General Laws c. 265, § 47, inserted by St. 2006, c. 308, § 8 (§ 47), requires any person who is “placed on probation” after conviction of a designated sex offense to wear a global positioning system (GPS) tracking device for the dura*560tion of his or her probation.1 In March, 2008, a judge in the Superior Court ruled that this statute applied to the defendant, and entered an order modifying the terms of the defendant’s probationary sentence to require that he wear a GPS device during his probation. The defendant appealed, and we granted his application for direct appellate review. We conclude that § 47 by its terms applies to the defendant, because he was “placed on” postconviction probation following the statute’s effective date of December 20, 2006. We further conclude, however, that § 47 is punitive in effect and, under the ex post facto provisions of the United States and Massachusetts Constitutions, may not be applied to persons who are placed on probation for qualifying sex offenses committed before the statute’s effective date.2 Because the defendant committed and was convicted of his qualifying sex offense before § 47 was enacted, the statute may not be applied to him. We therefore vacate the order of the Superior Court judge and remand for further proceedings.3
Facts and background. We recite the essential facts reflected *561in the record, which are not disputed. On December 3, 1997, the defendant pleaded guilty in the Superior Court to a charge of indecent assault and battery on a child under fourteen, G. L. c. 265, § 13B, and was sentenced to twenty-five years’ probation. The defendant also pleaded guilty to a charge of rape of a child, G. L. c. 265, § 23, for which he was sentenced to a term of from seven to ten years in State prison.4 In May of 2006, the defendant was released on parole, but because he was also serving his probationary sentence, the probation department assumed responsibility for his supervision beginning in July of 2006. On October 19, 2006, a probation surrender notice issued to the defendant, alleging that he had violated several conditions of his probation that required him to attend counselling and treatment programs. On December 20, 2006, G. L. c. 265, § 47, took effect. See St. 2006, c. 303, § 8. On January 5, 2007, a Superior Court judge (first Superior Court judge) held a probation revocation hearing, and found the defendant to be in violation of his probation. That probation was revoked, and the judge sentenced the defendant to two and one-half years in a house of correction, with one year to serve and the balance suspended for six years, with the defendant to be on probation during the suspended portion of the sentence. Although § 47 was then in effect, the judge did not order the defendant to wear a GPS tracking device as a condition of this probation, and there was no mention of § 47 at the revocation hearing.
Subsequently, the probation department requested that the conditions of the defendant’s probation ordered on January 5, 2007, be amended to include a requirement that the defendant wear a GPS device, pursuant to § 47. A hearing was held on February 22, 2008, before a second Superior Court judge. The judge concluded that § 47 applies to sex offenders who commence a term of probation after the statute’s effective date. He further concluded that because, in his view, the “commencement of probation is the triggering event,” the statute was not impermissibly retroactive as applied to the defendant, whose current probationary term did not begin until after § 47 became effective. Accordingly, the second judge ordered that the defendant be required to *562be monitored by the probation department with a GPS tracking device during his probationary term “in accordance with” § 47.
Discussion. The defendant’s appeal from the judge’s order focuses entirely on § 47. He claims that the statute does not apply to him because he was “placed on probation” when he was originally sentenced to probation in 1997, long before the statute’s effective date. His primary argument, however, is that, in any event, § 47 cannot be applied to him, because in the circumstances of his case, § 47 operates as an unconstitutional ex post facto law.5
1. Does § 47 apply to the defendant? Section 47 states in relevant part:
“Any person who is placed on probation for any offense listed within the definition of ‘sex offense’, a ‘sex offense involving a child’ or a ‘sexually violent offense’, as defined in [G. L. c. 6, § 178C], shall, as a requirement of any term of probation, wear a global positioning system device, or any comparable device, administered by the commissioner of probation, at all times for the length of his probation for any such offense. The commissioner of probation, in addition to any other conditions, shall establish defined geographic exclusion zones including, but not limited to, the areas in and around the victim’s residence, place of employment and school and other areas defined to minimize the probationer’s contact with children, if applicable. If the probationer enters an excluded zone, as defined by the terms of his probation, the probationer’s location data shall be immediately transmitted to the police department in the municipality wherein the violation occurred and the commissioner of probation .... If the commissioner or the probationer’s probation officer has probable cause to believe that the probationer has violated this term of his probation, the commissioner or the probationer’s probation officer shall arrest the probationer pursuant to [G. L. c. 279, § 3]. Otherwise, the commis*563sioner shall cause a notice of surrender to be issued to such probationer.” (Emphasis added.)
