation condition that protects the public from the defendant may not advance the likelihood of his rehabilitation. The success of probation as a correctional tool depends on judges having the flexibility at sentencing to tailor probation conditions to the circumstances of the individual defendant and the crime that he committed. Commonwealth v. Lapointe, 435 Mass. 455, 459-460 (2001).
Under Mass. R. Crim. R 29 (a), 378 Mass. 899 (1979), the power of a trial judge to revise or revoke a criminal disposition is severely limited. Under rule 29 (a), a trial judge may revise or revoke a sentence only on her own motion or the written motion of a defendant filed within sixty days after the imposition of sentence,6 and “may not take into account conduct of the defendant that occurs subsequent to the original sentencing.” Commonwealth v. Barclay, 424 Mass. 377, 380 (1997). However, the addition of reasonable conditions to a defendant’s probation does not constitute a revision or revocation of a sentence under rule 29 (a). See Buckley v. Quincy Div. of the Dist. Court Dep’t, 395 Mass. 815, 818-819 (1985) (Buckley).
Under our common law, a judge has authority to modify or add conditions of probation “to serve ‘the ends of justice and the best interests of both the public and the defendant.’ ” Id. at 817, quoting Burns v. United States, 287 U.S. 216, 221 (1932).7 Just as judges have considerable discretion at sentencing in establishing the terms of probation, they also have the discretion to modify those conditions “as a proper regard for the welfare, not only of the defendant but of the community, may require.” Buckley, supra at 818, quoting Commonwealth v. McGovern, 183 Mass. 238, 240 (1903).
[17] Where a defendant has violated a condition of his probation, a judge’s authority to modify or add conditions of probation is nearly unlimited should the judge decide not to imprison the defendant but to return him to probation.8 See Commonwealth v. Durling, supra. In such cases, the defendant is essentially being sentenced anew on his underlying conviction, and the judge may impose any conditions of probation that could have been imposed at his original sentencing. See Cory, supra at 564 (sentence following probation revocation attributed to original conviction rather than violation of probation); Commonwealth v. Wilcox, 446 Mass. 61, 65 (2006).
Where, as here, a defendant has complied with his conditions of probation, a judge’s authority under the common law to modify or add conditions of probation is significantly more limited. Where a probation condition is stated only in general terms, a judge may modify the condition to provide more detailed guidance to the defendant and probation officer as to what is required. See Commonwealth v. Delisle, 440 Mass. 137, 145 (2003) (where defendant originally required to participate in substance abuse counselling and therapy as ordered by probation department, modification to require defendant to attend particular program “only a specific directive predicated on the original condition that he participate in an unspecified program”). Similarly, where a condition is ambiguous, the judge may modify the condition to provide the necessary clarity. See Commonwealth v. Power, 420 Mass. 410, 421 (1995), cert, denied, 516 U.S. 1042 (1996) (“defendant is entitled to know what conduct is forbidden by her probation condition”); Commonwealth v. Lally, 55 Mass. App. Ct. 601, 603 (2002) (“probationers are entitled to reasonably specific conditions that provide clear guidelines as to what and when their actions or omissions will constitute a violation of their probation”). In both these circumstances, the scope of the original probation conditions is not changed; all that is added is specificity and clarity.
[18] Where a defendant has performed so well on probation and made such rehabilitative progress that the conditions imposed on him should be relaxed, a judge may eliminate or modify a probation condition, even if this diminishes the scope of the original probation conditions. See generally 1 N.P. Cohen, Probation and Parole § 16:12, at 16-17 (1999).