At issue is the meaning of the phrase, “is placed on probation.”
In seeking to interpret a statute, the starting point is its language. Commonwealth v. Welch, 444 Mass. 80, 85 (2005). Because the Legislature used the present tense verb “is placed” before the word “probation,” we conclude that § 47 by its strict terms applies to sex offenders who are convicted of designated or qualifying sex offenses and who are sentenced to probation after the effective date of the statute, regardless of whether the crimes at issue were committed before or after the statute’s effective date.6 The statute applies to the defendant in this case, because he was “placed on probation” on January 5, 2007 — the date the first Superior Court judge found him in violation of his previous probationary sentence, revoked that probation, and imposed the new sentence with a new probationary term of six years.7
2. Is § 47 an ex post facto law! Because we conclude that *564§ 47 applies to the defendant, we must consider the defendant’s claim that as so applied, the statute is an unconstitutional ex post facto law.8 An ex post facto law is, in this context, one that “changes the punishment, and inflicts a greater punishment, than the law annexed to the crime, when committed.” Commonwealth v. Bargeron, 402 Mass. 589, 590 (1988), quoting Calder v. Bull, 3 U.S. (3 Dali.) 386, 390 (1798).9 “To prevail on this sort of ex post facto claim, [the defendant] must show both that the law he challenges operates retroactively (that it applies to conduct completed before its enactment) and that it raises the penalty from whatever the law provided when he acted.” Johnson v. United States, 529 U.S. 694, 699 (2000). See Miller v. Florida, 482 U.S. 423, 430 (1987), quoting Weaver v. Graham, 450 U.S. 24, 31 (1981) (“A law is retrospective if it ‘changes the legal consequences of acts completed before its effective date’ ”).
The defendant received his most recent probationary sentence — and the GPS monitoring requirement imposed under § 47 as a term of that probation — as a consequence of the violation of his prior probation. Penalties for violation of the terms of supervised release, including the penalty of additional supervised release, are attributed to the original conviction rather than to the violation. Johnson v. United States, supra at 700-701. Cf. Krochta v. Commonwealth, 429 Mass. 711, 714 (1999) (“[probation] revocation does not punish the defendant for any crime *565charged subsequent to the imposition of probation”). Thus, because the probationary sentence the defendant received in 2007 relates back to a criminal offense he committed in or before 1997 (when he was originally convicted), § 47 has a retrospective application to him. It is necessary, therefore, to consider whether § 47 constitutes punishment or, what is to the same effect, is penal. See Opinion of the Justices, 423 Mass. 1201, 1225 (1996).
This consideration involves a two-part inquiry. First, we must try to discern whether the Legislature explicitly or implicitly intended to denominate the statute a civil remedy or criminal penalty. See Smith v. Doe, 538 U.S. 84, 93 (2003), quoting Hudson v. United States, 522 U.S. 93, 99 (1997) (“The courts ‘must first ask whether the legislature, in establishing the penalizing mechanism, indicated either expressly or impliedly a preference for one label or the other’ ”). See also Commonwealth v. Bruno, 432 Mass. 489, 500 (2000) (Bruno) (“Whether a statute was intended to be criminal or civil depends on the Legislature’s intent, which is a matter of statutory construction”). If the Legislature intended to denominate the statute as criminal, that ends the inquiry. If, however, the Legislature evinced an intent that the statute be civil, the second part of the inquiry comes into play: is the statutory scheme “ ‘so punitive either in purpose or effect as to negate [the State’s] intention’ to deem it ‘civil.’ ” Smith v. Doe, supra at 92, quoting Kansas v. Hendricks, 521 U.S. 346, 361 (1997). In such circumstances — i.e., where there is an evident legislative intent to create a civil regime — the party challenging the statute carries a very heavy burden to show that the statute is punitive in one of these ways. Smith v. Doe, supra.
For the first part of this inquiry, which focuses on legislative intent, examination of the statute’s text and structure is key. Id. Courts have identified certain factors of text and structure that may signal an intent to create a civil or regulatory scheme. See, e.g., id. at 93-96 (looking to explicit statement of civil intent contained in statute; statute’s lack of safeguards associated with criminal process; placement of statute within civil or criminal code); Kansas v. Hendricks, 521 U.S. at 361 (looking to statute’s placement in probate, rather than criminal, code; statute’s denomination as “civil commitment procedure”); Bruno, 432 Mass, at 500 (looking to denomination of “civil commitment”; placement in public welfare chapters; statute’s title, “Care, Treatment *566and Rehabilitation of Sexually Dangerous Persons”; stated purposes of “protecting] forthwith the vulnerable members of our communities from sexual offenders” and “care, custody, treatment and rehabilitation” of offenders).
Section 47 contains not one of these recognized indicators that the Legislature intended a civil categorization: it has no statement of civil intent in its preamble, title, or text; it is imposed only in the criminal context, when an offender is sentenced to probation; and it is placed in a criminal code (G. L. c. 265, “Crimes Against the Person”). Moreover, the structure of the statute suggests a penal or punitive intent: the GPS requirement must be uniformly imposed on every defendant sentenced to probation for certain sex offenses, and for precisely the period of his or her probationary sentence, without regard to present dangerousness, and even if there are no exclusion zones that can reasonably be applied to the defendant.10,11 It bears emphasis as well that probation itself serves as a disposition of and punishment for a crime; it is not a civil program or sanction. See Commonwealth v. Power, 420 Mass. 410, 414-415 (1995), cert, denied, 516 U.S. 1042 (1996); Commonwealth v. Durling, 407 Mass. 108, 111 (1990). Cf. MacLean v. State Bd. of Retirement, 432 Mass. 339, 343-344, 348 (2000).
Text and structure may thus suggest in some respects a criminal intent, but § 47 is not without indications of civil intent as well. The statute imposes a condition of probation, and while *567probation is a type of criminal sentence with goals that include punishment, deterrence, and retribution, ‘.‘[t]he primary goals of a probationary sentence are rehabilitation of the probationer and protection of the public.” Commonwealth v. Power, 420 Mass, at 414. Rehabilitation and public protection may suggest more of a civil than a criminal orientation.12 In any event, the text of § 47 being silent as to the statute’s intended categorization as criminal or civil, we conclude that consideration of the Legislature’s intent by itself does not offer a clear guide in this case. Cf. Smith v. Doe, 538 U.S. at 110 (Souter, J., concurring in judgment) (concluding that indications whether Alaska’s sex offender registration law was civil or punitive were “in rough equipoise”); id. at 114-115, 116-118 (Ginsburg, J., dissenting) (agreeing with Justice Souter that whether Alaska’s registration law was intended to be regulatory or penal was “unclear”). Accordingly, we consider the punitive effects of § 47 on sex offenders serving probationary sentences. Because this is not a case where we are asked to “reject the legislature’s manifest intent,” Kansas v. Hendricks, 521 U.S. 346, 361 (1997), or “ ‘negate [the State’s] intention’ to deem [the statutory scheme] ‘civil,’ ” Smith v. Doe, supra at 92, quoting Kansas v. Hendricks, supra, we evaluate the punitive effects of the GPS requirement without placing a heightened burden on the defendant. See Smith v. Doe, supra at 107 (Souter, J., concurring in judgment); id. at 115 (Ginsburg, J., dissenting), citing Kennedy v. Mendoza-Martinez, 372 U.S. 144, 168-169 (1963). Cf. Doe v. State, 189 P.3d 999, 1008 n.62 (Alaska 2008) (“imposing a heightened presumption requiring ‘clearest proof’ of punitive effect could threaten rights protected by the Alaska *568Constitution and might be inconsistent with the responsibilities of this court”).
In Smith v. Doe, supra, the United States Supreme Court, in analyzing the Alaska sex offender registration law, referred to the seven factors listed in Kennedy v. Mendoza-Martinez, 372 U.S. at 168-169 (Mendoza-Martinez factors), as “useful guideposts” in determining whether a sanction is punitive in effect. Smith v. Doe, 538 U.S. at 97, quoting Hudson v. United States, 522 U.S. 93, 99 (1997). See Powers v. Commonwealth, 426 Mass. 534, 539 & n.9 (1998) (acknowledging, although not fully applying, Mendoza-Martinez factors). Cf. Bruno, 432 Mass, at 502 n.12. The seven factors or guideposts are:
(1) “whether the behavior to which [the sanction] applies is already a crime”;
(2) “whether it comes into play only on a finding of sci-enter”;
(3) “[w]hether the sanction involves an affirmative disability or restraint”;
(4) “whether its operation will promote the traditional aims of punishment — retribution and deterrence”;
(5) “whether it has historically been regarded as a punishment”;
(6) “whether an alternative purpose to which it may rationally be connected is assignable for it”; and
(7) “whether it appears excessive in relation to the alternative purpose assigned.”