A judge may add or modify a probation condition that will increase the scope of the original probation conditions only where there has been a “material change in the probationer’s circumstances since the time that the terms of probation were initially imposed,” and where the added or modified conditions are not so punitive as to significantly increase the severity of the original probation. Buckley, supra at 818-819 n.5, 820. See Commonwealth v. Morales, 70 Mass. App. Ct. 839, 844 (2007), quoting Buckley, supra at 818 n.5 (“Where a judge is asked to modify the conditions of probation, the judge retains the discretion to include additional reasonable restrictions consistent with the underlying basis for the modification, ... so long as the modification of the terms is not ‘so drastic’ that it amounts to a revision of the terms of the sentence” [citation omitted]).9’10 Because such [19] modification imposes new burdens on the defendant, even though he has not violated any condition of his probation, we require “proof of some basis for modification commensurate with the additional conditions.” 1 N.P. Cohen, Probation and Parole, supra at § 16:12, at 16-18. Where a modified or additional condition is so punitive as to increase significantly the severity of the original probation, even a material change in circumstances is not sufficient under our common law to justify what is essentially a new, harsher sentence. See Buckley, supra at 818-819 n.5; Commonwealth v. Morales, supra. Cf. ABA Standards for Criminal Justice, Sentencing § 18-7.2, at 237 (3d ed. 1994) (sentencing court may modify conditions of sanction to fit present circumstances of offender, but modification “may not increase the overall severity of an offender’s sentence”).
There are two reasons for this limitation. First, the constitutional guarantee against double jeopardy protects a defendant not only against a second prosecution for the same offense after acquittal or conviction but also against “multiple punishments for the same offense.” Aldoupolis v. Commonwealth, 386 Mass. 260, 271-272, cert, denied, 459 U.S. 864 (1982), quoting North Carolina v. Pearce, 395 U.S. 711, 717 (1969). See Commonwealth v. Bruzzese, 437 Mass. 606, 614 (2002). The double jeopardy clause “ ‘represents a constitutional policy of finality for the defendant’s benefit’ in criminal proceedings.” Aldoupolis v. Commonwealth, supra at 274, quoting United States v. Jorn, 400 U.S. 470, 479 (1971) (plurality). See United States v. Fogel, 829 F.2d 77, 88 (D.C. Cir. 1987) (“primary purpose of the clause is to protect the finality of judgments”). After a sentence is final, the time to revise and revoke under Mass. R. Crim. P. 29 (a) having expired and any direct appeal having [20] concluded, a defendant may not be sentenced again for that same conviction. Aldoupolis v. Commonwealth, supra at 274-275. Cf. United States v. Fogel, supra at 87 (“If a defendant has a legitimate expectation of finality, then an increase in that sentence is prohibited by the double jeopardy clause”); Burns v. United States, 552 F.2d 828, 831 (8th Cir. 1977) (general rule that Federal court cannot increase legal sentence once execution of sentence has begun). “In this Commonwealth, the subject of double jeopardy generally has been treated as a matter of common law rather than as a question under the Constitution of the Commonwealth.” Aldoupolis v. Commonwealth, supra at 271 n.14, quoting Commonwealth v. Therrien, 383 Mass. 529, 532 (1981). Where a new or modified probationary condition is so punitive in effect that it significantly increases the severity of the original probation, we forbid such a punitive modification under our common law.11 See Buckley, supra at 818-819 n.5; Commonwealth v. Morales, supra at 844. See also Lippman v. State, 633 So. 2d 1061, 1064 (Fla. 1994) (“double jeopardy protection against multiple punishments includes the protection against enhancements or extensions of the conditions of probation” unless probationary condition is violated).