Kennedy v. Mendoza-Martinez, 372 U.S. at 168-169.13,14
The first four of these factors by themselves establish that the *569GPS requirement has a pronounced punitive effect. The first and second ask whether the sanction “comes into play only on a finding of scienter” and “whether the behavior to which it applies is already a crime” — questions that together consider whether the sanction is administered as a criminal penalty, that is, in response to criminal conduct. In imposing GPS monitoring as a mandatory condition of a probationary sentence for convicted sex offenders, § 47 more than meets this standard. The GPS requirement applies only to those persons who commit and are convicted of certain crimes; it applies to every person who is convicted of those crimes and receives a probationary term as part of the criminal proceeding’s disposition; and it applies for the precise duration of the probationary sentence imposed.15 A sanction bearing these qualities, if it also imposes a significant limitation on liberty, carries a strong presumption of punishment.16 See Smith v. Doe, 538 U.S. at 112 (Stevens, J., dissenting); id. at 115-116 (Ginsburg, 1, dissenting) (sanctions were punitive where requirements were “comparable to conditions of supervised release or parole,” and triggered by past crime alone rather than current dangerousness).
Limitation or burden on liberty is the subject of the third *570Mendoza-Martinez factor, which asks “[w]hether the sanction involves an affirmative disability or restraint.” The GPS device burdens liberty in two ways: by its permanent, physical attachment to the offender, and by its continuous surveillance of the offender’s activities. As to the permanent, physical attachment to the offender, our observations about the Commonwealth’s statutory sex offender registration system is instructive. We have said that the registration requirement “presents an ‘importantly distinct kind of constitutional danger,’ because it ‘forces an action on the person required to register. It is a continuing, intrusive, and humiliating regulation of the person himself.’ ” Doe, Sex Offender Registry Bd. No. 8725 v. Sex Offender Registry Bd., 450 Mass. 780, 792 (2008), quoting Doe v. Attorney Gen., 426 Mass. 136, 149 (1997) (Fried, J., concurring). As “continuing, intrusive, and humiliating” as a yearly registration requirement might be, a requirement permanently to attach a GPS device seems dramatically more intrusive and burdensome. There is no context other than punishment in which the State physically attaches an item to a person, without consent and also without consideration of individual circumstances, that must remain attached for a period of years17 and may not be tampered with or removed on penalty of imprisonment. Such an imposition is a serious, affirmative restraint.18
The intended function of the GPS device, continuous reporting of the offender’s location to the probation department, also represents an affirmative burden on liberty. While GPS monitor*571ing does not rise to the same level of intrusive regulation that having a personal guard constantly and physically present would impose, it is certainly far greater than that associated with traditional monitoring.19 And the impact of such intrusion is of course heightened by the physical attachment of the GPS bracelet, which serves as a continual reminder of the State’s oversight.
The GPS requirement thus places significant restraints on offenders. While these liberty burdens, as is true of probation generally, may serve a goal of rehabilitation, the GPS requirement and its burdens — again like probation generally — also satisfy the fourth Mendoza-Martinez factor, by “promoting] the traditional aims of punishment — retribution and deterrence.” Kennedy v. Mendoza-Martinez, 372 U.S. at 168. See Commonwealth v. Power, 420 Mass, at 415.
We briefly discuss the remaining three Mendoza-Martinez factors. The fifth factor asks “whether [the sanction] has historically been regarded as a punishment.” While there is no historical version of the GPS device (it is not the same as a ball and chain, for example, any more than it is the same as a weekly reporting requirement to the probation department), there are many historical cases where those convicted of a crime were required to wear some item for the length of their sentence, or suffer some long-term modification to their bodies. Such requirements are consistently viewed as punitive.
*572The final two Mendoza-Martinez factors are respectively “whether an alternative purpose to which [the sanction] may rationally be connected is assignable for it” and “whether [the sanction] appears excessive in relation to the alternative purpose assigned.” The GPS requirement, with its instruction to designate exclusionary zones that are off limits to the sex offender, supports the rational purpose of keeping offenders away from their past victims and, more generally, the purpose of limiting opportunities for reoffense. The sanction appears excessive, however, to the extent that it applies without exception to convicted sex offenders sentenced to a probationary term, regardless of any individualized determination of their dangerousness or risk of reoffense. See Smith v. Doe, 538 U.S. at 116-117 (Ginsburg, J., dissenting) (sanction excessive in relation to nonpunitive purpose, where keyed to offense rather than offender’s particularized risk of reoffense).
Weighing these factors together, we conclude that, as a result of the substantial burden on liberty § 47 imposes as part of the sentence for certain crimes, the statute is punitive in effect. And because § 47 operates retroactively with respect to the defendant, its application to him is impermissible under the ex post facto provisions of the United States and Massachusetts Constitutions.
The fact that sentencing judges prior to the enactment of § 47 had the discretionary power to impose GPS monitoring as a condition of probation does not affect our analysis. It was settled long before § 47 was enacted that “[j]udges are permitted ‘great latitude’ in imposing conditions of probation . . . .” Commonwealth v. Lapointe, 435 Mass. 455, 459 (2001), quoting Commonwealth v. Pike, 428 Mass. 393, 402 (1998). A judge therefore does not implicate ex post facto provisions by exercising that discretionary power in an individual case. Cf. Buckley v. Quincy Div. of the Dist. Court Dep’t, 395 Mass. 815, 818 (1985), quoting Commonwealth v. McGovern, 183 Mass. 238, 240 (1903) (“terms and conditions [of probation] may be subject to modification from time to time as a proper regard for the welfare, not only of the defendant but of the community, may require”). It is, however, quite a different matter for the Legislature to remove discretion from the judge by imposing a greater *573minimum punishment than the crime previously required; such a law may only be applied prospectively. See Lindsey v. Washington, 301 U.S. 397, 400-402 (1937) (ex post facto clause barred application of amended law that changed discretionary maximum sentence of fifteen years to mandatory fifteen-year sentence to defendant who committed crime before effective date of law’s modification); Commonwealth v. Davis, 380 Mass. 1, 15 (1980) (“legislation which would have the effect of changing a discretionary sentence to a mandatory one could not be validly enforced with respect to crimes antedating the law”). See also Miller v. Florida, 482 U.S. 423, 432-433 (1987) (application of new presumptive sentencing guidelines violated ex post facto clause, even though defendant could have received same sentence under old guidelines).
Conclusion. The order of the Superior Court requiring the defendant to be monitored by the probation department with a GPS tracking device during his probationary term is vacated, and the case is remanded to that court for further proceedings consistent with this opinion.
So ordered.
“A GPS [global positioning system] device, as implemented by the Commissioner of Probation . . . , consists of two pieces of electronic equipment: an ankle bracelet, which is permanently attached to the probationer, and a GPS-enabled cellular telephone, which communicates with the ankle bracelet and transmits the probationer’s current location to the probation department.” Commonwealth v. Raposo, 453 Mass. 739, 740 (2009).
The defendant does not challenge prospective application of G. L. c. 265, § 47, and so we do not consider the constitutionality of the statute as prospectively applied.
The judge’s order may be moot. The Commonwealth reports that, since this case was argued, the defendant was found to be in violation of his probation, based in part on failure to comply with GPS monitoring requirements and in part on failure to undergo a required mental health evaluation, and was sentenced to serve the balance of his sentence without further probation. The Commonwealth nevertheless requests that we address the issues raised by the defendant’s appeal, and the defendant has not opposed that request. In light of the fact that the defendant’s probation was revoked for reasons that related in part to the GPS requirement, the case may still present a live issue. In any event, the case is fully briefed and argued, and the issues raised are certain to arise again. Because of the significant public interest in deciding those issues, we exercise our discretion to reach the merits of the defendant’s appeal regardless of mootness. See Newspapers of New England, Inc. v. Clerk-Magistrate of the Ware Div. of the Dist. Court Dep’t, 403 Mass. 628, 629 n.4 (1988), cert, denied, 490 U.S. 1066 (1989); Wellesley College v. Attorney Gen., 313 Mass. 722, 731 (1943). See generally Commonwealth v. McCulloch, 450 Mass. 483, 486 (2008), and cases cited.
The rape charge is not at issue in this case.
The Commonwealth does not disagree that the judge’s order that GPS monitoring be added as a term of probation was premised on the judge’s conclusion that G. L. c. 265, § 47, mandated this result, and that therefore the issues raised in this case are whether § 47 applies and can be applied to the defendant.