Second, where a guilty plea is tendered in return for, and contingent on, a sentence recommendation, “the judge shall inform the defendant that he will not impose a sentence that exceeds the terms of the recommendation without first giving the defendant the right to withdraw his plea.” Mass. R. Crim. P. 12 (c) (2), as appearing in 399 Mass. 1215 (1987).12 The effect of the rule is to give defendants “pleading guilty pursuant to a [21] sentencing bargain more certainty about their fate.” Reporters’ Notes (revised, 2004) to Mass. R. Crim. P. 12, Mass. Ann. Laws Court Rules, Rules of Criminal Procedure, at 1428 (LexisNexis 2008-2009). Under this rule, where the recommendation includes conditions of probation, a judge may not add a new or modified probationary condition that is so punitive in effect that it significantly increases the severity of the recommended sentence without giving the defendant the opportunity to withdraw his plea. See Commonwealth v. Christian, 46 Mass. App. Ct. 477, 481 n.4, S.C., 429 Mass. 1022 (1999) (“Conditions of probation may be a subject of negotiation in connection with a plea bargain, i.e., prior to sentencing. A trial judge may reject a proffered plea bargain, but if the judge accepts the bargain, the judge is bound to sentence in accordance with its terms”). As a result of this rule, a judge must “pay careful attention to dispositions involving probationary terms or a suspended sentence to ensure that they conform to the legitimate sentence expectation of the defendant.” Reporters’ Notes (revised, 2004) to Mass. R. Crim. P. 12, supra at 1429. See K.B. Smith, Criminal Practice and Procedure § 23.1, at 268 (3d ed. 2007). It would be unfair and contrary to the spirit of rule 12 (c) (2) for a judge to accept a plea bargain and impose the recommended sentence, and then, after the defendant has lost the opportunity to withdraw his plea, increase the sentence by adding a new or modified probationary condition so severe as to significantly increase the recommended sentence.
Here, the judge correctly found that there had been no material change in the defendant’s circumstances after the terms of probation were initially imposed that would justify the proposed additional probation condition of GPS monitoring and exclusion zones. The defendant had spent the nineteen years since he was sentenced in 1990 either incarcerated or civilly committed, and had just been found by a jury to be no longer sexually dangerous. In the short time he was on probation, he was fully compliant with the conditions of his probation. As a probationer under “maximum supervision,” he was meeting with his probation [22] officer once every two weeks and participating in various prescribed forms of daily mental health and sex offender treatment through the DMH. The probation department failed to identify any material change in his circumstances that suggested he was more sexually dangerous at the time of the modification hearing in 2009 than the sentencing judge would have understood him to be at the time of sentencing in 1990. Even if the earlier finding that he was a sexually dangerous person would constitute such a material change in circumstances, the more recent finding that he was not sexually dangerous neutralized that material change.
The judge also correctly concluded that, even with a material change in circumstances, the law did not authorize her to add GPS monitoring and exclusion zones, where neither monitoring nor geographic exclusion had been included as a probation condition by the sentencing judge. In Cory, supra at 570, we recognized that a GPS device burdens a defendant’s liberty in two ways: “by its permanent, physical attachment to the offender, and by its continuous surveillance of the offender’s activities.” Wearing the device at all times is a “serious, affirmative restraint” that is “dramatically more intrusive and burdensome” than being required to register as a convicted sex offender, which itself is “continuing, intrusive, and humiliating.” Id., quoting Doe, Sex Offender Registry Bd. No. 8725 v. Sex Offender Registry Bd., 450 Mass. 780, 792 (2008). “To the extent that the ankle bracelet portion of the GPS device is potentially visible to the public, it may have the additional punitive effect of exposing the offender to persecution or ostracism, or at least placing the offender in fear of such consequences.” Cory, supra at 570 n.18. In addition, the GPS device constantly tracks and reports a defendant’s location to the probation department, which is a far greater intrusion on the defendant’s liberty than that associated with traditional probation monitoring. See id. at 570-571. Finally, the coupling of GPS monitoring with geographic exclusion zones “could dramatically limit an offender’s freedom of movement,” by restricting not only where he can live and go, but also where he can pass through when traveling from one place to another. See id. at 571 n.19. Having concluded in Cory, supra at 572, that GPS monitoring constitutes a “substantial burden on liberty” that is “punitive in effect,” we conclude [23] here that the additional probation condition of GPS monitoring, paired with geographic exclusions, is so punitive in effect as to increase significantly the severity of the original probationary conditions and therefore may be imposed only after a finding of a violation of a condition of probation.
Conclusion. For the reasons stated, we affirm the judge’s denial of the probation department’s request to add a probationary condition that would require the defendant to wear a GPS monitoring device and not enter geographic exclusion zones that would encompass all parks, playgrounds, schools, and libraries.
So ordered.