We therefore reject both the defendant’s and the Commonwealth’s reading of § 47. As has been stated, the defendant argues that for purposes of § 47, he was “placed on probation” in 1997, when he received the twenty-five year probationary sentence, and therefore long before § 47 was in effect. In light of the facts that (1) the probationary term the defendant was serving pursuant to the 1997 sentence had been revoked and a new probationary sentence had been imposed after § 47’s effective date; and (2) the challenged order imposing the GPS monitoring requirement pursuant to § 47 applied only to the new probationary sentence, the defendant’s argument fails. The Commonwealth’s argument is that § 47 governs sex offenders serving sentences of probation on and after the effective date of § 47, and it does not matter whether they were sentenced before or after that date. This assertion finds no support in the language the Legislature chose to use in § 47. If the Legislature had intended the statute to apply to all qualifying sex offenders currently on probation, presumably it would have written “any person who has been placed on probation,” or else omitted the words “is placed” altogether.
The second Superior Court judge pointed out, correctly, that our cases have sometimes used the phrase “placed on probation” to refer to the time that the defendant was sentenced to a term of probation, see, e.g., Commonwealth v. Bruzzese, 437 Mass. 606, 610 (2002); and, at other times, have used the same phrase to refer to the time the defendant began serving a probationary sentence. See, e.g., Commonwealth v. Delisle, 440 Mass. 137, 138 (2003). As indicated in the text supra, we adopt here the former meaning — the time that the defendant is sentenced to probation — but the choice makes no difference in this case, because the effective date of § 47 preceded both the imposition of the probationary sentence in 2007 and the later commencement of the probationary term under that sentence.
The Commonwealth argues that the defendant did not raise his ex post facto challenge to the statute below. The defendant argues that he did preserve it when his attorney argued, in essence, that it was not fair to apply the new statute to the defendant; certainly the judge considered the question of retroactive application. In the circumstances, we consider the issue raised. See Commonwealth v. Cancel, 394 Mass. 567, 573 (1985). Cf. Commonwealth v. Welch, 444 Mass. 80, 81, 93 (2005) (noting that in certain circumstances court will consider constitutional issue even where not necessary to particular outcome).
The United States Constitution prohibits ex post facto laws in art. I, § 10, cl. 1, which reads: “No state shall . . . pass any ... ex post facto law.” Article 24 of the Massachusetts Declaration of Rights provides: “Laws made to punish for actions done before the existence of such laws, and which have not been declared crimes by preceding laws, are unjust, oppressive, and inconsistent with the fundamental principles of a free government.” We have treated the meaning and scope of the ex post facto provisions in the Federal and State Constitutions as identical. See Commonwealth v. Bruno, 432 Mass. 489, 492 n.4 (2000).
An offender might, for example, have had a single adult victim who is now deceased; have been convicted of possessing child pornography in his home; or have committed two acts of lewd and lascivious behavior on a public sidewalk. See G. L. c. 6, § 178C (“[S]ex offense” defined). In no case would an exclusion zone be relevant to monitoring the offender.
Relying on Smith v. Doe, 538 U.S. 84, 104 (2003), the dissent argues that the statute’s burden on all sex offenders, for the duration of their probationary sentence and without regard to present dangerousness, has no bearing on the statute’s intended denomination as punitive or civil. Post at 576-577. The dissent’s reliance on Smith is doubly misplaced. First, Smith v. Doe deals with sex offender registration, which the United States Supreme Court explicitly distinguished from the greater restraint imposed by probation or supervised release. Smith v. Doe, supra at 101. Second, the part of Smith v. Doe relied on by the dissent discusses punitive effects, a very different analysis; by the time the Court reached the issue of punitive effects, it had already determined that Alaska’s sex offender registry law was explicitly civil in intent, based entirely on factors lacking in the present case. Id. at 93-96.
The dissent argues that, because “[t]he principal goals of probation itself are the rehabilitation of the probationer and the protection of the public” (emphasis in original), we should conclude that the Legislature affirmatively intended to denominate § 47 a civil regulation. Post at 574. While it is true that probation has rehabilitative and protective purposes (as all punishment does, to some extent), it is nevertheless a punishment. See Commonwealth v. Power, 420 Mass. 410, 414-415 (1995), cert, denied, 516 U.S. 1042 (1996). The dissent does not suggest, nor can it, that a statute imposing a mandatory period of probation for a crime could be applied retroactively. Cf. Commonwealth v. Talbot, 444 Mass. 586, 597 (2005). We do not disagree with the dissent that “[djetermining the most effective way to control problems affecting public safety is left to the Legislature.” Post at 574. However, where the Legislature chooses to control a problem through punitive means, its solution must be applied prospectively.
We place the factors in the order we will discuss them, rather than in the order they appear in Kennedy v. Mendoza-Martinez, 372 U.S. 144, 168-169 (1963) (Mendoza-Martinez).
The dissent begins its analysis of punitive effects by stating the position that the final two Mendoza-Martinez factors are dispositive, “absent an express showing of an intent to punish.” Post at 577. This view, which the dissent supports with a reference to Justice Rehnquist’s opinion for the Court in Schall v. Martin, 467 U.S. 253, 269 (1984), would effectively overrule *569Mendoza-Martinez. The Supreme Court has not applied Schall v. Martin, supra, in such a fashion. See Smith v. Doe, supra at 97-105 (considering all seven Mendoza-Martinez factors in ex post facto analysis of sex offender registry law explicitly denominated civil). Moreover, it is not correct to suggest, as the dissent does, that the Supreme Court has determined that “the most important” of the Mendoza-Martinez factors is whether the statute may be rationally connected to a nonpunitive purpose (emphasis added). See post at 580. In Smith v. Doe, supra, the Court described this factor as “a [m]ost significant factor" in connection with its analysis whether the Alaska sex offender registration statute violated the ex post facto clause. Id. at 102. It went no further.
It is true that the GPS requirement will not apply to offenders whom sentencing judges choose not to sentence to a term of probation. The discretionary nature of the sanction does not remove it from the realm of punishment, however. A sentence imposing incarceration itself is often discretionary, and is certainly so in the case of the defendant’s crime of indecent assault and battery on a child. See G. L. c. 265, § 13B.
By way of analogy, a statute expanding the length of time that probation must run would clearly be punitive. Cf. Commonwealth v. Talbot, 444 Mass. 586, 597 (2005) (“Lifetime community parole constitutes ‘an enhanced penalty for sex offenders,’ ” and therefore cannot be imposed ex post facto). A statute expanding the burden imposed during the time probation is to run is punitive for similar reasons.
As the defendant’s case shows, the term of probation in sex offense cases may be quite long; he was initially sentenced to twenty-five years of probation. Cf. G. L. c. 6, § 178H (a); G. L. c. 127, § 133DV2 (violation of sex offender registration requirement by certain offenders punishable by parole supervision for life, including GPS monitoring).
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. Cf. Smith v. Doe, 538 U.S. 84, 115 (2003) (Ginsburg, J., dissenting) (affirmative restraint established by “profound humiliation and community-wide ostracism” of sex offender registry); Note, Who Are the People in Your Neighborhood? Due Process, Public Protection, and Sex Offender Notification Laws, 74N.Y.U. L. Rev. 1451,1467-1469 (1999) (describing acts of violence against registered sex offenders). Cf. also N. Hawthorne, The Scarlet Letter 58 (1871) (“Ah, but... let her cover the mark as she will, the pang of it will be always in her heart”).
Section 47 creates an additional affirmative disability or restraint by requiring the Commissioner of Probation to “establish defined geographic exclusion zones including, but not limited to, the areas in and around the victim’s residence, place of employment and school and other areas defined to minimize the probationer’s contact with children, if applicable.” The record does not specify what exclusion zones, if any, were applied to the defendant, so it is difficult to evaluate the extent of this burden. However, such exclusion zones could dramatically limit an offender’s freedom of movement. See, e.g., Towns Push Sex Offender Limits, Boston Globe, June 15, 2008 (over ninety per cent of Weymouth within 1,500 feet of school, park, or daycare center); Levenson, Sex Offender Residence Restrictions: Sensible Crime Policy or Flawed Logic?, 71 Fed. Probation 2, 4 (2007) (near-total lack in urban areas of residences not within 1,000 feet of school, park, or childcare center). Cf. Doe v. Schwarzenegger, 476 F. Supp. 2d 1178, 1181 (E.D. Cal. 2007) (applying sex offender residency law retroactively “would raise serious ex post facto concerns”). A geographic exclusion zone, in contrast to a residency prohibition, not only prevents offenders from living in areas that would allow contact with children, but even from passing through such areas while driving to another destination.